Nuisance breaker tripping on the primary side of a 2kva transformer

[gtbaca]

I have a 480 volt single phase 2kva transformer with protection only on the primary side with a 8 amp D curve breaker. I sized the breaker using NEC 450.3B. 2000/480 = 4.2 amps. Since it is less than 9 amps I use 167% of 4.2 which equals 7.014 amps reason for 8 amp breaker. I get these nuisance trips. Not sure if the trips are associated with inrush current from the transformer or a load turning on during the cycle of the equipment? Not sure what to look for. I was told to up the breaker to 10 amps. If I do this, I would think I would need to protect the secondary side. The trips are not always on start up.

 

[meternerd]

Have you used a "peak" recording ammeter or recorder to see what the current draw is? Is the load using a neutral? Is it connected to the primary neutral? If it is, that may be a problem.

 

[ptonsparky]

I would certainly want to know what your load is on the secondary side. Put a meter on it.

 

[Ingenieur]

what is the sec v
what are the loads, lights, motors, etc
can't be too much at 2 kva

 

[big john]

If the trips aren't always at start up then this is load related.

Because inrush only occurs when the transformer is initially excited and that current doesn't ever reappear until after the transformer is de-energized again.

So something on the low side is overloading you.

 

[kwired]

Have you used a "peak" recording ammeter or recorder to see what the current draw is? Is the load using a neutral? Is it connected to the primary neutral? If it is, that may be a problem.

He said it was 480 volts single phase primary, so should just connect to two "phase conductors" of the supply, you possibly thinking about connecting the wye point of three phase transformer to supply side neutral, which sometimes is a problem?

My guess is problem with what is connected to secondary or the breaker used just doesn't have the right trip curve to hold when energizing, but sounds like it isn't just when energizing when it trips. Maybe even secondary load has a high starting surge that is too much for the primary device, in such a case with an inductive load you not only have starting surge of the transformer you also have a starting surge of the secondary load.

 

[Ingenieur]

I looked at the curve
takes 120 A to trip <6 cycle
that is 25 x xfmr rated 4.7 A

looks load or load switching related to me

 

[Russs57]

These days I wouldn't overlook the possibility of breaker malfunction. I have had Square D tell me that because they are required to use a certain percentage of recycled material some problems happen.

Might try a pair of fast blow 8 amp fuses. If they hold, and breaker trips, I'd say you have your answer.

 

[kwired]

These days I wouldn't overlook the possibility of breaker malfunction. I have had Square D tell me that because they are required to use a certain percentage of recycled material some problems happen.

Might try a pair of fast blow 8 amp fuses. If they hold, and breaker trips, I'd say you have your answer.

Said fuses and breaker may not have (more like likely don't have) same trip curve and such test doesn't confirm something is wrong with the breaker.

 

[ptonsparky]

Make this simple, keep putting in different breakers or fuses until the smoke is out of the transformer.

 

[kwired]

Make this simple, keep putting in different breakers or fuses until the smoke is out of the transformer.

How many transformers should one let the smoke out of before checking out the connected load?

 

[Ingenieur]

Make this simple, keep putting in different breakers or fuses until the smoke is out of the transformer.

jumper around the cb and call it a day

 

[Sahib]

Make this simple, keep putting in different breakers or fuses until the smoke is out of the transformer.

or, better, until code allowance for breaker over sizing limit is reached. What prevents OP from trying that? The OP over ambition to use primary protection of the transformer for over load protection of the secondary load does not work in his case.

 

[rian0201]

This might be off topic. Have the transformer tested. There might wrong in the transformer's insulation.

Or plot the fuse curve with the transformer's damage curve.

Or put the transformer near an allspark it might transformer to an autobot..hehe

Sent from my vivo 1606 using Tapatalk

 

[kwired]

Is the secondary a two wire system? If not you need secondary protection anyway.

 

[Jraef]

To kwired's point, the NEC allows for primary only protection, because the NEC doesn't know what TYPE of transformer you have and is only concerned for safety, not functionality. It's not a design guide, it's a set of minimum standards.

The REASON why you see MOST Control Power Transformers fused on both the primary and secondary is that by fusing the secondary, you are allowed to use 250% OCPD on the primary side and the magnetizing inrush current of CPTs is typically higher than on standard transformers. That then is because CPTs are designed specifically to not allow as much voltage drop on the secondary when a large coil inrush takes place, but the trade off is a higher inrush then initially energized. It doesn't blow every time because there are other factors in determining the peak inrush current, such as the point in the incoming sine wave at the exact moment of energization, but 167% is almost assuredly going to pop once in a while.

So if this is not a control power application with inductive coils on the secondary side to worry about, change to a distribution transformer, or more simply, just put protection on the secondary side and a larger breaker on the primary.