ETA Automotive

Low Voltage & Automotive Circuit Protection

Much needs to be said about this oft neglected subject. We would never even think of connecting the utility mains to our house without proper circuit protection—also, we must observe electrical codes. Low voltage applications (under 24V), we say, are safe—or are they? perhaps safe from electrical shock, but not short circuits. Few know that the short circuit current of even a tiny alkaline 9V battery can exceed 5A—how much more a huge lead-acid battery such as is found in all automobiles and many solar systems. With lead-acid storage batteries, the short circuit current could easily exceed 1000A—enough to destroy wiring if not properly protected. Generally we think of protecting the connected device, but neglect the interconnecting wire.

Some websites are so paranoid about automotive modifications that they will not even allow discussions on electronic circuits for automotive applications. However, note that your automobile represents a substantial investment and deserves sensible circuit protection. While I have never seen an electrical fire in a vehicle, I have seen a harness melt-down. Such is very expensive and/or laborious to replace. Circuit protection is generally inexpensive and always should be observed.

Circuit protection devices include fuses, fusible links, circuit breakers and PTC current limiters. The following is a concise list of the most inexpensive devices available from DigiKey.

Inline Fuse

Littlefuse 155 100 Series
Littlefuse inline fuse, 20A, DigiKey F062-ND $2.77 each (Includes 20A fuse)

MPD 10A Inline Fuse
Memory Protection Devices inline fuse, 10A, DigiKey BF301-ND $1.16 each

The inline fuse, though klunky in appearance is the least expensive means of protection. If connected close to the power source, it provides protection for both the wiring and the connected device—something that circuit breakers cannot do as well due to physical location. The Littlefuse inline fuse is available with or without pig-tails. Generally inline fuses come with a loop that may be cut and spliced as required. Note the maximum load current specification.

One caution is that if the fuse is removed, the contact could accidentally short to ground, or if reassembled without the fuse, the contacts could short together—keep this in mind.

Fusible Link

Fusible Link

These are made by Littlefuse. Your automotive shop is your best source of fusible links. While DigiKey no longer offers them, they have a very nice FAQ page

The fusible link is simply a wire gauge selection that is (4) wire sizes smaller—e.g. to protect an AWG #12 wire, an AWG #16 link would be used. The link wire has special, thick, high-temperature thermosetting insulation (not thermoplastic that melts) that is capable of preventing harness meltdown. Unlike a fuse, they are intended to protect wiring rather than equipment. Short circuit current is high and melting time is much longer than an actual fuse. They are regularly used in automotive applications. Recently, I had to protect a cable connecting two batteries in a recreational vehicle—one battery located in the engine compartment, the other in the rear. Because there were two power sources, the solution was a fusible link on each end of the cable.

Aircraft style circuit breaker

TE Connectivity Aircraft Breaker
TE Connectivity /Tyco, W23 Series 50A, 50VDC Circuit Breaker, DigiKey PB411-ND

This is an aircraft style circuit breaker. They are available in current ratings from 1 to 50A. While these are expensive at about $24 each, they provide the function of an On /Off switch as well. If you find some at a flea market, grab them. Some have a toggle switch actuator—I like these better.

Rocker switch circuit breaker

Qualtek Rocker Breaker
Qualtek 736 series circuit breaker, 15A, 125VAC, DigiKey Q279-ND, $2.15 each

This is a good buy at $2.15 each. While this one is not specifically rated for 12VDC, it will provide good protection because 12V is easy duty—the TE Connectivity competitor rates theirs at 50VDC. The probable reason why it is not specified for DC is that some versions offer a neon indicator lamp that requires 115VAC. It is also a possible marketing decision not to go after the low voltage market and thus save the additional cost of agency approvals for the DC rating.

The following rocker breaker does have the DC rating.
TE Connectivity /Tyco W51 series, 15A, 50VDC, DigiKey PB1366-ND, $3.60 each
They are available in current ratings from 5 to 20A.

120/240VAC branch circuit protection circuit breaker

Why not? For experimentation this will work fine and they are easy to obtain. Current behaves the same in 12V circuits as it does in 120/240V circuits.

Automotive circuit breaker

ETA Automotive
E-T-A 1610 series, 10A, 12VDC, DigiKey 302-1242-ND, $7 each

Automotive circuit breakers are of the automatic reset type that plug into the fuse box. While they offer circuit protection, they are relatively expensive and generally are not practical for hobbyist applications.

Testing circuit breakers

Since there is nothing to replace, it is fun to throw a short across the breaker and see how quickly it trips.

PTC resettable fuse

Raychem 15A polyswitch
Polyswitch RHE series, 15A, 16V, DigiKey RHEF1500-ND, $1.42 each

The PTC (positive temperature coefficient resistor) is a great circuit board level device. Because it never actually blows, it never needs replacement. They are inexpensive and come in a great variety of package styles and current ratings. The 15A version shown is one of the larger ones available.

