Instantaneous trip elements

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mbrooke

mbrooke

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Are these required to be enabled or present in molded case circuit breaker and power breakers? I'm hypothetically thinking about them in regards to selective coordination.
 
ron

ron

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By default, they are enabled in power breakers, and always enabled (not able to be disabled in molded case breakers).
For power breakers in Switchgear with 30 cycle ratings, theoretically instantaneous trips can be disabled, but arc flash enrgy with skyrocket and be sure that instantaneous override doesn't get enacted.
 
mbrooke

mbrooke

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ron said:
By default, they are enabled in power breakers, and always enabled (not able to be disabled in molded case breakers).
For power breakers in Switchgear with 30 cycle ratings, theoretically instantaneous trips can be disabled, but arc flash enrgy with skyrocket and be sure that instantaneous override doesn't get enacted.
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So it has to do with the magnetic withstand of equipment? I'm having massive selective coordination issues.
 
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petersonra

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mbrooke said:
So it has to do with the magnetic withstand of equipment? I'm having massive selective coordination issues.
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Take this with a grain of salt because I am not an expert in such things but as I understand it ANSI rated switchgear generally uses breakers that don't open until 2 seconds after a short circuit event. This allows the downstream breakers to open long before the switchgear opens in the event of short circuit event.

as I understand it UL listed panel boards and switchboards do not have such a long time delay on the instantaneous side. they will trip pretty quick on a short circuit if the trip current is in the instantaneous range.

Coordination involves finding downstream breakers that will trip first on a reliable basis. Often the trip curves are close enough that it's difficult to make this happen especially if the breakers are similar size.

You might be able to split up your downstream feeder circuits and use multiple breakers with lower trip settings. If you have an upstream breaker that's 200 amps and a downstream breaker that's 150 they're probably going to have overlapping trip curves and you won't have any way to reliably coordinate them.
 
PhenixFord

PhenixFord

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petersonra said:
Take this with a grain of salt because I am not an expert in such things but as I understand it ANSI rated switchgear generally uses breakers that don't open until 2 seconds after a short circuit event. This allows the downstream breakers to open long before the switchgear opens in the event of short circuit event.

as I understand it UL listed panel boards and switchboards do not have such a long time delay on the instantaneous side. they will trip pretty quick on a short circuit if the trip current is in the instantaneous range.

Coordination involves finding downstream breakers that will trip first on a reliable basis. Often the trip curves are close enough that it's difficult to make this happen especially if the breakers are similar size.

You might be able to split up your downstream feeder circuits and use multiple breakers with lower trip settings. If you have an upstream breaker that's 200 amps and a downstream breaker that's 150 they're probably going to have overlapping trip curves and you won't have any way to reliably coordinate them.
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I've recently learned more about this since working in the industrial maintenance industrial. I can't remember the terminology used for this type of engineering. But it is quite important in a manufacturing facility when replacing breakers in sub panels.
 
ron

ron

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Selective coordination is a very involved topic. Each manufacture has literature on the topic as it is more involved than just what you see on a time current coordination curve.
There are tested combinations when the breakers in question are the same manufacturer and the know short circuit current is less than the tested combination in the published tables. For example, Schneider has this document, even if the instantaneous curves overlap on a TCC. https://download.schneider-electric...File_Name=0100DB0501.pdf&p_Doc_Ref=0100DB0501
 
mbrooke

mbrooke

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ron said:
Selective coordination is a very involved topic. Each manufacture has literature on the topic as it is more involved than just what you see on a time current coordination curve.
There are tested combinations when the breakers in question are the same manufacturer and the know short circuit current is less than the tested combination in the published tables. For example, Schneider has this document, even if the instantaneous curves overlap on a TCC. https://download.schneider-electric...File_Name=0100DB0501.pdf&p_Doc_Ref=0100DB0501
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Thanks

But The way I see it is a bi-metal strip alone would allow for coordination provided there is a 2:1 ratio of each down stream device?
 
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topgone

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mbrooke said:
Thanks

But The way I see it is a bi-metal strip alone would allow for coordination provided there is a 2:1 ratio of each down stream device?
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Been there, done that but failed big time!. You cannot coordinate breakers even if you have more than 2:1 ratio between the two! Even a 3200A main breaker failed to coordinate with a 630A branch breaker! Remember that TCCs have almost the same instantaneous trip times. That said, if your system faults and the fault is seen by both breakers, it will be a battle which breaker trips first. The system I inherited blacked-out when such a thing happened!
 
mbrooke

mbrooke

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topgone said:
Been there, done that but failed big time!. You cannot coordinate breakers even if you have more than 2:1 ratio between the two! Even a 3200A main breaker failed to coordinate with a 630A branch breaker! Remember that TCCs have almost the same instantaneous trip times. That said, if your system faults and the fault is seen by both breakers, it will be a battle which breaker trips first. The system I inherited blacked-out when such a thing happened!
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But whats doing this- the delay aspect or the instantaneous aspect?
 
ron

ron

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A regular ul listed circuit breaker in a panelboard, switchboard or switchgear will not be thermal only.
Thermal only, would be like overloads in a starter.
Magnetic only (instantaneous), is like a molded case switch in a starter.
2:1 would only work for a breaker if it coincidentally was on a tested combination table from a particular manufacturer.
 
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