Fuse Question

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Fuse Question

Postby EdStiles » Mon Oct 08, 2007 7:30 pm

In the "Good Old Days" before we had polarized plugs, it didn't matter where the fuse was located (hot or neutral side if the line) - there was a 50 % chance you had it right.

What is the recommended place for the fuse today? I have been putting them on the "hot" side immediately after the power switch and before the CL80 or CL90 surge resistor. Logic is that if the chassis has a short or defect, the blown fuse will keep the chassis from having AC on it.

Any suggestions?

Thanks,

Ed
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Postby TomMcNally » Mon Oct 08, 2007 8:30 pm

The fuse should be in the hot side of the line, BUT before
the switch also ... the switch could fall apart and go to ground,
especially in the case of a metal toggle switch or a
switch with neutral going to it for a pilot light.

The schematics you see with the fuse in the neutral and
the switch in the hot lead are bogus. Not very safe.
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Postby erichayes » Mon Oct 08, 2007 8:56 pm

This is somewhat apropos, since we're talking about potentially "hot" chassis:

A quick, qualitative check for potential on the chassis of an unfamiliar piece of gear (including one you've just built but haven't fired up yet) is to rub a knuckle on a bare part of the chassis. If the metal feels like it has very fine burrs on it, you have AC on the chassis. If it's an old 2 wire unpolarized unit, reverse the power plug and repeat the knuckle test. If the metal feels normal, there's probably a cap going from one side of the line to chassis that's leaky--although I've run into power transformers that are leaky, too.

If you do the test on something with a 3 wire line cord and get burrs, you need to check the building's wiring as well as the unit in question.
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hot side.

Postby EWBrown » Tue Oct 09, 2007 6:47 am

I've always used this approach:

First the fuse, then the current limiter, then the switch, then to the "hot" side of the primary, all in series in the "hot" side of the power line. The neutral should go directly to the other side of the power trannie primary.

The fuse should be the first thing to "go" if the switch, ICL or power trannie fails or shorts out, so it's best to have it closest to the hot side AC powerline.

/ed B in NH .
Last edited by EWBrown on Thu Oct 11, 2007 8:11 am, edited 1 time in total.
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Postby dhuebert » Tue Oct 09, 2007 8:15 am

Hey, I have something to contribute. If the switch is first and the fuse blows the gear is dead but the switch is live which is dangerous for inattentive fingers. (ask me how I know)

Don
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Postby Gingertube » Wed Oct 10, 2007 9:33 pm

I did my electronics training in a major hospital. It was a medical equipment requirement that both active and neutral were fused. I've got into the habit and build everything this way now. Gives added protection in those older houses where you can't always rely on the active and neutral being wired to the correct wall outlet pins.

For guitar amps where power wiring in venues is often really sus its almost essential to fuse active and neutral in your amp.

If it only ever saves your life once !!!!!

Cheers,
Ian
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Postby Slartibartfast » Thu Oct 11, 2007 5:06 am

On another Vacuum Tube forum, there was the discussion of using GFCI outlets for powering vintage tube gear. This had more to do with transformerLESS inputs on desktop tube radios and such, where there is no isolation from the line voltage. There was even the concern of a short to frame in a power transformer.


When I am switching my Dynaco amp on or off, I just make sure my other hand is not touching anything else. The one arm behnd the back, like they taught in lab class.
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Postby dhuebert » Thu Oct 11, 2007 7:29 am

It was a medical equipment requirement that both active and neutral were fused


Interesting. I worked in a research lab in a hospital and built electronics that would occasionally find its way into the intensive care unit. Any such gear had to be CSA approved beforehand, so for $50 a guy would come do a brief inspection and put a sticker on it. Nobody ever said anything about two fuses to me. The only mod I ever had to make was to put a separate post for chassis ground.

Don
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Postby EWBrown » Thu Oct 11, 2007 7:50 am

I used to do product safety testing at a former job, the USA / Canadian, and European, and Australian safety standards are somewhat different, especially for medical equipment. Basic consideration is the implementation of 120VAC, 60Hz vs various forms of 220-240VAC, 50 Hz.

It's pretty much of a major "NO-NO" to fuse the neutral, according to US / Canadian electrical codes...

I remember that the med gear had these fancy (and very expensive) "see-through" 3-prong plugs. That way, the internal conections could be easily inspected before each use.

Audiophile "magic snake oil" power cords aren't the only expensive and overpriced ones out there ...

/ed B in NH
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Postby erichayes » Thu Oct 11, 2007 2:16 pm

Back in the '80s, a Canadian version of an Akai VCR came into my shop for cleaning and I noticed that it was festooned with fuses. I'd seen schematics of Pioneer and Kenwood receivers and had noticed that the Canadian versions always had 2 to 3 times as many fuses as the US and General Export versions had.

I made a snide remark about how you could tell a Canadian version of something by the number of fuses to the customer. Turns out he was a deputy fire marshal for Vancouver BC (you guys look remarkably like us). He politely pointed out that, with a national population less than that of some of our states, it was rather difficult to put a fire station on every other block. Their reasoning was that it was probably better to blow a fuse and piss someone off than it was to arrive at a house that had already burned to the ground.
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Postby dhuebert » Thu Oct 11, 2007 2:41 pm

Gar used to gripe about having to put a second fuse inside the machine too.

Don
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Postby dcriner » Thu Oct 11, 2007 5:34 pm

If you put fuses on both the hot and neutral legs, what does that protect against?

If there is simple overload within the amp, then it doesn't matter where the fuse is - it will blow - but unless it is located in the hot leg, the amp may still be energized.

If there is a short to ground within the amp, then only a fuse located in the hot leg will blow. A fuse in the neutral leg will not blow.

I suspect that a fire chief who insists on fuses on both the hot and netural legs perhaps doesn't fully understand this.

Or maybe there is a theory that two fuses in series are twice as good as a single fuse. If so, put both of them in the hot leg.
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