Earthing

Q Why are installations earthed?

A In general an earthing system needs to satisfy three demands:
- it must conduct lightning and short-circuit currents without introducing intolerable step and touch-voltages
- it must protect the occupants, equipment and fabric of the building against damage due to short-circuit currents and lightning strikes
- it must provide a suitable environment for electronic equipment by providing a low impedance path to interconnect equipment.

Although requirements for these three aspects are very different and are often specified separately, the implementation of them requires an integrated systems approach.

Q What steps should I take to deal with earth leakage currents?

A Earth leakage currents arise mainly from the EMC filters built into electronic equipment with switched mode power supplies.  Standards limit the leakage current from non-fixed equipment (i.e. equipment that plugs into a standard socket outlet) to less than 3.5 mA.  When a lot of electronic equipment is in use, the total leakage current in the protective conductor can become significant.  If there is a break in the CPC the earthed conductive parts of all equipment connected to the isolated section will rise to about half the supply voltage.  To reduce this risk, special rules apply when the leakage current in a CPC is likely to exceed 10 mA.   In the current edition of BS7671, these regulations are contained in clause 534.7, replacing section 607 of previous editions. 

2008 Spring Wiring Matters

Pub 142 Section 607 in Residential Installations

Q What is TN-C-S?

A TN–C-S is the most common type of earthing employed in the UK.  The name, defined in French in European standards, indicates that the earth (Terre) and Neutral are connected together by the supplier, that the earth and neutral are Combined on one conductor in the supply system and that they are Separated at the consumer’s point of common coupling.  This separation of earth and neutral is maintained throughout the installation.  In other words, the neutral is treated is the same way as the phase – insulated and isolated from earth throughout. 

This is important because keeping the neutral and earth separate within the building reduces stray currents in the earthing system, and improves electromagnetic compatibility.

Other types of earthing system are described in section 3.2 Earthing on LV Systems and Within Premises in Pub 119 Earthing Practice.