Page 116 - Mechanical Engineers Reference Book

References 2/57
or alteration may be carried out safely. This is achieved by resistance could easily be reduced by damp conditions. In
isolating switches which have no automatic features. Various cases where work is carried out within confined metal enclo-
circuits and motors should not ‘share’ an isolation switch sures, mains voltage equipment must not be used. A double-
unless it is clear that under no circumstances will it be wound transformer with a secondary centre tap to earth is
necessary, or convenient, to use one circuit while the other is allowable in these cases. This transformer gives 50 V for
being serviced. lighting and 100 V for portable tools.
Isolation switches should be capable of being locked in the These few notes on general electricity safety are by no
‘off position but not in the ‘on’ one. If the isolation switch means extensive or authoritative. Reference should always be
cannot be locked it should be possible to remove the fuse on made to the full guide to the reguations.” Further recom-
the power line so that the line cannot be energized by mended reading on electrical safety guidelines can be obtained
inadveitant closing of the isolation switch. from references 13 and 14.

2.4.5 Earthing References
The external metal casing of electrical apparatus and cables 1. Hughes, W. Electrical Technology, sixth edition. revised by
~
must bta earthed for three reasons: I. McKenzie Smith, Longman, Harlow (1987)
2. Bell, E. C. and Whitehead, R. W., Basic Electrical
1. To prevent the casing rising to a dangerous voltage if there Engineering & Instrumentation for Engineers, third edition,
is a fault such as a short circuit between the conductor and Granada, St Albans (1987)
the casing; 3. Bell, D. A., Fundamentals of Electric Circuits. third edition,
Reston Publishing, New York (1984)
2. To conduct any current away by a safe path; 4. Gray, C. B., Electrical Machines and Drive Systems.
3. To ensure that the faulty circuit is automatically discon- Longman Scientific & Technical, Harlow (1989)
nected from the supply by drawing sufficient current to 5. Sen, P. C., Principles of Electric Machines and Power
blow the protective fuse or operate the circuit breaker. Electronics, John Wiley, Chichester (1989)
6. Orthwein, W., Machine Component Design, West Publishing.
Earthing consists of connecting the metal casing by means of New York (1990)
a conductor to an earth electrode. The earth electrode may be 7. Kenjo, T. and Nagamori: S., Permanent-Magnet and
a buried pipe or other such conductor which is known to be Brushless dc Motors, Monographs in Electrical and Electronic
making, an effective connection to the ‘general mass of the Engineering, Clarendon Press, Oxford (1985)
earth’. Where the earth connection to a casing is made with a 8. Kaufman, M. and Seidman, A. H., Handbook of Electi,onics
nut and bolt a spring washer or other similar locking device Calculations for Engineers and Technicians, second edition,
McGraw-Hill, New York (1988)
must be used. Earthing is a legal requirement and must be 9. Watson, J., Mastering Electronics, Macmillan, London (1983)
effective at all times.
10. Milne, J. S. and Fraser, C. J., ‘Development of a
mechatronics learning facility’, in Mechatronic Systems
2.4.6 Double insulation Engineering, Vol. 1, pp. 3140, Kluwer Academic, New York
(1990)
Although the electricity regulations require all portable appa- 11. Horowitz, P. and Hill, W., The Art of Electronics: second
ratus used at normal mains voltage to have an earthing edition. Cambridge University Press, Cambridge (1989)
conductor, these can introduce their own hazards. As a result 12. Health and Safety Executive, Memorandum of Guidance on
‘double-insulated’ or ‘all-insulated’ apparatus is made which the Electricity at Work Regulations, 1989, Health & Safety
series booklet HS(R)25, HMSO (1989)
does not require earthing. Double insulation means what its 13. Imperial College of Science and Technology, Safety
name says, and all live conductors are separated from the Precautions in the Use of Electrical Equipment, third edition,
outside world by two separate and distinctive layers of insula- Imperial College Safety Booklets (1976)
tion. Each layer of insulation would adequately insulate the 14. Reeves. E. A,, Handbook of Electrical Installation Practice,
conductor on its own, but together they virtually negate the Vol. 1, Systems, Standards and Safery, Ed. E. A. Reeves,
probability of danger arising from insulation failure. Double Granada, St Albans (1984)
insulation avoids the requirement for any external metalwork
of the equipment to be protected by an earth conductor. Further reading
Fitzgerald, A. E., Higginbottom, I). G. and Grabel, A,. Basic
2.4.7 Low-voltage supplies Electrical Engineering, fifth edition, McGraw-Hill, New York
(1981)
Portable tools (particularly hand inspection lamps) can be a Nasar, S. A,, Handbook of Electric Machines, McGraw-Hill, New
source of danger because they are subject to severe wear and York (1987)
tear and are likely to be used in confined spaces where the skin Say, M. G., Alternating Current Machines, Pitman, London (1983)