Circuit Breaker Keeps Tripping: MCB vs RCD, Fault Finding & Megger Testing

Summary

A tripping circuit breaker is one of the most common electrical faults in domestic and light commercial properties. The distinction between an MCB trip and an RCD trip is fundamental — they protect against different fault conditions, and the diagnostic approach differs accordingly. Many homeowners and even some tradespeople confuse the two, leading to either ineffective fault finding or, worse, resetting the device without resolving the underlying fault.

Modern UK consumer units (since Amendment 2 to BS 7671, effective 2019) typically use either split-load boards with a separate RCD protecting groups of circuits, or individual RCBOs (combining MCB and RCD in one device) on each circuit. The type of protection fitted determines how the consumer unit responds to different fault types. An RCBO will trip a single circuit on an earth fault; a split-load RCD will trip all circuits on its side of the board when any one of them has an earth fault.

Megger testing (insulation resistance testing) is the definitive tool for diagnosing wiring insulation faults. A 500V DC test applied between conductors (L-N, L-E, N-E) reveals degraded insulation before it causes a visible fault. This is essential after flood damage, in old properties with rubber-insulated wiring, and after any suspected cable damage. The test must only be conducted on an isolated circuit with all sensitive equipment (electronic controls, LED drivers, smart switches) disconnected or bypassed.

Key Facts

• MCB trip current — Type B MCBs trip instantaneously at 3–5× rated current; Type C at 5–10×; Type D at 10–20×; sustained overcurrent trips the thermal element within seconds to minutes
• RCD sensitivity — 30mA RCDs are standard for socket outlet and lighting circuits; 300mA for fire protection at the main incomer; 10mA for some special locations
• RCD trip time — a 30mA RCD must disconnect within 300ms at 30mA, and within 40ms at 150mA (5×IΔn)
• RCBO — an RCBO provides both overcurrent (MCB) and earth fault (RCD) protection in one device; trips the individual circuit only, not all circuits on a shared RCD
• Split-load consumer unit — has two RCDs, each protecting half the circuits; an earth fault on one circuit trips all circuits on that RCD
• Nuisance tripping — RCDs can trip on cumulative leakage (many appliances each with small leakage), or on transient voltage spikes; a 30mA RCD with 10A of connected load may trip if total leakage exceeds 30mA
• Insulation resistance test voltage — 500V DC for circuits up to 500V; 250V DC for SELV and PELV circuits; per BS 7671 Table 61
• Insulation resistance pass value — minimum 1MΩ between all conductors; values above 200MΩ are excellent; values below 2MΩ warrant investigation
• Megger before LED equipment — always disconnect LED drivers, dimmers, and electronic controls before IR testing — 500V DC will destroy most electronics
• Short circuit test — a dead short (L-N, or L-E) produces maximum prospective fault current; measured at consumer unit by PSCC test; not required for routine fault finding
• Overloaded circuit — a 32A ring final circuit rated cable (2.5mm²) connected to a 13A MCB will not trip on modest overload; the MCB rating must match the design current of the circuit
• Type B vs Type C — Type B is standard for domestic circuits; Type C is used where high motor start currents are present (e.g., compressors, refrigeration); Type D for industrial motors

Quick Reference Table

Detailed Guidance

Decision Tree

BREAKER KEEPS TRIPPING — START HERE
             |
             v
WHICH DEVICE has tripped?
             |
      -------+-------
      |               |
      v               v
   MCB only        RCD or RCBO
   (no RCD)        tripped
      |               |
      v               v
Is it a       Go to RCD section
short circuit
or overload?
      |
      v
RESET MCB — does it trip immediately?
      |
  YES | NO
      |  \
      v   v
Short    Thermal overload
circuit  -- check total load
      |  connected to circuit
      v
Disconnect all loads
      |
Still trips immediately?
      |
  YES | NO
      v   \
Cable fault  Reconnect loads one
(L-N or L-E  by one until trip
short)       -- faulty appliance
      |
IR test: 500V Megger
L-N, L-E, N-E
      |
<1MΩ: cable fault    >1MΩ: transient
(replace or repair)   -- check connections

============================
RCD TRIPS — HALF-SPLIT METHOD
============================
             |
             v
Remove all plugs from sockets on circuit
Turn off all switches (lights etc.)
             |
             v
Reset RCD -- does it hold?
             |
         YES | NO
             |  \
             v   v
      Appliance  Wiring fault
      fault       (see below)
             |
      Plug in appliances one at a time
      until RCD trips -- faulty appliance
             |
             v
      Is it tripping at random / no load?
             |
         YES | NO
             v   \
      Nuisance    Fixed fault
      trip --
      check
      total
      leakage

MCB Tripping — Overcurrent Diagnosis

When an MCB trips, the first question is whether the total load on the circuit exceeds the MCB rating. A 32A MCB on a ring final circuit can serve up to 32A of simultaneous load. In practice, a ring final circuit serves a room or floor and may have 10+ sockets; the connected load at any moment depends on what is plugged in.

Calculating connected load: Add up the wattage of all appliances connected to the circuit. Divide by 230V to get amps. If the total exceeds the MCB rating, you have an overload. Solution: redistribute loads across circuits, or investigate whether a new circuit is needed.

