How to calculate the KA rating for a circuit breaker if we
know the KVA rating of the generator feeding it?
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Answer Posted / ravishankar
How to correctly size kA ratings of circuit breakers
Introduction
One of the most important safety aspects to consider when
designing an electrical circuit is to correctly size the
circuit breaker fault current rating (commonly called the
kA rating). Unfortunately many electricians are unaware of
this concept, and it remains one of the most common design
faults found in electrical circuits. In the last 10 years
it has become very common in domestic wiring, because of
changes in the way power is distributed to new houses.
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.
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, if not thousands 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.
It is outside the scope of this document to show how to
calculate the short circuit current, but tables for this
can be found on the internet. Another source is page 9-20
of the NHP Circuit Breaker Products catalogue.
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.
Most common mistakes
The most common mistake on large installations is that the
kA rating of the circuit breaker was not taken into account
when designing the circuit. Instead, the cheapest circuit
breaker is chosen that meets the required standard current
draw. The author has seen this himself a number of times
(in fact, the example used above in the document did
actually occur - a 20kA circuit breaker was used on the
site when it needed a minimum rating of 26kA).
It is becoming more and more common for larger transformers
to be used to power domestic installations. This has
created a situation where the short circuit currents are
much higher than they used to be. A typical transformer to
power a street of houses may have been 100kVA. Now it is
not uncommon to see 300kVA or even 500kVA transformers.
This can result in short circuit currents of 20kA and
above. To make matters worse, a number of well known
manufacturers sell cheap low quality circuit breakers that
have a rating of only 3kA. Electricians are continuing to
use these to wire these without understanding the full
ramifications of their decision.
Conclusion
The kA rating of a circuit breaker is a very important
safety aspect to consider when designing a circuit. Without
it, there is a good chance that a serious accident will
occur. It only takes a few minutes to do the calculations
when you have the correct tables.
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