LV circuit Breaker Fault Level Ratings

1.       The main parameters

2.       What does it all mean?

3.       Applying the ratings

The main parameters

Just to be clear, we are only talking about fault ratings (not rated current).  On this, it's probably easiest to start of with the IEC definition of the relevant ratings:

I_cu - 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

I_cs - 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

I_cw - rated short-time withstand current
The rated short-time withstand current of a circuit-breaker is the value of short-time withstand
current assigned

I_cm - short-circuit breaking (or making) capacity
a breaking (or making) capacity for which the prescribed conditions include a short circuit

What does it all mean?

While the definitions are pretty straight forward, perhaps a little discussion is worthwhile. 

I_cu 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 derate 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 I_cs is lower than the I_cu.

Difference between I_cu and I_cs

When tested against the standard, circuit breakers under go the following tests:  

I_cu is subject to an O-t-CO sequence. The breaker is then certified safe by a simple dielectric test. 

I_cs is subject to an O-t-CO-t-CO sequence. The breaker is then subject to both dielectric withstand and temperature rise tests.

O	-	breaking operation
CO	-	making operation followed by breaking operation
t	-	time interval (short as possible, but minimum of 3 minutes)

Tests are carried out at the specified fault current.

I_cs 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 I_cs, the standard proposes ranges of 25%, 50%, 75% and 100%.

I_cw 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 I_cw 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).

I_cm 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 I_cu.  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. 

Applying the ratings

To end the post, a short discussion on applying the ratings in selecting a circuit breaker. 

It should now be obvious that all the fault parameters need to be considered in selecting the correct circuit breaker. I think a lot of people reading this have seen specification clauses like, "circuit breakers shall be rated at 50 kA for 3 seconds", with no other details.  When I see statements like this my first impression (rightly or wrongly), is that I'm reading a poor specification.  I would encourage everyone to be specific in their specification of what they require and address I_cu, I_cs, I_cw and I_cm as part of the specification.

I've also noticed that in general for most of the big manufacturers the majority of their ranges have an I_cs equal to the I_cu.  If  I'm presented with a breaker where this is not the case, I immediately become sceptical on the quality of the device.  The device may well be suitable for the application, but I would be more vigilant on my appraisal of the circuit breaker.  I tend always specify that the I_cs should equal the I_cu - to my mind, what is the point of a circuit breaker if it can't be readily reused after tripping.

Something else to bear in mind is that the rating vary depending on the voltage.  For low voltage, we are talking anything from 1000 V down to zero.  The lower the voltage the higher the rating (for example, a Schneider NW12 H2 breaker has an I_cu = 100 kA at 415 V, and this drops to 85 kA at 525 V).  I_cw is also dependent on time (the longer the fault the lower will be the rating).  Often I_cm is larger than I_cu or I_cs - make sure you are looking at the right figures and if you are interested in the I_cu it should be I_cu you are being shown and not I_cm.

As a final bit of advice - always consult manufacturers catalogues and technical data.  Manufacturer's produce a whole range of devices, and you need to select the correct one for your application.  There is no point paying for higher rating if you don't need to.  At the same time compromising on ratings to have a cheaper circuit breaker jeopardizes safety and operational reliability.





Courtesy : Steven McFadyen, myelectrical.com