Fuses and circuits

[Charles Bishop]

While working in an older unit, I was helping to track down which fuse controlled which circuit. The fuses were in a closet, in two separate fuse boxes. These were the screw in/screw out fuses.

There were several porcelein fuse holders and each contained 2-4 fuses. I unscrewed one fuse and noticed that the ceiling light in the bedroom went out. However, when I unscrewed the other fuse in the porcelein fuse holder (there were two) the same light went out. The same thing happened for the light in the closet.

Why would two fuses control the same light?

The porcelein fuse holders were a 4" square block for those that held two fuses. Wires went into the back or sides of the fuse holder.

Oh, also, the wiring looks to be 12 or 14 ga (cloth covered, older wire so it's hard to tell), and the fuses are all 30A. I'm assuming I should mention this to the owner and mention that he should get the correct sized fuses in?

[Speedy Jim]

Many, many years ago, it was fairly common to fuse *both* the Hot and the Neutral lines. Not considered safe today.

You are probably not in a position to dictate to the owner what he should do about all these problems. But at the very least present him with some kind of statement of what was found, for your own benefit if nothing else.

Jim

[RBM]

Speedy Jim has hit the nail on the head. There was a time when you could get screw in shunts to replace the fuses on the neutral side. In any event those neutrals should not be fused now.

[Jeff Wisnia]

Well, I've learned something new again in this thread. When I posted my first reply I thought for sure that neutrals were NEVER intentionally fused, because why would anyone feel a need to do that?

Was there a time when neutral wasn't intentially at at ground potential?

If someone can tell me WHY they felt the need to fuse them "back then" my curious mind would sure like to learn more.

Jeff

[Terry]

snip

snip

What do you think he meant by "I should mention this to the owner?"

[Big_Jake]

Anyone have the thought that one of the fuses may have been the "main" and the other fuse the "circuit"?

JK

[RBM]

Not necessarily, if the service was only 30 amp, it was common to have plug fuses

[Speedy Jim]

Well, not always. I can recall clearly one service which was rated at 30 Amps 120/240V. It used 2 plug fuses for the Mains protection and a bank of 4 plug fuses for the 4 branch circuits (no Neutral fusing).

Much more interesting though was a large branch circuit panel in an elegant mansion built in 1900. This panel came off a 3-wire feeder rated at 60 Amps. The interesting part was that most of the branch circuits used an Edison 3-wire circuit where the Neutral was shared. BUT.....they fused the shared Neutrals!! When one of those Neutral fuses opened, the circuit voltage on any one branch could be anything from near Zero all the way up to 240V !

Jim

[RBM]

Here's on with a non fused neutral:

[Terry]

Mains are not screw in. They are pull out type.

[Charles Bishop]

[snip]

I have the dwg I made on site in front of me now. I'll see what I can do with ASCII art - 0 indicates a screw in fuse, X and empty space in the fuse block, and the numbers are my reference numbers:

1 0 0 5 2 0 0 6 3 0 0 7

X 0 8

4 0

0 9

0 10

Not as good as I'd like, but it will do. Pairs 1, 2, then 3, 4, and 5, 6, then 7, 8 and finally 9, 10 are in their own, separate, fuse block. All but 9, 10 are porcelein blocks with space for two or 3 fuses. The one with three (3, 4 pair) only has two in it. 9, 10 has a metal box, but still with screw in fuses. I know that 1, 2 and 5, 6 both turn off separate ceiling lights, but each one of the pairs turns off the same light. It does sound as if the neutral is fused. I don't have any notes on whether the other pairs behave similarily, but could check. It's a small 1 bedroom unit in an older building in San Francisco, CA.