Fuses for safety

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Hi, First of all I love this site. I haven't installed my BRAND NEW Digitrax Super Empire builder yet, but hope to REALLY soon. In preparation for it I bought a transformer that outs out 16 volts at 8 amps. That should be good enough. I need to put this in a box and put in an ON/OFF switch and fuse it. I was going to fuse it at 1 amps in the input and 6 amps (maybe 5 amps)at the output for saftey. The only problem is most Radio Shack fuses are set for 250 volts. Do I use the math and multiply the amp x volts for the power rating and use an appropriate fuse like that? For instance 8 x 16 =128 watts. so a .5 amp 250 volt fuse should work? It has been SOOOOO long since I figured this stuff I forgot the real criteria. Can you provide any insight as to not over fuse or under fuse? In other words what size fuse to use for input at 112 and 1 amp and output at 16v at 8amp? Thanks and regards, Vic bitleris

-- Victor Bitleris (bitlerisvj@hotmail.com), August 06, 2003



This is a great question. Thanks for asking it. We have gotten several really good ones lately.

First, let us answer, what is a fuse for? It is to prevent catastrophic failure - fire. It would be nice if it could protect electronics. Sometimes it does just that. Generally though, electronics will die long before a fuse blows. So it's real purpose is to prevent fire. There is an old joke: What is a transistor for? To protect the fuse!

Your first question was: Can I use a fuse rated for 250V and if so, how?

Yes you can. You do not need to do anything special. A fuse blows on the current drawn. The voltage has nothing to do with it other than this is the maximum voltage the manufacturer of the fuse feels that it is safe to use their fuse on. (On higher voltages, the fuse may explode or arc over; for example.) So as long as the voltage rating of the fuse exceeds the voltage you will be using the fuse on, you are good to use it. The only thing you need to worry about at that point is the current.

Your second question was: What should be the rating of the fuse given that you have a 16V, 8A, output? (You mention 6 and 5 as well.)

A book could be written on this topic. At the ideal limit, you would use a fuse that was just high enough that it didn't blow when the device was working properly. As a practical matter, you don't have to knock yourself out finding out precisely what that number is.

You can use a fuse up to double over what you think you need. If it still blows at turn on and you hit the 2 times limit, try a slo-blo. In general, for a fast-blo, I would start with 1.5 times what you need. For a slo-blo, start with 1.25 times over what you need. This is merely a starting point.

In general, a slo-blo will allow you to use a fuse closer to the operating limit of the device being protected. Just realize that it is slower blowing - meaning you are definitely preventing fire and not protecting electronics.

With boosters that have their own automatic shutdown electronics, the only purpose for the fuse is to protect against catastrophic failure of the booster. If the booster is working, it will shut down on its own long before the fuse blows. So you can probably use a fuse close to the INPUT current of the booster (not the output. The input might be a little higher. Check the manufacturers suggestions or specifications.)

For my garden boosters operating at 8A, 18V out, I use 2A slo-blo's on the input and 10A fuses (one has a 10A circuit breaker) on the output. I had to use slo-blo's on the input because turn-on surges were blowing fast-blo fuses. In the example you provided, you could use the same. For a 5A output, you could try the 6A. You might still use the 2A slo-blo. (Notice how my input fuses are 2 over, not 1.25 over as my starting point suggested. It's hard to have a general rule that applies well to everything!)

Note: If you are 2 times over your expected need and even a slo-blo blows, I would definitely suspect a problem.

Here's another "trick." When a fuse blows, look at it. Did the wire melt and sag? Or did it melt so badly that the metal is plated on the inside of the glass? If it sagged, it didn't blow terribly high over it's rating. If the metal is flashed on the inside of the glass, either the fuse is MUCH too small or something is SERIOUSLY wrong.

-- Allan Gartner (bigboy@WiringForDCC.com), August 06, 2003.

Allan's advice is good. However think some more about the fire protection purpose of the fuse. Fuses generally are not fast enough to protect more sensitive electronic components from failure, but they should blow open if the electronic device (booster, power supply, or your wiring) fails in order to prevent a possible fire. Fuses are rated to cary or blow on amperes (amps). But the voltage rating specifies the maximum circuit voltage that it is designed to interrupt or clear. Selecting fuses that are too small in amp rating may simply cause nuisance fuse blowing. Selecting a fuse that is not rated to operate at the actual voltage in the circuit may cause it to not clear when it needs to, to prevent a fire hazard.

The transformer itself is quite rugged and will put up with much abuse. But miswiring or a short circuit on the output side can cause it to fail in a matter of seconds. Fusing both primary and secondary will help prevent disaster. Remember that the current expected on the primary (115Vac) side will be different than that on the secondary, low voltage side. Rated primary current for your example will be will be 8amps X 16/115 (the sec/pri voltage ratio) or 1.1 amps. A 2 amp slo-blow fuse on the primary would be a reasonable choice. But for the purpose of preventing a fire at the end of the (most likely) 18 ga lamp cord between the transformer and wall outlet, a 5 amp fuse or circuit breaker would be just as good. [18 ga lamp cord is good for 5-7 amps.]

However, be aware that any fuse or circuit breaker on the primary side MUST BE RATED FOR 125 Vac OR MORE. You can use 250 V fuses here, but never use a low voltage automotive type fuse or circuit breaker on 115 V primary wiring!

Fusing of the secondary side is not quite as critical if there is only one secondary winding and you do fuse the primary. A 10 amp slo- blow fuse or CB will protect your 16V 8A transformer from sustained overloads. The voltage rating for a fuse or CB on the secondary side needs to be only 16 Vac or more. Automotive type fuses or CBs will meet this requirement. So will equipment rated for 125 or 250 Vac.

To fuse a multi-secondary transformer properly... Fuse each secondry winding according to its own amp and volt rating. Fuse the primary winding at the sumation of all secondary VxA loads, at the primary voltage.


-- Don Vollrath (dvollrath@magnetek.com), August 07, 2003.

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