AC Coil Overvoltage

[Zipfrog2022]

If 120VAC was applied to a 24VAC coil would this create a short between neutral and the hot leg secondary side of a 120VAC power supply “ 480VAC step-down transformer to 120VAC? If so would this likely blow fuses on the primary side of the transformer and cause 110VAC boards to fry? The arc marks and burning on this board stack looks like there was a dead short to the grounded mounting bracket for the board stack that then arced between the mounting bracket and three of the boards in the stack. Several components on the boards have blown and there are burn marks across the boards in the same area of the board stack mounting bracket. I have found no bare exposed wires and have gone through all the wiring to confirm everything is landed in the correct locations. The only thing I have found in this control head is that someone installed a reversing contactor with 24VAC coils when it should have been 120VAC coils.

Any thoughts on this would be appreciated. I’m just not sure if such a high overvoltage would actually cause a short between the two sides to cause all this damage or if the coils would have just burned up.

The other thing is my primary fuses “480VAC side blew not the secondary fuses on the 110VAC side of the power supply. One is extremely burned inside the other actually must have exploded charred glass fragments from the fuse were all over the inside of this control head.

Any thoughts on this would be appreciated.

Thanks

 

[gar]

230211-0530 EST

Zipfrog2022:

For many electrical components they will be applied with a voltage that is somewhat near their maximum continuous rating.

A device designed to operate at a nominal 24 VAC might operate at a continuous voltage of 30 V for a long time without failure. For a linear load this is almost 60% more power dissipation than at 24 V. If you apply 120 V to a 24 V device, then the power dissipation at 120 V will be about ( 120/24 )^2 times the value at 24 V. This calculates at 120 V to be about 25 times the power dissipation at 24 V. You will fry the relay coil.

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[ptonsparky]

If 120VAC was applied to a 24VAC coil would this create a short between neutral and the hot leg secondary side of a 120VAC power supply “ 480VAC step-down transformer to 120VAC? If so would this likely blow fuses on the primary side of the transformer and cause 110VAC boards to fry? The arc marks and burning on this board stack looks like there was a dead short to the grounded mounting bracket for the board stack that then arced between the mounting bracket and three of the boards in the stack. Several components on the boards have blown and there are burn marks across the boards in the same area of the board stack mounting bracket. I have found no bare exposed wires and have gone through all the wiring to confirm everything is landed in the correct locations. The only thing I have found in this control head is that someone installed a reversing contactor with 24VAC coils when it should have been 120VAC coils.

Any thoughts on this would be appreciated. I’m just not sure if such a high overvoltage would actually cause a short between the two sides to cause all this damage or if the coils would have just burned up.

The other thing is my primary fuses “480VAC side blew not the secondary fuses on the 110VAC side of the power supply. One is extremely burned inside the other actually must have exploded charred glass fragments from the fuse were all over the inside of this control head.

Any thoughts on this would be appreciated.

Thanks

All of the above. I don't know if a full and complete autopsy report is worth the effort.

Hopefully someone has enough funding in the 'Shtuff Happens' account.

 

[synchro]

Was this equipment working previously, or was it newly installed? If the latter, could the transformer have been wired incorrectly?
Perhaps the 24 VAC coil was not the only problem, and some other issue caused the extensive damage.

I can only speculate, but maybe there was a failure of the insulation in the transformer and a resulting fault between the primary and secondary windings. And either the secondary was not properly bonded to ground, or arcing caused an open circuit on the neutral side of the secondary winding. Then this allowed a primary-to-secondary fault to cause a large overvoltage on the 120V circuits, and only the primary fuses blew because they have a smaller current rating than the fuses on the secondary.

If finding the root cause of the failure is important to you, resistance measurements could be made on the transformer to see if any discrepancies can be found indicating that it had failed. But as ptonsparky has said, it may not be worth it given the extensive damage and the difficulty that this causes in identifying a root cause that initiated a sequence of destruction.

 

[Besoeker3]

If 120VAC was applied to a 24VAC coil would this create a short between neutral and the hot leg secondary side of a 120VAC power supply “ 480VAC step-down transformer to 120VAC?

At much over the 120Vac would saturate the supply and very likely blow fuses. It isn't something done, or not intentionally ..............

 

[LarryFine]

A lower-voltage load has a lower impedance than its higher-voltage version, so it could indeed overload the power source and do possible damage.

 

[kwired]

A lower-voltage load has a lower impedance than its higher-voltage version, so it could indeed overload the power source and do possible damage.

But it likely will have a higher impedance than a "short circuit" though, possibly enough the fuse doesn't blow as quickly as it does in a short circuit condition. Then comes whether or not the fuses used were actually correct ones to protect said transformer.