This page covers protection devices.
- 1 Summary
- 2 Introduction
- 3 Fuses
- 4 Over-voltage Protection
- 5 Thermal fuses
- 6 External links
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.
- 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.
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 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.
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.
Miniature Circuit Breakers (MCBs)
Older houses have a fusebox where the supply comes into the house, each fuse containing a piece of fuse wire that you can replace. In newer properties the fusebox contains MCBs instead.
An MCB is an electromechanical device which performs the same function as a fuse. The current passes through a coil, generating a magenetic field. If the current exceeds the rated value for the device then the magnetic field releases a spring which snaps a pair of contacts apart, breaking the circuit. There may also be a bimetallic strip which heats up with an excessive current and can also release the spring. This is designed to respond in slower time to a persistent current which may not be quite sufficient to trigger the main mechanism. A lever allows the contacts to be re-closed, reloading the spring.
MCBs are not normally found in equipment and appliances though they may be fitted to extension leads or adapters.
Residual Current Devices (RCDs)
These are easily confused with MCBs (though they may be combinded with an MCB in a single device) but serve a different purpose.
Normally, all the current flowing out of one pin in an electrical socket is expected to flow back into the other. If there is a difference, the balance must have gone elsewhere, possibly through somone's body with lethal consequences.
An RCD contains a double coil. If the current from the live wire goes in a clockwise direction the current returning to the neutral goes anticlockwise and their magnetic fields cancel. A third winding on the same coil detects any magnetic field created by an imbalance and triggers a circuit which releases a spring to open a pair of contacts.
Surges in the mains supply can damage delicate electronic equipment such as computers or cause it to malfunction. These can be caused by faults in the supply grid, by lightning strikes or by electric motors. These could be the motors in domestic equipment such as washing machines and vacuum cleaners or could be heavy electrical machinery such as the motors driving lifts or large air conditioning equipment, but in both cases, the equipment itself should have its own over-voltage protection which should be services if it's causing problems.
Also known as Voltage Dependent Resistors or VDRs, and one type as Metal Oxide Varistors or MOVs, varistors normally have a very high resistance. Placed across the supply input to an appliance they draw practically no current at all, unless the voltage exceeds a threshold. In this case their resistance drops dramatically, diverting the surge away from the appliance itself.
A varistor may momentarily draw a very heavy current, and if this is too great or is more than instantaneous the device is likely to fail. Each varistor therefor has a maximum rating for the size of surge it can withstand. It would take a very large one to survive and provide protection against a lightning strike.
Spark gaps and neons
Single-use thermal fuses
Resettable thermal fuses
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