1.2.1 Conductor sizing

The sizing of the conductor is based on several considerations as detailed below.

1.3.1 Parabola

Parabola is defined as the shape assumed by a cable suspended between two support points, when the cable is supporting a uniform horizontal load. For example, the cables used for supporting the deck in a suspension type bridge take the form of a parabola.

1.3.2 Catenary

Catenary is the natural shape assumed by a conductor whose weight is constant per unit of arc length, when the conductor is suspended freely between two support points. The shape assumed by OH conductors therefore is a catenary. However, the formulae related to the calculations of catenary are more involved and complex than that of a parabola. If the Sag (described later in the section) of the conductor is less than 9% of the span length, then the difference between a parabola and a catenary is observed to be less than 1%. Hence, for all practical purposes of calculation, the shape of the OH line is assumed to be that of a parabola.

1.3.3 Span

Span is the horizontal distance between the two supporting points of the overhead conductor. The Span of the section is dependent on:

• Nature of Terrain
• Adequate clearance between the conductors
• Permissible tension under maximum mechanical loads

In general, smaller spans are used in urban areas attributable to reasons of conductor clearances, right of way, pole locations and constraints in providing stays. Longer spans are mostly used in rural areas.

In practical situations all the spans of an OH line are hardly of the same length, hence the use of the Ruling span. Ruling span is defined as the assumed uniform span which would most closely resemble the variety of spans in a particular section of the line. Sag and clearances are calculated based on the Ruling span and used for spotting the structures. The condition that needs to be satisfied is that the spans in the line should not be more than twice or less than half of the Ruling span.

Ruling span is given by the equation:

Wind span is defined as the length of span that determines the transverse load on the support structure due to force of wind on the conductor. Wind span is equal to half of the sum of the adjacent spans. Refer to Figure 1.1.

Weight span is defined as the length of span between the lowest points of the catenaries on either side of a structure. Weight span determines the vertical loading on the structure due to the weight of the conductor. Refer to Figure 1.1.

1.3.4 Sag

Sag is defined as the vertical distance between the point where the overhead conductor is attached to the support poles and the lowest point in the conductor as shown in the Figure 1.2.

Factors affecting Sag are:

• Conductor load per unit length (including wind and ice loads)
• Span
• Conductor tension
• Temperature
• Levels of conductor supports

Sag is given by the formula:

S = WL²/8T

Where:

S is the Sag of the conductor at mid span (m)
W is the weight of the conductor per unit length (N/m) including wind and ice loads.
Conductor weight is normally provided by the manufacturer in terms of Kg/km. This should be converted to N/m using the formula W_N = (9.81 X W_Kg)/1000
L is the horizontal span length between supports
T is the tension of the conductor at the lowest point of conductor (N)

1.3.5 Slack

Slack is defined as the difference in the lengths between the straight line across the conductor supports and the distance along the conductor length. For a level span, the slack is given by:

K = (8 x S²) /(3 x L)

Where

K = Slack (m)
S = Mid span Sag (m)
L = Length of Span (m)

1.3.6 Conductor length

The length of the conductor is given by the equation:

L = S x (1 + (W x D)/ (3 x T))

Where

S is the horizontal length of the Span
D is the Sag of the conductor
W is the weight of the conductor per unit length (including wind and ice loads)
T is the tension at the lowest point of the conductor

1.3.7 Swing of conductor

The horizontal swing is provided by the following equation:

S_W = P X D X L² /8 X T

Where S_W is the horizontal swing at mid span in m
W is the conductor weight in N/m
P is the wind pressure under final conditions in Pa
D is the conductor diameter in m
L is the length of the span in m
T is the final tension in N

1.3.8 Conductor Tension

Conductor tension is affected by the various following factors;

3.1 Introduction

Various steps are involved in the design of an indoor MV metal enclosed switchboard. The steps include:

• Obtaining the basic inputs from various agencies
• Reviewing the specifications of various available types of equipment in the market
• Carrying out a preliminary design and presenting the same as a single line diagram
• Drawing up detailed bid specifications
• Inviting bids
• Selecting a given make/model that best suits the application after a proper technical and cost evaluation process
• Obtaining the complete data of the selected product required for interfacing
• Preparing detailed drawings based on the data

3.2 Standardisation

An important aspect that is not to be missed is that while choosing the equipment, due consideration must be given to makes, types and ratings that are already being used in the other existing units of the plant or those under advanced planning stage. The main purpose is to reduce variety, to use existing skills of operation and maintenance personnel and to conserve the inventory of spares and consumables. This consideration naturally will not be applicable if there are any complaints in the existing makes/types of equipment attributable to manufacturing quality.

In order to properly identify the equipment in question, it should be given a designation and should be referred in all documentation. The industry may already use some standard naming convention, which must also be adopted for the new equipment.

3.3 Inputs

The following are the basic inputs required for a MV switchboard with example data that are typical of industrial systems.

Site conditions

Electrical system

The Figure 3.1 shows the overall distribution diagram indicating a switchboard, the loads which it feeds and other feeder details.

Existing equipment data to be used as the basis of specifications

3.4 Design step-1: Basic equipment configuration

Based on the requirements given in earlier sections, a configuration is to be decided and represented in the form of a basic single line diagram of the switchboard MVDS-03 as shown in Figure 3.2. In this scheme the following configuration has been indicated.

Switchgear with two incomers; one from normal source and another from the emergency source with the latter normally in OFF position.

A sectionalizer to segregate the essential loads, which are on a separate section of the bus. Interlocking to ensure that the emergency section incomer can be fed only from either the incomer of that section or the sectionalizer. Normally sectionalizer will be ON.

For better protection against inadvertent paralleling due to interlock failure, bus voltage will be checked and will be used to block the closing if the bus is live.

All panels except the motors will be breaker controlled. Motors will be contactor controlled. Both circuit breakers and contactors will be vacuum type. Panels will be of draw out design. All features will be as in existing switchboards.

Additional transformer and motor feeders required for later expansion are shown in dotted lines. The loads of these feeders are considered for equipment sizing.

3.5 Design step-2: Ratings of equipment and devices

In this step, the ratings of feeders and other associated devices are to be decided by calculation using the input data.

