View Single Post
  #1   Report Post  
Posted to rec.crafts.metalworking
Ignoramus22165 Ignoramus22165 is offline
external usenet poster
 
Posts: 3
Default Phantom voltage in a disconnected busbar?

I was recently pulling some extremely small amount of MCM 500 copper
cables. Of course, it was legitimately purchased from the owner of
that building.

There was this one particular panel that was always live (hooked up to
service drop) and under power.

The panel had two 800A switch boxes. The boxes were fed through panel
busbars, with a big handle switch, then fuses, then terminals for MCM 500
cables.

Both boxes were in the same condition, namely:

1) The switch was in the OFF position
2) The outgoing cables were already removed.
3) The whole panel was 480 volts

There was literally NO incoming power that was not disconnected,
back-fed, or anything.

And yet, on one box out of two, my "Sperry voltage tester" registered
live voltage.

I then re-checked it with a voltmeter and found "40 volts" (sic). And
thi sis in a 480 volt panel!

I needed to touch those busbars (switched off, remember) to unbolt and
remove the fuses, since the fuses are $100 for 3. Out of abundance of
caution, I brought in a wooden pallet, stood on wood, used a socket
extension and was very careful not to touch anything, and everything
worked out OK.

But I am still wondering how, on a disconnected busbar, I would read
that phantom voltage. Was that just electromagnetic induction?

It is still a mystery to me.

The biggest question is how come it was only on one of the two
identical boxes.

i