DIYbanter - View Single Post - shock in electrical panel learned something new

Ralph Mowery

In article , says...

Motor contactors have to be rated for the locked-rotor current, which is
about 6 x the run current.
Circuit breakers have to be rated for the available-fault-current.

I have seen pictures of high rated circuit breaker contacts with
arc-chutes and magnetic blowouts. I haven't see that on contactors, but
you likely had some real big motors where you worked.
[you picked a great time to retire.]

In the same type of motor starter, many contactors will have a
transformer across 2 legs before the contactor that converts the 480 to
120 volts and the coil will be 120 volts. That is usually for long runs
to switches. The fuses for that transformer are not long at all.

Seems a little surprising there are not even start/stop buttons on the
door in the picture. The only motor control is the disconnect.

Wouldn't work so well to run 480 coil voltage outside the controller
(switches and sensors would have to be rated 480), and it would have to
be fused. in both legs.

Everything I remember was 120V control like you describe.

I retired about 7 years ago from a small plant that had about 40 acers
under roof and had over 3000 people there at one time.

Dealt with many things. From small motors you could hold in one hand to
some around 300 HP. Not sure how how the big compressor motors were as
I dealt very little with them. They operated on 4160 volts. They had
fuses that were about 6 inches across and 2 feet long. The fuse in the
primary of the control circuit that dropped the 4160 to 120 volts was
about 2 inches in diameter and almost a foot long. Rated for only 1
amp,but 6000 volts.

Some of the big contactors that I dealt with were rated around 300 amps
at 480 3 phase had 480 volt coils but there was an interposing relay
that was fed with 120 volts to run outside the cabinet.

Some of the big resistance heaters that had 300 amp fuses and 480 volts
had 3 contactors in series. One would pull in and about 10 seconds
later the other would pull in. This was a safety so that if one
contactors contacts welded together, the one that pulled in first not
under load should release. They were both wired to seperate
thermocouple safety switches to if it got too hot they would release and
not have a thermo meltdown. We made ployester out of raw materials, so
it could be a really bad thing if it all melted or caught fire.