For each outgoing/incoming feeder

• Current rating based on standard values
• Fault withstand capacity and short time rating
• CT ratio for metering and protection and CT class
• Metering requirements
• Protective devices to be used with range/characteristics where applicable
• Termination of outgoing/incoming feeders based on cable data

For the switchboard as a whole

• Busbar current rating and short time withstand capacity
• Rating of incoming feeders
• Protection of busbars
• Voltage transformer configuration
• Measuring instruments for each bus section
• Category of enclosure (IP protection)

Feeder sizing for the motor feeder shown in Figure 3.2

The current of a motor under rated load condition (full load current or FLC) can be calculated based on the formula below.

Power factor (pf) and efficiency values at rated load are to be considered in the above calculation.

Where pf and efficiency values are not readily available a pf of 0.85 and efficiency of 0.9 can be assumed, being typical values.

An overloading factor of 1.1 can be taken into account since a motor can operate indefinitely at a small overload. (This will also be useful in case the motor has to work at the lowest possible voltage limit of -10% for long periods). Thus the feeder rating can be calculated as:

Feeder rating = FLC * Overload factor

This is the minimum current rating that the feeder and switching device must be designed for. For example a 6600V motor of 320 kW will have FLC of 36 amperes approximately and will need a feeder rating of 40 amperes at a minimum.

The rating of the selected equipment must not be less than the current arrived at as above. There may be recommended derating factors by switchgear manufacturers for operations at ambient conditions other than those for which the equipment is rated. Thus an equipment rated for operation at 40 Deg C in open air may require derating when placed inside an enclosure and at an ambient temperature of 45 Deg C. The equipment rating at specified condition is given by:

Feeder rating < Standard rating * Temperature derating * Enclosure derating

The rating of the switching device (breaker, isolator, contactor etc.) must be selected on the above basis. Starting current of a cage motor can be assumed as 600% of FLC (app 220A). This value taken along with the starting time will be useful to select the motor HV fuse. Fusing characteristic of typical HV fuses are provided by different manufacturers and can be used for selection. (A motor starting calculation may be also performed to assess the voltage drop and starting time for motor DOL starting to verify that the bus voltage does not drop over 10% during starting).

Typical fuse characteristic curves are shown in Figure 3.3 and 3.4. The highest value of normal current (in this case starting current) should not initiate melting curve in Figure 3.3 within the starting time of the motor. Fuse range 2R (70 amps) is the minimum rating of fuse satisfying both conditions. The fuse will melt in a range of 15 to 35 seconds at a value of 100 times the “R’ number, in this case 200 Amps. For starting current of 220 amps, the fuse blows out after 12 seconds as per melt time curve (3.3). If the starting time is likely to be more than this value, a higher fuse rating will be necessary.

The lowest current under fault condition (in this case 5000A as given in the section on inputs) should result in fusing with the delay given by the curve in Figure 3.4. The manufacturer’s data indicates a maximum interrupting capacity of 50 kA (symmetrical) for this fuse and is therefore safe at the maximum fault level given (15 kA) in this case.

CT current must be as close as possible to the rated current and a value of 40/5 will be suitable. Protection CT can also have the same ratio and a 10P10 class is adequate as the main function is to operate overload and other protections but not short circuit faults (which are cleared by fuse). Rated CT burden must be adequate to supply the devices (metering or protection as the case may be). 15VA burden is normally specified and is adequate provided no long wiring is involved (example: for meters mounted remotely from switchgear).

Minimum metering will be provided on the panel in the form of an ammeter with a selector switch to read the current in any of the phases. A meter with compressed scale graduated 0-40-240 Amps of primary current (40 to 240 is the compressed scale range) is required.

Protection against overload (thermal image type), single phasing, earth fault and stall condition will be provided using a comprehensive motor protection relay. Numerical relay with adjustable thermal curves is a preferred choice.

Protection against loss of voltage by an undervoltage relay will be required to ensure that the circuit is disconnected if the supply voltage fails. The protection should be able to distinguish between transient sag in the supply and a power failure and should trip for the latter condition only. Instantaneous undervoltage relay in tandem with a timer relay can achieve this objective.

Feeder sizing for the transformer feeder shown in Figure 3.2

The transformer rating given is 1000 kVA. Full load primary current can be calculated by the formula:

Substituting, we get a current of 87.4 amps. A factor of 1.1 can be considered for short time overloading and the feeder rating will therefore be 96 amps.

The feeder will be switched using a circuit breaker of vacuum type. The minimum current rating of the circuit breaker after applying derating should be more than the feeder rating. The minimum rating of circuit breaker viz., 400A is amply suited for this purpose.

Circuit breaker interrupting capacity of 30 kA is selected as already applied in the system and has a sufficient reserve capacity considering the maximum fault level of 15 kA. The CT selection should be done considering the capability for withstanding the short circuit level of 15 kA for duration of 1 second (this is the period normally considered for breaker-fed feeders). Also, the protection relay setting requirements must be kept in mind.

IDMTL relay in combination with instantaneous high-set element is normally chosen. The instantaneous element should be set so as not to operate for terminal short circuits on the transformer secondary. For this rating of transformer, the impedance is normally about 5% and therefore the secondary short circuit current may be as high as 20 times the rated current (around 1.8 kA). As the instantaneous elements may not provide setting as high as 2000% it will be convenient to consider 200/5 as the CT ratio. A relay range of 400 to 1600% and CT class and 10P15 class for protection will be appropriate.

Earth fault protection will be provided using an instantaneous relay of 20 to 80% range using the CT summation connection of all three phases. The preferred form of relaying is a numerical relay with adjustable characteristic with the required IDMTL and instantaneous current relay functions.

Metering is normally limited to an ammeter with phase sector switch. An energy meter can be added for monitoring process energy consumption if called for. Usually, ammeters may have to be additionally provided near the local control point of the motor, in which case CT burden should consider the leads also into account. Transformer-mounted protection devices such as oil/winding temperature alarm/trip and Buchholz relay will be wired to the switchboard for alarm/tripping.

3.6 Switchboard

The various ratings of the switchboard are calculated now.

Design output

The data computed above is incorporated in the final single line diagram as shown in Figure 3.5.

Calculation 6:

The earthing system of a high voltage switchyard consists of a grid with conductors buried in the ground in the form of a number of 20 metre square meshes (refer to Figure 5.7). The earth resistance of the grid was measured to be 0.9 ohms. When a fault current of 10 kA flows into the ground through the grid what will be the earth potential rise (EPR) of the substation earth grid with respect to true ground?

