# Time Current Curves of Circuit breakers



## Girish (Dec 12, 2012)

Are all circuit breakers designed according to International/National Standards?

I have to do an arc flash analysis. I have seen a Merlin Gerin compact NS1250 N in a plant. The latest version of ETAP library does not have the same listed. One of my friends says you can take an equal rated breaker from another manufacturer. He says that these breakers follow IEC 60947 and the time current curves will be matching for similar rated ones (circuit breakers). I am not sure about that statement. Please share your experience/views.

Thank you in an advance.


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## Flyer_PE (Dec 12, 2012)

I would be very careful about making a general assumption like that. I do a lot of work in older facilities where the equipment predates some of the standards.

If you can get the curves from Merlin Gerin, compare them to the curves in the ETAP library. At least that will give you a basis for which curves you are using in the program.


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## Girish (Dec 13, 2012)

Flyer, have you in you experience compared the TCC of any two breakers from different manufacturers, found any similarity (I mean the level required to perform arc-flash).


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## Flyer_PE (Dec 13, 2012)

The shapes of the TCCs will be similar for any given device. The thing I would be looking for is a curve that matches the clearing time in the instantaneous region as well as the current value where the breaker transitions from instantaneous to thermal. Those values will vary between breaker manufacturers and models.


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## ventilator (Dec 13, 2012)

I have used breakers from other manufacturers in arc flash studies but have always obtained the TCCs for the breakers in question and made sure they are the same. A lot of times the same trips are used across different brands (like a lot of old westinghouse matches cutler hammer) but I always check. This can be tricky depending on the type of trip units, thermal vs digital, but you need to make sure they are truly comparable devices. I've never used Etap, but SKM will build you a model for your breaker if you can provide them with the TCC from the manufacturer. Maybe Etap would do the same if you can find the TCC.

Basically, I wouldn't just trust that they should have been made to conform to a standard without checking with the manufacturer for TCCs first.


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## akpak (Dec 14, 2012)

What type of breakers are you referring to? If they are high-voltage breakers used in power distribution substations, then the breakers themselves don't have a time current curve (TCC). The breaker has a certain clearing time (6~10 cycles). It is the overcurrent *relay* that has the TCC. So your total fault clearing time is your relay operate time (tcc)+relay response time(few cycles for electromechanical relays)+breaker clearing time(few cycles). The latter two are pretty much constant for all fault current levels. Your relay operate time is based on pickup and time dial settings.


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## Girish (Dec 15, 2012)

Thank you all for your views. That just clears my doubt. This means that the International/National standards do not define/regulate curves (as points in TCC) for the manufacturers to use while designing these LV breakers. So ETAP/SKM can be contacted for the respective TCCs. Correct me if I am wrong!

@akpak : Merlin Gerin NS1250 N is an LV breaker. My apologies for not mentioning it in the post.


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