3 Phase breaker trip

[pimpdaddydog123]

We have a single phase welder which we want to connect to a 3 phase welding socket.

The socket has an integrated RCD and is wired back to a 3 phase breaker.

we have connected the welder across 2 phases but when we plug it in it immediately trips the breaker.

Do 3 phase breakers trip on phase imbalance as this is the only cause of tripping I can find, there are no earth faults and the welder doesn’t seem to be drawing too much current.

thanks I’m advance chaps.

 

[ericmark]

What voltage? 230 or 400?

 

[ericmark]

Single phase welders come as 230 volt and 400 to connect a 400 volt between two phases is OK, but 230 connects phase to neutral.

 

[pimpdaddydog123]

It is an old 400v welder, I measured the resistance of the transformer primary at about 20 ohms so it should only be drawing about 8 amps and it’s a 63 amp breaker.

 

[ericmark]

In rush could take out a 63 amp MCB and the resistance says nothing it's AC.

 

[pimpdaddydog123]

I agree about the inrush being a possibility but surely the resistance will lower than the impedance or have I got that wrong lol!

 

[AdrianUK]

Do 3 phase breakers trip on phase imbalance.......

No, they don't. Three phase breakers are simply three single phase breakers mechanically coupled so that if one breakers decides to trip then they ALL trip.

There is no problem with a phase-phase load (or even a single phase load if a neutral exists) being supplied through a three-phase breaker.

 

[AdrianUK]

It is an old 400v welder, I measured the resistance of the transformer primary at about 20 ohms so it should only be drawing about 8 amps .....

Think you may need to double check you math here

 

[ericmark]

400/20 = 20 amp. And yes one would expect the load to be less than 20 not more, but I have been caught out where some relay has dropped in once powered up.

 

[pimpdaddydog123]

You’re right chaps, my maths was a bit out!!!
Do you know if some breakers do trip on phase imbalance though?

something in the back of my mind tells me that such breakers do exist. I will check it on Monday once I can get the model number.

 

[RF Lighting]

No. No circuit breaker will trip on phase imbalance. They operate on over current only. If you draw more than the rating of the device it will trip and that is it.

As has been said, number one thing to check is that the welder is definitely a 400V welder. If so then check the rating and type of the circuit breakers as inrush would be the number 2 cause.

 

[JohnW2]

As has been said, number one thing to check is that the welder is definitely a 400V welder. If so then check the rating and type of the circuit breakers as inrush would be the number 2 cause.

We've been told that it's a 63A breaker, but no what type.

On the face of it, even if it's a B63, if (as seems to be the case) the running current is under 20A, I would have doubted that the inrush would be greater than 315A (well, 189A for 3In), wouldn't you?

Kind Regards, John

 

[RF Lighting]

It’s quite possible. We are dealing with a big inductive load which will be effectively a dead short at switch on.

I’ve got a 110V transformer which even with no load connected on the secondary side trips my 50A C type submain when it is switched directly on line.

The resistance the OP states seems unlikely. The primary coil should be effectively a short circuit when tested with an ohm meter. When powered but not actually welding the current draw should be next to zero.

 

[JohnW2]

It’s quite possible. We are dealing with a big inductive load which will be effectively a dead short at switch on.

Yes, but doesn't it behave as a big inductive load which is in series with (not in parallel with) the resistive component of the impedance? If so, since we've been told that the resistance is about 20Ω, then the impedance couldn't be less than that at switch-on, which would equate to less than 20A at switch-on (at 400V), even if the inductive reactance were zero, wouldn't it?

... or am I wrong about how an 'inductor with resistance' behaves?

Kind Regards, John

 

[pimpdaddydog123]

Thanks for the comments chaps, it is an interesting subject.

I measured the resistance with a Fluke MFT set to low resistance.

The breaker is a Schneider NSX 100F.