Consumer Unit Tripping on Startup: How Do You Find the Fault?

Summary

"It trips as soon as I switch it back on" is a different fault from "it trips at random". A random trip points to an intermittent fault or a leakage that builds over time; a startup trip means something is wrong at the moment current first flows. Startup trips cluster after power cuts, after a new appliance is plugged in, after a new consumer unit is fitted, and on cold damp mornings. Each of those clues narrows the cause before you lift a cover.

The first job is to identify which device is tripping. A main switch on a split-load or dual-RCD board is just a switch and does not trip; what customers call "the main switch" is often an RCD incomer (100mA or 30mA, common on TT installations), an RCBO, or an AFDD. Each device trips for a different reason: an MCB trips on overcurrent, an RCD on earth leakage, an RCBO on either, an AFDD on arcing signatures as well as both. Knowing which device went tells you which measurement to take.

This article gives a decision tree, the startup-specific causes that general tripping guides miss, and the testing sequence. For the broader half-split method on RCDs and MCBs see rcd tripping and tripping breaker.

Key Facts

• MCB instantaneous trip bands (BS EN 60898-1) — Type B: 3 to 5 × In; Type C: 5 to 10 × In; Type D: 10 to 20 × In. A 16A Type B trips magnetically somewhere between 48A and 80A, which a motor or large transformer can exceed on starting.
• Inrush examples — induction motor direct-on-line: 6 to 8 × full-load current; toroidal transformer: up to 20 × or more for one half-cycle; LED driver or switch-mode power supply bank: capacitor charging surge of tens of amps for microseconds; electric vehicle charger: usually soft-start, rarely the culprit.
• RCD trip thresholds (BS EN 61008 / 61009) — a 30mA RCD must not trip below 15mA (0.5 IΔn) and must trip at 30mA within 300ms, at 150mA (5 IΔn) within 40ms. Most trip between 18 and 25mA in practice.
• Leakage design limit — BS 7671 regulation 531.3.2: the sum of protective-conductor currents on a circuit under normal operation should not exceed 30% of IΔn, so 9mA on a 30mA RCD.
• Appliance leakage — IT and audio equipment to BS EN 62368-1 may leak up to 3.5mA each through filter capacitors; a home office with ten devices can approach the design limit on its own at the moment they all power up.
• Neutral-to-earth faults — do not trip an RCD until load current flows, because the diverted neutral current is what creates the imbalance. This is the classic "fine with everything off, trips when anything is switched on" signature.
• Borrowed neutrals — a lighting circuit using the neutral from another circuit across two different RCDs trips one or both as soon as the light is switched on. Common after old two-way wiring is split across a new dual-RCD board.
• Insulation resistance — BS 7671 Table 61: minimum 1.0MΩ at 500V DC for circuits up to 500V. Below 2MΩ warrants investigation even where it passes.
• Safe isolation — HSE GS38 for test equipment; prove dead with an approved voltage indicator and proving unit. See safe isolation procedure.
• Supply reconnection surges — after a DNO outage, every fridge compressor, boiler pump, immersion contactor and storage heater in the house starts in the same second. A board that holds in normal use can trip on reconnection without any fault present.
• Condensation — an outside socket, shed supply or junction box that has lost its heating effect during a power cut can hold enough moisture to trip the RCD on re-energisation and then dry out and pass.
• Type AC vs Type A RCDs — a Type AC RCD can be blinded by DC components from inverters and some appliances, and may trip or fail to trip erratically at startup. Since Amendment 2 (2022), carried through to BS 7671:2018+A4:2026, Type A is the minimum for most domestic circuits. See rcd types explained.
• Faulty device — RCD mechanisms do fail, especially cheap imports and devices that have been tripped hundreds of times. Test with an RCD tester at 0.5 IΔn, 1 IΔn and 5 IΔn before condemning the installation.