The centre of the mesh has a voltage of 5 kV with reference to true ground and the voltage gradient between the centre point and the mesh is assumed to be uniform (linear).

What will be the touch and step voltage that will be impressed on the persons A and B respectively? A is touching the structure S which is connected to the mesh and B stands on the ground within the mesh as shown. All external impedances can be neglected.

Answer:

The Sag of the conductor is given by:

Sag = WL²/8T

Where

W = 1.893 N/m

L = 200 m

T = 5368 N

Therefore

Sag of the conductor = (1.893 x 200 x 200) / (8 x 5368) = 1.76 m

Answer:

The slack of the conductor for the above Sag calculation is given by

Slack = 8 x 1.76 x 1.76/(3 x 200)

= 0.04 m

Answer:

The length of the conductor is given by the equation:

L = S x (1 + (W x D)/ (3 x T))

Where

S is the horizontal length of the Span
D is the Sag of the conductor
W is the weight of the conductor per unit length (including wind and ice loads)
T is the tension at the lowest point of the conductor

The length of the conductor seen in the earlier example is given by:

L = 200 X (1 + (1.893 X 1.76)/ (3 X 5368))
= 200.04 m

Answer:

The increase in conductor length with temperature is given by the formula:

ΔL = α x T x S

Where α is the coefficient of thermal expansion of the conductor
T is the increase in temperature in deg C
S is the length of the conductor in m

Therefore the increase in length of the conductor considered in the earlier example for a 30°C temperature rise is:

ΔL = 13.9 X 10^-6 X 30 X 200.04
= 0.08 m

Answer

Armoured, XLPE insulated, three core cable is chosen over PILC (Paper insulated lead covered) for its superior insulation properties, higher current ratings and temperature of 90°C (compared to PILC at 70°C), higher tolerance to movement, low charging current and also for its cost effectiveness. Three core cable is chosen as cost effective option and preferred over three numbers of single core cables unless the required cable size is greater than 300mm² (to be confirmed by calculations below).

The Cable that is selected is 95mm² XLPE, obtained from the provided Table of the cable manufacturer. The current rating of the 95mm² cable is 290A as per the Table.

Neglecting cable impedance, the fault current to be carried by the cable = 11.7kA

Answer

Calculating Earth Potential Rise:

EPR calculated using a current dispersion factor (S_f) of 1.

Figure 5.7 shows the ground voltage gradient with the calculated EPR. The voltage difference between the grid and the centre point of the mesh (in the soil) is the difference between EPR and the voltage at the centre of the mesh.

Voltage difference	= 9000 – 5000
	= 4000 Volts

The distance from the centre point of the mesh and the grid (Point S) is 10 m since the mesh size is 20 x 20m.

The ground voltage gradient between the centre point and the mesh is linear, therefore the voltage will increase at a rate of 400V per meter.

• Person A is standing 0.75m away from structure S, therefore the touch potential he will experience will be 300V.
• Person B is standing 5m inside the mesh with their feet 1m apart. The step potential he will experience will be 400V.

Prerequisites

The following information is required / desirable before starting the calculation:

• A layout of the site
• Maximum earth fault current into the earthing grid
• Maximum fault clearing time
• Ambient (or soil) temperature at the site
• Soil resistivity measurements at the site (for touch and step only)
• Resistivity of any surface layers intended to be laid (for touch and step only)

Earthing Grid Conductor Sizing

Determining the minimum size of the earthing grid conductors is necessary to ensure that the earthing grid will be able to withstand the maximum earth fault current. Like a normal power cable under fault, the earthing grid conductors experience an adiabatic short circuit temperature rise. However unlike a fault on a normal cable, where the limiting temperature is that which would cause permanent damage to the cable’s insulation, the temperature limit for earthing grid conductors is the melting point of the conductor. In other words, during the worst case earth fault, we don’t want the earthing grid conductors to start melting!

The minimum conductor size capable of withstanding the adiabatic temperature rise associated with an earth fault is given by re-arranging IEEE Std 80 Equation 37:

Where A is the minimum cross-sectional area of the earthing grid conductor (mm²)

ρ_t is the energy of the maximum earth fault (A²s)

T_m is the maximum allowable (fusing) temperature (°C)

T_a is the ambient temperature (°C)

α_r is the thermal coefficient of resistivity (°C ^{– 1})

ρ_r is the resistivity of the earthing conductor (μΩ.cm)

TCAP is the thermal capacity of the conductor per unit volume(Jcm ^{– 3}°C ^{– 1})

The material constants T_m, α_r, ρ_r and TCAP for common conductor materials can be found in IEEE Std 80 Table 1. For example. commercial hard-drawn copper has material constants:

T_m = 1084 °C
α_r = 0.00381 °C ^{– 1}
ρ_r = 1.78 μΩ.cm
TCAP = 3.42 Jcm ^{– 3}°C ^{– 1}.

As described in IEEE Std 80 Section 11.3.1.1, there are alternative methods to formulate this equation, all of which can also be derived from first principles).

There are also additional factors that should be considered (e.g. taking into account future growth in fault levels), as discussed in IEEE Std 80 Section 11.3.3.

Touch and Step Potential Calculations

When electricity is generated remotely and there are no return paths for earth faults other than the earth itself, then there is a risk that earth faults can cause dangerous voltage gradients in the earth around the site of the fault (called ground potential rises). This means that someone standing near the fault can receive a dangerous electrical shock due to:

• Touch voltages – there is a dangerous potential difference between the earth and a metallic object that a person is touching
• Step voltages – there is a dangerous voltage gradient between the feet of a person standing on earth

The earthing grid can be used to dissipate fault currents to remote earth and reduce the voltage gradients in the earth. The touch and step potential calculations are performed in order to assess whether the earthing grid can dissipate the fault currents so that dangerous touch and step voltages cannot exist.

Step 2: Surface Layer Materials

Applying a thin layer (0.08m – 0.15m) of high resistivity material (such as gravel, blue metal, crushed rock, etc) over the surface of the ground is commonly used to help protect against dangerous touch and step voltages. This is because the surface layer material increases the contact resistance between the soil (i.e. earth) and the feet of a person standing on it, thereby lowering the current flowing through the person in the event of a fault.

IEEE Std 80 Table 7 gives typical values for surface layer material resistivity in dry and wet conditions (e.g. 40mm crushed granite = 4,000 Ω.m (dry) and 1,200 Ω.m (wet)).

