Fender 3 conductor conversion - two fuses?

[glebert]

OK, this is an idea I'm considering, let me know if its stupid. It seems like one of the issues when updating a Fender (and perhaps others) from an unpolarized two conductor cord to a grounded cord is that the fuse is on one conductor and the switch on the other, creating a situation where the fuse does nothing if current is flowing to a point other than the return line. I think most people rewire so the fuse and switch are both on the hot conductor (this is what I did on mine). About to do a friends original 6G3 and wondering if an alternative would be to add a second inline fuse that is on the same conductor as the switch. Presumably this would have the same current rating as the primary fuse.

Also, removing death cap, of course.

TIA,
Greg

[Helmholtz]

You only have one mains voltage. Neutral is connected to safety earth in the house wiring, so no voltage between.
Means there can only be one and the same mains current in both mains wires (including fault currents to chassis).
So no matter on which side you wire the mains fuse, it always sees the total mains current.

No sense adding a second fuse as the fuses would actually be wired in series.

[Tom Phillips]

I consider that the only proper way to do a three wire power cord upgrade is to re-wire to proper current electrical code standards with the switch and the fuse in the hot line. In addition, few "Amp Techs" get the chassis ground connection right.
Of all the "conversions" that come across my bench, I find that less than 5% are done correctly.
I always fix the situation when I work on the amp.

[glebert]

You only have one mains voltage. Neutral is connected to safety earth in the house wiring, so no voltage between.
Means there can only be one and the same mains current in both mains wires (including fault currents to chassis).
So no matter on which side you wire the mains fuse, it always sees the total mains current.

I don't know that I am understanding what you are saying here. Assuming the safety earth is correctly wired, it seems to me that it is in parallel with neutral, so current flowing to safety earth isn't going through the neutral conductor and so a fuse there wouldn't be seeing all the current.

Maybe the issue is that everyone has a different scenario(s) in mind when it comes to potential faults, and applying those scenarios to an old circuit is challenging. Once you start thinking about "what if the outlet is wired incorrectly?" maybe it all becomes folly.

[Helmholtz]

I don't know that I am understanding what you are saying here. Assuming the safety earth is correctly wired, it seems to me that it is in parallel with neutral, so current flowing to safety earth isn't going through the neutral conductor and so a fuse there wouldn't be seeing all the current.

You have a fuse wired to the hot conductor and that fuse must pass any mains current. There can be only one mains current.
If that fuse opens there's no mains voltage in the amp except before the fuse.

[glebert]

You have a fuse wired to the hot conductor and that fuse must pass any mains current. There can be only one mains current.
If that fuse opens there's no mains voltage in the amp except before the fuse.

If the fuse is on the hot conductor in a traditional Fender circuit then the switch would be on the neutral. To me this would not be safe as the primary of the transformer (and wiring prior to the switch) is always hot even when the amp is off.

[Helmholtz]

If the fuse is on the hot conductor in a traditional Fender circuit then the switch would be on the neutral. To me this would not be safe as the primary of the transformer (and wiring prior to the switch) is always hot even when the amp is off.

I thought you had them both wired on the hot side according to your national safety standard?

But anyway, no matter on which side the fuse is wired, it always serves its main purpose, which is to protect the mains cable and PT from catching fire.

[SoulFetish]

I thought you had them both wired on the hot side according to your national safety standard?

But anyway, no matter on which side the fuse is wired, it always serves its main purpose, which is to protect the mains cable and PT from catching fire.

This is correct in that a fuse in line with either mains conductor will open the circuit (current through one must pass through the other). However, in North America, neutral must never be fused. This is an explanation I wrote in a previous thread:
51573-proper-len-txf-primary-wiring
"...in North America, the neutral conductor is the designated load/current return conductor. Further, both neutral and earth conductors must be able to carry fault currents and be rated accordingly. For this reason, they (Live and Neutral) are not interchangeable and must maintain their polarity (in our application).
This is the reasoning for why the live conductor is fused, and why the neutral should not be. First, the neutral needs to maintain a closed circuit for fault currents to trip the breaker in case of a fault. If the neutral was fused, the fuse interrupts this function.
The other reason the neutral is not fused is, even though the circuit would be broken, there might still exist a live voltage hazard inside the equipment potentially putting someone at risk of shock. This is why the live connection is fused right at the entrance to the device generally."

[Helmholtz]

My statement was that a second fuse doesn't make sense technically.
You're adding that your national safety standard doesn't allow for a fuse in the neutral wiring.

[glebert]

To clarify, my thought was not that adding a second fuse increased safety, but would allow the switch and a fuse to be on the hot line without disrupting the original wiring as much. SoulFetish gives a good reason why having a fuse on the neutral wire is a bad idea.

Thanks!

[pdf64]

… no matter on which side the fuse is wired, it always serves its main purpose, which is to protect the mains cable and PT from catching fire.

Consider the JTM45, the mains transformer primary circuit design of which might be seen to be taken somewhat directly from the 5F6A Bassman, but with a safety earth connection to the chassis. ie the live feed gets the amp’s power switch, the neutral feed gets the mains fuse and voltage selector.
https://el34world.com/charts/Schemat...5_readable.pdf
If a fault developed in the primary circuit, such that mains fault current flows to earth, there’s no mechanism to cause the mains fuse (on the neutral feed) to blow.

In regions where it’s permitted, it seems a good idea to put mains fuses in both the live and neutral feeds. Because some regions use unpolarised connector system, so it’s undefined which is live, which is neutral.
Also, even in the UK, where polarised and earthed mains connectors have been standard for 7 decades, it’s not uncommon to find mains outlets in which live and neutral are reversed.

[Helmholtz]

If a fault developed in the primary circuit, such that mains fault current flows to earth, there’s no mechanism to cause the mains fuse (on the neutral feed) to blow.

Good point!
That would be a job for the RCD.

Our safety earth plugs are reversible/unpolarised.
RCDs (30mA) are required since 2007.

[Mick Bailey]

Reading this thread made me recall an interesting effect present in some UK mains supplies many decades ago; the neutral wasn't connected to earth at the property (as with PME) and had about 6v or more between it and earth, but at a decent current and certainly enough to run the common 6v vehicle bulbs still available in the '60s. Free electricity, but illegal, as it constituted 'unlawful abstraction of electricity' due to being unmetered. No RCDs in common use back then, either.

[SoulFetish]

I had to look up what an “RCD” was. Looks like it’s another name for a “GFCI”, which is more commonly used over here. It’s such a simple and brilliant circuit, and has no doubt saved countless lives.

[g1]

Now arc-fault protection (AFCI) outlets are becoming prevalent and required here.