Short Circuit
This is current bypassing the load and returning to the source through a very low resistance path. Because the resistance drops to almost zero, the current rises to hundreds of amperes within a small fraction of a second, the fuse blows or the protection circuit shuts down. When a battery is shorted directly, there is a risk of fire and explosion.
The symptoms are typical: the fuse blows the moment the device is switched on, the protection keeps tripping, or sparking and blackening are seen at some point. By Ohm's law, when the path resistance falls to the milliohm range, the current is limited only by the internal resistance of the battery.
- Common causes: insulation chafed on a sheet metal edge, a core that has worked loose and touches the neighbouring terminal, water inside the box, a diode fitted the wrong way round, a connector fitted with the wrong pin order.
- Diagnosis: switch off the power, disconnect the suspect line at both ends and measure between positive and negative with an ohmmeter. A reading below a few ohms confirms the short circuit.
- Narrowing down: split the circuit in two and measure each part separately; at every step the search area halves.
Fitting a bigger fuse is not the solution to a short circuit; it only lets the cable burn. If the fuse keeps blowing, the system must not be energised until the source is found.
Context in which this term is used: Installation, redundancy and field continuity