Fuse before switch or vice versa?

[Axtman]

I believe that the fuse should go before the power switch when hooking up the black (hot) wire of the power cord.

The only exception would be if there is a convenience outlet. Then the outlet should be wired before the fuse. That way equipment plugged into the outlet (drawing too much current or faulty) won't blow the amp fuse.

That said I have seen a few modern or redrawn schematics where the power switch is the first thing on the black wire then the fuse.

[g1]

The 'plus' of putting the switch first is in case someone tries to change the fuse having turned off the switch, but forgetting to unplug the unit. Now there is now power at the fuse holder.

[Axtman]

Interesting. I had never thought of that.

I do know it is good to not run the power directly to the "tip" of the fuse holder. That way someone does not get shocked putting the fuse in.

[Chuck H]

Interesting. I had never thought of that.

I do know it is good to not run the power directly to the "tip" of the fuse holder. That way someone does not get shocked putting the fuse in.

Well you only need to be holding the fuse to connect to the "bottom" of the fuse holder. Maybe most guys actually stick a fuse into the cap and then insert it, but I'll bet there are plenty of cases where someone just sticks the fuse in with their fingers and then tries to install the cap.

Fender tends toward putting the switch first. Marshall and others tend toward putting the fuse first. I lean toward putting the switch first. I think the switch shorting to the lever is less likely than someone holding the end of the fuse while inserting it. Maybe it would be better to have two fuses.?.

[Enzo]

When people like Fender wired the chassis, they used layout convenience, not protocol. The mains cord came in and solder one end to the fuse holder, the other end to the switch.

Actually, if you hold a fuse and insert it, if the tip inside is hot, your could get a shock, but if the sleeve is hot, you will touch that as you go in. SO either way you can get a shock if the unit is powered. And that is part of why modern fuse holders have the fuse farther recessed.

[Chuck H]

I wasn't referring to the vintage amps with the single pole switch on the hot and the fuse on the neutral. I looked at a bunch of later model Fender amps which do switch both AC leads and fuse the hot.

[nevetslab]

When people like Fender wired the chassis, they used layout convenience, not protocol. The mains cord came in and solder one end to the fuse holder, the other end to the switch.

Actually, if you hold a fuse and insert it, if the tip inside is hot, your could get a shock, but if the sleeve is hot, you will touch that as you go in. SO either way you can get a shock if the unit is powered. And that is part of why modern fuse holders have the fuse farther recessed.

In fact, they've gone as far as not engaging the AC Mains until the fuse is fully inserted and the cap turned to lock it. Turning the cap engages the circuit. I gotta look to see if it's what is in the new Fender amps or one of the other mfgrs. I constantly find those fuse holders (having the plastic 9/16" hex nut) are loose.

[galaxiex]

On vintage amps where I have to install a 3 wire line cord to replace a 2 wire,
I like to connect the line hot (black) to the switch.

That way if the chassis is out (for repairs/mods whatever) and plugged in,
as long as the switch is off, the only hot point in the chassis is the black wire on the switch,
and that I always try to insulate with heat shrink.

I know, I know, if I'm working in there the unit should be unplugged.

Sometimes you just want to make a small tweak, or move a test lead etc,
that takes 2 seconds so you switch off without unplugging, make the change and flip back on.

BUT... there have been times I've done that on amps that have the line hot going places before the switch,
and I've accidentally touched/brushed against a line connected point that is prior to the switch and not insulated.
Got a little tingle and that causes the startle effect,
so you jerk your hand away,
knock over the soldering iron,
burn things,
small parts go flying....
I could go on....

Yeah... shoulda unplugged it....

[R.G.]

There is a proper way. The black wire should be wired first to the fuse holder. The purpose of the fuse is not to protect the circuits. It's to stop the chassis from becoming hot (in three wire setups) and to stop the flow of electricity from starting a fire.

The problem of a fuse end being live to the black wire when replacing a fuse is a well known issue in safety circles. It's mitigated by wiring the black wire first to the cylinder terminal, not the tip. That ensures that removing an intact fuse does not leave the open end of the fuse live, as noted earlier in the thread. It is uncommon for repair techs to know this, so a fair amount of work has gone into remaking fuse holders so it's more difficult to electrocute yourself replacing a fuse. More modern fuse holders are designed to insulate the outside end of the fuse until after the fuse clears the holder. On new construction, I always use an IEC input module with an integrated fuse holder. These are designed to make fuse shocks highly unlikely.

Logically, if you're trying to enclose as much of the amp as possible inside the fuse, the fuse holder should be very first terminal the hot wire touches, just as the chassis safety ground point should be the first thing the green wire ground touches.

