Open delta wild leg fault scenario

[djb65]

I have two transformers connected in an open delta configuration providing 120/240V loads. The two transformers are 50kVa and 25kVA with the 50kVA center tapped. I recently experienced some significant damage to my 3 phase panel. Turns out that the neutral and one of the phase conductors shorted and open circuited at the utility pole. The resultant fault also opened the 50kVA cutout (12A). The 25kVA transformer and cutout (3A) were unaffected, possibly continuing to provide power to the building via the "wild leg" conductor. My question is this, the 3 phase panel took a serious beating, lots of arcing damage, is it possible that the panel backplane was exposed to utility line voltage (7200V to ground in this case)?

 

[waross]

Hi djb65
It is possible but not likely.
A contributing factor would be poor wiring practices by the installers of the transformer bank. I have seen many installations where a conductor starts at the primary bushing, loops through the secondary neutral connector and thence to the system neutral/ground conductor. Although the possibility of losing the ground connection with this type of installation is higher than if seperate connections to neutral/ground are employed I have not yet seen a failure put high voltage on the secondary.
To get line voltage on the secondary, you have to blow off all the ground and neutral connections to the transformer.
If the failure is such that the transformer primary neutral is left connected to the trqnsformer secondary neutral, but all connections to ground and the system neutral are lost it is possible to get primary voltage floating on the secondary windings.
This would also put 7200 volts to ground on the transformer secondary windings. The transformers would probably fail.
The meter would probably be destroyed.
The 7200 volts would be floating on the secondary of the 25 KVA transformer and arcing from the wild leg to ground would be expected as well. The 240 volts would flow in the air ionized by the 7200 volt arc and the primary fuse on the 25 KVA transformer would be expected to blow also.
A phase to ground fault in a panel fed from a 50 KVA transformer can do a lot of damage without any help from 7200 volts. In fact, in many cases the higher current available at 240 volts will do much more damage than a 7200 volt arc.
Although it is possible, I don't think that it happened. I may be corrected on this if I have made some erroneous assumptions about what equipment was not damaged.
respectfully

 

[jghrist]

I'll agree with waross; possible but unlikely. A more likely scenario would be that the fault started in the customer's panel and caused the damage at the transformer. Maybe poor connections at the transformer.

Often the utility will provide no secondary protection, so if the fault was ahead of the service entrance protection, the primary fuse would blow to clear the fault. This does not mean that there was a primary fault on the transformer.