The Warm Heater, the Broken Earth Pin and the 16 A MCB
During a pre-vocational practical session, a student connects a 2000 W room heater and a 750 W electric iron to a 230 V, 15 A three-pin socket through a multi-plug adapter. The socket's earth pin is broken and the metal body of the heater gives a mild shock when touched. The distribution board has a 16 A miniature circuit breaker (MCB) and no RCCB. The heater's flexible cord is 1.0 mm² and feels slightly warm after ten minutes. The supply voltage measured at the socket is 228 V. The instructor stops the session and asks you, as the senior student, to assess the installation before anyone uses the socket again. The workshop has a megger, a multimeter, a test lamp and a spare 15 A socket, but no spare 2.5 mm² cable.
1. Calculate the total load current and decide whether the 16 A MCB would trip. Explain why the student received a shock even though the MCB did not operate.
Read the worked answer
Total current I = P/V = (2000 + 750)/228 = 12.06 A. A 16 A MCB has a thermal trip threshold of about 1.13 × 16 = 18.1 A, which it must not trip below for at least one hour, and a magnetic trip of roughly 5–10 × 16 = 80–160 A. A steady 12 A therefore does not trip it. The shock is not an overcurrent event: it is a leakage current from a damaged insulation or heating element to the metal body. Because the earth pin is broken, the body is not held at earth potential, so the leakage raises it toward 228 V. The current through the student was limited by body and floor resistance, giving a mild tingle, but a lower-resistance path (wet hands, damp floor) could be lethal. An MCB protects cables against overload and short circuit; it cannot detect a 5–30 mA earth leakage. Only an RCCB/ELCB of 30 mA sensitivity responds to that.
2. Explain the function of earthing and of a 30 mA RCCB in this situation, and say which single protective measure you would insist on before the socket is used again.
Read the worked answer
Earthing connects the metal body to the earth electrode through a low-resistance conductor. If insulation fails, the leakage current flows to earth through that path, the body stays near earth potential, and the fault current is large enough to blow the fuse or trip the MCB. With the earth pin broken, the body is floating: no fault current flows, no protective device operates, and the body sits at a dangerous voltage until someone touches it. A 30 mA RCCB continuously compares phase and neutral current. Any imbalance above 30 mA (typically tripping within 30 ms) means current is escaping to earth, so it disconnects both poles. It protects people even when the earth path is poor. Before reuse I would insist on restoring a tested earth connection: repair or replace the socket, verify earth continuity to the distribution board with a megger, and confirm the electrode resistance is acceptable. Fitting a 30 mA RCCB is the second must-do, not a substitute for earthing.
3. Besides earthing, list the corrections you would make to the heater connection and the checks you would record before clearing the socket.
Read the worked answer
Corrections: (1) Remove the multi-plug adapter. It encourages two high-current appliances on one 15 A outlet; give the iron its own 5 A or 15 A point. (2) Replace the 1.0 mm² heater cord. It carries 8.8 A and is already warm; a 1.5 mm² flexible cord gives margin and less voltage drop. (3) Replace the damaged socket and the heater plug, and inspect the element and internal wiring for scorching. (4) Replace the 16 A MCB with one sized to the cable, and add a 30 mA RCCB at the board. Checks to record: insulation resistance of the heater between live parts and body (a megger reading in the megohm range, not a low value); earth continuity from socket to board; tightness of terminals; supply voltage; and a functional test of the RCCB using its test button. Only after all readings are satisfactory should the socket be cleared for use, and the student should be re-briefed on not touching a suspect appliance.