Introduction to Health and Safety at Work

Residual current devices
If electrical equipment must operate at mains voltage, 
the best form of protection against electric shock is the 
residual current device (RCD). RCDs, also known as earth 
leakage circuit breakers, monitor and compare the cur-
rent fl owing in the live and neutral conductors supplying 
the protected equipment. Such devices are very sensi-
tive to differences of current between the live and neutral 
power lines and will cut the supply to the equipment in a 
very short period of time when a difference of only a few 
milliamperes occurs. It is the speed of the reaction which 
offers the protection against electric shock.
RCDs can be used to protect installations against 
fi re since they will interrupt the electrical supply before 
suffi cient energy to start a fi re has been dissipated. For 
protection against electric shock, the RCD must have a 
rated residual current of 30 mA or less and an operating 
time of 40 milliseconds or less at a residual current of 
250 
mA. The protected equipment must be properly 
protected by insulation and enclosure in addition to the 
RCD. The RCD will not prevent shock or limit the current 
resulting from an accidental contact, but it will ensure 
that the duration of the shock is limited to the time taken 
for the RCD to operate. The RCD has a test button which 
should be tested frequently to ensure that it is working 
properly.
Double insulation
To remove the need for earthing on some portable 
power tools, double insulation is used. Double insulation 
employs two independent layers of insulation over the 
live conductors, each layer alone being adequate to insu-
late the electrical equipment safely. Since such tools are 
not protected by an earth, they must be inspected and 
maintained regularly and must be discarded if damaged.
Figure 12.9 shows the symbol which is marked on 
double insulated portable power tools.