Quick Reference Table

Detailed Guidance

Decision tree: startup trips

START: Device trips immediately on energising or on reset
│
├─ Which device trips?  (MCB / RCD / RCBO / AFDD / RCD incomer)
│
├─ MCB trips ───────────────────────────────────────────────────────┐
│    ├─ Trips with ALL loads unplugged and switched off?           │
│    │     YES → Dead short L–N or L–E on fixed wiring.            │
│    │           Isolate, IR test L–N, L–E, N–E at 500V.           │
│    │           <1MΩ → locate by half-split at JBs/accessories.   │
│    │           ≥1MΩ → swap MCB (faulty device).                  │
│    │     NO ↓                                                     │
│    └─ Trips when one appliance starts?                           │
│          YES → Inrush. Clamp the start current.                  │
│                > 5×In → correct circuit design: Type C MCB       │
│                (check Zs still meets Table 41.3), soft-start,     │
│                or split loads.                                    │
│          NO  → Cumulative start on reconnection: stagger loads,  │
│                check circuit loading, consider dedicated circuits │
│
├─ RCD / RCBO / RCD incomer trips ───────────────────────────────────┐
│    ├─ Trips with all MCBs on that RCD switched OFF?               │
│    │     YES → Faulty RCD, or fault between RCD and MCBs          │
│    │           (crossed neutral, damaged busbar). Test RCD;       │
│    │           inspect board wiring.                              │
│    │     NO ↓                                                      │
│    ├─ Trips with MCBs on but NO loads connected?                   │
│    │     YES → L–E fault on fixed wiring. IR test each circuit    │
│    │           L–E at 500V with neutrals disconnected.            │
│    │     NO ↓                                                      │
│    ├─ Holds with no load, trips as soon as ANY load is applied?    │
│    │     YES → N–E fault. IR test N–E on each circuit with the    │
│    │           neutral lifted from the bar. Also check for a      │
│    │           borrowed neutral between RCD groups.               │
│    │     NO ↓                                                      │
│    └─ Trips only when several loads start together?               │
│          YES → Cumulative leakage. Earth leakage clamp on each    │
│                circuit's L+N together at power-up. Sum > 9mA →    │
│                re-distribute circuits, fit RCBOs, replace leaky   │
│                appliances. Check for Type AC RCD on inverter/     │
│                EV/PV circuits.                                     │
│
├─ AFDD trips ───────────────────────────────────────────────────────┐
│    └─ Isolate, inspect every termination on the circuit for a     │
│       loose connection FIRST. Then test with suspect appliance    │
│       removed. See [afdd nuisance tripping](/knowledge/fault-finder/electrical/afdd-nuisance-tripping)
│
└─ After the fix: full RCD test (x0.5, x1, x5), IR, Zs, record on
   Minor Works or EIC; replace any device that has been tripped
   repeatedly under fault.

Step 1: Interview before you test

Three questions save an hour:

1. When did it start? After a power cut, a storm, a new appliance, new lighting, a new board, or heavy rain. Each points somewhere different.
2. What exactly do you reset, and what happens? "The big switch on the left" on a dual-RCD board is an RCD, not the main switch. Ask them to photograph the board with the tripped device.
3. Does it hold if you switch everything off first, then reset, then switch things back on one at a time? If yes, the fault is a load or cumulative leakage, not the fixed wiring, and the customer may already have found the circuit for you.

Step 2: Identify the device and isolate

Photograph the board. Note the make and whether the RCDs are Type AC, A or F, and their rated residual current. Isolate at the main switch, lock off, prove dead to GS38 on all line and neutral terminals. If the board is a TT installation with a 100mA time-delayed incomer and 30mA RCDs downstream, note which tripped: an incomer tripping with the 30mA devices holding suggests leakage spread across several circuits or a fault upstream of the 30mA devices.

Step 3: Dead tests by half-split

With the supply isolated:

• Insulation resistance at 500V DC between L–N, L–E and N–E on each circuit on the tripping device, with lamps and electronic loads disconnected and neutrals lifted from the neutral bar so each circuit is tested alone. Record results; anything under 1MΩ fails, anything under 2MΩ is suspicious.
• N–E test is the one people skip. It is the only test that finds the fault that holds with no load and trips with load. Lift the neutral, test N–E at 500V. A reading in kilohms rather than megohms is your fault: typically a nicked neutral in a back box, a neutral trapped under an earth terminal in a downlight, or water in an outside fitting.
• Continuity between circuits. With neutrals lifted, a reading between one circuit's neutral and another's indicates a borrowed neutral. Two-way lighting across landings and kitchens split onto separate RCDs is the usual place.
• Board wiring. On a new board that trips from day one, check every neutral is on the correct RCD's neutral bar. One neutral on the wrong bar trips the RCD the moment that circuit carries current.

Halve the circuit at a convenient point (a junction box, an accessory mid-run, the first downlight) and test each half until the fault is in one leg. See test and inspection procedures.

