Control Measures and Safe Systems of Work: Isolation, Purging, Permit-to-Work, Entry Procedures
This domain tests the practical controls that make an entry safe: the hierarchy of control, isolation and lockout, purging and ventilation, the permit-to-work, and the entry procedure itself.
Hierarchy of control
Avoid entry - remote cameras, long tools, external cleaning; engineer out the hazard - isolation and ventilation; administrative controls - permit, training, standby person; PPE last.
Isolation
Identify every energy source: process gas, liquid and steam; mechanical; electrical; gravity; pressure; thermal; stored material. Use positive isolation with spades, blanks or double block and bleed; lockout and tagout with personal locks; verify zero energy by testing, draining, venting and trying to start. Isolate drains and vents that could backflow, and consider adjacent plant and gas migration.
Purging and ventilation
- Purge with fresh air, never with oxygen.
- Use continuous mechanical ventilation with supply and exhaust, drawing air from a clean source.
- Ventilation may not remove gas generated by sludge or gas trapped in pockets; re-test after ventilation and monitor continuously.
- Use explosion-proof ventilation equipment where a flammable atmosphere is possible.
Permit-to-work
The permit should identify the space, task, hazards, isolation points, test results, PPE, rescue plan, authorised persons, validity period and signatories. Display it at the entry point, sign entrants in and out, and close it at completion. A permit is not a substitute for testing.
Entry procedures
Toolbox briefing; entry log; standby person; agreed communications; maximum number of entrants; correct lighting and tools; harness and lifeline where retrieval is required; no lone working.
Worked example
A permit is issued for a mixing tank with an agitator. The isolation list includes the MCC breaker locked with the entrant's personal lock, a mechanical pin on the agitator, the product line blanked and the drain isolated. The supervisor verifies zero energy by attempting a local start, tests the atmosphere and records the results on the permit. At shift change the permit is closed and a new one issued with fresh testing.
Traps that cost marks
- "Switching off" without lockout or verification.
- Relying on a single valve to isolate a line.
- Ventilating but not re-testing.
- Signing a permit without visiting the space.
- Leaving the permit open after the job is finished.