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CIRCUIT BREAKER FAULT RATINGS
Introduction
The IEC low voltage circuit breaker Standard IEC 60947-2, Low voltage switchgear and
controlgear - Part 2: Circuit-breakers has been around for many years now, however it's surprising
how often fault ratings are misunderstood.
So what is the kA rating?
The value of the kA rating determines how much current the circuit breaker can withstand under
fault conditions. The circuit breaker only has to withstand this for a brief period of time, usually the
time it takes for the circuit breaker to trip. For example, a value of 6kA means that the circuit
breaker can withstand 6,000 amps of current during the brief time it takes to trip.
The main parameters
Icu - ultimate short-circuit breaking capacity - a breaking capacity for which the prescribed
conditions according to a specified test sequence do not include the capability of the circuit
breaker to carry its rated current continuously.
Ics - service short-circuit breaking capacity - a breaking capacity for which the prescribed
conditions according to a specified test sequence include the capability of the circuit breaker to
carry its rated current continuously.
Icw - rated short-time withstand current - the rated short-time withstand current of a circuit
breaker is the value of short-time withstand current assigned.
Icm - short-circuit breaking (or making) capacity - a breaking (or making) capacity for which the
prescribed conditions include a short circuit
What does it mean?
Icu is really the maximum perspective fault which a circuit breaker can clear (with the fault current
being expressed as rms for ac).
This is verified by testing in accordance with the standard and is applicable at a specific set of
electrical and environmental conditions. If these conditions change then it may be necessary to
de-rate the circuit breaker. After clearing a fault the circuit breaker does not have to remain
serviceable and could be dangerous to operate. This point is particularly important in circuit
breakers when the Ics is lower than the Icu.
Ics is the maximum perspective fault current which the circuit breaker can clear and still remain
serviceable.
The standard does allow some minor welding of the contacts to take place, so after a large fault it
would still be necessary to inspect the breaker. When specified as a percentage of Ics, the
standard proposes ranges of 25%, 50%, 75% and 100%.
Icw is the perspective fault withstand rating (rms for ac).
Circuit breakers may be subject to through fault which they are not intended to clear. While not
clearing these faults, the breaker will still need to withstand the thermal and mechanical stress
imposed by the fault current. The longer a fault is present the more the effects build up and Icw
always has a time element associated with it (i.e. 50 kA for 1 second). The standard specified
preferred time ranges of 0.05, 0.1, 0.25, 0.5 and 1 second (although 3 seconds is also often used
in practice).
Icm is the peak current which the circuit breaker can safely break or make.
It is expressed as the maximum perspective peak current at a rated voltage, frequency and power
factor and is always greater than Icu. From a safety aspect this is particularly important as it will be
the primary mechanism to protect the operator if the circuit breaker is closed on to a fault. All
ratings are derived under specific electrical and environmental conditions and are verified with the
circuit breaker in free air. As soon as the breaker is enclosed in in any kind of panel or cabinet the
ratings change and need to be re-assessed as part of the assembly testing.
Why is the kA rating so important?
Under fault conditions (such as a short circuit) much more current flows through the circuit than
what it was designed for. A circuit that was designed for a maximum of 20A may suddenly be
drawing hundreds of amps, the circuit breaker will trip if this occurs.
However, what if during a short circuit there is more current flowing through the circuit than the kA
rating of the circuit breaker? In this case the circuit breaker will fail, often in either one of two ways.
One possibility is that the contacts in the circuit breaker will weld, thus preventing the circuit
breaker from tripping. The best case scenario for this is that the cable in the circuit is damaged.
The worst case is that a fire is started. Another possibility is that the circuit breaker explodes, as a
result of the copper in the circuit breaker overheating and turning into dangerous plasma. This
could be very dangerous to people nearby, for example the electrician turning the circuit breaker
on after a fault.
How to calculate the correct kA
The maximum amount of current that can flow through a circuit is determined by the size of the
transformer feeding the circuit and 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 typical 500kVA transformer has a short circuit current of 35kA at its terminals. The
cable run from the transformer to the main breaker is 10m and is run with 90mm2 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 26kA.
In this case, a 20kA circuit breaker cannot be used in the installation.
Cascading
Fortunately, not every circuit breaker in the installation needs to be rated above 26kA. Cascading
is what happens when you place a smaller kA rated circuit breaker on the load side of a larger kA
rated circuit breaker (for example, a 6kA circuit breaker downstream from a 20kA circuit breaker).
In these cases, the larger circuit breaker limits a certain amount of the fault current, thus enabling
you to safely use smaller rated circuit breakers downstream.
You can determine what size circuit breakers can be cascaded from the manufacturers. These are
usually listed as cascade tables. You need to consult these tables, because you can't just use any
smaller breaker size downstream. For example, the cascade tables may show that you can use a
6kA breaker downstream from a 20kA breaker. However, you probably can't use a 3kA breaker the 20kA breaker just doesn't provide enough protection.
For more information about cascading see our other post here
Conclusion
The kA rating of a circuit breaker is a very important safety aspect to consider when designing
switchgear. Without it, there is a good chance that a serious accident will occur.