Abstract :
The object of the paper is to recommend a simple, effective and co-ordinated scheme whereby any portable and transportable industrial electrical equipment can be used with a high degree of safety. In the early sections conclusions are drawn regarding the severity of the shocks which may be expected at different voltages and under different conditions, and a special note is made of the value from a safety point of view of limiting the duration of any medium-voltage shock to a few thousandths of a second. Typical a.c. supply systems are then studied to determine the voltage likely to appear on the earthed metalwork of protected equipment under fault conditions. Following a critical review of possible safety measures it is suggested that the use of extra-low voltage should be confined to handlamps, and that all portable tools should be supplied at 110 volts from fixed transformers in which the mid-point of the secondary is earthed. For transportable equipment, which may require considerable power, low voltage is unsuitable, and the use of medium voltage is recommended, provided that it is controlled by a circuit-breaker designed to trip instantly if any break should occur in the equipment earth-connection or in the event of an earth fault on the transportable equipment or its flexible cable. Two simple and inexpensive types of control units, developed for the Engineering Equipment Users Association, are described. Finally, stress is laid on the importance of regular maintenance and the use of reliable equipment.
