Mains fuse in the neutral wire

[wpaulvogel]

I agree with both of you about the “house wired wrong” scenario. The device/amp needs to be wired properly though, in the event that it is plugged into a proper outlet.
And yes, neutral doesn’t mean ground. It’s just current return, sometimes it is connected to earth early in the return path but it doesn’t have to be.

[Tony Bones]

And yes, neutral doesn’t mean ground. It’s just current return, sometimes it is connected to earth early in the return path but it doesn’t have to be.

"Current return"? What does that mean? Exactly the same current flows in both wires - opposite directions, of course, but there's absolutely no way to distinguish between the two. There's no need or advantage to assuming that one is more earthy than the other, so just don't.

[wpaulvogel]

Well one is more earthy than the other. The neutral wire is connected to earth somewhere in the circuit. Whether it is at the power generation facility or in your home. It is referenced to earth and said to have 0 potential.
It should be considered current return. It completes the circuit after the load.
And true it makes no difference which wire.

[Tony Bones]

Well one is more earthy than the other. The neutral wire is connected to earth somewhere in the circuit. Whether it is at the power generation facility or in your home. It is referenced to earth and said to have 0 potential.

Right, but what benefit is there in knowing this, or which of the two wires is which?

[wpaulvogel]

When everything is working properly there is no significant difference for a single phase circuit.

[wpaulvogel]

I thought of something. The neutral isn’t protected for current overload in the house or on the feed from outside. The hot leg is protected. It’s nice to have the protection in the right place.

[Tony Bones]

That's a good point. Only one of the two wires is protected with a circuit breaker. I'm not sure how that should matter to us at the guitar amp end of things, but it's a reasonable point.

I apologize if I sound argumentative. I'm just trying to convince myself one way or the other that the two wires really should be treated differently, for either performance or safety reasons. I know that we're conditioned to be aware that one wire is neutral while the other is hot, and maybe that made sense with two-wire power. But, with the inclusion of a third earth ground I'm not sure it does us any good.

Thanks for entertaining the Socratic Method.

[pompeiisneaks]

That's a good point. Only one of the two wires is protected with a circuit breaker. I'm not sure how that should matter to us at the guitar amp end of things, but it's a reasonable point.

I apologize if I sound argumentative. I'm just trying to convince myself one way or the other that the two wires really should be treated differently, for either performance or safety reasons. I know that we're conditioned to be aware that one wire is neutral while the other is hot, and maybe that made sense with two-wire power. But, with the inclusion of a third earth ground I'm not sure it does us any good.

Thanks for entertaining the Socratic Method.

That point that wpaulvogel made is one I was trying to make, if the amp is designed to have the fuse on the hot side, and the house is wired wrong, then the amp is dangerous as the fuse could blow, but still not stop 120V being somewhere on the chassis due to a short to chassis. Some designs fuse both sides to protect against this kind of wiring error. I don't know if there are other ways of creating that kind of protection.

~Phil

[wpaulvogel]

Tony Bones, I didn’t want an argument either, I really thought you were testing me for my electrical knowledge. If we were talking about multiphase (3 phase) the neutral positioning is 100% necessary and there are no earth ground connections, just 3 hot and 1 neutral. Don’t hook those up incorrect. I used to put up tower cranes and wire them, fix them and operate them. I used to install and operate concrete pump booms on top of high rise buildings. Both use 3 phase. Usually the motor just runs backwards but sometimes fuses blow from crossed phase or crossed neutral.
Now I just work on Toyota’s, master tech. Still certified for cranes and pumps and AWS, CWE welding. Too tired for all that stuff now. Just do guitars and amps in my spare time now.

[Phil_S]

Assuming this is authoritative and not some fake news:
https://incompliancemag.com/article/dou ... -the-line/
Single Phase Equipment

For single-phase plug-and-socket-connected equipment, the plug and socket may or may not reliably maintain polarity, depending on the electrical code and the socket configuration.

Supposedly, the NEMA 5-l5R socket maintains the polarity of the building wiring, with the wide blade being the neutral conductor. However, there are several versions of the NEMA 5-l5P plug, some with wide blade and some without. Therefore, some plugs allow polarity reversal, while others do not.

