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Protective conductor between MV/LV transformer and the Main Low Voltage Switchboard (MLVS)

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General rules of electrical installation design
Connection to the MV utility distribution network
Connection to the LV utility distribution network
MV and LV architecture selection guide for buildings
LV Distribution
Protection against electric shocks and electrical fires
Sizing and protection of conductors
LV switchgear: functions and selection
Overvoltage protection
Energy Efficiency in electrical distribution
Power Factor Correction
Power harmonics management
Characteristics of particular sources and loads
PhotoVoltaic (PV) installation
Residential premises and other special locations
ElectroMagnetic Compatibility (EMC)
Measurement

These conductors must be sized according to national practices

All phase and neutral conductors upstream of the main incoming circuit-breaker controlling and protecting the MLVS are protected against short-circuit and fault current by devices at the MV side of the transformer. The conductors in question, together with the PE conductor, must be dimensioned accordingly. Dimensioning of the phase and neutral conductors from the transformer is exemplified (for circuit C1 of the system illustrated in Fig. G69).

Recommended conductor sizes for bare and insulated PE conductors from the transformer neutral point, shown in Figure G61, are indicated below in Figure G62.

The kVA rating to consider is the sum of all (if more than one) transformers connected to the MLVS.

Fig. G61PE conductor to the main earth bar in the MLVS

The table indicates the c.s.a. of the conductors in mm2 according to:

  • The nominal rating of the MV/LV transformer(s) in kVA
  • The fault-current clearance time by the MV protective devices, in seconds
  • The kinds of insulation and conductor materials

If the MV protection is by fuses, then use the 0.2 seconds columns.

In IT schemes, if an overvoltage protection device is installed (between the transformer neutral point and earth) the conductors for connection of the device should also be dimensioned in the same way as that described above for PE conductors.

Transformer rating in kVA (230/400 V output) Conductor material Bare conductors PVC-insulated conductors XLPE-insulated conductors
Copper t(s) 0.2 0.5 - 0.2 0.5 - 0.2 0.5 -
Aluminium t(s) - 0.2 0.5 - 0.2 0.5 - 0.2 0.5
≤ 100 c.s.a. of PE conductors SPE (mm2) 25 25 25 25 25 25 25 25 25
160 25 25 35 25 25 50 25 25 35
200 25 35 50 25 35 50 25 25 50
250 25 35 70 35 50 70 25 35 50
315 35 50 70 35 50 95 35 50 70
400 50 70 95 50 70 95 35 50 95
500 50 70 120 70 95 120 50 70 95
630 70 95 150 70 95 150 70 95 120
800 70 120 150 95 120 185 70 95 150
1,000 95 120 185 95 120 185 70 120 150
1,250 95 150 185 120 150 240 95 120 185

Fig. G62Recommended c.s.a. of PE conductor between the MV/LV transformer and the MLVS, as a function of transformer ratings and fault-clearance times

This table is the result of the application of the adiabatic method described in Sizing of protective earthing conductor with:

  • i2 short-circuit level at the transformer LV side,
  • t is the tripping time of the MV device for this short circuit current.