Protection components

This page covers protection devices.

Summary

Various devices provide protection against excessive and potentially dangerous or damaging currents, voltages or temperatures. Some are single-use and must be replaced when blown but others are self-resetting.

Safety

Protection devices protect you against fire and malfunction and may limit collateral damage following an initial fault. They must always be replaced by a device with a similar (ideally, identical) rating, and must never be bypassed or deactivated.

Introduction

The commonest and most familiar protection device is the fuse, which prevents an excessive and potentially dangerous current from flowing, but devices also exist to absorb an excessive voltage which might cause damage or malfunction, as well as to cut the supply in the case of overheating, such as a kettle boiling dry.

Fuses

Fuses are used to shut off an excessive current in a fault condition. If you were to replace a fuse with a piece of wire such a fault condition would create a serious fire hazard.

Conventional wire fuses

A fuse can simply consist of a thin piece of wire with a low melting point. Above a certain current the wire will heat up and melt, breaking the circuit.

Some equipment naturaly draws a heavy "inrush" current when first switched on. After a second or less the current drops to a muc lower running level. A "slow blow" fuse is often fitted in this case, which will not blow if its rated current is briefly exceeded.

In most cases a "quick blow" fuse is fitted, which will cut off the current as fast as possible.

Polyfuses

In cases where it might be difficult to change a fuse or where occasional faults might not be unexpected (for example if a motor temporarily jams) a polyfuse might be fitted, also known as a polyswitch or polymeric positive temperature coefficient (PPTC) device.

This consists of an insulating polymer loaded with carbon particles. Normally, many of the carbon particles are in contact with one another and provide a low resistance conducting path through the device. If too much current flows it heats up, the polymer expands and the carbon particle are no longer in contact. The current can no longer flow untill it cools doen again.

Thermal fuses

Single-use thermal fuses

Single-use thermal fuses.

Resettable thermal fuses

Resettable thermal fuses.

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