Sparks Electrical News

Selecting the correct Circuit Breaker (CB) for your distribution panel is crucial for the longevity of the installation as well as the safety of those maintaining and occupying the premises. This article addresses the selection of key breaker attributes such as voltage, current and kA rating.

Rating considerations

Circuit Breaker Voltage Rating

How to select the right circuit breaker for your installationThe voltage rating of a CB is determined by the highest voltage that can be applied across any two conductors in the circuit. It is important to select a circuit breaker with enough voltage capacity to meet the end application. A single phase ac circuit in South Africa is generally rated at 230 V and a single pole CB rated at 230 V can be used. A three phase ac circuit operates at 400 V and requires a Triple Pole CB rated at 400 V.

Circuit Breaker Current Rating

The next rating to consider is the amperage or ‘operating current’ of the breaker. CB’s are designed to operate at 100 percent of the required load. However, in order to offset the effects of heat generated by the system, it is good practice to select a CB at approximately 125 percent of the required load. For example: If a supply of 250 A is available from the transformer, the breaker of choice for the main incomer should be rated at 25 0A in order to protect the transformer. However, the feeder breakers feeding a 25 A load should be rated at 32 A.

Circuit Breaker kA Rating

Finally the ‘kA rating’ or ‘fault level’/’rupturing capacity’ of the CB should be taken into account. The kA rating of the CB indicates the maximum short circuit current that the CB can withstand without arcing or catastrophic failure. This current can be upwards of 100 times the required load and has the potential to cause major damage to property and personnel. For example: A circuit breaker rated at ‘6 kA’ means that the circuit breaker can withstand 6 000 amps of current during the brief time it takes to trip.

Why is it so important to choose the correct kA rating?

If the short circuit current is greater than what the CB can withstand, the contacts in the CB can weld together, preventing it from tripping. Another possibility is that the CB can explode, spewing dangerous plasma.

So how do I calculate the correct kA?

The maximum current that can flow through a circuit is determined by the size of the transformer feeding the circuit as well as the length of the cable run from the transformer.

This is often called the downstream short circuit current. This will determine the maximum

kA rating required for the main circuit breaker.

For example: A 500 kVA transformer that has a short circuit current of 35 kA at its terminals.

The cable run from the transformer to the main breaker is 10m and is run with 90 mm² cable. The resistance in the cable limits how much current comes from the transformer, and so after calculations it was determined that the short circuit current at the end of the cable would be 26 kA. In this case, a 20 kA circuit breaker cannot be used in the installation.

SABS Approved Dealers

When selecting a CB, it is vital for it to be SABS or IEC approved. This provides the assurance that the CB’s have been tested to strict quality standards and will operate in a safe manner as required. Well-known brands such as ABB, Schneider and CBI are all SABS approved and are regarded as high quality devices. Switchboard Group is a registered supplier of these products and the leading manufacturer of LV panels is South Africa.

In conclusion a CB should be selected based on the nominal current, kA rating, number of poles required and whether the CB is SABS approved.

By Brendon Swanepoel, 2nd Year Electrical Engineering Student, University of the Witwatersrand. Brendon is completing Switchboard Group’s six week Learnership and Training program offered to students looking to further their practical skills.



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