Electrical, mechanical and control systems
Domain 7 tests the machinery: power supply, controls, hoist, brake and the devices that stop a fault becoming a fall.
Power supply
- Dedicated supply through a lockable isolator and a residual current device; cables protected from damage, plugs and sockets industrial type, earth connection sound.
- Keep cables off the roof edge and out of water; do not use damaged or taped leads.
- Control circuits are at reduced voltage (commonly 110 V or less); the pendant is safer than mains at the operator's hands but still must be treated as live equipment.
Controls
- Pendant with up/down, emergency stop and, where fitted, a deadman's handle that stops motion when released.
- Directional interlocks prevent simultaneous opposite commands.
- Emergency stop is for emergencies, not routine stopping.
- No bypassing, taping or rewiring of controls.
Hoist and brake
- Electric motor through a gearbox to the rope drum; the brake is spring-applied and electrically released, so it fails safe on power loss.
- Test the brake each shift with a light load just above the ground: it should hold without drift.
- Watch for abnormal noise, overheating, oil leaks and slack or uneven rope spooling.
Protective devices
- Overspeed device - stops or brakes the platform if descent speed exceeds the set limit.
- Slack-rope device - stops the hoist when rope tension is lost, preventing unspooling and jamming.
- Tilt limiter - stops the platform if it tilts beyond the set angle.
- Overload limiter - prevents lifting above the rated load where fitted.
- Upper and lower limit stops and an audible alarm.
Worked example
The platform is lowered onto the ground and the ropes go slack; the slack-rope device stops the hoist and the alarm sounds. The correct response is to raise the platform slightly, allow the ropes to tension, and reset only by the manufacturer's procedure. Bypassing the device to continue working risks rope unspooling and jamming in the drum.
Traps
- Resetting a limiter without finding the cause.
- Testing the brake by switching off at height with a full load.
- Treating the emergency stop as the normal way to stop.
- Ignoring a tilt alarm.
- Running cables through standing water.