Leaking Geyser, Tripping RCCB and a Request to Bypass It
A customer's 2 kW, 230 V storage water heater has started tripping the 30 mA RCCB in the distribution board about 20 minutes after switch-on, once the water is hot. The house wiring is about 12 years old. A local repairer disconnected the RCCB's earth link 'to test', and the heater then ran without tripping; he has quoted a low price to replace the thermostat and says the RCCB is over-sensitive. The customer asks an authorised workshop for a second opinion. On arrival, the technician isolates the circuit, locks the MCB, proves the voltage tester on a known live source, and measures: element cold resistance 25 Ω, insulation resistance from live to earth 0.4 MΩ at 500 V DC, and earth continuity from the heater body to the distribution-board earth 0.6 Ω. The element flange gasket shows hard white scale and slight deformation. The customer is worried about cost and asks whether the RCCB can simply be left bypassed.
1. Interpret each measurement and identify the most likely fault. Explain why the heater stopped tripping when the earth link was removed.
Read the worked answer
Element resistance: R = V²/P = 230² / 2000 = 26.5 Ω, so the measured 25 Ω is normal for a 2 kW element and the element is not open or shorted.
Earth continuity of 0.6 Ω is acceptable; the body is earthed.
Insulation resistance of 0.4 MΩ at 500 V DC is low. A healthy heating element should normally read at least 1 MΩ, and manufacturer data should be checked. At 500 V the leakage is about 1.25 mA, but at 230 V the cold leakage is small; as the element heats, the damp, scaled magnesium-oxide filling becomes more conductive and leakage rises, which explains the trip after about 20 minutes.
Removing the earth link stops the RCCB seeing earth leakage, but it also removes the protective earth. The heater body can then become live, and the RCCB will not operate on a body fault. That 'test' made the appliance more dangerous, not better.
2. Decide between repairing and replacing the element, and set out the safe procedure and the tests required before the heater is returned to service.
Read the worked answer
The element is the likely fault, and the gasket is deformed and scaled, so replacing the element and gasket together is the correct repair. Replacing only the thermostat would not fix the leakage. If the element cannot be replaced separately, the heater should be replaced.
Procedure:
- Isolate at the MCB, lock off, prove dead, and drain the tank.
- Remove the element flange, clean the mating surface, fit a new gasket and element of the same rating (2 kW, 230 V), and tighten to the manufacturer's torque.
- Refill the tank completely before switching on; a dry element will fail immediately.
- Verify insulation resistance again at 500 V DC, live to earth, and record the value; it should be well above 1 MΩ.
- Check earth continuity, the thermostat setting, the safety cut-out, and the RCCB trip test. Run the heater and re-check for leakage after it reaches temperature.
- Complete a job card with readings and advise the customer to service the tank and check the anode.
3. Write your response to the customer's request to leave the RCCB bypassed, including safety, cost and disposal points.
Read the worked answer
The RCCB must not be bypassed. It is a 30 mA residual-current device that protects people from electric shock; bypassing it removes the protection for the whole circuit, not just the heater. If the heater body becomes live through a failed element or a water leak, the result can be fatal. It also leaves the installation non-compliant and could affect insurance and the customer's liability if someone is injured.
The correct repair costs more than the 'low-price' thermostat change, but it addresses the measured insulation failure. A thermostat swap would leave the leakage path in place, so the tripping would return once the tank heated.
Explain the measured 0.4 MΩ result and show the customer the old element and gasket. Warn that the earlier earth-link removal must be reversed and tested before use.
Dispose of the old element, gasket and packaging through an authorised e-waste channel, and give a receipt and job card. Offer a maintenance reminder for the tank and anode.