How to test an automotive circuit breaker?

An automotive circuit breaker (sometimes referred to as an AGC or automatic-reset) is a device that

helps prevent damage from short circuits and allow easy and quick clearing of a fault.

The electrical loads in cars today are almost all controlled by complex electronic control units

(ECU), which monitor system voltage, over-current, undervoltage, etc., via sensors mounted on the car's wire harness.  When one of these faults occur it triggers a ‘circuit breaker' unit that goes open circuit after conducting for a set period of time.

The purpose of this article is to show how to test an automotive circuit breaker so we can determine if it is good or bad . When testing any device, you should always test it under load and see if the device performs as expected, i.e., does it trip in a normal time frame when presented with a fault?

Testing an automotive circuit breaker is relatively simple but due to the nature of how they are used, testing one that has been ‘in-service' is not easy.  We will use a new (never thrown) automotive circuit breaker for our tests, rather than one removed from an actual vehicle because there are too many variables involved in using such a part. We need to get several things out of the way before we start though:

1 – Circuit breakers do not protect against shorting – they only protect against overloads or an open line.  If you have a short to ground, the circuit breaker won't help you.  You need a fuse for that.

2 – Circuit breakers will only protect circuits up to what they are rated for (15A, 30A, etc.)

3 – An automotive circuit breaker is not a fuse replacement . They are designed to be used in place of fuses and have different ratings/timing.  They should never be used where an actual fuse would go because it could be catastrophic depending on the fault presented at the time it goes open-circuit.

The first thing we do when testing an automotive circuit breaker (ACB) is apply power directly across the ACB terminals in order to trip it open-circuit and reset it while checking the current draw.  This is not the normal way an ACB would be used in a car, but it helps us verify that the ACB does what we expect it to do – trip and reset.

Testing automotive circuit breakers under load There are many ways to test an ACB, so I will cover some of them here and then give you a few options for your own testing.

Terminal block method Connect a 15A fuse between one of the ACB terminals (to be labeled '15' below) and ground as shown below:

Connect the other terminal (to be labeled ‘0'), which should read 0V, to power positive (+12V).   This step shows how an automotive circuit breaker closes . To test the ACB, simply remove the fuse from terminals 15 and 0 and replace it with a wire jumper:

Apply power to +12V.  The ACB should trip (open circuit) after about 10 seconds but will reset in less than 3 seconds.  The current draw of the breaker during its on-time period was around 2 Amps (I could not measure peak current), which is slightly lower than I would expect for a 30A ACB.  If you don't have access to three 12V batteries, an automotive battery works just fine since they are designed for this type of application.   *warning – connecting your meter clips directly to a car battery may result in breaking them off or throwing your multimeter out of calibration.

Testing automotive circuit breakers under load – switch method *I added this in after the original draft to clarify ACB operation (thanks to Al Eberhardt for helping clarify what a typical 30A ACB's current draw should be)

There is a simple way to test an ACB once it has closed that does not require batteries as described above.  As shown below, connect your power supply directly across the terminals:

With alligator clips on one end of the power supply and connected across terminals 15/0, apply 12V briefly (less than 2 seconds).  The current drawn by the breaker at this point will depend on how long it was energized and what duty cycle you use but should be around 1.5 to 2 Amps. 

The ‘switch' method is not as good as the first way described, but it works if you don't have batteries or just want a quick check of an ACB for something like troubleshooting factory wiring in a vehicle that has an intermittent problem (for example: does it trip when fuse #10 on the dash goes?).  

Resistors equal 5-6 Ohms per ampere so at 2A, we need 10-13 Ohms of load (the resistor needs to be rated for this amount).  If you don't have a 12V supply rated 10-13A you can use three 6V sources in parallel – two car batteries and one DC power supply should do it.  

ACB continued… If you need a simple circuit breaker in your car that can be used instead of fuses, the main thing to remember is that they protect only parts of the factory circuits (generally ignition/battery or powertrain) and should never be installed where an actual fuse would go .  They have different ratings from regular automotive fuses because their on-time period needs to cover most or all of a short's energy so that they trip at lower current levels than fuses.  If there are other ACBs nearby on a shared load, such as in an older vehicle with a lot of relays – be aware there may not be enough capacity left over after one goes open-circuit to protect other circuits.  It's always good to reset a tripped ACB, but it may not be enough (especially if you have been driving for 30-60 minutes).  