If the MCB trips immediately on reset (before any load is connected), the fault is a short circuit — either line-to-neutral or line-to-earth. To locate it:

1. Isolate the MCB and prove dead
2. Disconnect all appliances
3. Perform a continuity test from line to neutral at the consumer unit output terminals — a reading close to 0Ω confirms a short
4. Use the ring circuit test methodology (if applicable) to identify which section of the ring contains the fault
5. Isolate sections by disconnecting at junction boxes until the short disappears

RCD Tripping — Half-Split Method

The half-split method is the fastest way to isolate an earth fault on a circuit with multiple loads:

1. De-energise the circuit (turn off all switches, unplug all appliances)
2. Reset the RCD
3. If it holds, the fault is in the equipment (plug in appliances one at a time to find the culprit)
4. If it still trips with no loads connected, the fault is in the fixed wiring

For fixed wiring faults:

1. Disconnect the circuit conductors from the RCD/MCB at the consumer unit
2. Perform an insulation resistance test (500V Megger) between L-E and N-E
3. A reading below 1MΩ indicates degraded insulation
4. Split the circuit at intermediate junction points to locate the faulty section

Megger (Insulation Resistance) Testing — Procedure

IR testing is a routine part of EICR (Electrical Installation Condition Report) testing and is used diagnostically when a wiring fault is suspected.

Before IR testing:

• Isolate the circuit completely (off at MCB and consumer unit)
• Disconnect ALL sensitive electronics: LED drivers, dimmer modules, electronic timers, RCD units, surge protection devices, any equipment with electronic controls
• Short out line and neutral at the furthest point of the circuit (to test the protective conductor separately from the live conductors)

Test sequence (BS 7671 Table 61):

Interpreting results:

• 200MΩ: excellent — no insulation concern

• 2–200MΩ: satisfactory — normal for older wiring
• 1–2MΩ: borderline — investigate further, especially if value is falling over time
• <1MΩ: fail — degraded insulation, find and repair or replace cable

Common causes of low IR values:

• Old rubber-insulated cables (pre-1960s) where the rubber has become brittle and carbonised
• Cables damaged by nails, screws, or mechanical impact
• Water ingress in external fittings, roof spaces, or flood-damaged properties
• Cables touching metal conduit without proper insulation
• Degraded flex to older appliances

Nuisance RCD Tripping

A 30mA RCD will trip if the sum of all leakage currents to earth on the circuit exceeds 30mA. Modern appliances each have small leakage currents (Class I equipment to earth, Class II equipment via Y-capacitors). On a circuit with many appliances — particularly a ring final serving an entire floor — cumulative leakage can exceed the threshold.

Typical appliance leakage currents:

• Washing machine: 0.5–3mA
• Dishwasher: 0.5–2mA
• Desktop PC: 1–3mA
• Television: 0.5–1mA
• Electric shower: 0.5–1mA (via RCD bond to earth)

If 10 such appliances are connected simultaneously, cumulative leakage may be 10–20mA — well within the 30mA threshold normally. However, an old appliance with degraded insulation may contribute 10–15mA itself, pushing the total over 30mA.

Solution: Use a clamp-on leakage current tester to measure the leakage current of each appliance individually. Replace or repair any appliance with >3.5mA leakage.

Frequently Asked Questions

Can I just tape the MCB in the on position?

Never. An MCB is a safety device. Forcing it to stay on when it is tripping means the circuit will not be protected from overcurrent or short circuit. This is both illegal (violation of EAW Regulations) and dangerous — an unprotected short circuit can cause a fire.

My RCD trips randomly in the middle of the night — what is this?

Random nighttime RCD tripping with no obvious load change is often caused by the refrigerator compressor cycling and the motor's switching transients triggering the RCD. It can also be caused by external voltage transients on the network. Check whether the tripping correlates with any appliances cycling. Consider fitting a 10ms time-delayed RCD (S-type) at the incomer if nuisance tripping persists — this is more resistant to transient triggers while still providing fault protection.

Do I need to hire an electrician to reset my consumer unit?

Resetting an MCB or RCD is not electrical work and anyone can do it. However, if the device trips repeatedly, an underlying fault needs diagnosis. Electrical fault finding and testing does require competence. If you're not able to perform the diagnostic steps above safely, hire a registered electrician (NICEIC, NAPIT, or ELECSA registered).

What is the difference between a 30mA and 100mA RCD?

A 30mA RCD provides personal shock protection — 30mA can cause cardiac arrest in sensitive individuals. A 100mA or 300mA RCD provides fire protection only — it will disconnect on a large earth fault that could start a fire, but may not disconnect fast enough to prevent electrocution. All socket outlet circuits in domestic premises must be protected by 30mA RCD per BS 7671 Regulation 411.3.3.

My MCB trips when I turn on a specific appliance — what's wrong?

Either the appliance is drawing more current than the MCB rating allows (overload), it has an internal short circuit, or the appliance has high inrush current that trips a Type B MCB. Check the appliance current rating label. If it's within the MCB rating, test the appliance on another circuit. If it trips other circuits too, the appliance is faulty. If it only trips this circuit, the circuit may be overloaded by other appliances or the MCB may be rated too low.

Regulations & Standards

• BS 7671:2018+A2:2022 (18th Edition IET Wiring Regulations) — Chapter 61 covers initial verification including insulation resistance testing

• BS 7671 Regulation 411.3.3 — 30mA RCD protection required for socket outlet circuits in domestic premises

• BS 7671 Table 61 — insulation resistance test voltages and minimum acceptable values

• Electricity at Work Regulations 1989 — duty to maintain electrical systems to prevent danger

• Building Regulations Part P — fault finding and remedial work in dwellings

• IET Wiring Regulations (BS 7671) — Chapter 61 verification procedures

• Megger: Guide to Insulation Resistance Testing — application notes on IR testing methodology

• NICEIC: Consumer Unit Guidance — split-load vs RCBO consumer unit advice

• Electrical Safety First — consumer guidance on tripping circuit breakers

• rcd tripping — detailed RCD tripping diagnosis including half-split methodology

• flickering lights — loose connections and neutral issues

• consumer units — consumer unit types, RCBO vs split-load, SPD requirements

• cable sizing — correct cable ratings to prevent overcurrent trips