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

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. G61 – PE 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.

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

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

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.