Chapter A General rules of electrical installation design

G40
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G - Sizing and protection of conductors
These conductors must be sized according to
national practices
6 Protective earthing conductor
(PE)
6.3 Protective conductor between MV/LV transformer
and the main general distribution board (MGDB)
All phase and neutral conductors upstream of the main incoming circuit-breaker
controlling and protecting the MGDB are protected 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 in Sub-clause 7.5 of this chapter (for circuit C1 of the
system illustrated in Fig. G65).
Recommended conductor sizes for bare and insulated PE conductors from the
transformer neutral point, shown in Figure G60, are indicated below in Figure G61.
The kVA rating to consider is the sum of all (if more than one) transformers
connected to the MGDB.
Schneider Electric - Electrical installation guide 2008
PE
Main earth bar
for the LV installation
Fig. G60 : PE conductor to the main earth bar in the MGDB
MGDB
The table indicates the c.s.a. of the conductors in mm 2 according to:
b The nominal rating of the MV/LV transformer(s) in kVA
b The fault-current clearance time by the MV protective devices, in seconds
b 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 Conductor Bare PVC-insulated XLPE-insulated
rating in kVA material conductors conductors conductors
(230/400 V Copper t(s) 0.2 0.5 - 0.2 0.5 - 0.2 0.5 -
output) Aluminium t(s) - 0.2 0.5 - 0.2 0.5 - 0.2 0.5
y100 c.s.a. of PE 25 25 25 25 25 25 25 25 25
160 conductors 25 25 35 25 25 50 25 25 35
200 SPE (mm 2 ) 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. G61 : Recommended c.s.a. of PE conductor between the MV/LV transformer and the MGDB,
as a function of transformer ratings and fault-clearance times.