Step 4: Live tests for inrush and leakage

Where dead tests pass, the fault is dynamic and must be measured live, with the customer's agreement and appropriate precautions:

• Earth leakage clamp. Clamp the line and neutral of each circuit together at the board (not the earth) and energise. The reading is that circuit's leakage. Repeat with each appliance switched on. Sum the circuits on each RCD. A total above 9mA on a 30mA RCD, or a single circuit above 5mA, is the problem. Dehumidifiers, old fridge-freezers with degraded heater insulation, immersion elements, outdoor lighting with water ingress and PV inverters are frequent offenders.
• Inrush clamp. Use a clamp with an inrush or peak-hold function on the circuit feeding the suspect appliance and energise it. Compare the peak with the MCB's magnetic band. A 2.5kW workshop compressor on a 16A Type B can easily show 90A for a few cycles.
• RCD test. Test the RCD at 0.5 IΔn (must not trip), 1 IΔn (must trip within 300ms) and 5 IΔn (within 40ms), on both half cycles. A device tripping at 0.5 IΔn or well under 18mA at 1 IΔn is over-sensitive and should be replaced.

Step 5: Fixes by cause

Step 6: Certification and the device itself

Fault repair on an existing circuit is not notifiable under Part P in England, but a replacement consumer unit or a new circuit is. Record the repair on a Minor Electrical Installation Works Certificate with the test results. Where an RCD or MCB has cleared a dead short, inspect it: BS EN 60898 devices are rated for a limited number of short-circuit operations and a device that has cleared several faults should be replaced. See part p notifiable work.

Frequently Asked Questions

Why does it hold when everything is off but trip as soon as I switch a light on?

That is the signature of a neutral-to-earth fault. With no load there is no current in the neutral, so nothing diverts to earth and the RCD sees no imbalance. The moment current flows, some of it returns via the fault to earth instead of through the RCD, and it trips. Test N–E at 500V with the neutral disconnected from the bar.

Can I change the Type B MCB for a Type C to stop a motor tripping it?

Only if the earth fault loop impedance still meets the lower maximum Zs for a Type C device in BS 7671 Table 41.3, so that the device still disconnects within 0.4s on an earth fault. Measure Zs, compare, and record it. If Zs is too high, use a soft-start or a dedicated circuit instead.

The consumer unit was fitted last week and has tripped every morning since. Is it faulty?

More likely a wiring error during the changeover: a neutral on the wrong RCD's neutral bar, or a pre-existing borrowed neutral that the old non-RCD board tolerated. Faulty new devices are rare. Ask the installer to return, test and correct it under their guarantee.

Does an RCBO board stop startup trips?

It stops one circuit's fault or leakage tripping other circuits, and it makes diagnosis far easier because the tripped device identifies the circuit. It does not cure a fault on that circuit. It is the right recommendation where cumulative leakage across many circuits is the cause.

Is it safe to keep resetting it until it holds?

No. Each reset under a genuine fault passes fault current and stresses the device, and if the cause is a damaged cable the repeated energising can start a fire. Isolate and test.

Regulations & Standards

• BS 7671:2018+A4:2026 — Requirements for Electrical Installations (IET Wiring Regulations, Amendment 4 published 15 April 2026; the A2:2022/A3:2024 edition is withdrawn from 15 October 2026), including regulation 531.3.2 (leakage limited to 30% of IΔn), regulation 533.3 (selection of overcurrent devices for inrush), Table 41.3 (maximum Zs for circuit breakers) and Table 61 (minimum insulation resistance)

• BS EN 60898-1 — circuit breakers for overcurrent protection for household installations; Type B, C and D tripping characteristics

• BS EN 61008-1 — residual current circuit breakers without integral overcurrent protection (RCCBs)

• BS EN 61009-1 — residual current circuit breakers with integral overcurrent protection (RCBOs)

• BS EN 62606 — arc fault detection devices (AFDDs)

• BS EN 62368-1 — audio/video and IT equipment safety, including touch current limits that set appliance leakage

• HSE GS38 — electrical test equipment for use by electricians

• Electricity at Work Regulations 1989 — duties when working on or near live conductors

• Building Regulations Approved Document P (England) — notifiable electrical work

• IET Guidance Note 3 — inspection and testing

• BS 7671:2018+A2:2022 (IET Wiring Regulations) — Institution of Engineering and Technology

• HSE GS38: Electrical test equipment for use on low voltage electrical systems — Health and Safety Executive

• Electricity at Work Regulations 1989 — legislation.gov.uk

• Approved Document P: Electrical safety, dwellings — GOV.UK

• IET Guidance Note 3: Inspection and Testing — IET

• rcd tripping — the general half-split method for persistent RCD trips

• tripping breaker — MCB overload and short-circuit diagnosis

• rcbo tripping — separating the RCD and MCB elements of an RCBO trip

• afdd nuisance tripping — arc-fault devices and appliance signatures

• rcd types explained — Type AC, A, F and B and where each is required

• safe isolation procedure — proving dead before dead tests

• maximum zs values and testing — checking Zs when changing MCB type