10 kva site transformer

[John robertson]

Hi got a 10kva transformer that keeps tripping. Im told its about 20 year old. Its fed off a type C 32 amp breaker. Single phase. Did continuity test between windings and all ok. Secondary has a centre tap so is that why I would get continuity between phase and earth? Not worked with much transformers so not really sure what else to check for. Supply cable is fine. Thanks in advance

 

[Lucien Nunes]

Tripping supply MCB or RCD or its own thermal breaker? Has it been working on that breaker before?
Does it only trip when switching on or while it is already on?
Is it likely to have been overloaded badly? Any sign of burning? Any possibility that the output wiring is shorted?

 

[Mikegh]

Could be it might not run off a C type assuming it's not faulty

Might need a D type

The 10kva rating obviously exceeds 32 amp but I doubt you're exceeding it, I think the 10kva is a "tool rating" and not a continuous rating

 

[pc1966]

10kVA is 43.5A at 230V so well above 32A!

Hager recommend a 63A D breaker for input protection to avoid tripping on inrush surges, though you probably should have something like a 45A breaker on the output (if 230V, different if 110V etc) for overload protection, assuming it really is 10kVA full rating.

EDIT: If site transformer then probably 55-0-55 so outputs would be various sockets on appropriate DP MCBs anyway.

 

[freddo]

is that why I would get continuity between phase and earth?

I assume you are not talking about the primary here?

 

[John robertson]

Hi guys, yeah it has been working for as long as they have had it, only started tripping the other day. They are just using grinders off it but usually just one so dont think its overloaded. I did see a 110 socket with the L and N points touching so sorted that but still trips when unplug everything. I was told before that when they were using the grinder they could feel it losing power. All connections are tight. And yes Secondary side was continuity between phase and earth.

 

[John robertson]

Tripping supply MCB or RCD or its own thermal breaker? Has it been working on that breaker before?
Does it only trip when switching on or while it is already on?
Is it likely to have been overloaded badly? Any sign of burning? Any possibility that the output wiring is shorted?

MCB. Yeah been working for as long as they have had it. No signs of burning, no in great conditions with dust etc though

 

[Lucien Nunes]

Sounds like it might have internal shorts if there is definitely no fault with the output socket wiring. Although that mostly happens when a transformer is overloaded and it doesn't seem like this one has been. The way to test for that in the workshop is to power it from a variac and ramp up the supply voltage watching the primary current, but obviously that's hard to do on site.

One possibility is that the inrush might always have been borderline for tripping the MCB, and the supply voltage has increased a little and now it routinely trips, although you would expect sometimes to be able to close the breaker or switch and then it should hold OK until it's switched off and on again. People speak of MCBs sometimes drifting in calibration and becoming overly sensitive; IMO that is often wishful thinking, I've never seen an MCB drift significantly, not the magnetic trip anyway.

 

[Mikegh]

AaaaaaaaQqaQ

10kVA is 43.5A at 230V so well above 32A!

Hager recommend a 63A D breaker for input protection to avoid tripping on inrush surges, though you probably should have something like a 45A breaker on the output (if 230V, different if 110V etc) for overload protection, assuming it really is 10kVA full rating.

EDIT: If site transformer then probably 55-0-55 so outputs would be various sockets on appropriate DP MCBs anyway.

I've fitted trafos and 110v distribution boards in the distant past

I think the arrangement was something like that with overload protection on the incommer at the 110v DB

Think they were like barebone trafos with no inbuilt protection