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7
Q
How does automatic
disconnection apply to the
LSC circuit?
The contractor must verify
that the earth fault loop
impedance does not exceed
the maximum tabulated
values in BS 7671, or any
values derived by applying the
appropriate formula specified
in BS 7671 for the overcurrent
protective device.
Manufacturers frequently
provide data derived from this
formula.
If the circuit is designed to
comply with BS 7671:2008
table 41.3 the maximum
earth fault loop impedance
for the 16 A type C circuit
breaker for 0.4 and 5 second
disconnection time would be
1.44 Ω (see figure 3).
Figure 2: An example showing the maximum permitted
voltage drop in installation
16 A
type C
circuit
breaker
"kilK" A 6
rotcudnoc mm 5.2
Nominal supply voltage is
assumed to be 230 V therefore
the maximum voltage drop
permitted = 3% ie: 230 x 3/100
= 6.9 V
(origin of
installation)
Figure 3: Verifying the maximum permitted earth fault
loop impedance
2.4 mm conductor
6 A "Klik"
16 A
type C
circuit
breaker
Maximum permitted
earth fault loop
impedance at LSC
outlet would be 1.17
Ω
Maximum permitted earth
fault loop impedance at
the furthest luminaire
would be 1.44
Ω