Compatibility questions

here was a time when electrical contractors only had to worry about one frequency in cables: 50 Hz. But with the increasing use of computers and ever more sensitive telecommunications, they must be aware of frequencies anywhere from 50 Hz to 2 GHz. Concern about radio frequency (rf) and electromagnetic compatibility (emc) is now a worldwide problem.

Power cables can emit electromagnetic waves even under normal conditions. Data cables can act as sinks for emc and suffer data corruption. Through all of this, safety remains an important consideration.

Incorrectly installed cables can emit electromagnetic waves that cause local electronics to fail, with sometimes disastrous results. They can also present short circuits to high frequency voltages, causing arcing and danger to personnel. The following examples demonstrate the problems that electromagnetic waves create:

• a computerised control system failed because transients in the mains power caused electromagnetic waves that interfered with the software;
  
• a vhf radio caused movement of a ship's joystick and a minor collision;
  
• train design must include protection from electromagnetic waves on the electrified rails;
  
• mains supply fluctuations interfered with the functioning of a microprocessor-controlled chlorine gas valve;
  
• inadequate filtering caused the loss of voltage to plcs on an oil rig;
  
• a handheld radio caused the failure of fire detection equipment;
  
• a crane's safe-load indicator was permanently changed when the operator extended their radio aerial;
  
• wheelchair brakes have failed in proximity to high electromagnetic waves;
  
• an aircraft compass malfunctioned in presence of a laptop computer;
  
• a cnc machine failed near the presence of an arc welder;
  
• a vhf radio disabled a gas detector.

Technical talk
So important is cabling to this area that the Institution of Electrical Engineers (IEE) dedicated a seminar to it recently in London. An international dimension was given by David Imeson, chair of the European EMC Competent Bodies.

In 1989 the EEC published its first emc directive, but this did not come into effect until 1996. The delay was due to European industry wanting more time to prepare for what was perceived as an entirely new technical requirement.

Imeson explained that a Simplified Legislation for the Internal Market (SLIM) procedure was directed at emc in 1998 and work on this continues. He predicted that it will be 2006 before legislation for this is finally concluded in all member states. But he reminded delegates that such legislation will have to be taken seriously. Penalties for ignoring legislation can be as high as £5000 or three months in jail. Indeed, one Cardiff manufacturer suffered a £6000 fine and had to endure the cost of adverse publicity, plus a redesign of their product.

But it was Keith Armstrong, of the UK's Cherry Clough Consultants, who was most concerned with the cabling aspects of emc and where it might relate to installers and contractors. He suggested six areas of concern:

• screened cables should be terminated at both ends to control radio frequency (rf) emissions;
  
• pigtails should never be used;
  
• single-ended terminations can expose electronics to damaging overvoltages;
  
• meshed earth bonding is better than single-point;
  
• parallel earthing prevents excessive currents when cable screens are terminated at both ends;
  
• copper communications between buildings.

Radio frequencies
"Achieving effective screening of rf is rather like plumbing," says Armstrong. "Any gaps or incomplete seals, including all couplings and joints, that would leak if the system was filled with water under pressure, would leak rf."

This effect is demonstrated by the better screening properties of continuous copper compared to braid or foil. Single-ended screen termination creates a gap at the open end, which compromises the performance of the whole cable. When the length of such cable exceeds one-sixth of a wavelength, the screen will begin to act as a resonant antenna – worse than no screen at all. It also means that there is no protection against magnetic fields with certain orientations.

Pigtails ruin the screening properties of rf. Their inductive properties coupled with screen capacitance can be disastrous at frequencies above 30 MHz.

According to Armstrong, single-ended screen termination, coupled with single-point earthing, can be particularly dangerous under fault conditions. Lightning, earth faults, switching large inductive loads and hv circuit breakage can cause spurious surge currents to flow through relatively high impedance paths. This causes large (possibly 66 kV) 'earth lift' voltages to be applied directly across possibly sensitive electronics.

"So we can see that the age-old practice of single-point grounding and its consequent requirement to only terminate cable screens at one end is poor for emc, poor for surge protection and reliability, and poor for safety," says Armstrong.

Radio frequencies are now part of the modern world maintains Armstrong. Because of this we must now provide meshed earth bonding (MESH-CBN, figure 1).

Rooms for computers or telecommunications equipment require mesh sizes down to 600 mm or less. Otherwise, mesh sizes should be under four metres (diagonally). Where mesh earthing is not possible, natural metalwork can sometimes be pressed into use: girders, structural metalwork, pipework, ducting, walkways, railings etc (figure 2).

Older buildings may have single-point earths, and installing MESH-CBN can be prohibitively costly. If this is the case, a locally meshed bonding may be employed, say for one room. Isolated from the building's main earthing system, the mesh is earthed only at a single point. Cables entering the room are either bonded directly to that point or employ surge protection or filters.

Parallel earth conductors
Dual-end screen termination can mean large screen currents flowing during fault conditions. However, Armstrong explains that meshed earth bonding lowers the earth lift between different items of equipment, so overheating can be avoided.

Meshed earth bonding is not always possible; it is not suitable for use in old buildings for example. In such cases the recommended solution is to use the parallel earth conductor (pec, figure 3).

Under normal conditions, "the largest currents flowing in an earthing system are at power frequency," points out Armstrong. "Given a choice of paths they will prefer to flow in the path of least impedance and at these low frequencies it is usually only resistance that matters." For this reason pecs need to have a much lower resistance than the cable's screen.

But under fault conditions inductive properties come into play. Lightning, for example, can produce high energies at anything from 10-500 kHz. For this reason it is important that the pec follows the path of its parent cable very closely.

If, however, a number of cable screens are bonded at both ends to the same items of equipment, they may act as their own pec. Sharing the screen currents between them, there may be no need for a separate pec.

Cable armour can also be used as a pec and, depending on the quality and assembly, it can give good responses at frequencies above 50 Hz. But, warns Armstrong, ordinary steel wire armour should not be relied upon alone to provide any benefits above 1 MHz.

If signal or data cables are to be strung between buildings, then it is best to use galvanic isolation up to 2 MV. Fibre optic cables, radio links, lasers and microwaves are also good candidates.

If copper must be used, then a certain amount of risk must ensue. "Each structure must have a complete MESH-CBN, which extends to the cables between them," says Armstrong, "and design should cater for the largest possible transient events."

Technical resources
All of the points suggested by Armstrong are outlined in the document: IEC 61000-5-2 1997: Electromagnetic compatibility (emc) - installation and mitigation guidelines - earthing and cabling.

This technical report covers guidelines for the earthing and cabling of electrical and electronic systems and installations. It is aimed at ensuring electromagnetic compatibility among electrical and electronic apparatus or systems. More particularly, it is concerned with earthing practices, and with cables used in industrial, commercial and residential installations.

It is intended for use by installers, users and, to some extent, manufacturers of sensitive electrical or electronic installations and systems, and equipment with high emission levels that could degrade the overall electromagnetic environment.

For more information see: www.compliance-club.com/archive1/ EMC%20for%20systems%20and%20installations%20-%20Part%202.pdf