Testing a fuse would seem to be fairly common task in our world and it normally is. So this begs the question, “why does this cause so much confusion?”
You are asking yourself, “Confusion?” YES. Misdiagnosis of a fuse is more common than you think and usually results in money being spent on service calls that could be avoided if the fuse would have been check properly on the first go-round.
Since most power fuses are the type that you cannot see the element, one needs to use a multi-meter in order to test most of the fuses we deal with. The following diagram shows the proper way to connect to the fuse in order to test it.
Now, this fuse is shown as “just a fuse” with no connection to a circuit. But in our world, the fuse is most always in the circuit and this is what introduces the confusion factor. So let me pose a question: what type of test would you chose to test this fuse in the circuit?
Most of you are going to say “ohmic”. That would be okay if this fuse were out of the circuit but in the circuit, this test (for the most part) does not work. This fuse in a drive for instance will read good ninety-nine times out of a hundred due to the fact that it has multiple paths from the input to the other side of the fuse. The ONLY way to properly test this fuse with an ohm meter is to remove it from the circuit. Testing it in the circuit is a waste of time and always leads to a misdiagnosis of the fuse status.
Now, there is a procedure for testing a fuse while it is still connected in the circuit. But this can only be done by doing a voltage test across the fuse. Remember that if a fuse is good and you are connected across the fuse like the above diagram, the voltage will take the path of “least resistance” which will be across the fuse and not the meter. This leads to a zero volt reading on the meter (or very close to it) which means the fuse is conducting and is good. If you are reading across a fuse in a 480 volt circuit and you read 480 volts (or somewhere in that neighborhood), the power is traveling across your meter and therefore the fuse is bad.
Voltage testing the fuse should only be done after it is determined that the unit can be SAFELY powered up. In most cases you can do this if the unit has tripped and is still powered up and you have on the proper PPE to perform this type of test. Powering up a unit that has failed to test the fuses is dangerous and should only be done after many other tests have been done to ensure that the unit can safely be powered up without further damage resulting from that act.
And that leads us to the easiest answer to the solution….ensure that the unit is powered down and that all incoming power has been removed. Then and only then, remove the fuses from the circuit and “bench test” them using the ohmic method and this will result in an accurate and safe test of the fuses.
I have made tens of thousands of dollars over the years going on service calls and finding a blown fuse in the first five minutes of being on-site. I have driven over 500 miles and this has happened and I have had electricians beg me to “say it was something else”. Please, testing is something that is difficult and requires knowledge and focus on the task. If you are unsure of what you are doing, ask questions of those that you can rely on for the right answers….I always do and so should you.