A Geyser That Trips the 30 mA RCCB Only Under Load
An electrician attends a first-floor flat where the 30 mA RCCB at the consumer unit trips only when the 3 kW storage geyser is switched on. Sometimes it holds for several minutes; sometimes it trips immediately. The geyser is fed by 4 mm² cable from a 32 A MCB. With the geyser isolated and cold, a 500 V insulation tester gives L-E 0.4 MΩ, L-N >200 MΩ and N-E >200 MΩ. The earth pit measures 12 Ω on a three-pole earth tester. The supply is 230 V. The element was replaced after the last monsoon, and there is slight damp staining inside the terminal cover. The owner's neighbour advises fitting a 100 mA RCCB and driving a deeper earth electrode. A second electrician proposes changing the 32 A MCB to 16 A and leaving everything else.
1. Calculate the leakage current corresponding to 0.4 MΩ at 230 V and decide whether it can explain a 30 mA RCCB trip. Show the resistance threshold for tripping.
Read the worked answer
At 230 V, the leakage through 0.4 MΩ is I = V/R = 230/400,000 = 0.575 mA. A 30 mA RCCB needs about 30 mA, corresponding to a resistance of 230/0.03 = 7.67 kΩ. The measured insulation resistance is roughly 52 times too high to explain the trip.
So the 0.4 MΩ is a real defect (below the usual 1 MΩ minimum) and must be corrected, but it is not the cause of the 30 mA trip. The tester must look for a lower-resistance path that appears only when the geyser is energized, such as a neutral-to-earth fault, damp tracking in the terminal block, or a borrowed neutral. A cold 500 V test can also miss a hot, damp fault, so the element should be tested hot or with a leakage clamp.
2. Evaluate the three proposals: fit a 100 mA RCCB, drive a deeper earth electrode, or change the 32 A MCB to 16 A. Which addresses the actual fault?
Read the worked answer
Fitting a 100 mA RCCB reduces protection and is not acceptable for a domestic final circuit where 30 mA additional protection is required; it may stop nuisance tripping but leaves a dangerous N-E fault. A deeper earth electrode addresses the 12 Ω reading, which exceeds the 8 Ω general recommendation, but the RCCB does not sense earth electrode resistance; it compares live and neutral current. Improving the earth is good for fault protection but will not stop this trip.
Changing the 32 A MCB to 16 A better matches the 13 A geyser load and 4 mm² cable, but an MCB responds to overload, not residual current. None of the three proposals finds the N-E fault. The correct action is to isolate, inspect and test the geyser circuit for N-E contact and verify other circuits for shared neutrals.
3. Give a logical test sequence to localize the fault without repeatedly resetting the RCCB.
Read the worked answer
First isolate and lock off. Disconnect the geyser and megger L-E, N-E and L-N at 500 V; a low N-E reading points to a N-E fault. Inspect the terminal block, element ends and cable for damp tracking and loose strands. Test the element separately.
Reconnect and, with the circuit energized, use a leakage clamp around L and N together at the geyser isolator; a residual current that rises with load confirms downstream leakage. Compare with the same test on other circuits to rule out a shared neutral. Test the RCCB with a proprietary tester for 30 mA trip time. Repair or replace the element or terminal block, dry and seal the enclosure, then re-test insulation and RCCB operation. Do not re-energize until N-E is clear.