The effective resistance of a person’s feet (with respect to earth) when standing on a surface layer is not the same as the surface layer resistance because the layer is not thick enough to have uniform resistivity in all directions. A surface layer derating factor needs to be applied in order to compute the effective foot resistance (with respect to earth) in the presence of a finite thickness of surface layer material. This derating factor can be approximated by an empirical formula as per IEEE Std 80 Equation 27:

Where C_s is the surface layer derating factor

ρ is the soil resistivity (Ω.m)

ρ_a is the resistivity of the surface layer material (Ω.m)

h_s is the thickness of the surface layer (m)

This derating factor will be used later in Step 5 when calculating the maximum allowable touch and step voltages.

Simplified Method

IEEE Std 80 Equation 52 gives the simplified method as modified by Sverak to include the effect of earthing grid depth:

Where R_g is the earthing grid resistance with respect to remote earth (Ω)

ρ is the soil resistivitiy (Ω.m)

L_r is the total length of buried conductors (m)

A is the total area occupied by the earthing grid (m²)

Schwarz Equations

The Schwarz equations are a series of equations that are more accurate in modelling the effect of earthing rods / electrodes. The equations are found in IEEE Std 80 Equations 53, 54, 55 and 56, as follows:

Where R_g is the earthing grid resistance with respect to remote earth (Ω)

R: is the earth resistance of the grid conductors (Ω)

R₂ is the earth resistance of the earthing electrodes (Ω)

R_m is the mutual earth resistance between the grid conductors and earthing electrodes (Ω)

And the grid, earthing electrode and mutual earth resistances are:

Where ρ is the soil resistivity (Ω.m)

L_c is the total length of buried grid conductors (m)

α΄ is  for conductors buried at depth h metres and with cross-sectional radius r metres, or simply r for grid conductors on the surface

A is the total area covered by the grid conductors (m²)

L_r is the length of each earthing electrode (m)

n_r is number of earthing electrodes in area

b is the cross-sectional radius of an earthing electrode (m)

K₁ and K₂ are constant coefficients depending on the geometry of the grid

The coefficient K₁ can be approximated by the following:

1. For depth : 
2. For depth : 
3. For depth : 

The coefficient K₂ can be approximated by the following:

1. For depth : 
2. For depth : 
3. For depth : 

Where in both cases, L/R is the length-to-width ratio of the earthing grid.

Step 4: Maximum Grid Current

The maximum grid current is the worst case earth fault current that would flow via the earthing grid back to remote earth. To calculate the maximum grid current, you firstly need to calculate the worst case symmetrical earth fault current at the facility that would have a return path through remote earth (call this I_k,e). This can be found from the power systems studies or from manual calculation. Generally speaking, the highest relevant earth fault level will be on the primary side of the largest distribution transformer (i.e. either the terminals or the delta windings).

Current Division Factor

Not all of the earth fault current will flow back through remote earth. A portion of the earth fault current may have local return paths (e.g. local generation) or there could be alternative return paths other than remote earth (e.g. overhead earth return cables, buried pipes and cables, etc). Therefore a current division factor S_f must be applied to account for the proportion of the fault current flowing back through remote earth.

Computing the current division factor is a task that is specific to each project and the fault location and it may incorporate some subjectivity (i.e. “engineeing judgement”). In any case, IEEE Std 80 Section 15.9 has a good discussion on calculating the current division factor. In the most conservative case, a current division factor of S_f = 1 can be applied, meaning that 100% of earth fault current flows back through remote earth.

The symmetrical grid current I_g is calculated by:

Decrement Factor

The symmetrical grid current is not the maximum grid current because of asymmetry in short circuits, namely a dc current offset. This is captured by the decrement factor, which can be calculated from IEEE Std 80 Equation 79:

Where D_f is the decrement factor

t_f is the duration of the fault (s)

T_A is the dc time offset constant (see below)

The dc time offset constant is derived from IEEE Std 80 Equation 74:

Where  is the X/R ratio at the fault location

f is the system frequency (Hz)

The maximum grid current I_G is lastly calculated by:

Step 6: Ground Potential Rise (GPR)

Normally, the potential difference between the local earth around the site and remote earth is considered to be zero (i.e. they are at the same potential). However an earth fault (where the fault current flows back through remote earth), the flow of current through the earth causes local potential gradients in and around the site. The maximum potential difference between the site and remote earth is known as the ground potential rise (GPR). It is important to note that this is a maximum potential difference and that earth potentials around the site will vary relative to the point of fault.

The maximum GPR is calculated by:

Where GPR is the maximum ground potential rise (V)

I_G is the maximum grid current found earlier in Step 4 (A)

R_g is the earthing grid resistance found earlier in Step 3 (Ω)

Mesh Voltage Calculation

The mesh voltage is the maximum touch voltage within a mesh of an earthing grid and is derived from IEEE Std 80 Equation 80:

Where :: ρ_s is the soil resistivity (Ω.m)

I_G is the maximum grid current found earlier in Step 4 (A)

K_m is the geometric spacing factor (see below)

K_i is the irregularity factor (see below)

L_M is the effective buried length of the grid (see below)

Geometric Spacing Factor K_m

The geometric spacing factor K_m is calculated from IEEE Std 80 Equation 81:

Where D is the spacing between parallel grid conductors (m)

h is the depth of buried grid conductors (m)

d is the cross-sectional diameter of a grid conductor (m)

K_h is a weighting factor for depth of burial = 

K_ii is a weighting factor for earth electrodes /rods on the corner mesh

K_ii = 1 for grids with earth electrodes along the grid perimeter or corners

 for grids with no earth electrodes on the corners or on the perimeter n is a geometric factor (see below)

Geometric Factor n

The geometric factor n is calculated from IEEE Std 80 Equation 85:

With 

n_b = 1 for square grids, or otherwise 

n_e = 1 for square and rectangular grids, or otherwise 

n_c = 1 for square, rectangular and L-shaped grids, or otherwise 

Where L_c is the total length of horizontal grid conductors (m)

L_p is the length of grid conductors on the perimeter (m)

A is the total area of the grid (m²)

L_x and L_y are the maximum length of the grids in the x and y directions (m)

D_m is the maximum distance between any two points on the grid (m)

Irregularity Factor Ki

The irregularity factor K_i is calculated from IEEE Std 80 Equation 89:

