Electricity + Control June 2017

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: http://www.aberdare.co.za/facts-figures [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.

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.s ½ /mm 2 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 mm 2 is the about same number of kiloamperes it can withstand for one cycle of current i.e. a 4 mm 2 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.

Acknowledgement Special thanks to Isak Kruger andWimpy Lyons for their valued input.

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 cbi@cbi-electric.com

Electricity+Control June ‘17

38