Protective conductor between MV/LV transformer and the Main Low Voltage Switchboard (MLVS)
From Electrical Installation Guide
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.
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|
|≤ 100||c.s.a. of PE conductors SPE (mm2)||25||25||25||25||25||25||25||25||25|
Fig. G62: Recommended 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.