Where n is the geometric factor derived above

Effective Buried Length LM

The effective buried length L_M is found as follows:

Where L_c is the total length of horizontal grid conductors (m)

L_R is the total length of earthing electrodes / rods (m)

Where L_c is the total length of horizontal grid conductors (m)

L_R is the total length of earthing electrodes / rods (m)

L_r is the length of each earthing electrode / rod (m)

L_x and L_y are the maximum length of the grids in the x and y directions (m)

• For grids with few or no earthing electrodes (and none on corners or along the perimeter):
• For grids with earthing electrodes on the corners and along the perimeter:

Step Voltage Calculation

The maximum allowable step voltage is calculated from IEEE Std 80 Equation 92:

Where :: ρ_s is the soil resistivity (Ω.m)

I_G is the maximum grid current found earlier in Step 4 (A)

K_s is the geometric spacing factor (see below)

K_i is the irregularity factor (as derived above in the mesh voltage calculation)

L_s is the effective buried length of the grid (see below)

Geometric Spacing Factor K_s

The geometric spacing factor K_s based on IEEE Std 80 Equation 81 is applicable for burial depths between 0.25m and 2.5m:

Where D is the spacing between parallel grid conductors (m)
h is the depth of buried grid conductors (m)
n is a geometric factor (as derived above in the mesh voltage calculation)

Effective Buried Length LS

The effective buried length L_s for all cases can be calculated by IEEE Std 80 Equation 93:

Where L_c is the total length of horizontal grid conductors (m)
L_R is the total length of earthing electrodes / rods (m)

What Now?

Now that the mesh and step voltages are calculated, compare them to the maximum tolerable touch and step voltages respectively. If:

• , and
• 

then the earthing grid design is safe.

If not, however, then further work needs to be done. Some of the things that can be done to make the earthing grid design safe:

• Redesign the earthing grid to lower the grid resistance (e.g. more grid conductors, more earthing electrodes, increasing cross-sectional area of conductors, etc). Once this is done, re-compute the earthing grid resistance (see Step 3) and re-do the touch and step potential calculations.
• Limit the total earth fault current or create alternative earth fault return paths
• Consider soil treatments to lower the resistivity of the soil
• Greater use of high resistivity surface layer materials

Step 1: Soil Resistivity

The soil resistivity around the site was measured with a Wenner four-pin probe and found to be approximately 300 Ω.m.

Step 2: Surface Layer Materials

A thin 100mm layer of blue metal (3,000 Ω.m) is proposed to be installed on the site. The surface layer derating factor is:

Step 3: Earthing Grid Resistance

A rectangular earthing grid (see the figure right) with the following parameters is proposed:

• Length of 90m and a width of 50m
• 6 parallel rows and 7 parallel columns
• Grid conductors will be 120 mm² and buried at a depth of 600mm
• 22 earthing rods will be installed on the corners and perimeter of the grid
• Each earthing rod will be 3m long

Using the simplified equation, the resistance of the earthing grid with respect to remote earth is:

Step 4: Maximum Grid Current

Suppose that the maximum single phase to earth fault at the HV winding of the transformer is 3.1kA and that the current division factor is 1 (all the fault current flows back to remote earth).

The X/R ratio at the fault is approximately 15, the maximum fault duration 150ms and the system nominal frequency is 50Hz. The DC time offset is therefore:

The decrement factor is then:

Finally, the maximum grid current is:

kA

Step 5: Touch and Step Potential Criteria

Based on the average weight of the workers on the site, a body weight of 70kg is assumed for the maximum touch and step potential. A maximum fault clearing time of 150ms is also assumed.

The maximum allowable touch potential is:

 V

The maximum allowable step potential is:

 V

Step 6: Ground Potential Rise (GPR)

The maximum ground potential rise is:

 V

The GPR far exceeds the maximum allowable touch and step potentials, and further analysis of mesh and step voltages need to be performed.

Step 7: Earthing Grid Design Verification

Mesh Voltage Calculation

The components of the geometric factor n_a, n_b, n_c and n_d for the rectangular grid are:

Therefore the geometric factor n is:

The average spacing between parallel grid conductors D is:

where W_e and L_g are the width and length of the grid respectively (e.g. 50m and 90m) n_r and n_c is the number of parallel rows and columns respectively (e.g. 6 and 7)

The geometric spacing factor K_m is:

The irregularity factor K_i is:

The effective buried length L_M is:

 m

Finally, the maximum mesh voltage is:

 V

The maximum allowable touch potential is 1,720V, which exceeds the mesh voltage calculated above and the earthing system passes the touch potential criteria (although it is quite marginal).

Step Voltage Calculation

The geometric spacing factor K_s is:

The effective buried length L_s is:

 M

Finally, the maximum allowable step voltage is:

 V

The maximum allowable step potential is 5,664V, which exceeds the step voltage calculated above and the earthing system passes the step potential criteria. Having passed both touch and step potential criteria, we can conclude that the earthing system is safe.

Soil resistance test data collected from substation site

Soil resistance measurement data at various locations at the site and resistivity calculation

Note – With 1 mtr spacing, we are not getting any values

Soil test result data from laboratory

Observations and corrective actions

It is observed from the earth resistance measurements at site that the soil resistivity is high of the order of several hundred ohms. It is also observed from the lab test report that the major proportion of the soil is made up of coarse particles and consisting of cobbles and gravel. The moisture content of the soil is also found to be very low of the order of 1.9%. As a result the, electrical resistance offered by the soil is high necessitating soil treatment. As such the soil condition is not ideally suited for earthing purposes in the proposed substation, without taking recourse to some corrective actions to improve the soil electrical resistivity conditions.

The soil is treated with Bentonite clay in order to reduce the electrical resistivity of the soil and improve electrical earthing conditions. Bentonite clay is a natural clay of volcanic origin and has the desirable properties of absorbing and retaining moisture for a very long time. In the event of higher temperatures, only the outer portion of the clay dries out retaining the internal moisture. In addition, the clay also does not cause any corrosion problems to the earth electrodes unlike salts, that are some times used for improving earth electrical resistance. The rocky soil was excaved and replaced by bentonite clay. After the bentonite clay treatment and initial watering, the soil resistivity dropped down to less than 10 ohm.metre. The number of vertical electrodes of 3 metres length was also installed in adequate numbers to reduce the electrode resistance and also to ensure that the bottom of the electrodes made contact with the underground water table.

