Chapter A General rules of electrical installation design

H28
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H - LV switchgear: functions & selection
4 Circuit-breaker
b Short-circuit level at MV terminals of transformer: 250 MVA
b Transformer MV/LV: 1,250 kVA 20/0.4 kV
b MV fuses: 63 A
b Cabling, transformer - LV circuit-breaker: 10 metres single-core cables
b LV circuit-breaker: Compact NS 2000 set at 1,800 A (Ir)
What is the maximum short-circuit trip current setting and its maximum time delay
allowable?
The curves of Figure H58 show that discrimination is assured if the short-time delay
tripping unit of the CB is set at:
b A level y 6 Ir = 10.8 kA
b A time-delay setting of step 1 or 2
1,000
200
100
10
0.2
0.1
0.50
0.01
t
(s)
1,800 A
Ir
Schneider Electric - Electrical installation guide 2008
NS 2000
set at
1,800 A
1 4 6
10 kA
Fig. H58 : Curves of MV fuses and LV circuit-breaker
8
Minimum pre-arcing
curve for 63 A HV fuses
(current referred to the
secondary side of the
transformer)
Isc maxi
31.4 kA
Step 4
Step 3
Step 2
Step 1
I