Tingling shock at a damp practical bay: why the MCB stayed silent
At a BSEH NSQF Power workshop, a first-year trainee is asked to replace a damaged 230 V single-phase switch board in a practical bay. The concrete floor is still damp from mopping. The nearest outlet is a 6 A three-pin socket fed from a 16 A MCB, and the trainee uses a two-core extension lead whose plug has the earth pin broken off. He stands on the damp floor in rubber-soled shoes and works on the board without switching off the supply. While tightening a terminal, his wrist touches the metal frame of the board and the live terminal at the same time. He feels a tingling shock. The 16 A MCB does not trip. Later, when he pulls the extension plug out of the socket, the 30 mA RCCB at the distribution board trips. The instructor asks you, as senior trainee, to write an incident note and to state what should have happened before work began.
1. Explain why the 16 A MCB did not trip at the moment of the shock, but the 30 mA RCCB tripped later when the plug was pulled out.
Read the worked answer
The two devices detect completely different things.
- MCB (16 A): it responds to overcurrent - a thermal element for sustained overload and a magnetic element for short-circuit currents many times its rating. A shock current of a few tens of milliamperes is invisible to it. It was never going to trip, and its silence says nothing about whether the installation was safe.
- RCCB (30 mA): it compares the current in the live and neutral conductors. Any difference is current returning to earth by another path, and it trips at roughly 30 mA.
At the moment of contact, current flowed through his wrist, body and the damp floor - the tingling proves that. It was probably below the 30 mA threshold, or the contact resistance of damp concrete limited it, so the RCCB held. When the plug was pulled out under load, the broken earth pin and the arcing at the plug created a brief, low-impedance earth path with an imbalance above 30 mA, so the RCCB operated. The lesson: a shock that does not trip an RCCB can still kill, and an MCB is not a shock-protection device.
2. State the immediate actions after the shock, and identify the root causes in this setup.
Read the worked answer
Immediate actions
- Do not touch the trainee or the board. Isolate at the distribution board, or remove the plug using insulated gloves if that is the only way.
- Give first aid: check response and breathing, look for entry and exit burns, watch for delayed cardiac effects, and arrange medical examination even if he feels fine.
- Tag the board and the extension lead out of service, inform the instructor, and do not re-energise until the circuit has been tested.
- Record the incident: time, circuit, devices, symptoms, and the fact that the MCB did not operate.
Root causes
- Live work with no isolation, lock-out or permit.
- Earth pin deliberately broken - the board frame and any faulty appliance had no earth path.
- Damp floor and no insulating mat.
- Two-core extension lead used for Class I equipment.
- RCCB not tested before use, and no supervision of a first-year trainee.
3. List the controls that should have been applied before any work started on that switch board.
Read the worked answer
- Isolate and lock out: switch off the relevant MCB, lock and tag it, and prove dead at the actual conductors with a voltage tester that has first been proved on a known live source.
- Keep the earth intact: use a three-core lead and a plug with the earth pin present; if the supply is temporary, use an RCCB-protected outlet and press the test button to confirm it trips.
- Control the environment: dry the floor or use an insulating mat, remove water sources, and improve lighting so terminals are clearly visible.
- Use the right equipment: insulated screwdrivers and gloves, correct-rated cable and socket, no daisy-chained extension boards.
- Supervision and procedure: a permit or supervised work instruction for a first-year trainee; one-hand technique where live work is unavoidable; no jewellery or watches.
- Test before touch: verify dead, verify the tester, then work. If the supply cannot be isolated, the work must not proceed.