Section 1

Section 1 of the Standard defines the scope of the Standard.

For the purpose of interpreting the standard, an electrical power installation is considered to be one of the following:

• Substation, including substation for railway power supply
• Electrical installations on mast, pole and tower
• Switchgear and/or transformers located outside a closed electrical operating area
• One (or more) power station(s) located on a single site
• The installation includes generators and transformers with all associated switchgear and all electrical auxiliary systems. Connections between generating stations located on different sites are excluded.
• The electrical system of a factory, industrial plant or other industrial, agricultural, commercial or public premises

The electrical power installation includes, among others, the following equipment:

• Rotating electrical machines
• Switchgear
• Transformers and reactors
• Converters
• Cables
• Wiring systems
• Batteries
• Capacitors
• Earthing systems
• Buildings and fences which are part of a closed electrical operating area
• Associated protection, control and auxiliary systems
• Large air core reactor

The Standard does not apply to the design and erection of any of the following:

• Overhead and underground lines between separate installations
• Electric railways
• Mining equipment and installations
• Fluorescent lamp installations
• Installations on ships and off-shore installations
• Electrostatic equipment
• Test sites
• Medical equipment, e.g. medical X-ray equipment

The standard does not apply to the design of factory-built, type-tested switchgear for which separate IEC standards exist. This standard does not apply to the requirements for carrying out live working on electrical installations. If not otherwise required in this standard, for low-voltage electrical installations the standard series IEC 60364 applies.

Section 2

Section 2 of the BS EN 61936-1 2010 Standard lists the normative reference Standards for the application of the Standard.

Section 3

Section 3 of the Standard lists the terms and definitions as applied to the Standard.

Section 4

Section 4 of the Standard specifies the fundamental requirements. Section 4.1 specifies the general requirements. Following are the general requirements:

• The equipment and installation shall be capable of withstanding the electrical, mechanical, climatic and environmental influences anticipated at the site
• The design shall take into account:
  • The purpose of the installation
  • Users requirements such as power quality, reliability, availability, and ability of the electrical network to withstand the effects of transient conditions such as starting of large motors, short power outages and re-energization of the installation.
  • Safety of the operators and the public
  • Environmental influence
  • The possibility for extension (if required) and maintenance

Preferences for specific maintenance features and identification of the safety requirements to be met are defined by the user. Where, specific design criteria is required, these may be agreed between the user and the manufacturer. The design of the installation shall take into account the working procedures of the user. Additional agreements between the manufacturer/ contractor/ planner and user/ orderer/ owner shall be followed for the design and erection of power installations.

Section 4.2 deals with the following electrical requirements for the installation:

• Methods of neutral earthing
• The importance and impact of the type of neutral earthing adopted
• The basis for the choice and selection of the type of neutral earthing
• Voltage classifications – Defining the nominal and maximum operating voltage of the system based on Tables 1, 2 or Annex A in the Standard
• nstallation shall be designed:
  • To carry currents safely under defined normal operation
  • To safely withstand the thermal and mechanical effects of short circuit currents. Installation protected with automatic devices against short circuit currents.
  • For rated frequency
  • So that interference due to corona lies within specified limits
  • So that generation of electric and magnetic fields is within acceptable level
  • With protection against overvoltages
  • Protection against harmful effects of harmonics

Section 4.3 specifies the following mechanical requirements.

The equipment and supporting structures, foundations shall withstand the anticipated mechanical stresses under normal and exceptional load conditions. Normal loads consist of dead load, tension load, erection load, ice load and wind load. Exceptional loads relate to the simultaneous acting of the dead load and tension load with the largest of occasional loads such as switching forces, short-circuit forces, vibration and loss of conductor tension.

Section 4.4 specifies requirements of the installation for climatic and environmental conditions. Installations, including the devices and equipment in the installations shall be designed for operation under the climatic and environmental conditions and the selection based on specification of factors such as the presence of:

• Condensation
• Precipitation
• Particles
• Dust
• Corrosive elements and
• Hazardous atmospheres

For the selection of a suitable indoor or outdoor equipment for the installation, the following factors are considered:

• Temperature
• Altitude
• Relative humidity
• Solar radiation
• Vibration
• Electromagnetic radiation

Special conditions related to altitude, pollution levels, temperature and humidity, vibration shall also be factored into the selection process.

Section 4.5 deals with any requirement of special requirements related to the following:

• Effects of small animals and micro-organisms
• Noise level
• Transport

Section 5

Section 5 deals with insulation requirements of the equipment and installation and the selection of insulation level. These requirements include minimum clearances to be maintained between live parts and earth and clearances between live parts of phases to avoid flashovers. The section also discusses about the selection of insulation level according to the established highest voltage for installation Um and/or impulse withstand voltage. The factors of methods of neutral earthing and rated withstand values are taken into consideration. The minimum clearances in air are provided in Table 1, Table 2 and Annex A of the Standard and apply for altitudes up to 1 000 m above sea level.

Section 6

Section 6 specifies the requirements of the equipment in the installation. The general requirements of equipment are as follows:

• Selection and installation of equipment should ensure safety in construction, safety and proper performance under normal and reasonably expected overload and abnormal conditions
• Compliance of the equipment with the relevant IEC standards
• Personnel safety which may include manuals and instructions, special tools, safe working procedures and safe earthing measures
• Specific requirements of the following are covered in the Standard:
  • Switching devices
  • Power transformers and reactors
  • Prefabricated type-tested switchgear
  • Instrument transformers
  • Surge arresters
  • Capacitors
  • Line traps
  • Insulators
  • Insulated cables (Temperature, Stress due to temperature changes, flexible reeling and trailing cables, crossings and proximities, Installation of cables, bending radius, tensile stress
  • Conductors and accessories
  • Rotating electrical machines
  • Generating units
  • Static converters
  • Fuses (Clearances needed and requirements for fuse replacements)
  • Electrical and mechanical interlocking

Section 7

Section 7 deals with installations comprising the following:

