Isolation Transformer

It could just be that the instantaneous trip setting of the breaker is set too low to allow for the magnetization inrush current of that transformer. That's not an uncommon problem. Transformer inrush can blow fuses or trip breakers, it can be tricky to coordinate the trip settings to prevent it.

The flash you see is likely just the contacts opening, that's normal, but some breakers contain the visible light from it better than others.

From what you report I think "main breaker" is on the supply side of the transfo?

What are the rated voltage and currents of transfo & breaker?

When you close onto a transfo it is certain that sometimes one phase will close at zero voltage. This leads to saturation so extreme that only primary winding resistance and its inherent (air cored) inductance limit the peak current which could exceed 20x rated current.

Most basic breakers have a (delayed) thermal overload and a magnetic instant trip at a fixed multiple of rated current, say 10x. But there is a tolerance on the "10x" e.g. 8 to 12.5x. There is also a tolerance on transfo impedance of around +/-20%.

It is not an easy design call and often a certain "no turn-on trip ever" solution needs a breaker with a rated current exceeding that of the transfo AND a magnetic trip at a higher multiple than the "standard" breaker.

You have not clarified if the trip occurs with the tugboat on shore or on-board power. If it is shore it is possible the cable size or run length is different between berths, giving higher currents.

The NSX100 data suggests it has an interchangeable trip unit - you report you changed the breaker. I note that the electronic trip unit has instantaneous trip >=12 In (compare 8 to 12.5 tolerance on standard) plus delayed 20ms trip at adjustable current, on top of the "thermal" Ir.

Did you change the trip unit or just the breaker?

Did the new breaker come with the same trip unit as the old one?

Is the transfo liquid or dry, what is accessible on it?

67model

Thank-you for your input it was most valuable .After further investigation i found the inrush current setting on the breaker was set too low so in the event of closing the breaker the making and instantaneously breaking of contacts caused the arcing. I wasn't aware of such a high inrush current related to isolation transformers.

Happy days, thanks again for the replies

Hi John,

We manufacture transformers and always recommend a motor start characteristic on the input breaker because the inrush current at switch-on can be up to 100 times the normal running current.

You state that it is a 100A transformer and a 100A breaker. That is bound to give you the inrush problems.

I suggest at least a 120 A breaker because the transformer can easily take a 20% overload - if designed correctly - at least for a short time. Then with motor start characteristics it is unlikely (not impossible) that the breaker will trip because it will allow a motor stall current for several seconds. The motor stall current is effectively identical to the transformer inrush current because a motor and a transformer are basically the same electrical circuit. By this time the transformer will have settled down - usually within about one or two seconds.

I can not explain why other installations do not have the same problem unless your incoming voltage might be a bit higher before the transformer is switched on or perhaps they have a higher installation wiring resistance which limits the current.

I hope this helps