Utility fuse blown

[karlr]

We received an emergency over the weekend for a reported partial power outage to a commercial property. Here is a little background of the electrical service.
120/208v 3ph 4w. Two Main disconnects, each rated at 1200amp. Here is the problem, indeed there was a partial power outage, loss of the B ph from the utility. It was a blown fuse in the utility transformer. It blew their fuse again when they replaced it. They tested the transformer and the conductors feeding the building, no issues. They turned the mains off and it would hold.
Through some very careful troubleshooting, we narrowed it down to two very suspect branch circuits for outdoor lighting. The mains and distribution breakers are all set at their minimum trip settings or just above the minimum. This circuit(s) bypassed every overcurrent protective device without tripping one of them. So it did not trip the branch circuit breaker, main breaker in the panel, breaker that feeds the panel and the main breaker. How is this possible?
I've seen similar situations where the main opened instead of the branch circuit due to a severe short circuit of a motor, but the utility fuse, that's new.
Any input would be greatlyappreciated.
Thanks in advance,

 

[mivey]

We received an emergency over the weekend for a reported partial power outage to a commercial property. Here is a little background of the electrical service.
120/208v 3ph 4w. Two Main disconnects, each rated at 1200amp. Here is the problem, indeed there was a partial power outage, loss of the B ph from the utility. It was a blown fuse in the utility transformer. It blew their fuse again when they replaced it. They tested the transformer and the conductors feeding the building, no issues. They turned the mains off and it would hold.
Through some very careful troubleshooting, we narrowed it down to two very suspect branch circuits for outdoor lighting. The mains and distribution breakers are all set at their minimum trip settings or just above the minimum. This circuit(s) bypassed every overcurrent protective device without tripping one of them. So it did not trip the branch circuit breaker, main breaker in the panel, breaker that feeds the panel and the main breaker. How is this possible?
I've seen similar situations where the main opened instead of the branch circuit due to a severe short circuit of a motor, but the utility fuse, that's new.
Any input would be greatlyappreciated.
Thanks in advance,

To state the obvious: the fuse was faster than the breaker at the experienced fault current.

A lot of times you will see neatly stacked coordination curves on one end of the current range but not on the other. A lot of times the curves coordinate well at low currents but cross at high currents.

Fuse curves and relay/breaker curves can differ in shape so you pick good coordination in the zone most important to you and let the mis-coordination happen elsewhere.

 

[meternerd]

You didn't say whether it is an overhead service with pole mounted transformers or an underground with a pad-mount transformer. Most utility WYE pad-mount transformers are common core, meaning there are three vertical sections connected with top and bottom horizontal sections connecting them together and the primary and secondary windings are wound on top of each other for better magnetic coupling. In this setup, when a load is connected, such as a step down transformer or motor load, there will still be voltage on all three secondary terminals even if one primary conductor is deenergized. The equipment may still operate, but voltages will not be correct and the resulting load currents can cause transformer fuses to blow. It happened quite a few times on our water pump stations and was only discovered during meter testing (most often on 277/480 services). Did the utility verify primary voltages on the transformer primary when no load was connected? It can be a head scratcher for most linemen. Just a thought.