©By: John T. Blair (WA4OHZ)
1133 Chatmoss Dr., Va. Beach, Va. 23464; (757) 495-8229

Originally written: circa Jan 1999
Last update: June 30, 2001 - fixed link back to index

Fuses
The following is an email that was posted to one of the mailing lists I belong to. Glen did nice job, explaining some of the differenced in the various types of fuses. I thought this might be of help to some of you.

John

Date: Mon, 06 May 1996 14:55:30 -0400
From: "Glen R. Wilson"
To: dan parslow
Subject: Re: Fuses

dan parslow wrote:

2. Lucas vs. 'Merican fuses: I've heard a rumor that the fuses you can buy in your local auto mart are not direct replacments for Lucas fuses...something to the effect that Lucas fuses are marked as to the amperage at which they'll actually blow, whereas your AutoPepZoneBoys type of 5-for-$1 fuses are designed to permit the marked load and blow somewhere higher up. Is this true? If so, what's the conversion factor?

Dan, Among fuses that are the same size (i.e., they fit the same holder) there can be many different types. Generally, there are fast blow, slow blow, and time-delayed fuses. This causes a lot of confusion even among people who work in electronics. A fuse described as fast-acting or fast-blow will generally go at the amperage specified (5 amps for a 5 amp fuse. These offer the most protection, but may not be what the auto manufacturer wants you to use. You see, many types of electrical circuits pull higher current when they are starting up than when they are in operation. In general, the harder a motor works, the more current it will draw. Slow-blow fuses will allow you to draw more than the rated current without blowing for some period of time dependent on how much of a current overload there is. A time-delay fuse will also allow a certain amount of overload before blowing, but will open after a designed-in time period when subjected to an overload (it is less dependent on the maximum current load than the slow-blow).

A good example might be a wiper motor. The motor might draw 8 amps (just to pick a figure) as the motor accelerates to normal speed and gets the wipers moving, but might only need to draw 4 amps to keep them moving. If you used a fast-acting 5 amp fuse, it would blow every time you turned the wipers on because the motor draws 8 amps briefly as it starts up. Fast-acting fuses are designed to quickly protect delicate circuits.

If you installed a 5 amp slow-blow fuse, there would be a difference. The 5 amp slow-blow fuse would have been designed to allow a brief surge in current (maybe 10 amps in this case) and would therefore let your motor start up at 8 amps without blowing. The 5 amp slow-blow would let you run indefinitely at 4 amps. The 5 amp slow-blow fuse would not, however, allow a sustained current of 8 amps. In fact, if your motor continued to draw 6 amps because the gears were not lubricated or a mechanical arm was bent, the 5 amp slow-blow fuse would open up to protect the motor. If it didn't, the motor would draw more amps than it was designed to withstand, and it would get hotter and hotter until the insulation on the motor windings melted causing a short and destroying the motor.

You might not be able to tell the difference between a slow-blow and a time-delayed fuse in actual practice. They would both give you some sort of brief allowance of excess current draw.

The key here, is to use the type of fuse specified by the equipment manufacturer. Specifications will generally include voltage, amperage, and fuse type. Most automotive fuses are rated for 24 volts since the power is generally supplied by a 12-volt battery or perhaps a 15-volt charge from the generator or alternator. Sometimes people make the mistake of using a 24-volt automotive fuse in some sort of home appliance which plugs into the wall and needs a 120-volt fuse. A typical auto electrical schematic might call for a 25-amp, slow-blow fuse. If it simply says 25-amp, they generally mean a fast-acting fuse, although some schematics are lousy. When in doubt, try to find a list of fuses in the operator's manual or parts list; these will often be more specific.

Don't assume that the type of fuse you find in the fuseholder is the type of fuse that should be in there. Someone may have put a fuse with a higher rating in there rather than fixing the problem.

Use the type of fuse specified by the equipment manufacturer.

If you look at the fine printing on the fuse, you'll often be able to tell the maximum voltage and the rated amperage but often not the fuse type. If you know the model number of the fuse you find, you'll still probably have to have a fuse catalog to tell what it is.

Slow-blow and time-delay glass fuses generally have more than a little piece of wire inside. Slow-blows often have some kind of spring. Time-delays may actually have a little resistor inside. However, there is an incredible variety of fuses out there, and it's pretty hard to tell from appearances exactly what kind of fuse you're holding in your hand.

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