Information and communications technology (ICT) Wiring installation considerations
Installers of permanent electrical wiring are required to ensure that new work meets current regulations. These regulations are designed to save lives in the case of faulty wiring or equipment. Further the practice is regulated to make our offices, factories and homes safe places to live.
Unfortunately modern equipment suffers from a wide range of other influences. Some of these may actually cause sensitive electronic chips to fail. Just wiring up and performing routine regulation oriented checks is inadequate when mission critical or crucial systems are involved.
By the time the wiring is being installed it is already too late. The retro-fitting of a star earthing system after a computer room has just been quite safely wired up involves a full strip out and re-install.
The regulations require that all permanent wiring is compliant with safe practice. Normally this means calculating fuse sizes and selecting wire sizes to create protection discrimination and quick disconnect.
The electrician is required to bond all exposed conducting elements in an equipotential zone together. As a practical matter this is normally done in a daisy chain fashion which is entirely safe. From a noise diversion point of view it can be a disaster. Just checking earth loop impedances does not mean that no earth loops exist in the protective wiring. A proper star plan must be evolved and the installer should prove the installation before connecting sensitive equipment. The regulations are covered by BS 7671.
Transients can occur naturally in our environment (lightning) and in heavy equipment switching. Specific attention must be given to the likelihood for such occurrences and a planned approach used to divert them as quickly as possible to earth.
Earth cabling should not be connected to more than one star point and all cables should radiate like spokes in a wheel.
Data cables between equipment must be checked for earth isolation at one end of the screen.
Data cables going outside the equipotential zone may require specific fitting of surge diversion boxes. This also applies to phone lines. Recommendations are given in BS 6651.
The static charge commonly experienced as a definite spark between humans and earthed objects in dry weather will destroy the silicon chips in a computer or telephone switch.
ICT equipment mains circuitry should never be available (preferably by the use of hardwiring or special plug tops) to alien loads such as the office kettle or vacuum cleaner.
As our systems become more complex and critical to our daily business operations all installations should be planned for best practice. Networked systems are particularly prone to problems appearing in unexpected places. Modem lines monitoring communications and transducer signals with long wires connected to the network are perfect aerials to collect harmful spikes.
To avoid earth loops
With the ever increasing use of Information + Computer Technology (ICT) equipment there is a greater chance of installation with unsatisfactory wiring.
The traditional UK electrician is taught to earth all metallic chassis. His training is directed towards making all installations safe. If single phase work is properly carried out he will check that the three conductors are definitely connected where they should be and definitely not where they should not be! Clearly he will also check for correct use of live and neutral and prove discrimination on fusing. He may also install added safety protection in the form of an RCCD.
These checks are no longer adequate. We have found that some wiring which is perfectly safe is quite unsuitable for ICT equipment. In some cases the use of mains noise protection equipment could be avoided by better wiring practice. However it is usually less expensive to retrofit protection rather than re-wire a whole building or even just the computer network. A better solution is to approach the problem at the planning stage. In addition to ensuring that the wiring is safe some specific attention must be made as early as the planning stage for wiring practice which ensures the best results for data processing equipment. After the installation is complete some extra checks should be carried out while the system is not connected to the supply and conductors may be temporarily disconnected.
The objective is to provide a totally safe installation without any earth 'loops'. An earth loop can give rise to surprisingly large currents in low impedance circuits.
These currents can cause mysterious data processing faults hum problems and failed communication ports on PCBs in EPoS equipment.
The diagrams show the incidence of an earth loop and approaches to wiring to avoid them. In simple terms all equipment must be earthed in only one place.The following checks should be done on the wiring of a system prior to considering if it is satisfactory for ICT use.
1 ALL existing statutory checks should be made.
2 The star point earth connections should be opened and all related equipment checked for isolation from earth.
3 On installations where there is galvanic isolation provided by a mains noise protecting device the galvanic isolation must be proved. Checks should be made for both earth and neutral conductors.
4 Once inter peripheral data or video lines are connected the earths of EACH peripheral need to be checked for single connection only. This is best done on a networked system by removing all mains plugs from their sockets and seeing if any plug earth pin is connected to any other)
Incidence of earth loop via data cable screen and peripheral earthed chassis
Preferred practice for wiring two conditioning products to the same star earth