What went wrong?

[thinkvoltage]

I work for an electric utility company and am trying to figure out what went wrong in the scenario below:

A new commercial building was in the middle of construction and we were called out to energize the brand new transformer we had installed to serve only this building. Upon energizing the transformer, a resident drove up and reported smoke in their hourse. A few minutes later, the fire department showed up and stated they were getting reports of smoke in several homes on the street and told us to shut off power to the transformer. Further investigation showed some burn damage on a water pipe about five feet from the transformer. The theory is an electrical ground fault travelled on the water pip and entered several homes nearby and damaged some of their appliances and electronic equipment. We checked the transformer and the primary connections and nothing was wrong. My question is, what could the electrician have done wrong on the secondary of the transformer that could have caused this event? The electrician ran the secondary cable from the transformer, to the new commercial building, and terminated the wires at the meter at the building and the secondary of the transformer (we witnessed the secondary terminations).

The electrician had not bonded to the water pipe in the building yet, so if the phase and neutral cable were switched, the main water pipe which it is believed carried the fault, could not have been energized.

 

[davidbeach]

What kind of connection at the transformer in question? What kind of connection at the service transformers of the affected customers.

 

[itsmoked]

How big is the service to the new building?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[PHovnanian]

The electrician had not bonded to the water pipe in the building yet, so if the phase and neutral cable were switched, the main water pipe which it is believed carried the fault, could not have been energized.

This seems odd. Most inspectors won't approve an installation for service until the grounding system (electrodes, conductors, bonding, etc.) is in place. What is your utilities and the inspection authorities process for verifying completed inspections and approvals and energizing new services?

 

[jghrist]

Check the ground-neutral connection at the new transformer. If there were a fault on the new transformer and the transformer neutral was tied to a ground rod 5' from the water pipe, but not metallically connected to the primary neutral, you could get return current flowing through the earth from the ground rod to the water pipe, then to the primary neutral through the neighboring service grounds. This may put a high voltage on the neighboring service grounds.

Is it a pole-mount or padmount transformer? Did any overcurrent device trip?

 

[alehman]

The building frame is probably electrically connected to the water pipe, even if not yet bonded at the service. If the same water pipe serves the houses and there is a ground fault in the building, with no water pipe connection at the service it could energize the pipe (and the frame of the building).

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[waross]

I spoke with the engineer who investigated an incident in Central America. Some of this is speculation because attempts were made to "cover up" before the engineer arrived. The system grounding was almost nonexistent. A primary conductor came off the pin and rested on the cross arm near the secondary neutral. Arcing and tracking and poor grounding of the secondary all combined to place several thousand volts common mode voltage on the 120:240 Volt secondary system. Fires were started in several buildings that were fed from the circuit by current passing through the insulation of the circuit conductors to the building structure. You may have had a similar situation. Examination of the ares where the smoke was generated would be enlightening.

Bill
--------------------
"Why not the best?"
Jimmy Carter