Discrimination and cascading in circuit breakers

[smithbrian]

Hello. I don't normally deal with this type of things so I am a bit confused with this issue. I have two circuit breakers, the upstream circuit breaker is rated for currents 100A-160A and the downstream circuit breaker is rated for currents up to 25A. It says the discrimination between the circuit breakers is up to 36kA and cascading up to 50kA.

My questions are as follows.

Q1. Does discrimination mean that for short circuit currents up to 36kA the downstream breaker will open first?

Q2. Does cascading mean that for short circuit currents up to 50kA the upstream breaker will open thus clearing the fault?

Please any help in this manner would be greatly appreciated.

 

[davidbeach]

Where in the world are you? In other words, what installation code and product standards apply?

 

[smithbrian]

I am in Australia. The electrical installation code here is called AS3000. I thought was a general question thought and the answer would be the same anywhere in the world.

 

[healyx]

Hi Brian,

I'm in Australia too.

Q1. Yes, but if the upstream breaker is adjustable, you'll need to make sure the short-time and/or overload portions of the curves do not overlap if the upstream breaker settings are not set to their maximums.
Q2. No. Cascading refers to the enhanced (reinforced) breaking capacity of the lower device. It has nothing to do with discrimination. It means the lower device can be installed in a location where fault currents are up to 50kA, even if the native breaking capacity is lower than this. Above 36kA, discrimination is not guaranteed.

 

[smithbrian]

Hello Healyx

I just had a few questions.

1. What happens if the time current curves do overlap?

2. If the downstream device is installed where the fault current is above 36kA less trhen or equal 50kA what is the point of installing it there if it cannot clear the fault?

 

[healyx]

Brian,

Q1. The curves will always overlap in the instantaneous portion, that is why we need the table figures. You need to check curves for overlap in the overload (long-time) and short-time(if applicable) portion. If they overlap in these regions you don't have discrimination (provided the maximum fault level is higher than the point at where they overlap, which is almost always the case in the LT/ST region).
Q2. It can clear the fault, but the upper device might clear it as well (both devices trip). So yes, a bit pointless, but you might consider that an acceptable risk depending on the installation. The manufacturer is just providing that info to assist, but you may decide not to go over 36 kA for that reason. Don't forget, just because 2 devices don't achieve discrimination according to the manufacturer doesn't mean they won't comply according to section 2.5.7.2.3 of AS3000. AS3000 has some pretty loose interpretations of what is deemed discrimination. I always make sure manufacturer discrimination is achieved.

What make of breaker are you using?

 

[oldfieldguy]

Instantaneous operations cannot be coordinated. In the event that two (or more) breakers see the same fault, both will try to operate to clear. For example, let us assume an 8 kA fault downstream of a breaker with an instantaneous trip setting of 2 kA and upstream of that breaker is its feeder set at 5 kA. Both breakers see the fault, both will attempt to operate. The fastest one wins, clearing the fault. If it is the breaker closest to the fault, you are happy. If it's not, and in clearing the downstream fault it isolated a lot more equipment, you have a problem.

I have seen a close-in fault on a 480-volt sump pump feeder clear an entire motor control center, shutting down a major production unit. Wasn't happy at all. Re-examined device settings and recommended changing the main breaker from instantaneous trip to short time delay.

old field guy

 

[healyx]

OFG.

Discrimination tables are published for the very purpose of proving coordination in the instantaneous region. This may be an IEC thing, but it is common practice.

 

[dpc]

FWIW: Apparently "cascading" is equivalent to "series rating" in the US. The term "discrimination" is generally never applied to breaker coordination in the US. We would call it selectivity.

Regarding coordination in the instantaneous region of MCCBs, this is not a common concept in the NEMA world. Since the requirement for selective coordination of emergency power system was added to the NEC (as pushed by the fuse manufacturers for many years), the circuit breaker manufacturers have been publishing information regarding selective coordination of molded case breakers in the instantaneous range. Of course, as with the series rating, this is only applicable where proven by test and this will always be for breakers from the same manufacturer at least in the US. I agree with old field guy that coordination in the instantaneous region is not the normal situation. Even when the published tables are used, there are limits since it depends on fault current levels within a certain range.