That does leave the problem of what to do for convenience when you work with an amp on a bench. Just my personal opinion, but if you do this more than very, very rarely, some kind of safety disconnect ON YOUR BENCH ought to be high on your list of specialized tools. A metal outlet box with one side having a duplex outlet and the other side having a DPST disconnect switch would be really smart. It completely removes the issue of what shocks you inside the amp, and you can make this very handy to use. A big, red mushroom disconnect switch would be useful in some situations. You would not be relying on a possibly defective or incorrectly wired switch inside the amp to save your life.

[trobbins]

Fuseholders compliant to standards nowadays don't allow finger touching anything live through the fuse, whether the active is connected to barrel or far end. And relatively recent updates to fuseholder and household appliance standards have made sure that the fuse holders perform better for fire safety (eg. glow wire or flame tests).

I'm certainly a fan of replacing plug-in cord connected vintage amps with IEC style panel mount sockets with integral fuse draws - can't go wrong - just wish there was a neat simple cheap punch/die to suit

[R.G.]

Fuseholders compliant to standards nowadays don't allow finger touching anything live through the fuse, whether the active is connected to barrel or far end. And relatively recent updates to fuseholder and household appliance standards have made sure that the fuse holders perform better for fire safety (eg. glow wire or flame tests).

Aye - there's the rub. In the USA you can sell fuse holders that are exact replacements/knock-offs of the old, original (and dangerous) fuse holders from the Golden Age Bad Old Days. And of course, given the mania for "original" in amps, fussy owners will often demand it look original.

IMHO, new design/construction really ought to use IEC connectors with integral fuses. It's a very slick solution.

[...] IEC style panel mount sockets with integral fuse draws - can't go wrong - just wish there was a neat simple cheap punch/die to suit

Sigh. Yeah, that would be great. I'm pretty sure that an IEC punch/die set exists, but it's certainly not going to be cheap. I'm fortunate in having a machinist friend that could, I'm sure, machine up a punch/die set for me, and then get it heat treated for use. But even so, I've always just drawn out the hole on the metal carefully, then drilled holes and used nibbler, saw (you can disassemble/reassemble a hacksaw into a hole) and file to make it fit. Including putting putting band-aids on the cuts and abrasions on my hands.

[Chuck H]

I've always just drawn out the hole on the metal carefully, then drilled holes and used nibbler, saw (you can disassemble/reassemble a hacksaw into a hole) and file to make it fit. Including putting putting band-aids on the cuts and abrasions on my hands.

That's the way I do it too Sometimes if I'm using a Hammond aluminum chassis (read, kinda thin gauge) I'll drill holes close to the perimeter and then use my Dremel tool to remove all but the corners and then finish up the corners with a round edge file.

For most of the amps I've made I did, indeed, use the vintage style fuse holders because I was shooting for that vintage-y look. That's been my reasoning for putting the switch in front of the fuse. I'll be looking into alternative fuse holders and putting the fuse first from here on.

[nevetslab]

I do similar, but using a Whitney XX Hole Punch, punching the corner holes first, then following the outline using over-lapping holes to nibble out the center, then finish the pattern with small flat files. The biggest problem with a fitting punch and die for the correct size is the tonnage required to knock that hole out. Always done on the flat with a production punch press before the flat is knocked out of the sheet, then run thru the time saver to debur and grain the material, and then fold it up.

[nevetslab]

In fact, they've gone as far as not engaging the AC Mains until the fuse is fully inserted and the cap turned to lock it. Turning the cap engages the circuit. I gotta look to see if it's what is in the new Fender amps or one of the other mfgrs. I constantly find those fuse holders (having the plastic 9/16" hex nut) are loose.

I found that Fuse Holder. It's a Littelfuse Series 345 Shock-Safe Fuse Holder. The spec sheet doesn't come right out and tell you the fuse doesn't make contact until you've pushed in the holder and rotated it into lock position, but, that's how it operates. Data sheet here:

Littelfuse_Fuse_Holder_345_Panel_Mount_Datasheet.pdf.pdf

[SoulFetish]

The problem of a fuse end being live to the black wire when replacing a fuse is a well known issue in safety circles. It's mitigated by wiring the black wire first to the cylinder terminal, not the tip. That ensures that removing an intact fuse does not leave the open end of the fuse live, as noted earlier in the thread. It is uncommon for repair techs to know this, so a fair amount of work has gone into remaking fuse holders so it's more difficult to electrocute yourself replacing a fuse.

Do you know how difficult it is to find industry data or info on which is the proper fuse terminal to wire the hot connection to?? I've seen it consistently wired both ways, so I tried to reason out likely shock scenarios (and apparently I did it wrongly). I would have thought that do to the spring action in most panel mount holders, the greatest risk seemed to be contacting a hot ring during the act of installing or removing an intact fuse?