Using a 32V fuse holder with 120V?

[GoingFastTurningLeft]

I am working on a temperature controller that switches 120V power using a relay. It's going to be used with a purpose-built 40W heating pad attached to the side of my beer fermenter in order to stop it from getting too cold. I'll probably use it with a fan in the summer months.

I assembled everything and it works with a 40W light bulb as a test load. One of the parts I used is a bussman in-line fuse holder that i'm using with a 2 amp glass fuse, I bought it at an auto parts store, part #BP/HHK.

This thing has #14 wire and is rated for 20 amps, so I figured it was OK. I just noticed on the back of the packaging that they only sell 1 part # rated for 250V and everything else is rated 32V. The part that is rated to 250V is a panel type fuse holder, not what I wanted.

Is the 32V rating just so people don't use them in 120V circuits, or is my device actually hazardous? I'm thinking it will be OK myself but I'm interested to hear people's opinions.

[sofaspud]

I don't recall seeing many inline fuse holders for 120V equipment. If any at all. It's probably not the safest thing to have.
With that said... just between you and I, and not your lawyer or insurance adjuster, I think you're alright. The fuse itself
is rated 250V isn't it?
I'm guessing their lawyers and marketing folks came up with those ratings.

[GoingFastTurningLeft]

Yes, your standard 250V/2amp fast-blow fuse. The inline fuse made it easier and safer to install in my case, since it is crammed into the space below the outlet in a single gang 'new work' box along with a pcb that holds the relay and switching transistor.

[gerty]

One mistake I see students make is attaching the line voltage to the side terminal of the fuseholder. What that does is shock you when you pull the fuse out, the line shouuld be connected to the end terminal so that the ring isn't hot. 32 volt fuseholders are typically used for automotive or low voltage controll applications.

[MrAl]

Hi there,


I would be very careful about this. Something that is rated for 32 volts has many parts
made with that low voltage in mind, including the wire insulation which may not be rated
for a higher voltage. When they make things these days they usually use the lowest
cost parts they can find, according to the required ratings. If they rated it at 32 volts
then they used parts that could handle 32 volts, but most likely not three times more
than that.
In short, i wouldnt use it, but if i had to for some reason, i would mount it in a place where
it could not be touched, even the wire leads, and turn off the power before opening it.

Dont those things have exposed ends when opened? That could be another problem.
One end will always be live, and the other end will still be live if the power is on because
it could feed through the device.

What else they make is a fuse holder that bolts down with one screw. That could be
mounted inside a case or something.

[haklesup]

Dont those things have exposed ends when opened? That could be another problem.
One end will always be live, and the other end will still be live if the power is on because
it could feed through the device.

I think that precisely is the difference between 32VDC and 120/240VAC in line fuse holders. It's more a safety thing than an electrical limit.

[GoingFastTurningLeft]

Only one of the terminals is spring loaded.

Well if it means anything, it's labelled Made in the USA. I doubt Cooper Bussmann would cheap out on something like insulation. I'd be surprised if anybody even makes 14 gauge wire with a dialectric strength under 300V.

I'm going to let it cycle some more tonight and then take it apart and inspect. The only thing I can see being an issue with it is the heat produced by the relay coil. Even then it's only on 5 minutes at a time. The on/off times will change, but it will probably never be higher than a 75% ratio since I need to allow for the applied heat to dissipate throughout 5 gallons of liquid and I'm trying to keep it between 64 and 70 F all the time.

[Lenp]

One hazards in using under-rated fuses and holders are that the integrity of the assembly may not withstand the enormous fault currrents high voltage dead short. In high voltage circuits arc over is also an important issue not seen in low voltage systems. A 10,000 AIC/AIR is a rating on some breakers!

Glass fuses rated at 32 volt fuse can, and have, exploded in a dead short on a line foltage circuit. Safety from shock is another issue since the in-line fuseholder likely has loose wires and contacts in the caps that you can come in contact with if you change a fuse and the circuit is energized.

Perhaps more than all the rest, the company didn't file for UL approvals because it is being rated for and sold to the low voltage market with lower risk and product liability insurance.

