OT half a fuse?

I had something really strange hahppen to me yesterday. We have(had) a 1.5 KVA transformer to drop 480 volts down to 110 volts. The 110 is for controls and some auxilary devices. I put my voltmeter on the low voltage and read 55 volts. I then checked the fuses. The secondary fuse is OK, but one of the primary fuses reads a resistance around 1.5 meg and there is avoltage drop across the fuse. About 235 volts. I change the fuse and got the same result. I know the fuse was good when I put it in, but as soon as I power up, I got what I would call half a fuse. Changed out the xfmr and all was well. Has anyone ever seen anything like this? I don't know!

Was the transformer output bonded to ground? If so the incoming side of the transformer would see approximate 277vac (if input is truly 480) or the 235 if input is closer to 440, with a 4:1 ratio would the transformer output approximately 1/4 of the 235, 57vac.

As an addition to the above its possible one side of the transformer has shorted to ground which would blow a fuse but also allow it to still be fed from the other side. The voltage seen on the input would be supply divided by sqrt of 3. This is more likely then the above.

I am sure but not certain that the secondary was grounded. I always ground the neutral on 110 ckts, but another man wired this panel. What bothers me is that I got a 1.5 meg reading on 2 fuses. That one really puzzles me. I physically removed the fuses and rang them out. I have a brand new Fluke meter and I am confident of what it tells me. Your explanation does make sense and I buy into it. The fuses? I guess I'll file them with "things I'll never know" Thanks
stv

You didn't check the fuse in circuit did you ?................

Unregistered said:

You didn't check the fuse in circuit did you ?

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He said he "physically removed the fuses and rang them out", but I'm wondering if he's holding the probes against the fuse ends with his fingers?...

IOW, measuring the resistance of his body?...



-Eric

You say that you ground the neutral on the secondary of the transformer. In the UK it is common practice to connect a centre tap on the secondary to ground giving a 55,0,55 V system. Are you sure that you have not connected two points to ground on this installation.

In the UK it is common practice to connect a centre tap on the secondary to ground giving a 55,0,55 V system. Are you sure that you have not connected two points to ground on this installation.

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Only for site electrics (tools etc), not control circuits in a panel.

The secondary fuse is OK, but one of the primary fuses reads a resistance around 1.5 meg and there is avoltage drop across the fuse. About 235 volts.

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Originally it was one fuse with a voltage across it, that tells me it was blown. If the transformer shorted to ground on one input that would put the 235vac on the primary and the output would be approximately 55vac.