January 2016
Intermediate to advanced
110 pages
3h 30m
English
Content preview from The IEE Wiring Regulations Explained and Illustrated, 2nd Edition
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Earthing
In this example, a 3-phase motor is fed using
25
mm
2
single
PVC conductors in trunking, the CPC being 2.5 mm
2
. The circuit
is protected by BS 1361 45A fuses in a distribution fuseboard. The
distribution circuit or sub-main feeding this fuseboard comprises
70
mm
2
PVC singles in trunking with a
25
mm
2
CPC the
protection being by BS88 160A fuses. The external loop im-
pedance Z
e
has a measured value of 0.2Ω. Will this circuit
arrangement comply with the shock risk constraints?
The formula Z
s
= Z
e
+
/?
1
+ i?
2
must be extended, as the (Ri
+ R
2
) component comprises both sub-main and motor circuit, it
therefore becomes:
Z
s
= Z
e
+
(Ä
1
+ Ä
2
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