Blown Fuse reading voltage? 3

[Michael8068]

Can someone explain what I encountered?

I was troubleshooting a 3Ø 480VAC motor. When I put my meter on each fuse, leg to ground the reading was ~280VAC, on any leg, top or bottom of the fuse. Which made me believe the fuse was good. But if I were to check phase to phase, at the bottom of the fuse, it was only 5 or 6 volts, leg to leg. Changing the fuses did resolve the issue, just left me confused.
The fuses were Buss, Low-Peak, Dual Element Time Delay LPJ-5SP Current Limiting.

Thanks in advance.

 

[JG2828]

I'm guessing the motor was connected in delta and the voltage to ground you were seeing was passing through one of the good fuses and through the motor windings to the load side of the bad fuse.

 

[waross]

I learned long ago to check the voltage across fuses as well as the voltage at the bottom of the fuses.
If the motor is connected (possibly running with an open fuse) or if there is a lot of capacitance in the downstream cable voltage readings with a high impedance meter at the bottom of the fuses may be unreliable as an indication of a blown fuse.
There may be a control voltage transformer in the circuit that will back-feed to the bottom of a blown fuse.
A voltage across the fuse indicates a blown fuse.
It is best to use both methods, measure the voltage at the load side of the fuses and measure the voltage across the fuses.
Neither method is 100% reliable but at least one method will indicate a blown fuse, depending on the downstream loading and capacitance.

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

 

[Crowbar1]

Is it an indicator type fuse?