5.10 - Combined functional and protective earthing

The previous section has made it clear that high earth leakage currents can cause difficulties in protection. The increasing use of data processing equipment such as computers has led to the need for filters to protect against transients in the installation which could otherwise result in the loss of valuable data. Such filters usually include capacitors connected between live conductors and earth. This has led to large increases in normal earth currents in such installations, and to the need for special regulations for them. Since these are special situations, they will be considered in detail in {7.8}.

Electrical disturbances on the earth system (known as 'earth noise') may cause
malfunctions of computer based systems, and 'clean' mains supplies and earth
systems may be necessary. A separate earthing system may be useful in such a case
provided that:

1. - the computer system has all accessible conductive parts earthed,

2. - the main earthing terminal of the computer earth system is connected directly to the main earthing terminal,

3. -all extraneous conductive parts within reach of the computer system are earthed to the main earthing terminal and not to the separate computer earth.

Supplementary bonding between the computer earth system and extraneous conductive parts is not necessary.