Electrical – Service Wire Burn

distribution-networktransformerwire

I have a Distribution Transformer rated 25kVA 14.4/240 kV installed in the system. One phase of the serving 3 phase line of our electric utility in 23kV primary side are faulted in Line to ground unfortunately the bundled line-to-line service wire going to our premises was burned. What would possibly happened?

I was wondering why among all the other customer in that common single phase transformer, only our service wire was burned that results into some equipment damage.

Best Answer

One hypothesis is the following series of events:

(1) A line on the primary side of the transformer faults to ground (neutral) as you describe.

(2) However, due to the neutral impedance (or a problem with the neutral), the fault current raises the neutral potential by several thousand volts.

(3) The primary-side utility neutral is typically connected to a grounding rod at the pole -- and also to the center tap of the secondary for split-phase service (120/240 VAC). You mention a line-to-line service, which implies that no neutral conductor (or grounded conductor) runs to the house. I'm not familiar with that configuration, but I'll assume there's no neutral going to the house.

(4) With the utility's neutral connected to the secondary center tap, the common-mode voltage of the secondary now rises by several thousand volts during the fault.

(5) At some point in the service entry or premises wiring, the high common-mode voltage of the service conductors arcs over to a grounded enclosure or other grounded conductor. The fault current from this arc burns the service wires and damages equipment.

(6) If the supply lines in your house arced to ground before your neighbor's, your neighbors may have been spared from any damage.