When a fuse blows or a circuit breaker trips, was it a short circuit or a ground fault that caused it? To the average do it yourself electrician, it's hard to tell by going to the fuse box or electrical panel. You open the fuse panel only to see a fuse blown, but that's no clue. Likewise, you go to the circuit breaker panel and see a tripped breaker, but why?
To help explain what happened, we first need to be familiar with the two terms; short circuit and ground fault. A short circuit occurs when a "hot" wire and a "neutral" wire actually touch each other. When this happens, a large amount of current flows, causing a fuse to blow or a circuit breaker to trip, not to mention the sparks and pop that is followed generally by a little smoke.
The cause of this unfortunate occurance may be as simple as a loose connection on one of these two wires nder a ternimal in a junction box. A wire slips off of under a terminal and lands in contact with the other wire. Sometimes, an appliance encounters an electrical problem and wires touch that were not meant to. And then there's the sneaky critter, the mouse, that chews the wire insulation and causes a short circuit between wires run in a bundle.
When a short occurs, a large current flow through a fuse or circuit breaker will open the circuit, blowing the fuse or tripping the breaker. Looking through the sight glass of the screw-in fuse (plug fuses), you'll notice a blackened glass if the fuse was blown. In the case of a cartridge fuse, there is likely no physical sign, as this fuse is concealed. But you can test it out of the circuit with an ohm meter or continuity tester to see if it is blown.
Now as for a ground fault, that occurs when the "hot" wire comes into contact with the ground wire or a grounded portion of a junction box or grounded part of an appliance or device. Similar to a short circuit, large amounts of current are forced to flow through the fuse or circuit breaker, causing a fuse to blow or a circuit breaker to trip. In the same way as the short circuit, you will be hard pressed to learn the cause without doing a little investigating on the circuit with the trouble.
As you may or may not know, some fuse can only handle the current capacity they are valued at, like 20 amps for a 20-amp fuse. However, and this may be the confusing part for some of you, some fuses are designed to handle a temporary surge of current beyond their rating for a short period of time, called time delay fuses. These are great for appliances and motor driven devices that normally need a larger cuurent to get a motor or pump turning, but then require less to keep them running, thus this fuse suits the purpose well.
As for short circuits and ground faults, we don't want either, but with the use of protective devices like fuses and circuit breakers, we can rest easy knowing that we have electrical safety devices guarding us at all times. Electrical shorts and ground faults can be caused by working on a circuit that wasn't turned off first, hopefully not, but it can be no fault of our own.
So the next time you wonder why your fuse blew or breaker tripped remember, the important thing is to have protective safety device in place to save both devices and the wiring throughout your home.