Confined-space entry to clear a blocked drain
A blocked 225 mm foul drain runs under a car park to a manhole 3.2 m deep. Two workers propose to lift the cover, lower a ladder, and one of them will descend with a drain rod to clear the blockage. The car park is busy, the manhole is in a corner, and the only access is through a pedestrian gate. There is no gas detector on site, no harness, and no rescue plan. The workers say they have 'done it many times before' and that the job will take 'ten minutes'.
1. Identify the hazards and explain why this entry must not proceed as proposed.
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The manhole is a confined space: 3.2 m deep, with restricted access and the potential for toxic gases (hydrogen sulphide, methane, carbon monoxide), oxygen deficiency, and sudden flooding if the blockage clears. The hazards include:
- Atmospheric: no gas detector, so the workers cannot know whether the air is safe. Hydrogen sulphide deadens the sense of smell at higher concentrations, so 'it smells fine' is not evidence.
- Access and egress: a ladder is not a safe means of escape if the worker becomes unwell. A harness and tripod or winch are needed.
- Rescue: there is no rescue plan, no trained rescuer, and no communication. Most confined-space deaths are would-be rescuers.
- Traffic and public: the car park is busy, and the open manhole is a fall hazard to the public.
- Flooding: clearing the blockage can release a surge of sewage.
The entry must not proceed without a risk assessment, a permit, atmospheric testing, forced ventilation, a rescue plan, and at least one person outside the space maintaining contact.
2. Describe the safe system of work that should be put in place.
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The safe system should include:
- A written risk assessment and a confined-space entry permit, signed by a competent person.
- Isolation of the drain where possible, and warning signs and barriers around the manhole.
- Atmospheric testing before entry and continuously during the work, for oxygen, hydrogen sulphide, methane, and carbon monoxide. Forced ventilation should run before and during entry.
- A minimum of two people outside, one of whom is the rescue-trained attendant, with a tripod, harness, and lifeline on the entrant.
- Communication by radio or voice, with a check-in interval.
- Rescue equipment at the surface and a rescue plan that has been rehearsed.
- Personal protective equipment: helmet, gloves, eye protection, and respiratory protection if testing shows it is needed.
- Consideration of a no-entry method: high-pressure jetting or rodding from the surface, which may clear the blockage without entry at all.
The best answer is often not to enter: use remote rodding or jetting from the surface.
3. If the blockage cannot be cleared from the surface, what additional controls are needed before entry?
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Before entry, the following additional controls are needed:
- A full permit-to-work, with a named entrant, attendant, and supervisor.
- Confirmation that the atmosphere is safe, with continuous monitoring and ventilation. If it cannot be made safe, entry with breathing apparatus and a trained team is required.
- A rescue plan specific to the manhole: tripod and winch, stretcher, first-aid-trained rescuers, and a means of calling emergency services.
- Isolation of the drain upstream and downstream to prevent a surge, or a plan to withdraw immediately if flow increases.
- A communication protocol and a maximum entry time.
- A check that the ladder is secure and that the manhole is not damaged.
Entry should only be made by trained, medically fit workers, and the permit should be closed and reviewed after the job.