50 kva transformer problem 5

[rickie]

Hello, Help We just installed a 50kva transformer going from 480 to 120/240 with a 100A breaker and when you throw the breaker we get a flash from the transformer. After speaking to manufactuer it is wired correctly and they said we needed a 225A breaker .We are getting the flash on the primary side, we do not see any evidence of anything burned.They claim it is end rush. On the secondary side is only a 200A panel. Thank You Rickie

 

[waross]

A flash from a transformer is bad. Check your connections.
Chances are that one or more of the connections is loose.
Probably, the electrician put the cable in the connector and tightened it. Not good.
The proper procedure is to;
1> Twist the cable back and forth in the connector after tightening and re-tighten. Repeat until it is not possible to tighten the connector any more.
2> Use channel-lock type pliers to take the twist out of the cable strands before inserting into the connector. Check by twisting the cable and retightening.

The cable strands take less space in the connector when they are straight than when they have the original twist. If the cable is inserted into the connector and just tightened once, the strands will tend to untwist and become very loose if the cable is disturbed, such as when installing the next cable. Twisting the cable encourages the strands to straighten out. You will be surprised at how loose a connection will become when the cable is first twisted.
Method "1" works best for larger cables which have different layers of strands twisted in opposite directions.
I have surprised many electricians over the years by giving their connections a little twist and dropping the cable out of the connector. I have invited electricians to tighten a cable connection as much as possible and then given the cable a twist and dropped it out of the connector.
respectfully

 

[Zogzog]

Did you test it? PI test? TTR? Or did you just wire it up and energize it?

 

[rickie]

All connections were torqued with a torque wrench in factory lugs. Line side H1 H2 Load side X1 X4 ground and neutral were X2 X3 bonded together per mfg . I am know expert at this so i might seem a little slow at times . Thanks Rickie

 

[l3city]

rickie:

Check configuration connections (delta-delta, delta-Y, etc)
Also, for breaker sizing, check NEC Art. 450, Table 450.3(b)
regards

 

[davidbeach]

Sounds like a single phase transformer, so no delta or wye.

 

[rickie]

Single phase yes. NEC says 100A breaker big enough, however mfg says we need 225A because they figure it at 225% instead of 125%. Thank You Rickie

 

[davidbeach]

Breaker size has nothing to do with anything flashing in the transformer. Inrush is a non-visible event. Something is seriously wrong.

 

[jghrist]

Is the breaker tripping? Are you sure the flash is from the transformer?

 

[rickie]

yes and yes

 

[swn1]

A visible flash in a low-voltage device that does not immediately turn into a puddle of slag is a series continuity problem. The advice from waross sounds like the voice of experience, look into it.

The inrush effect means you have a very high instantaneous current at energization, so V=IR across the high-resistance joint is giving you a little spark around it.

De-energize and measure the series resistance of each segment of the power delivery chain. Preferably with a "ductor" (DLRO) using at least ten amps through current.

Check for loose connections and possible spark gaps at or between the tap arrangement.

Pull the cover off, put a video camera on a tripod, start the camera, turn off the lights, and energize the transformer.

Double-check your ground conductor integrity and verify high-to-low insulation integrity before putting it in service.

 

[advidana]

I would just replace the transformer.

 

[rickie]

A new one is on the way. Thank's Everyone for your advice.