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The drawing into ventilated electrical tools of conductive dusts

OC 492/2

1 This OC revises and replaces NIGM 10A/1990/8. The attached Information Document (ID) outlines risks and precautions associated with the use of such tools and makes reference to appropriate parts of the Electricity at Work Regulations. The ID may be distributed to interested parties outside HSE.

2 There is a growing tendency for portable electrical tools to be used under circumstances where conductive dusts or other substances are likely to be drawn into the tools so causing electrical danger. Very often the use of electrical tools in such circumstances is inappropriate.

3 Inspectors who find portable electrical tools being used where conductive dusts are present, should advise the employer of the hazard and recommend that the electrical tool is replaced by a hydraulic or air-powered tool, or (failing that) by an electrical tool operating at a reduced voltage.

29 November 1991


Disc No: FOD1C\Editors\J106\10.91\DH\GK

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Health and safety executive

Information document

HSE 492/2


1 This document contains internal guidance which has been made available to the public. The information may not be applicable in all circumstances and any queries should be directed to the appropriate enforcing authority.

2 Risks and precautions associated with the use of certain types of electrical equipment are outlined and reference is made to appropriate parts of the Electricity at Work Regulations.


3 At all times, the insulation properties of the insulating components used in hand-held equipment must be such that risk of injury is not likely to occur. The popular types of equipment in use are Class II (ie double-insulated or all-insulated machines) and Class I machines (ie those with earthed metal enclosures). Class II machines are more readily available from suppliers of tools. Both types require ventilation to prevent overheating of the drive motor and gear box. Ventilation is achieved by the use of a fan blade fitted to the drive motor shaft. In some designs a proportion of the airflow is diverted to remove debris from the tool's cutting blade.

4 Filters are not normally provided on portable industrial tools to prevent ingress of dust and in normal dry, clean conditions the airflow should be sufficient to blow any dust through the machines. However, where moisture or oily deposits are present in the machine or if the dust itself has adhesive properties there will be a risk of dust deposits forming within the machine and on its surface. When this occurs in Class II machines it is possible that live parts will become connected to the hand-held parts through the deposits on the insulating surface. This effect could allow current to flow from the hand-held parts to earth mass (or any earthed metal) through the person holding the machine. The degree and nature of the deposited contamination would influence the level of current flow and thus the severity of any electric shock received.

5 The effect is less serious when it occurs with Class I machines because it is likely that any conducting path will connect to earthed metal on the machine thus causing the protective device to switch off the machine or (if the contaminant resistance is high) the voltage on the metal parts would be maintained at about earth potential.

6 On fixed electrical machinery the protection is provided by enclosing the machine sufficiently to prevent ingress of contaminants. This is achieved by manufacturing the machine to the appropriate index of protection (IP) rating. This is not the normal practice with hand-held machines. Further guidance on IP ratings for electrical equipment is contained in IEC 529 2nd Edition, BS 5420 and BS 5490.

7 Regular maintenance of portable tools is included in the requirements of the Electricity at Work Regulations 1989 and in normal dry, reasonably clean factory conditions, routine visual inspection and less frequent electrical testing should be sufficient to prevent risk of injury.

8 In damp environments, or where the process being carried out will produce conductive dusts, the risk of injury from contamination is high. Regular electrical testing of the machines is necessary to ensure that the build up of dust is not sufficient to degrade the insulation. In some circumstances it has been estimated that this might be required at least once per working day and is unlikely to be achieved. In such circumstances electrically powered machines should not be used unless the manufacturer or supplier can demonstrate that the machines have been manufactured to an acceptable standard for such use. The use of hydraulic or air powered tools might be a better practice. In Guidance Note PM32 The safe use of portable electrical apparatus(ISBN 011 885590 5 - available from HMSO) one of the alternatives given is to use machines that operate at reduced voltages, eg with 110-volt, centre tapped to earth supplies or safety extra-low voltage (SELV) supplies. Where conductive dusts are also present, the use of 110-volt machines might not be a safe alternative.

9 In the "at-work" situation, the Electricity at Work Regulations 1989 are relevant to the use of hand-held machines. Regulation 6 relates to adverse or hazardous environments and requires that where it is reasonably foreseeable that electrical equipment will be exposed to contamination it should be constructed, or protected as necessary, to prevent danger so far as is reasonably practicable. Regulation 6 also refers to the danger from flammable dusts which might be relevant to hand-held machines.

10 The user should carry out an assessment of the work being undertaken to decide whether or not contamination is foreseeable.

11 In the documents which accompany machines, suppliers or hirers should state which environmental conditions are incompatible with the manufacturing standard.

November 1991

Updated 2008-05-13