Apparently conflicting demands surrounding IT and telecoms installations have left many confused as to how to achieve a safe, legally compliant set up. John Lane unravels the complexities
07 October 2005
Earthing is a complex issue. When it comes to safe installation there are two important principles to uphold: keep it simple and keep it safe. Special demands of IT and telecoms have become confused with the apparently competing demands of protective earthing, high-integrity earthing, clean earthing and telecommunications earthing.
The situation is further complicated by recommendations from some US manufacturers that may not be compatible with the UK regulations. In any installation scheme the first priority must be electrical safety and the UK IEE wiring regulations - BS7671:2001 (2004 edition) - constitutes the starting point. The intention of these regulations is that, in the event of a fault, no part of the installation will become live and that the fault current will: flow back to the neutral star point of the supply; be of a short duration; and be large enough to trip the fuse or circuit breaker.
The regulations have led to extensive bonding to earth of any metal work that could conceivably become live including ceiling grids, parts of desks and water pipes. With all this earth metal about we need to get the protective earthing right. It is relatively straightforward to ensure a fault current will flow back to the neutral star point if there is only one supply.
Take the case of a lamp plugged into a desk power strip which in turn is plugged into a floor box and an under-floor power track. The installation will have a protective earth conductor from the desk lamp to the riser distribution board and eventually the supply transformer. Also the metal parts of the desk, including wire management, will be bonded to the desk power strip's earth terminal.
The complication arises if we now wish to run the desktop PCs from a separate supply supported by an uninterruptible power supply (UPS). Typically this will involve a second under-floor power track and a second riser distribution board. If the fault occurs in the PC we must ensure the fault current flows back to the neutral star point of the UPS output.
Employing two earthing systems - 'clean' earth for the UPS power and or 'dirty' earth for the non-UPS power - can leave one unclear as to what should be bonded to which supply and also risks a significant voltage difference between the metal case of the PC and metal parts of the desk. There are several possible solutions including: using a local UPS under the desk for each PC; running everything on the desk from the UPS including the desk lamps; or installing an isolation transformer in the riser closet so that the neutral of the UPS supply for that floor can be connected to the normal supply earth bar in the riser.
There should only be one earth system with protective earthing considerations having priority. Modern computer equipment usually has a switch mode power supply to convert 230 V AC to 3 or 5 and 12 V DC. In order to limit the amount of electrical noise leaking out from the switch mode power supply, a filter is usually fitted on the mains input terminals just inside the computer case. This filter comprises inductors in the live and neutral conductors and capacitors between these phase conductors and earth. The filter gives rise to an earth leakage current (typically 3 mA per PC).
The requirements of IEE Wiring Regulations Section 607 in BS7671:2001 apply to individual items of equipment where the earth leakage current exceeds 3.5mA and to all final circuits where the accumulated earth leakage is expected to exceed 10mA. In effect all new electrical installations feeding more than three or four PCs or similar equipment require high-integrity earthing.
A current of 10mA or more will give a nasty shock and can be fatal. If the normal protective earth wire feeding one or more PCs becomes disconnected the fault will not be noticed until someone disconnects a device on the faulty circuit from one on good circuit, such as a PC from a printer. They would then get an electric shock. To prevent this happening it is crucial that the protective earth never becomes disconnected in circuits where there is an earth leakage current. High-integrity earthing is intended to minimise the chance of a protective earth cable being disconnected by effectively having two earth cables.
Manufacturers of under-floor power track and floor boxes have developed their standard products and now offer a high-integrity earth version. High-integrity earthing is now mandatory in the UK for all new installations feeding IT equipment.
BS 6701:2004 is a standard for telephone wiring in offices and other premises and calls for a telecoms earth to be provided. Typically this is a 10 sq mm conductor with a cream sheath and labelled:
"Telecommunications earth". BS 6701 requires a dedicated connection direct to the main building earth terminal. Still, the advent of ISDN30 digital trunks delivered over fibre or copper has seen the need for this earth all but disappear. It was traditionally used for earth signalling on analogue trunk lines between the public exchange and a Private Branch Exchange (PBX).
Similarly the telecoms earth was extended from the main distribution frame to distribution points on each floor and was essential for the correct operation of the recall button. However, more modern PBXs no longer need the earth connection: digital telephone and Voice over IP (VoIP) handsets have never needed an earth connection.
However, BS 6701 still applies and BT requires the 10 sq mm conductor with a labelled cream sheath to be provided whenever services are delivered over copper cables.
John Lane is a director at IT consultants Cundall Johnston and Partners