Safety firstSimon Barrowcliff examines the implication of the Low Voltage Directive Nine months into the mandatory implementation of the EMC directive most manufacturers and importers of electrical products should have at least addressed the potential impact of the directive and the specific measures to demonstrate compliance with it. For many products this has involved testing and in some cases retrospective modification to bring
the product within emission or immunity limits.
However, whilst attention is focused on the immediate task of meeting the EMC requirements, what of the implications for product safety and the Low Voltage Directive (LVD)?
Currently in the UK, manufacturers and importers of electrical products can choose to satisfy the safety requirements as laid out in either the Electrical Equipment Safety Regulations 1989 (SI 1989/728) which implement the 1973 version of the LVD)
or the 1994 Regulations (SI 3260) which implement the 1993 amendments. The significant differences between the two sets of regulations is the introduction of CE Marking requirements in the 1994 version. After 1996, the 1989 Regulations are no longer an option and all applicable product falls within the requirements of the 1994 Regulations.
One method of satisfying the Regulations is to conduct testing against a harmonised specification. There are a number of commonly used specifications including EN 60 065 (BS 415), EN 60 335 (BS 3456), EN 60 950 (BS 7002) and EN 61 010. Lists of LVD recognised specifications are periodically listing in the Official Journal of the European Communities.
Power line and signal line emissions are often attenuated using capacitors or capacitor and inductor networks. These may be proprietary filter assemblies or combinations of discrete components. When these components are in primary circuits they can have a significant impact upon product safety.
A prime example is the application of additional Y capacitors. Manufacturers should check the effect on earth leakage current when additional Y capacitors are added or values changed. The cumulative effect of Y capacitors within an entire equipment may cause safe limits to be exceeded. Leakage current limits are given in the text of the harmonised standards.
X capacitors and other across the line suppression components (e.g. VDRs) may also compromise safety. For example, many harmonised standards require IEC 364-14 approved X capacitors to ensure protection against fire hazards. For pluggable equipment, care should also be taken in the selection of X capacitors and bleed resistors to ensure that a hazardous residual voltage does not remain on the plug pins, once it has been removed from the socket.
Ferrite cores add inductance to a line, provide roll off' for capacitors and act as absorbers' of RF energy. The addition of these components can reduce wiring and component space such that safety insulation is bridged or exposed to greater temperatures in its relocated position. Copper EMC bands' applied to windings or ferrite cores for suppression purposes can also alter the thermal properties of transformers or inductors.
Thermal effects are associated with current carrying wound components. In primary circuits, these currents are protected by safety insulation which may be degraded by excess temperatures. Components and insulation should be considered against working temperature requirements at European voltage levels (230V) and also at North American levels (110V) when currents can be double those measured at 230V.
There is a further factor effecting components. Inadequate fuse or circuit protection of filter components can introduce unexpected fire hazards and therefore consideration should be given to the potential risks during the design and testing processes.
A conductive coating applied to the inside of plastics material enclosures reduces electric field emissions by emulating a Faraday cage. It is an accepted and widely used method for solving this particular EMC problem. However, the solution has safety implications, and consideration should be given to the method for adhering the coating to the enclosure, as well as the flame retardant properties of the coating and its chemical effect on the flame retardant properties of the enclosure.
Electric field emissions can sometimes only be prevented by a modification to the design of the enclosure, either by RF sealing' of all joints or by eliminating wave guiding apertures of a particular size. Two safety factors may be effected - accessibility to moving or live parts and disruption to the thermal profile of the equipment. In the former case, accessibility with the appropriate test probe (e.g. test probe or test pin) should remain in accordance with the safety specification.
Some EMC modifications require re-routing of conductors or earth (ground) paths. Any routing should take account of sharp edges or hot parts of the equipment if insulation damage or short circuit faults could occur. Some specifications prohibit contact between wires at hazardous live potentials and safe to touch parts. In such cases, additional tying, ducting or sleeving may be required.
When it is an earth wire which is being added or moved, additional considerations apply. Safety earth continuity (typically up to 25A current capacity) must be ensured in Class I equipment. Where functional earthed parts are used in Class II equipment, double insulation barriers should be maintained between EMC grounds and hazardous live parts.
It is essential that manufacturers refer back to safety design parameters, product safety specifications and previously obtained test reports/approvals before introducing EMC modifications. It would be most ironic should a manufacturer end up in court under the Low Voltage Directive (LVD) as a direct consequence of their diligent efforts to satisfy the requirements of the EMC directive.
Simon Barrowcliff is director of the safety division of TRL EMC