Time current coordination between 3 pole breaker vs 1-pole breaker

[OCPD]

Hello all,

I have question on time current coordination (TCC) done in software's available. I keep on having discussions with people on this. Is it correct to show coordination of a 3 pole breaker with a single pole breaker on TCC in a 3 -phase system? For example a panelboard has a main of 225A and the branch breakers are all single pole 20A. On the TCC will the trip curves showing coordination between 225A/3P and 20A/1P be correct?


I think the three pole breaker trips all three poles for an overcurrent on a single phase, it does not require all three phases to have an overcurrent. Therefore, if there is a fault downstream of one of the single-pole breakers, one phase of the three-pole breaker will also see this same current and could trip if not coordinated properly.

People usually confuse the phase domain with sequence domain and I get mix comments on it. I really would like to know if it is correct to show 3 pole breaker and single pole breaker on a TCC are correct? If correct how to explain this to people who dont agree. If not correct. Why?

 

[ron]

I think the three pole breaker trips all three poles for an overcurrent on a single phase, it does not require all three phases to have an overcurrent. Therefore, if there is a fault downstream of one of the single-pole breakers, one phase of the three-pole breaker will also see this same current and could trip if not coordinated properly.

Agreed

 

[mgookin]

And if the fault is of a magnitude sufficient to trip both the branch and the main, the main will trip first if it has a faster trip curve. This stumps many when it happens. They don't understand why a 200A breaker tripped before a 20A breaker.

 

[steve66]

To be honest, I've never even thought that there might be a difference.

But when we graph the TCC of a three pole breaker, aren't we really just graphing the trip curve of a single pole on that breaker?

So there shouldn't any problem with coordinating 1 pole breakers with 3 pole breakers. If there is space between the curves, they should be coordinated, and if there isn't space, they aren't coordinated.

The same should apply for 2 pole breaker with either 3 or 1 pole.

 

[Jraef]

To be honest, I've never even thought that there might be a difference.

But when we graph the TCC of a three pole breaker, aren't we really just graphing the trip curve of a single pole on that breaker?

So there shouldn't any problem with coordinating 1 pole breakers with 3 pole breakers. If there is space between the curves, they should be coordinated, and if there isn't space, they aren't coordinated.

The same should apply for 2 pole breaker with either 3 or 1 pole.

Yes, TCCs are per pole, not per breaker. If you have a 3 pole breaker a 2 or 1 pole breaker and ask for the TCC, it is the same one. Each pole in the breaker has it's own trip elements, they sense the current in THAT pole only. When it needs to trip, the elements acts on a common trip bar (in multi-pole breakers) and opens them all, but only one of them need sense the over current.