7 tells that:
● for line conductors equal less than 16mm2
, the protective con-
ductor should equal the line conductor;
● for line conductors greater than 16mm2
but less than 35mm2
,
the protective conductor should have cross-sectional area of
16mm2
;
● for line conductors greater than 35mm2
, the protective conductor
should half the size the line conductor.2 Maximum Earth Fault Loop Impedance MCB Type Maximum Measured Earth Fault
Loop Impedance (Ω) When Overcurrent Protective Device MCB Type 60898
MCB rating (A)
6 50
For 0.Basic Electrical Installation Work
250
The maximum permitted value given Table the Site Guide for 20A MCB
protecting socket outlet 2.87 2.92
.4
of the IEE Regulations).4s disconnection (Ω) 7.
Example 1
A 230 ring main circuit socket outlets wired 2.2. If
the cross-section the CPC complies with Table 54. earth fault loop impedance test identifies Zs
Table 12.4s.44 1.7 the IEE Regulations,
there need carry out further checks.6 2. The circuit earth fault loop
impedance less than this value and therefore the protective device will operate within the
required disconnection time 0.5mm single PVC copper cables a
plastic conduit with separate 1.15 0. Where line and protective con-
ductors are made from the same material, Table 54.3 1.1.
However, where the conductor cross-section does not comply with this
table, then the formula given Regulation 543.5 CPC.3Ω shown Table 12.
Protective conductor size
The CPC forms integral part the total earth fault loop impedance, it
is necessary check that the cross-section this conductor adequate.84 1.3 must used:
S
I t
k
2
2
(mm )
where
S cross-sectional area mm2
I value maximum fault current amperes
t operating time the protective device
k factor for the particular protective conductor (see Tables 54.2 54.67 4
