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The ATC blade-style fuse has become very common in small boat electrical wiring. These fuses are color coded and marked with their Ampere rating in a clear manner. Visual detection of an open fuse link requires removing the fuse from its holder and making a close visual inspection of the fuse link area between the blades.

Marine electrical manufacturer BLUE SEA SYSTEMS is offering an ATC blade style that contains an LED indicator that will illuminate when the fuse link opens. This should help locate an open fuse in a power distribution panel of many circuits. Blue Sea Systems calls them easyID™ fuses.



These illuminated ATC fuses are available in ratings of 3, 5, 7.5, 10, 15, 20, 25, 30, and 40-Amperes. They cost $4 for a package of two fuses. For more information see the Blue Sea Systems webpage about these fuses.

These fuses are also available from COOPER-BUSSMAN, who also refer to them using the same trademarked name, easyID™, and they are probably the actual manufacturer of them. The easyID fuses from Cooper-Bussman are also available in styles ATC, ATM, and Maxi. They also offer illuminating fuse holders that can be wired in-line. For more information see the extensive product information pages on the website of Cooper-Bussman.

The Cooper-Bussman fuses can often be found at automotive parts retailers like AutoZone, and they sell at the same $4 for two fuses. There is also an assortment package containing twenty fuses for $21.

Do you know how these work?

I have a 12-Volt panel in my recreational vehicle that, when circuit is blown, an LED indicator lights up next to the bad fuse. But, it only lights up if there is a bad circuit. Slots not used do not light up with a fuse in place.

Because the illumination circuit is contained within the ATC fuse, and since the fuse is only a two-terminal device, I presume the illuminated ATC fuses work as follows:

--the LED is wired across the fuse link, with a series resistor or some other means to limit current;

--when the fuse link is closed, the LED circuit carries almost no current because its path has enormously higher resistance than the fuse link path;

--when the fuse opens, the LED circuit replaces the fuse link, and all current flows through the LED circuit;

--a limited amount of current flows, just enough to illuminate the LED, probably about 0.010-Ampere, mainly limited by the series resistor;

--the load on the circuit has to sink the current flow back to the negative circuit; if there is no load on the circuit, there can be no current flow, and the LED cannot illuminate.

Let's look at a circuit which is being protected by a 10-Ampere fuse. The resistance of the load in a 13.2-Volt DC circuit drawing 10-Amperes would be

R = E / I

R = 13.2 / 10

R = 1.32-Ohms

If the LED has a forward voltage drop of 1-Volt, then 12.2-Volts should drop across the series resistor when 0.010-Ampere flows to illuminate the LED. The series resistor will thus be

R = E / I

R = 12.2 / 0.01

R = 1220-Ohm

When the fuse link opens we have a circuit with a resistive voltage divider consisting of three elements:

--1220-Ohm series resistor
--fixed 1-Volt drop across LED
--1.32-Ohm original circuit load

If the applied voltage is 13.2-Volts and 1-Volt drops across the LED, that leaves 12.2-Volts to divide across the two resistances. The voltage divides in proportion to the resistance in each series branch compared to the total resistance of the series branches. Thus the voltage across the original load would be

E = 12.2-Volts × 1.32/(1.32 + 1220)

E = 12.2-Volts × 0.0011

E = 0.013-Volt

This very small voltage impressed on the original load should have no harmful effect. The most harm that can occur with these fuses is probably to your wallet if you have to buy a lot of them.

Fuse nomenclature is straightforward. I think AT stands for auto, and the last letter indicates type or size:

ATC = fuse ink is enclosed

AT0 = fuse link is in an open area

ATM = fuse size is mini