In continental Europe, the socket wiring for the common 220 V, 16A plug is not polarized, and the equipment fuse location would be variable. In the UK and Australia, sockets and plugs are polarized, and the equipment fuse location would be constant.

The point is that each plug, socket, and building wiring is an independent situation which must be separately evaluated as to whether polarity reversal is possible. This in turn would make the equipment fuse location constant or variable.

The General Case for Single-Phase Plug-and-Socket-Connected Equipment

For single-phase, single-voltage plug-and-socket-connected equipment, single fusing ONLY provides protection for both faults when polarity reversal is not possible. If polarity reversal is possible, then a single fuse can only provide protection against phase-to-ground faults 50% of the time.

For single-phase, single-voltage plug-and-socket-connected equipment, double fusing ALWAYS provides protection for both kinds of faults regardless whether polarity reversal is possible or not.

However, there are two hitches to double fusing.

First, when operating on a polarized system, some safety authorities insist that fusing be provided only in the phase conductor such that all of the equipment is de-energized for protection of the serviceman. This seems to require one fuse only.

However, this can be accommodated by using two, different value fuses. Select the fuse for the phase conductor (when connected to a polarized system) for proper overcurrent protection. Select the fuse for the neutral conductor to be one size larger than the phase conductor fuse. Thus, when connected to a polarized system, the smaller fuse properly opens for phase-to-neutral and for phase-to-ground faults. When connected to a non-polarized system and with reverse polarization, the smaller fuse provides protection for phase-to-neutral faults, and the larger fuse provides protection for phase-to-ground faults.

Second, some safety authorities insist that fusing be provided only in the phase conductor as required for building wiring. Any fuse in the neutral is cause for non-compliance of the equipment. The only solution here is to change our building codes and regulations to exempt single-phase plug-and-socket-connected equipment.

It seems to me, if you play out, carry a tester with you and use it every time before you plug in.

[R.G.]

It's simpler than that.

First, if there is no polarity in the wall socket and plug, the amp plug can be inserted either way round. This means that no matter what reasoning you use for fusing one side or the other, it will be wrong about 50% of the time.

Second, If one of the AC power wires is close to earth potential ("neutral"), then the fuse would be protecting the house wiring, not the amp, and then only 50% of the time. Even then, the "hot" wire is the one supplying the current, so a fuse in the hot wire also protects the earth return/neutral wire.

Second-sub-a, if one wire isn't neutral, very near earth potential, you need >two< fuses to protect against ground faults, so the single fuse in the neutral wire is still no protection 50% of the time.

A better answer is to have the house wiring protect the house wiring, by making the hot and neutral wires the same gauge, fusing the house in the "hot" side, which is where the power is sourced, and forcing the amp to use the "right" hot and neutral, and a protective fuse to keep the amp from starting a fire in the hot side.

Edit>> the other posts got in before I could belatedly hit "submit"

[R.G.]

And yes, every guitar player that gets outside their bedroom ought to have a tester plug (e.g. Home Depot, ~$10).

Actually, every guitar player that plays in public is tempting fate if they don't have a gigging bag with spare cords, extra (and pre-tested) tubes, spare fuses, a set of strings, small tools to fix things that could break or not function, and that plug-in tester.

It takes some level of maturity to realize that things simply WILL go wrong. Being prepared for it is part of being competent.

[wpaulvogel]

Phil, great article, very good information
R.G., great points
The important thing I want to say is that I do my best to build amps or whatever with everyone’s safety in mind. We can’t always prepare for every situation, but if we make it a point to put things together as best as can be then we know we’ve done our part correctly. Enjoy

[R.G.]

OK, that bit of information helps.

If you're building amps:
1. Make them 3-wire.
2. Safety ground the chassis and all metal parts that the user can touch in any conceivable situation. Check this with an ohmmeter at a minimum.
3. Learn how to terminate the 3rd wire properly. There's a batch of "how to" on this one item
4. Solve any hum/grounding issues that pop up with isolated jacks, isolated speaker jacks, and good/proper ground wiring inside the chassis.

[Tony Bones]

ypaulvogel, I completely get what you're saying about installing 3-phase equipment.

For guitar amps and similar gear, I'm thinking that a 2 pole power switch and 2 pole circuit breaker (like this) would completely eliminate the problem along with worrying about whether or not the plugs in public spaces are wired correctly. (The 3-wire power cord goes without saying.)