Thread: Is this a fuse?
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Jeff Liebermann Jeff Liebermann is offline
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Default Is this a fuse?

On Thu, 11 Jul 2019 10:43:34 -0400, Bob Engelhardt
wrote:

On 7/10/2019 10:06 PM, Jeff Liebermann wrote:
I couldn't find anything similar, partly because you didn't supply the
maker and model number of the variac.


It's a Ward Leonard (no model number) - yeah, I never heard of them
either and I didn't find anything on the web. They did make variacs for
theatrical lighting, so that's probably where this came from.


Google finds plenty of hits on the company name, but none involving a
variac. Probably parts of a system as you suggest.

The lead colored material is probably zinc steel. Try checking with a
magnet.


It's non magnetic. Following your powerstream lead, I'm guessing it's
tin, or lead-tin.


Ok, it's not iron. It's also not 63/37, 60/40 or 50/50 Sn/Pb, also
known as solder. I would guess 100% tin with the dull coloration that
looks like lead be tinning, or some kind of corrosion.

...
Therefore, the cross sectional area is:
0.063 * 0.033 = 0.0021 sq-in = 2700 circular-mils


The bridge measures .017 x .068 in; 1472 circ mils (.038 equiv diam)

...
https://www.powerstream.com/wire-fusing-currents.htm
shows that #16 AWG will fuse at: ...


AWG 18 is .0403 diam & tin fusing current is 13A. Which is considerably
less than the 20A that the variac is rated for resistive.


I would expect the fuse to be rated at MORE than 20A. Also, a 0.017
in thick spade lug is rather flimsy and will probably make a lousy
push on tab terminal. For 20A, it really should be thicker or the tab
terminal will arc, loosen, and/or fall apart. Tin is easily soldered.
I would therefore expect wires to be soldered to the tabs and not use
push on terminals.

I doubt that you're going to find a replacement fuse. ...
Or, you could just replace it with an external 35A cartridge fuse.


Yeah, I put in a piece of 14ga copper & will use an external fuse. For
now, I'll just rely on the 15A breaker that it's plugged into.


That will work. I would heavier wire to a fuse socket. However, I'm
at a loss as to how to rate the fuse. My initial guess(tm) would be
20A or slightly larger to correspond to the stated rating. However,
if the fuse were too large, the variac winding would probably blow
before the fuse, which kinda defeats the purpose of having a fuse.
Therefore, I would measure the variac copper wire diameter, calculate
the fusing current from the Powerstream chart, and size the fuse to
blow at LESS than wire fusing current.

The confusing thing to me was how inconvenient this fuse's replacement
was. The mounting block had to be removed (2 screws) & the fuse was
held by another 2 screws, each with a sleeve, flat washer, lock washer,
& nut. What were they thinking?


My guess(tm) is if the fuse blew, the variac would be considered
totalled and require a factory replacement. That fuse was obviously
not intended for customer replacement. My conspiracy theory is that
someone forgot to design a fuse into the original device, and had the
variac winding blow up in the field. UL or the equivalent, probably
wouldn't like Ward Leonard modifying the design without having the
device recertified. Not wanting to go through that ordeal process
again, WL installed a fuse where UL wouldn't see it and said nothing
about the field failures.

Thanks for your in-depth reply.


Y'er welcome.

--
Jeff Liebermann
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Skype: JeffLiebermann AE6KS 831-336-2558