TO BE READ BY ALL ELECTROSCHEMATICS ENTHUSIASTS!
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.
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.
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
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 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
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
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:
http://www.flamecorp.com/Sensata/PDF/Sensata,%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