Originally Posted by
GCE
To Answer the original question I would say that section 6.121.1.1 (3) gives you the calculation to get to maximum resistance values of the earth continuity conductor -
3) where the maximum length of the conductor may be determined using
the following equation
Lmax=
0,8×Uo×Sph
ρ(1+m)×Im
where
Uo is the phase voltage of 230 V,
ρ is the resistivity at normal working temperature in ohm.mm2/metre
which is 23 × 10-3 for copper which value increases above 120 mm2
with R+15 % (150 mm2), R+20 % (185mm2), R+25 % (240mm2) and
R+30 % (300 mm2),
Lmax is the maximum length of the conductor in metres (m);
Sph is the size of the phase conductor in square metres (mm2);
Spe is the size of the earth continuity conductor in square metres
(mm2);
Im is the instantaneous or short time tripping current in amperes (A).
For most general purpose circuit breakers it would suffice to use
10 multiply by the rated nominal current In for the instantaneous
tripping point of the circuit breaker, however the precise number could
be obtained from the manufacturer's information or from the
instantaneous current setting of adjustable circuit breakers, thus
𝐼𝑚 = 𝐼𝑛 × 10
Correction factor m =
𝑆𝑝ℎ
𝑆𝑝𝑒
= 1 [for table 6.28 (a)]
f) using the same formulae above where the earth continuity conductor
cross sectional area is 50 % of the phase conductor, thus m = 2,
table 6.28(b) for the maximum length of copper earth continuity
conductor for general purpose circuit breaker apply.
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