Nuisance RCCB Tripping in a Two-Storey House
A 30 mA, 63 A two-pole RCCB protects the entire single-phase installation of a two-storey house. It has tripped about once a week for two months, always in the evening and mostly within minutes of the 3 kW storage geyser switching on; the MCB never trips. A clamp meter on the earthing conductor reads 4 mA with only lights and fans running, and jumps to 30–34 mA when the geyser is on. Insulation resistance of the whole installation at 500 V is 60 MΩ; the geyser element, tested disconnected and cold, reads 200 MΩ. The geyser terminal box is damp, and a multimeter between the live terminal and the metal body reads 6.8 kΩ. The owner has been advised by a neighbour to fit a 100 mA RCCB, or better, to 'just remove it'.
1. Use the readings to explain the trips and identify the fault. Why was the cold insulation resistance of the element misleading?
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The RCCB is behaving correctly. A 30 mA device trips when about 30 mA reaches earth. The clamp meter reads 30–34 mA with the geyser on, matching the trip threshold step for step - this is not nuisance tripping, it is a real earth-leakage path.
Locating it: 6.8 kΩ from live terminal to metal body at 240 V gives I = 240 / 6800 ≈ 35 mA, which agrees with the clamp reading. The fault is in the damp terminal box, not in the heating element.
Why the 200 MΩ was misleading: the element was tested cold and disconnected. The leakage appears only when the box is warm and moist, when the supply is on and the termination is at full potential - the test conditions did not reproduce the service conditions. Likewise, the whole-installation 60 MΩ proves only that the wiring insulation is sound; a 6.8 kΩ tracking path is invisible to a 500 V insulation test on the installation as a whole if it is bypassed by the test connections, and in any case a resistance of a few kilo-ohms sits well outside the range that test is read for.
Rectification: isolate, dry out the box, replace the gasket, gland and any discoloured termination, re-terminate, verify body earthing continuity, then re-energize and confirm leakage is below 5 mA with the geyser heating.
2. Evaluate the two pieces of advice the owner received - fit a 100 mA RCCB, or remove the RCCB altogether.
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Fitting 100 mA: 30 mA is chosen because it is in the range that limits the duration of a shock to a level a healthy adult can survive; 100 mA may not open the circuit before ventricular fibrillation occurs in a vulnerable person. A 100 mA device is a fire/property protection device, not a personal-protection device. In a bathroom and other wet areas, 30 mA is what the wiring code and utility practice require. Fitting 100 mA would hide the fault and remove shock protection from the very appliance most likely to cause a shock.
Removing the RCCB: this removes earth-leakage protection from the whole installation. The MCB will not help - it responds to overload and short circuit, not to 35 mA flowing through a person or through a damp wall. The metal body of the geyser would simply sit at line potential waiting for someone to touch it.
Verdict: both pieces of advice treat the symptom and create a bigger hazard. The correct action is to repair the identified fault, which costs little, and keep the 30 mA device. Record the leakage test after repair as evidence.
3. The owner also wants a second RCCB added so that 'the whole house doesn't go dark'. Explain discrimination and describe how you would arrange the protection.
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Discrimination means that when a fault occurs, only the device nearest the fault operates. Two identical 30 mA non-delayed RCCBs in series do not discriminate: the fault current is seen by both, and either - or both - may trip. Adding a second 30 mA unit upstream would not keep the lights on.
Arrangement I would use:
- Upstream (main): a 100 mA type S (time-delayed) RCCB, if the supply authority or the installation requires a main earth-leakage device - it provides fire protection and selectivity, not personal protection.
- Downstream: separate 30 mA RCCBs on the wet-area and high-risk circuits - geyser/bathroom, kitchen, outdoor socket, and the pump or air-conditioner if applicable. Each of these groups also has its own MCB for overload and short circuit.
- Wiring discipline: the neutral of every downstream circuit must be taken from the same RCCB that feeds its phase. A borrowed neutral from another group creates a standing imbalance and will trip the device continuously - a common cause of 'the new RCCB keeps tripping'.
- Earthing: the protective conductor must never be used as a neutral, and the body earthing of the geyser must be continuous and tested.
- Verify: after wiring, test each RCCB with its test button and with a leakage tester on each protected group, and record the trip times.