Electricity Supply Regulation 26 indicates that the level of earth leakage current should not normally exceed one ten thousandth part of the installation maximum demand (for example, 10 mA earth leakage current for an installation with a maximum demand of 100 A). Data processing equipment is likely to have a higher leakage current than this, so special regulations become necessary. Foremost is the requirement that where earthing is used for functional purposes (to allow the filters to do their job) as well as protective purposes, the protective function must take precedence. When the earth leakage current is high, serious shocks are likely from accessible conductive parts which are connected to a protective conductor which is not itself solidly connected to the main earth terminal.
Any piece of equipment having a leakage current exceeding 3.5 mA must be fitted with a label adjacent to the primary power connection which reads
"HIGH LEAKAGE CURRENT
earth connection essential
before connecting the supply"
Stationary equipment with an earth leakage current exceeding 3.5 mA must be permanently connected, or an industrial plug and socket to BS EN 60309-2 must be used. When a socket outlet circuit may he expected to feed data processing equipment with normal earth leakage current of more than 10 mA, or if the earth leakage current for a circuit feeding fixed stationary equipment is greater than 10 mA, earthing must be through a high integrity protective system complying with at least one of the following:
1. - a protective conductor of cross-sectional area at least 10 mm²
2. - duplicated protective conductors having separate connections and each of at least 4 mm² cross-sectional area
3. - duplicated protective cross-sectional areas of all the conductors is at least 10 mm², in which case the metallic sheath, armour or braid of the cable may be one of the protective conductors, provided that it complies with the adiabatic equation
4. - duplicated protective conductors, one of which can be metal conduit, trunking or ducting, whilst the other is a 2.5 mm² conductor installed within it
5. - an earth monitoring device is used which switches off the supply automatically if the protective conductor continuity fails
6. - connection of the equipment to the supply through a double wound transformer, a protective conductor complying with one of the arrangements 1 - 5 above connecting exposed conductive parts to a point on the secondary winding.
The reason for these precautions is that if the circuit protective conductor should become open circuit, leakage current could flow to earth through a person touching exposed conductive parts with possibly lethal consequences.
The alternative to items 1 to 6 above is the use 13 A sockets to BS 1363. In all cases the protective conductors must have a minimum size of 1.5 mm² with both ends of the ring connected to separate earth terminals at the distribution board. There must be no spurs on ring circuits, or branches on radial circuits, which must have a duplicate protective conductor. Where twin or triple socket outlets are used, they must be provided with two earth terminals, one for each protective conductor. Where protective conductors are not in the same sheath, conduit or trunking as circuit conductors, they must be 4 mm² rather than 1.5 mm².
Where an installation having more than one item of stationary equipment with earth leakage current exceeding 3.5 mA is protected by an RCD the sum of the earth leakage currents due to data processing equipment must not exceed 25% of the device tripping current. Where this requirement cannot be met, connection must be via a double wound transformer as in item 6 above. For other installations, no specific figure for the leakage current as a percentage of tripping current is given; the requirement is that the normal leakage current will be unlikely to cause unnecessary tripping. The 25% limit means effectively either that a single RCD with a high operating current must be used, or that the installation must be subdivided to allow a number of lower rated RCDs to be used.
If the Electricity Supply Company does not provide an earthed terminal and an installation electrode is required (TT system), the result of multiplying the total earth leakage current in amperes and twice the resistance of the earth electrode in ohms must not exceed 50 (volts).
For normal TT systems, there is no need to double the earth electrode resistance, so in the case of data processing equipment the earth electrode resistance must effectively be half its value for other installations. Data processing equipment must not he connected to a system which is not earthed in the normal way (IT system).
