A domestic 30 mA RCCB that trips after rain, and a tempting shortcut
A two-storey house wired in 2011 has a 30 A, 63 A two-pole RCCB as the main incoming device, with six MCB final circuits downstream and no earth-leakage protection on individual circuits. The owner reports that the RCCB trips two or three times a week, mostly at night, and reliably after rain. Nothing smells hot, no MCB trips, and resetting restores supply. Your insulation resistance test at 500 V with all loads disconnected gives: lighting circuit 60 MΩ, kitchen sockets 55 MΩ, bedroom sockets 48 MΩ, geyser 0.4 MΩ, outhouse and verandah sockets 0.3 MΩ, garden pump 12 MΩ. A three-pole earth tester reads 11 Ω on the earth electrode, against the utility's requirement of 5 Ω or less for this service. The owner's suggestion is to change the 30 mA RCCB for a 100 mA unit 'so it stops tripping', and he says a neighbour will do the swap cheaply this weekend.
1. Interpret the insulation resistance results and explain why only two circuits are under suspicion even though the whole house loses supply.
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Read the numbers against a benchmark. Good modern PVC wiring should read well above 1 MΩ at 500 V, and new work usually reads in the tens of megohms. Lighting at 60 MΩ, kitchen at 55 MΩ, bedroom sockets at 48 MΩ and the pump at 12 MΩ are all healthy and are not the cause.
The two failing circuits. The geyser at 0.4 MΩ and the outhouse/verandah sockets at 0.3 MΩ are an order of magnitude below any acceptable figure. These are exactly the circuits exposed to moisture - a geyser element and its terminal box, and outdoor sockets - which fits the owner's report that trips follow rain.
Why the whole house goes dark. All six final circuits sit behind one 30 mA RCCB, so a single leaky circuit trips the incomer and kills everything. The fault is local, the symptom is global. This is not a fault in the RCCB; the device is doing precisely its job.
Cumulative leakage. Even where no single circuit fails, the total standing leakage of many electronic loads must stay well below 15 mA, roughly half of the 30 mA rating. If a clamp leakage meter shows more, the circuits must be split, not the device uprated.
Before condemning anything, repeat the test with a 250 V setting on any circuit containing surge-protective devices, and confirm the 0.3 MΩ reading is not just a damp socket that dries out. Retest dry to distinguish permanent damage from seasonal moisture.
2. The owner wants a 100 mA RCCB fitted instead. Explain, in terms he can act on, why this must be refused.
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The 30 mA is not a nuisance setting; it is a physiological limit. Currents of roughly 30 mA and above, flowing through a person for a fraction of a second, can cause sustained muscle contraction and can push the heart into ventricular fibrillation. The 30 mA RCCB exists to disconnect before that happens, within about 30 ms at rated residual current. Its job is to protect the person holding the appliance, not the wiring.
What 100 mA would mean. Raising the threshold to 100 mA leaves the household unprotected against the leakage that actually kills - a faulty geyser element, a wet outdoor socket, a damaged extension lead. It converts a device that protects life into a device that mainly protects property. The house would stop tripping, and the reason it stopped tripping would be that the danger is now allowed to persist.
The 'cheap weekend job' risk. The work must be done by a licensed person and, if the incoming device and earthing are altered, the installation should be re-tested and the certificate updated. An unlicensed cash swap removes both your liability cover and the owner's insurance position, and no test record would exist after a fire.
Give a better answer. Fit 30 mA RCBOs on the geyser, outhouse and pump circuits, keep the 30 mA incomer, repair the two faulty circuits, and bring the earth electrode from 11 Ω down to 5 Ω or less. The tripping stops because the leaks are gone, not because the alarm was silenced.
3. Set out a logical sequence for the repair and the re-testing, and say what the owner should be told to do between now and then.
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Step 1 - make safe. Isolate the geyser and the outhouse socket circuit at their MCBs and label them 'DO NOT USE'. The rest of the house can stay live because those circuits tested healthy.
Step 2 - find the leak. Open the geyser terminal cover and examine the element and its gasket for moisture tracking; megger the element alone, then the wiring with the element disconnected, so you know which of the two has failed. For the outhouse, inspect glands, back boxes and socket outlets for water entry, corroded contacts and damaged cable sheath.
Step 3 - repair, then prove. Replace the failed element or the water-damaged accessories, dry the boxes, re-make glands with suitable IP-rated entries, and re-test at 500 V. Do not re-energise until the circuit reads at least 1 MΩ, and in practice aim far higher.
Step 4 - fix the earthing. An 11 Ω electrode against a 5 Ω limit means the earth path cannot be relied on to clear a fault. Add a second electrode spaced at least its own driven length away and bond it, or use a chemical/maintenance-free electrode, then re-measure with the three-pole method and record the value.
Step 5 - verify the device and its discrimination. Press the RCCB test button (that only proves the mechanism), then use an RCCB tester to confirm trip time at half, one and five times rated residual current. Replace any device that fails.
Step 6 - advise the owner. Tell him to keep the two circuits off, to have the installation re-tested annually, and to test the RCCB button monthly. Give him the written test results so the repair is documented.