If an ACB goes open circuit – find out what caused it and correct the issue.  This is pretty easy to do because the fault will either show up on power or ground somewhere.  For example, if one of your vehicle's relays was energized at 12V when it shouldn't have been, and the factory wiring fed that relay from +12V then there is a good chance its now damaged the ACB when you removed power so that it can no longer close properly.  

A system I helped design in the early 1990s was protected with automotive circuit breakers and we had an unusual problem where they would go open-circuit while the vehicle was stopped.  It turned out to be a relay that fed power to our system but when it got hot due to other loads (it was located in front of the radiator) it didn't work properly so we eliminated it.  This relay operated at 12V and fed +12V directly into one of our ACBs and caused enough current spike when it opened that the breaker could not reset itself properly, resulting in an open-circuit condition.  

I hope you find this article useful – I did my best to provide basic information that can save you time and effort in your own projects.  If people find it helpful, I will add more articles soon.

*I know some of my readers aren't familiar with automotive circuit breakers so here is a description: if you have ever used a home appliance like an air conditioner or refrigerator that has an overload protection switch on the motor (circuit breaker in the box) then its basically the same thing – except it protects car electrical systems from shorts rather than overloading.  Otherwise, as noted in this article, they are not to be confused with fuses unless you want them to be burnt toast!  

* I usually use four-wire looms for items that connect directly into 12V DC power because stranded wire can fit into tight spaces and be bundled together.  If you use twisted pair wire (two wires with different colored plastic insulation) do not confuse them for ground wires – they should be used for power/positive 12V only.  

* If you use low-voltage ACB's they are polarized so its important to keep the breaker in the right orientation or one side will always be open circuit when it is supposed to be closed.  Also, if you buy a lot of cheap ACBs from China pay attention to how the contacts are installed on each PCB because sometimes they accidentally swap sides!  

It is well-known that automobile electrical circuits differ in some ways from other electrical circuits. But what are those differences? And how does the average automotive electrician apply them to daily work? Here I collect a few for your consideration.

A major difference between automotive and industrial/commercial wiring systems lies in the arrangements of distribution buses and their grouping, as shown below:

Service bus (in addition to bus No. 1) is common. The circuit breaker panel usually has a service (that is, main) bus as well as an auxiliary bus or two, but this hasn't changed much since World War 2 ended in 1945!!! A quick look at the circuit layout will show you why they did it this way ​— ​no doubt because it is more efficient.

The left of the figure shows a conventional industrial/commercial system (the older narrow-bus design). Both main and auxiliary service buses are present, with all circuits connected to them. The modern wide-bus design on the right has only a service bus (some systems have an additional two or three) but uses such busses efficiently.

Actually this “efficiency” isn't a human factor at all; it's a simple mechanical optimization problem that electrical engineers have long solved with little thought. We will not go into details as to how clever designers use fewer components and wiring to save cost and space, but we can tell you what they did: They simply consolidated distribution buses as much as possible in order to minimize the number of sizes and types of conductors used. This is the reason for having only one bus in a modern wide-bus system instead of two, as shown below:

Let's see how this works by analyzing the circuit arrangements.

Ignore service bus No. 2 for now; it will be explained later. The same circuit breaker serves both subpanels A and C (on left) while B and D are served by another circuit breaker mounted nearby (not shown). So any given main distribution cable within subpanel A carries power to three load panels (or possibly more if the layout is really busy). In contrast, an industrial/commercial wiring practice dictates that one main cable alone should not supply power from the panel to multiple load panels. This, of course, is so common sense it seems silly to state the obvious. However this rule isn't followed in most automobile electrical systems as it would significantly increase wiring cost and complexity by requiring additional conductors (three for each connection instead of one). Again we will not go into technical details on how they get around this problem because such detail is well beyond the scope of this article.

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