Electrical safety

Regarding electrical equipment designed to optimise patient safety, which one of the

following statements is TRUE?

a) Under single fault conditions, type I, CF equipment should have a leakage current in

the order of 5mA

b) Type BF equipment is safe because the patient circuit is earthed

c) To promote patient safety a theatre suite should have an uninterruptible power

supply (UPS)

d) Class III equipment is defined as that which operates at ‘safety extra-low voltage’ of

less than 12V

e) A current-operated earth-leakage circuit breaker relies on an unacceptable current

causing disintegration of a fuse that then breaks the circuit

Answer: C

Explanation

A patient’s safety may be jeopardised by a lack of electricity as well as an excess of it.

UPS ensure continued operation of safety-critical equipment in the event of a mains

power failure. Rather frustratingly for the purposes of exams, values for allowable

leakage currents vary according to source. However, the order of magnitude is incorrect

in Option (a) – any device potentially forming a direct electrical link with the heart

and thus with the potential to cause microshock, should have leakage currents in the

order of microA, not mA. The ‘F’ of type BF equipment indicates a floating circuit – one

in which there is no electrical contact with any other circuit or earth. The only current

that flows in it is induced by a step-down transformer. Safety extra-low voltage (SELV)

is less than 24V. A current-operated earth-leakage circuit breaker (COELCB) incorporates

equal coils of live and neutral wires inducing a self-cancelling magnetic field. In

the presence of an unacceptable current the magnetic fields no longer match: one

dominates and operates a solenoid that breaks the circuit.