Usage:
Reference: Understanding "Shorts," "Opens" and "Intermittents; and How To Troubleshoot A Short-To-Ground In Power Wiring in this issue.
Fuses and circuit breakers are often reported as the CAUSE of failure. This is very seldom correct. Open fuses and circuit breakers are most often the RESULT of an electrical system overload. These components are electrical "flow control valves" connected in series with a given circuit to protect wiring from high current damage.
Fuses are made of an electrical conductor with a low melting temperature that melts "OPEN" during an overload condition. Circuit breakers on earthmoving products use a bimetallic disk that closes the circuit between the two breaker terminals (generator sets use a different type). During overload, the disk heats up and snaps ("oil cans") to open the circuit. On a manual reset breaker, the push button snaps the disk into a position that again closes the circuit between the breaker terminals. The disk in an automatic-reset breaker will snap closed when it cools down.
NOTE: Fuses and circuit breakers are almost never the CAUSE of failure. When troubleshooting the actual cause, report (on a SIMS or warranty claim) the part number of the harness or component that caused the fuse to blow or breaker to trip.
Fuse Failure Modes
Open
An open fuse is the result of system overload. Fuses with the element visible can give a clue as to the cause. Testing on a questionable fuse only requires a 6V7070 Multimeter (or equivalent) on the 200 ohm scale. A good fuse will read less than approximately 0.2 ohms. An open fuse will read "OL" for open circuit.
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- 1. The glass or plastic housing is smoked or blackened. This is caused by gross circuit overload far beyond the fuse rating. This is usually caused by a short circuit-to-ground from:
...Incorrect troubleshooting as with a screwdriver.
...A component short as in a forward warning horn.
...A harness short where a wire (conductor) is pinched to make a connection to ground.
- 2. The glass or plastic housing is clear showing the fuse element simply melted open. This is caused by a gentle overload somewhat greater than the fuse rating.
- 1. The glass or plastic housing is smoked or blackened. This is caused by gross circuit overload far beyond the fuse rating. This is usually caused by a short circuit-to-ground from:
Will not fit fuse holder
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- 1. Incorrect type or size of fuse for holder.
- 2. Supplier tolerances are incorrect.
- 1. Incorrect type or size of fuse for holder.
Breaker Failure Modes
Open
An open circuit breaker is an automatic or manual breaker that will not reset (reads "OL" on 6V7070 Multimeter). This usually occurs from gross circuit overload far beyond the breaker current rating. This is usually caused by a short circuit-to-ground from:
...Incorrect troubleshooting as with a screwdriver.
...A component short as in a forward warning horn.
...A harness short where a wire (conductor) is pinched to make a connection to ground.
Failure of a circuit breaker to reset is often caused by a melted disk or a disk that was so hot during overload that it is warped and cannot be reset to a closed position.
Shorted
A shorted circuit breaker, one that will not open under an overload, is rare and might be found in a circuit with wiring and/or harness damage. This may possibly be caused by an overload condition that welded the disk in place.