Element 11: Electricity

11.1 Hazards and risks

Electric shock occurs when current passes through the body. Its effects range from a tingle, to muscle contraction (the person may not be able to let go), breathing difficulties, heart disturbance (ventricular fibrillation) and death.

Severity depends on voltage, frequency, duration of contact, body resistance (wet skin lowers it) and the path the current takes through the body. Electrical burns can result from direct contact or from a nearby arc.

Incidents and electrical fires are likely when: equipment is unsuitable; inadequately maintained; defective or damaged; used in wet environments; or overheated (for example when charging portable equipment).

Secondary effects include falls from height after a shock. Other hazards: working near overhead power lines and striking underground cables during excavation.

Study tip: A small current through the heart can kill, so the path matters as much as the voltage.

Practice questions for 11.1

11.2 Control measures

Protect conductors with insulation and enclosure; use equipment that is strong enough and suited to the job and place; and use competent people.

Protective systems and their advantages and limits:

• Fuses and circuit breakers (protect against overload and short circuit, not against shock)
• Earthing (gives a safe path for fault current)
• Isolation of the supply (the safest way to work on equipment)
• Double insulation (no earth needed, but damage to the casing can expose live parts)
• Residual current devices (cut the supply quickly in a fault, but they need testing and do not prevent all shocks)
• Reduced and low voltage systems (they limit the voltage, and so the harm that a fault can cause)

Safe systems of work: no live working unless there is no alternative; isolate and lock off; locate buried services before digging; and protect against overhead lines. Have emergency procedures for an electrical incident: do not touch the casualty until the supply is off, call for help, give first aid.

Inspection and maintenance:

• User checks before use
• Formal inspection and tests of installations and equipment
• Frequency based on the risk
• Records kept
• Portable appliance testing (PAT), which has advantages (finds faults) and limitations (only shows the condition at the time and does not find all faults)

Study tip: No single device is enough, so layer your protection.

Practice questions for 11.2