Fusing AC

[philba]

The recently posted fuse question reminded me of a question of my own. I looked through several fuse makers' sites and the answer didn't leap out at me so I thought I'd ask it here.<p>I've seen several designs that use a fuse in the AC line. Most of the the time, the fuse is in-line with the Hot AC line (L, I believe). This makes sense since a short to earth ground would be caught as well as a short within the equipment. However, I've seen several projects that have the fuse on the Neutral line. I think this is a problem or do I miss understand how this works? Sure it will catch a short in the equipment itself but a short to ground won't blow the fuse. If so this an issue for a non-polarized cord. Maybe it's just playing the odds - short to ground being a lower likelihood thing. Obviously, a GFI circuit will catch that kind of problem, too.<p>By the way, fuses are much more complex beast than I ever imagined. This is especially true if you are designing equipment with transient pulses above the nominal current rating of the fuse.<p>[ October 17, 2005: Message edited by: philba ]</p>

[haklesup]

I'm not sure what circuits you have seen with fusing on the neutral but it would be essentially equivelent to fusing the hot line if the equipment did not have any large metal components that could act as a ground or if the device was double insulated etc. In these cases, the choice may have been based on mechanical placement and fit.<p>Also in devices that accept connections from other devices, it might be prudent to fuse against shorts from one device into the neutral or common ground node of the other. <p>All very hypothetical in the absence of a specimin schematic to pick apart.<p>Even a wire seems a complicated bit when you try to engineer it to its limits.

[Enzo]

There may be a specific ratiionale for fusing the neutral instead of the hot in some piece of gear, but in general you should fuse the hot. If the neutral opens as it would if fused, then the mains voltage remains on your circuit. The current path is interrupted, but the voltage is still there with all its attendant safety hazards on the circuit components.<p>If you fuse the hot, when the fuse opens, the only place the mains voltage remains is the hot end of the fuse holder. The rest of the circuit is cold.

[philba]

Thanks, that confirmed what I thought.

[Donald S. Lambert]

I've seen a couple of instances in which a non-polarized plug could be plugged into an outlet on a contraption which would leave that item with the electronics still hot unless both lines were fuzed in case of a short.<p>I also had a basement in a house I'd bought that had the wires ran in conduit in which the conduit was the ground. One day the conduit pulled apart and I had light bulbs pop all over as 220 hit them. I checked the building code and conduit wasn't required if the lines were on the ceiling. So I put in regular romax with a ground, which took two romax to do it. But it was far safer.
Greybie

[Dean Huster]

It was not the least bit unusual for equipment manufactured under military contract and a lot of commercial equipment to fuse both sides of the line even with a polarized and 3-wire-grounded plug. They also switched both sides of the line with the power switch.<p>Whether outside an equipment box in the NEC-controlled world or inside an appliance, you should NEVER switch or fuse the neutral only. Power switches and fuses are ALWAYS installed on the hot side of the line (black wire in older U.S. line cords, brown wire in international power cords) and there's nothing inherently wrong with fusing both sides of the line. But if you ever do switch the neutral with the power switch, you must also switch the hot side as a 2-pole ganged-switch operation.<p>As mentioned, the fuse blowing on a neutral side would still leave the circuitry hot and dangerous. A single fuse situation would be an especially bad problem with a non-polarized plug where the fuse blew when it was plugged in one way and then you plugged it in the other way. For that reason, your projects should always have at least a polarized plug and preferably a 3-wire grounded plug with the ground securely connected to the outer hull.<p>Dean