• General requirements – Choice of circuit arrangement, documentation, transport routes, lighting, operational safety and labelling, aisles and access areas, lighting, operational safety, labeling
• Outdoor installations of open design – Minimum phase to phase, phase to earth clearances and restriction of access to danger zones
  • Protective barrier clearances
  • Protective obstacle clearances
  • Boundary clearances
  • Minimum height over access area
  • Clearances to buildings
  • External fences or walls and access doors
• Indoor installations of open design
• Installation of prefabricated type-tested switchgear comprising of:
  • General requirements
  • Additional requirements for gas-insulated metal-enclosed switchgear related to:
    • Design
    • Erection on site
    • Protection against overvoltages
    • Earthing
• Requirements for buildings – Buildings shall comply with national building codes and fire regulations. Where such national standards do not exist, the Standard provides guidelines in section 7.5 on the following:
  • Structural provisions
    • Specifications for walls
    • Windows
    • Roofs
    • Floors
  • Rooms for switchgear
  • Maintenance and operating areas
  • Doors
  • Draining of insulating liquids
  • Air conditioning and ventilation
    • Ventilation of battery rooms
    • Rooms for emergency generating units
  • Buildings which require special consideration
  • High voltage/low voltage prefabricated substations
  • Electrical installations on mast, pole and tower

Section 8

Section 8 relates to safety measures. The topics covered under this section are:

• General requirements as follows:
  • Installations shall be constructed in such a way as to enable the operating and maintenance personnel to circulate and intervene within the framework of their duties and authorizations, according to circumstances, at any point of the installation
  • Specific maintenance work, preparation and repair work, which involve working in the vicinity of live parts or actual work on live parts, are carried out observing the rules, procedures and work distances as defined in national standards and regulations

• Protection against direct contact
  • Measures for protection against direct contact
    • Recognized protection measures (Protection by enclosure, protection by barrier, protection by obstacle, protection by placing out of reach)
    • Design of protective measures (Protective barriers such as solid walls, doors or screens (wire mesh), protective obstacles such as covers, rails, chains and ropes, walls, doors and screens)
    • Protection by placing out of reach

• Protection requirements
  • Protection outside of closed electrical operating areas
  • Protection inside closed electrical operating areas
  • Protection during normal operation
• Means to protect persons in case of indirect contact
• Means to protect persons working on electrical installations
  • Equipment for isolating installations or apparatus
  • Devices to prevent reclosing of isolating devices
  • Devices for determining the de-energized state
  • Devices for earthing and short-circuiting
  • Equipment acting as protective barriers against adjacent live parts
    • Insertable insulated partitions
    • Insertable partition walls
  • Storage of personal protection equipment
• Protection from danger resulting from arc fault
• Protection against direct lightning strokes
• Protection against fire comprising of the following:
  • General – Relevant national, provincial and local fire protection regulations shall be taken into account in the design of the installation.
  • Transformers, reactors
  • Outdoor installations
  • Indoor installation in closed electrical operating areas
    • Indoor installations in industrial buildings
    • Indoor installations in buildings which are permanently occupied by persons
    • Fire in the vicinity of transformers
  • Cables
  • Other equipment with flammable liquid
• Protection against leakage of insulating liquid and SF₆
  • Insulating liquid leakage and subsoil water protection
    • General requirements
    • Containment for indoor equipment
    • Containment for outdoor equipment
  • SF₆ leakage
  • Failure with loss of SF₆ and its decomposition products
• Identification and marking
  • General
  • Information plates and warning plates
  • Electrical hazard warning
  • Installations with incorporated capacitors
  • Emergency signs for emergency exits
  • Cable identification marks

Section 9

Section 9 provides details of protection, control and auxiliary systems. This section comprises of the following sections:

• Monitoring and control systems
• DC and AC supply circuits
  • General requirements
  • AC supply
  • DC supply
• Compressed air systems
• SF₆ gas handling plants
• Hydrogen handling plants
• Basic rules for electromagnetic compatibility of control systems
  • General – Deals with the protection of control circuits against electromagnetic interference
  • Electrical noise sources in high voltage installations
    • High frequency interferences are produced by
    • Low frequency interferences are produced by
• Measures to be taken to reduce the effects of high frequency interference
• Measures to be taken to reduce the effects of low frequency interference
• Measures related to the selection of equipment
• Other possible measures to reduce the effects of interference

Section 10

Section 10 pertains to earthing systems.

General – This clause provides the criteria for design, installation, testing and maintenance of an earthing system for ensuring the safety of human life and that the integrity of equipment connected and in proximity to the earthing system is maintained.

Safety criteria – Flow of current into the body of a human in the region of the heart can result in ventricular fibrillation. The curve for obtaining the current limit is provided in IEC/TS 60479-1:2005. This body current limit when converted to voltage limits is used for comparing with the calculated touch and step voltages in the design of the earthing system taking into consideration the following factors:

• The proportion of current flowing through the region of the heart
• Impedance of the body along the current path
• Resistance between the body contact points to remote ground including shoes or gravel
• Duration of the fault

The fault occurrence, fault current magnitude, fault duration and presence of human beings are probabilistic in nature. Generally, meeting the requirements of touch voltage satisfies the step voltage requirements, since the tolerable step voltage limits are much higher than touch voltage limits due to the different current path through the body.

For installations where high-voltage equipment is not located in closed electrical operating areas, a global earthing system should be used to prevent touch voltages resulting from HV faults exceeding the low voltage limit.

Following are the various functional requirements of the earthing system.

The earthing system:

• Shall be capable of distributing and discharging the fault current without exceeding the thermal and mechanical design limits based on backup protection operating time
• Shall maintain its integrity for the expected installation lifetime with due allowance for corrosion and mechanical constraints
• Shall avoid damage to equipment due to excessive potential rise, potential differences within the earthing system and flow of excessive currents in auxiliary paths not intended for carrying parts of the fault current
• Along with appropriate measures, shall maintain the step, touch and transferred potentials within the voltage limits based on normal operating time of protection relays and breakers
• Shall contribute to ensuring electromagnetic compatibility among electrical and electronic apparatus of the high-voltage system in accordance with Standards

Where high- and low-voltage earthing systems exist in proximity to each other and do not form a global earthing system, part of the EPR from the HV system can be applied on the LV system either by interconnection of all HV with LV earthing systems or by separation of HV from LV earthing systems. However, compliance shall be met with reference to the requirements of step, touch and transfer potentials within a substation and at LV installation supplied from that substation.

In locations where the LV system is totally confined within the area covered by the HV earthing system, both earthing systems shall be interconnected even if there is no global earthing system. In locations where the earthing system of the HV installation is part of a global earthing system or connected to a multi-earthed HV neutral conductor in a balanced system, then full compliance is ensured. In locations not having a global earthing system, the minimum requirements for interconnection of low-voltage and high-voltage earthing systems based on EPR limits specified in the Standard shall be used to identify those situations where interconnection of earthing systems with low-voltage supply outside the high-voltage installation is feasible.