Interrupting capacity, voltage rating and AC vs DC

The interrupting capacity rating is how much current the circuit breaker can safely interrupt in a short circuit condition for a specified source voltage. For 120 /240VAC branch circuit protection, it is generally 5kA (5000A). When any circuit is opened an electrical arc occurs. AC tends to extinguish the arc when the applied voltage passes through zero volts, while DC tends to maintain the arc and is much more difficult to interrupt when the voltage exceeds about 48VDC. For fusing DC circuits with significant voltage (50 to 700VDC), a semiconductor fuse is generally specified. For 12VDC, interrupting capacity is generally not an issue because low voltage systems have so much series resistance, that the current rarely exceeds about 2kA, and 12VDC is insufficient voltage to maintain an arc across open contacts.

FYI, check out this link for a thermal breaker that is rated for low voltage marine installations:,%20F.pdf (Sensata was formally Airpax Corp.

Circuit breaker curves

The curve is the graphical presentation of trip time vs current—the higher the current, the lower the tripping time. Some are delayed acting so that moderate inrush over-current, such as occurs when some circuits are closed, will not trip the breaker. Some have magnetic actuation that is much faster than thermal—so fast that it may have to be mechanically delayed slightly via a hydraulic damper to prevent nuisance tripping (“magnetic /hydraulic”). The curve is generally not an issue unless the load is closely matched to the circuit breaker current capacity.

Disconnecting an automotive battery

This is done simply by first unbolting the ground terminal. Why the ground terminal? Unlike the hot terminal, the ground terminal has no voltage to ground, so there will be no SPARKS should the wrench accidentally bridge the terminal to chassis. After the ground terminal is disconnected, the hot terminal may be safely disconnected.

Lead-acid battery explosion

Keep flames away and avoid making any arcs near the battery. The hydrogen gas generated by a battery can explode inside and/or outside the battery. This happened to me—fortunately the acid did not spray in my face, but it looked like my sweatshirt got hit with buckshot. Battery compartments always need ventilation.

Fuel pump disconnect switch

This is an inertial crash switch that turns off the fuel pump in the event of a crash thus preventing the electric fuel pump from delivering a large amount a fuel in the event of a fuel line rupture. This is why cars often need to be towed after only a bumper cruncher.

For the Future

Ground fault interrupter

Glossary of undocumented words and idioms for our ESL friends

Bumper cruncher –minor automobile crash


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  • ton c

    In my car, there are 30 Amp fuses which I imagine protect very high current circuits. How can you insert a half capacity 15 Amp device like the PTC? Wouldn’t the latter shut down the circuit which would be operating within limits? Is it possible to use parallel PTC devices in such situations?

    • Jim Keith

      Identical PTC devices will load share when connected in parallel thus increasing the current rating –note that each must have the same thermal resistance to the ambient air temperature, so the lead lengths must be the same and each must have adequate physical clearance around the device. I forgot to mention that these things run hot when tripped. But it is sure great to have it reset itself when the circuit is opened for a minute or so.

  • Jim Keith

    Very interesting story–hot wiring an electric lift truck! I heard of a similar story where employees desiring to take a break found a way to stop the machine by disconnecting a control circuit in a wire-way under a floor plate. This was in a steel mill–rough people! Unexpected human intervention can bring a new dimension to troubleshooting.

    Ya, I did not get into semiconductor fuses because they are not generally used in automotive applications. However, electric vehicles will become more common in the future and large battery packs require proper fuse protection. It is always a race to what blows first: the semiconductor fuse or the semiconductor it is attempting to protect. According to Murphy’s Law, the semiconductor blows to protect the fuse.


    This is very informative but if you want to protect then a 20 Amp fuse can be too strong and some it will be too weak! For the a;ternator I found on “Hyster” lift trucks that it needed a 100 amp fuse and this must be a semiconductor type I had some problem with these links so I did fit these fuses for reliability. On a battery lift truck I had the problem that the wiring would burn to smithereens but the fuse 10 amps was intact. I had to repair the wiring under repeat job and my boss was getting uneasy about it. I then fitted a 100 amp fuse but I hid it in the wiring with pieces of scrap wire just being taped on the sides so that the buldge did not attract attention. Suddenly the customer said that the truck is dead as a dodo. The manager and I went then together to this truck and the fuses were intact but there was no current on the circuit! I then slit the wiring loom open where I had my “secret” fuse and the fuse was burnt black and the glass section so overheated that it cracked . I then said that I will pay BUT how can a 10 amp fuse burn a 100 amp and itself did not blow? It boiled down that on night shift they want to use the truck without the keys and then made a connection around the switch with bare piece of wire and this made a short circuit and blew the fuse but not the 10 amp on the switch He was mad at me for hiding this fuse now I had a independent witness and that my boss wanted then to go to court. He had to pay for several trips I had to do and all the parts damaged So make sure of the current needed and add depending on what you want to protect plus about 30 per cent to prevent nuisance blowing of the fuses

  • Jim Keith

    Just a little follow up. When testing my Engine Running Detector, I observed that my Toyota has (2) fusible links located where the wires exit the battery box.

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