6.6.2 - Electromagnetic compatibility (EMC)

All electrical equipment must be selected and installed so that it will not affect the supply or cause harmful effects to other equipment. One of the harmful effects is electromagnetic interference (EMI). Whenever current flows in a conductor it sets up a magnetic field; a change in the current will result in a corresponding change in the magnetic field, which will result in the induction of electromotive force (voltage) in any conducting system subject to the field. Whilst induced voltages will usually be very small, they may be considerable when rates of change of current are heavy (for example, circuits feeding lift motors) or when there is a lightning strike in the vicinity.

The effects are most pronounced when large metal loops are formed by circuits (perhaps power and data circuits) which are run at a distance from each other but have common earthing and bonding. Power and data cables need to follow common routes to prevent aerial loops which will be subject to induced e.m.f., but with sufficient spacing to prevent interference between them (see{Table 6.4}). The emission standard is BS EN 50081 and the immunity standard BS EN 50082.

The designer needs to consider EMC when planning an installation, and may decide that some of the following measures are appropriate:

providing surge protectors and filters for sensitive equipment, proper separation of power and other cables to limit electro-magnetic interference (EMI),
using bonding connections which are as short as possible, screening sensitive equipment and bonding of metal enclosures, and avoiding inductive loops by using the same route for cables of different systems.

This list is far from exhaustive.