If high-voltage and low-voltage earthing systems are separate, the method of separating earth electrodes shall be chosen such that no danger to persons or equipment can occur in the low voltage installation. This means that step, touch and transfer potentials and stress voltage in the LV installation caused by a high-voltage fault are within the appropriate limits.

In locations where the LV systems are located in the zone of influence of the HV substation earthing, special consideration should be given. Common earthing system can be used for industrial and commercial installations since it is not possible to separate earthing systems due to the close proximity of equipment.

Section 10.3 discusses about the design of earthing systems.

The steps in the designing of an earthing system are as follows:

1. Collection of data – earth fault currents, duration, layout etc.
2. Making initial design of earthing system based on functional requirements and determining if it is part of a global earthing system
3. If it is not, determining the characteristics of the soil
4. Determining the current discharged into soil from earthing system, based on earth fault current
5. Determining the overall impedance to earth, based on the layout, soil characteristics, and parallel earthing systems
6. Determining the earth potential rise
7. Determining the permissible touch voltage
8. Checking whether the earth potential rise is below the permissible touch voltage and the minimum requirements for interconnection of low-voltage and high-voltage earthing systems based on EPR limits are met
9. If the limits are not met, determining if touch voltages inside and in the vicinity of the earthing system are below the tolerable limits
10. Determining if transferred potentials present a hazard outside or inside the electrical power installation; if yes, proceed with mitigation at exposed location
11. Determining if low-voltage equipment is exposed to excessive stress voltage and if so proceeding with mitigation measures which can include separation of HV and LV earthing systems
12. Determining if the circulating transformer neutral current can lead to excessive potential differences between different parts of the earthing system and if so, proceeding with mitigation measures

After the above criteria are satisfied, the design can be refined, if necessary, by repeating the above steps. Detailed design is necessary to ensure that all exposed conductive parts, are earthed. Bonding should be made between the earthing system and the structural earth electrodes. Metallic structures with cathodic protection may be separated from the earthing system.

The various following types of earth faults at the different voltage levels, within and outside the installation should be analysed to determine the worst case fault situation. Power system faults:

• Three phases to earth
• Two phases to earth
• Single phase to earth
• Where applicable: phase to phase via earth

High voltage and current surges occur due to lightning and switching operations on long cable sections, operating GIS disconnectors or carrying out back-to-back capacitor switching. The earthing system should also include the earthing for the lightning protection system.

Where more than one building or location are involved, interconnection should be made for the earthing system of each. Protection measures must be taken to prevent damages to sensitive equipment connected between different buildings or locations during lightning strokes. Non-metallic media, such as fibre optic cable, should be used where possible, for the exchange of low-level signals between such locations.

Section 10.4 requires that protective measures shall be taken to ensure the safety of persons during fault conditions where construction work involves an existing earthing system.

Section 10.5 is on measurements on earthing systems. The adequacy of the earthing design can be verified through measurements after the construction. These measurements may include earthing system impedance, prospective touch and step voltages at relevant locations and transferred potential, if appropriate.

Section 10.6 is about maintainability of earthing systems. Maintainability of earthing systems comprise of the following:

• Inspections – Considerations should be given in the construction of the earthing system to facilitate maintenance inspections
• Measurements – The design and installation shall allow the carrying out of periodic measurements and tests on earthing

Section 11

Section 11 deals with inspection and testing. This section of the Standard discusses about the need and requirement for inspection and testing of, equipment with technical specifications and of installations for compliance with the Standard. The points of inspection and testing should be agreed between the user and the supplier of equipment. The inspections and testing comprise of:

• Verification of specified performances
  • This section discusses about the tests that may need to be carried out to verify the performance of the equipment forming part of the installation and the defining of the details these tests
• Tests during installation and commissioning
  • The various points on which the user and supplier of the equipment should agree with reference to the tests during installation and commissioning are detailed in this section
• Trial running
  • The purpose of the trial run and the requirements of the Agreement between the user and supplier are detailed in this section

Section 12

Section 12 deals with operation and maintenance manual.

Each installation should have:

• Operation manual describing the normal, emergency, electrical installation
• Set of up-to-date drawings and operating diagrams on the premises

Manufacturers of major components of installation should provide operation and maintenance manuals and test and in-service reports. Emergency routes to the nearest hospital and emergency phone numbers should be displayed in a visible location in the installation.

Annex-A

Annexure-A of BS EN 61936-1 2010 provides values of rated insulation levels and minimum clearances for various levels of voltages, based on current practice in some countries.

• Tables A.1 and A.2 provide the values of rated insulation levels and minimum clearances in air for 1 kV < Um < 245 kV for highest voltage for installation Um
• Table A.3 provides values of rated insulation levels and minimum clearances in air for Um > 245 kV for highest voltages for installation Um

Annex B

Annexure-B of BS EN 61936-1 2010 provides the method of calculating permissible touch voltages.

Annex C

Annexure-C of BS EN 61936-1 2010 provides details on the Permissible touch voltage according IEEE 80.

Annex D

Annexure-D of BS EN 61936-1 2010 provides a Earthing system design flow chart.

Annex E

Annexure-D of BS EN 61936-1 2010 provides the Protection measures against direct lightning strokes. Following methods have been listed as providing sufficient protection level for avoiding direct lightning strokes with a high degree of certainty but without detailed studies of insulation coordination.

• Shield wires
• Lightning rods

Annex ZA

Annexure-ZA of BS EN 61936-1 2010 provides special national conditions (National characteristic or practice that cannot be changed even over a long period, e.g. climatic conditions, electrical earthing conditions). For the countries in which the relevant special national conditions apply these provisions are normative, for other countries they are informative.

Annex ZB

Annexure-ZB of BS EN 61936-1 2010 provides A-deviations which are National deviation due to regulations, the alteration of which is for the time being outside the competence of the CENELEC national member. This European Standard does not fall under any Directive of the EC. In the relevant CENELEC countries these A-deviations are valid instead of the provisions of the European Standard until they have been removed.

Annex ZC

Annexure-ZC of BS EN 61936-1 2010 provides Normative references to international publication-96 with their corresponding European publications. The listed referenced documents are indispensable for the application of BS EN 61936-1 2010.