Consider that bare fuse clips are often soldered to PCB's for line voltage protecton and they aren't likely to be as controlled as a manufactured fuse holder. Here the manufacture takes the responsibility for the design.

So,
will it work? Maybe, but, like once said. "You pays your money and you takes your chances"

Len

[CeaSaR]

Hopefully the "new work" box is all metal and not the plastic kind. That way you can ground the
box properly AND contain any possible fire hazard. Technically, the circuit that the controller is
plugged into should trip before real major problems occur in the event of a short in your work. If
you have any concerns, you can mount the box separate from everything else (isolating it) with
a "pre-box" kill switch.

As for the power rating, I am assuming (I know the saying!) that the 32V is for DC, right? At 20A,
the unit is rated for 640 W. Your 250V 2A fuse should pass a max of about 90W before blowing. I
know that AC and DC ratings are way different, but it seems like there is enough leeway there.
Can anyone elaborate on how to compare these "apples to oranges"?

Just my $1.50 ($0.02 adjusted for inflation).

CeaSaR

[Robert Reed]

Wattage probably does not enter into it since the fuse is a current operated device only. If the OP is talking about the standard 1/4" x 1.25 " glass housed fuse, I have wondered about those voltage ratings from time to time myself. A close inspection reveals very little difference between the two and the only thing that comes to mind is greater arc over when it opens. A 32 VDC inductive load has much greater potential for that than an ACV load would have. I don't know if the filaments are such that the AC version completely disintegrates vs only a small break in the DC version that might make them superior for arc over reduction. The plastic fuse holders look to be identical as the line operated panel mount style. For my use, I would not be concerned about the difference- remember- I said my use! Of course like anything else it depends on the criticality of the application. There was a time when line plugs used to incorporate a fuse holder right into their housings and in fact many of the Xmas string lights still do.

[CeaSaR]

So the theoretical difference between the 2 systems (in this particular case) is the design/construction
of the fuse itself? That and the possible voltage breakdown of the insulation/holder?

As for the exposed contact safety factor, a cutoff switch prior to the fuse will take care of that.

CeaSaR

**Remember the old panel mount fuses on older equipment that could be changed by twisting off
the top and pulling the old glass tube fuse out and putting it back in? They were similar to what you
described except the "top contact" was in the lid and made contact on the side of the "screw" part of
the lid. Both wires are inside the unit away from hands. Safety came in the form of the power switch
being in-line before the fuse.

[MrAl]

Hi again,


Another thing to keep in mind is the rated pull strength between opposite wires.

Also, i noticed that some of the 250v rated units are waterproof...a nice feature
in case it ever falls into a body of water or gets worked with wet hands.

They have a 250v rated unit on Digikey for about 2.50 USD. Maybe go with that one.

[Bear]

i would not use it unless you want to give your insurance company a reason not pay for your house if it burns down. They will look for any reason to not pay. You may want to check the fine print of your life insurance policy.

Take care all

Dennis

[GoingFastTurningLeft]

Of course the box is plastic, otherwise the PCB under the outlet would short out The devices I will be switching with it don't have ground prongs so I didn't use a grounding power cord.

The more I discuss this part of the project with you N&V guys and a co-worker the more doubts I have. This is my first mixed 5VDC/120VAC project and I'd rather not learn any lessons the hard way.

Functionally, the circuit is fine, its implementation just could be a little safer. Its too bad that I won't be able to use my original design, as I wanted everything related to the 120V to fit in a narrow gray plastic outlet box.

It looks like I'm going to have to bite the bullet and move up to one of those ugly metal 5"x5" boxes and find a single outlet cover for it. At least then I'll be able to fit a 5VDC power supply in there.

Thanks for all the opinions and ideas.

[sofaspud]

Interesting that the OP gets talked out of this, while another thread discusses homemade GFCIs. But, hey, it's all good.
On a side note, I was recently at the Radio Shack Outlet Store in Fort Worth and picked up a bunch of inexpensive AGC
fuses. The packaging for one type states, "Fast-Acting, Automotive, 5-Amp, 32-Volt Fuses." The fuses in the package
are actually rated 5A 250V. So go figure.