Why Put a Deliberately Weak Link in a Circuit? Something Has to Fail First

Current flowing through any conductor heats it, and the heating rises with the square of the current, so a fault drawing several times the intended current heats the wiring dramatically rather than proportionally. Cables inside walls, inside appliances and inside equipment have a current they can carry indefinitely without overheating, and exceeding it melts insulation and starts fires. The protective device is a deliberately weakened point in the circuit, rated to fail before the cable it protects reaches a dangerous temperature, so a fault destroys a component costing almost nothing rather than the installation. It protects the wiring rather than the appliance, which is the point most often misunderstood.

Several forms exist and each suits a different job:

• •A wire element in a glass or ceramic cartridge, the familiar domestic form
• •Sand-filled cartridges, which quench the arc and handle very large fault currents
• •Slow-blow types, which tolerate a brief surge such as a motor starting
• •Fast-acting types, used where sensitive electronics must be protected
• •Resettable devices, which increase their resistance when hot and recover when cool
• •Circuit breakers, which trip mechanically and can be reset

Fitting the wrong rating creates a hazard in whichever direction the error runs. Too low and the device fails during normal operation, which is merely annoying and which leads directly to the real danger, since the usual response is to fit a larger one. Too high and the protection no longer operates before the cable overheats, so the circuit continues delivering current through a fault until something else fails, which is what causes fires. The practice of replacing a blown element with wire, foil or a coin defeats the protection entirely and has killed people. The correct rating is determined by the cable and the load rather than by whatever stops the device operating.

The number printed on one of these devices is not the current at which it fails, which is the most common misunderstanding. It is the current the device carries indefinitely without failing, and interrupting a fault requires substantially more, typically around twice that figure for a sustained period or far more for an instant. Every type therefore has a characteristic relating current to the time taken to operate, which falls steeply, so a small overload takes minutes and a dead short takes milliseconds. Designers use those characteristics to arrange that the device nearest a fault operates before the ones upstream, which keeps a fault in one appliance from disconnecting a whole building, and getting that coordination right is a routine part of electrical design.

These devices protect equipment and wiring rather than people, and confusing the two is dangerous. The current that kills a person is a small fraction of what is required to melt even the smallest domestic element, so a person touching a live conductor receives a fatal shock while the protection remains entirely undisturbed. Protecting people requires a different device that compares the current flowing out against the current returning and disconnects within milliseconds when they differ, indicating that current is leaking to earth through something, which in a domestic setting is frequently a person. Modern installations use both, since each addresses a hazard the other does not, and neither substitutes for the other.

Heating rises with the square of the current, so a fault drawing several times the intended current overheats cables dramatically, and a deliberately weak element rated to fail first destroys something worthless instead of the installation. It protects the wiring rather than the appliance. Fitting too high a rating removes the protection, and these devices do not protect people, which requires a device detecting current leaking to earth.