Electricity + Control June 2017

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ELECTRICAL PROTECTION + SAFETY
If the circuit breaker is to be used to provide isolation to part of
the installation, it has to be rated as a switch-disconnector. Without
the disconnector function, it does not provide adequate isola-
tion to allow safe maintenance work on the switched-off
circuit. A sealing or locking provision can be provided
to ensure that a circuit under maintenance is not
unintentionally re-energised.
Conclusion
A circuit breaker is a critical safety device to protect
people and property where electricity is distributed.
As discussed, its primary function is to protect the
distribution system from reaching damaging tempera-
tures, whether caused by gradual heating from an overload or
the near-instant effects of a short-circuit fault. When correctly rated,
these over-current devices quietly and reliably protect our homes,
industries and people.
References
[1] Aberdare Cables. (Jan 2008). ‘Cables Facts and Figures’. [online]
Available at:
[2] SANS 60947-2, 2012. Low-voltage switchgear and controlgear:
circuit breakers.
[3] ,SANS 10142-1, 2012. The wiring of premises: low-voltage instal-
lations.
Acknowledgement
Special thanks to Isak Kruger andWimpy Lyons for their valued input.
lation testers can directlymeasure the prospective short circuit current
for a complete installation, but for other systems, calculation based
on transformer impedance and feeder cable characteristics
must be used.
Cable manufactures often publish K-factors to
plug into the equation I=K×A/√t, where I is the fault
current, A is the conductor cross-section and t the
duration of the fault current (typical K=115 A.s½/mm2
for copper in PVC). A general rule of thumb provided
in SANS 10142 is that a PVC insulated cable’s cop-
per cross-sectional area in mm2 is the about same
number of kiloamperes it can withstand for one cycle
of current i.e. a 4 mm2 cable will withstand 4 kA for 20 ms.
For practical reasons, there are some additional concessions
to keep LV installations economical.
If the source prospective fault level is very high, it might be eco-
nomically unfeasible to use many distribution breakers that is rated
to that level. In such cases, a cascading system can be implemented.
Cascaded systems are combinations of high rated upstream short-
circuit protection with defined energy limiting capability, paired with
lower rated downstream distribution breakers and lighter cables.
It has the desirable effect that, during a short circuit fault, the up-
stream current-limiting breaker opens and limits the energy to a level
that the downstream breakers (and cables) can withstand. Inversely,
the upstreambreaker will not trip for a branch circuit’s overload where
the downstreambreaker trips. Circuit breaker manufacturers will have
listed combinations of breakers that are tested to provide adequate
discrimination and withstand capability when used in such pairs.
Additional considerations
Thermal-magnetic circuit breakers depend on the ambient tempera-
ture. For example outdoor distribution panels, that see temperature
variation from sub-zero to 60°C, might change the trip-level by ±15%
or more. In such a case, hydraulic-magnetic technology offers the
advantage of consistent trip levels.
Wynand Visser joined CBI-
electric: low-voltage in 2012,
after obtaining his B.Eng and
M.Sc.Eng from Stellenbosch
University. He currently works
as design engineer for CBI, is involved in
testing and certification operations and sits
on SABS Technical Committee 73.
Enquiries: Tel. +27 (0) 11 928 2000 or email
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