7.8.1 - Introduction

Data processing equipment is extremely common. Such computer based equipment are used in the office for word processing, handling accounts, dealing with wages and so on. In the factory they control processes, and in the retail shop they are used for stock control, till management and many other purposes. Even in the home we are starring to see their use for security, for temperature control, for domestic banking services, etc. Telecommunications equipment is also becoming more widely used.

All such equipment have a common danger of failing and losing their stored data if subject to mains disturbances such as voltage spikes and transients. They are protected from such failures by feeding the supplies to them via filter circuits, which are designed to remove or reduce such voltage variations before they reach the sensitive circuitry. A simple filter is shown in {Fig 7.8} and will almost always include resistive and capacitive components which are connected from live conductors to earth. This will give rise to increased earth currents, driven by supply voltage through the resistive and reactive components. When the circuit, including the filter, is switched on, higher earth currents will usually flow for a very short time whilst capacitors are charging.

Data processing and telecommunications equipment of this kind therefore has high levels of earth current in normal use, although in most cases the earth current produced by a filter will not exceed 3.5 mA. Where a number of equipments containing filters are fed from a single circuit, the total supply current may be low due to the small demand of each device, but the total earth leakage current may well be very high. Consequently, special regulations apply to the protective and earthing conductors of such circuits.

Fig 7.8 Mains transient suppression filter