Circuit Protection Faults · Kettering & Northamptonshire · NAPIT-Registered

Fuse Keeps Blowing or MCB Keeps Tripping?

Q: Why does my MCB keep tripping?

An MCB trips when the circuit draws more current than its rated capacity — either because the circuit is overloaded, or because a short circuit has occurred. A repeated trip that does not resolve with appliance removal usually means the fault is in the fixed wiring. This requires insulation resistance testing to locate.

Q: What is the difference between an MCB trip and an RCD trip?

An MCB trips on overcurrent — too much current caused by overload or short circuit. An RCD trips on earth fault current — current leaking to earth, indicating insulation failure or a neutral-earth contact. Knowing which device has tripped determines which type of fault to test for.

Q: Is it safe to keep resetting a tripping breaker?

One reset with all appliances unplugged is safe and useful. If the breaker trips again immediately with nothing connected, there is a wiring fault that must be located before re-energising. Repeated resetting accelerates damage at the fault point and creates a fire risk.

Q: Can a neutral-earth fault cause repeated RCD trips?

Yes. A neutral-to-earth fault causes the RCD to detect a current imbalance and trip, even with no faulty appliance present. It requires insulation resistance testing across all conductor pairs and circuit tracing to locate.

A breaker that trips is doing its job — but the reason it trips is the thing that needs finding. Resetting it doesn't fix the fault. We trace the root cause with proper test equipment and fix it right first time.

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Before the Diagnosis Starts

Which Device Tripped — and Why That Matters

The first question isn't "how do I reset it?" — it's "which device tripped, and what does that tell us?" In a UK consumer unit, three types of protective device can trip, each on a different principle. Knowing which one is the starting point for every proper diagnosis.

Overcurrent Protection

MCB — Miniature Circuit Breaker

Trips when the circuit draws more current than its rated capacity. The cause is either overload (too many appliances) or a short circuit (live-to-neutral contact). An MCB does not detect earth faults — only excess current.

Earth Fault Protection

RCD — Residual Current Device

Trips when it detects an imbalance between line and neutral current — meaning current is leaking to earth somewhere on the protected circuit group. On older boards, one RCD may protect several circuits simultaneously.

Combined Protection

RCBO — Combined MCB + RCD

A modern device combining both functions in one unit. If an RCBO trips, the fault is confined to that single circuit — which makes fault location considerably more straightforward than on an older split-RCD board.

This distinction shapes everything that follows. An MCB that trips with nothing connected indicates a short circuit on the fixed wiring. An RCD that only trips with one specific appliance plugged in points to that appliance or its circuit. Getting this right at the start avoids hours of unnecessary investigation.

What We Look For

Common Causes We Check For

Circuit overloadEach domestic circuit is designed for a maximum sustained load — typically 32A for ring final circuits, 16A for some radial socket circuits, 6A for lighting. Plugging in too many high-draw appliances pushes the circuit beyond the MCB's rating, and it trips. If the circuit holds when load is reduced, overload is the likely cause. If it trips at modest loads, the fault is elsewhere.

Short circuitA direct contact between live and neutral conductors — within a socket, fitting, appliance flex, or section of damaged cable — causes a near-instantaneous surge of current that trips the MCB. A short will cause the MCB to trip again the moment you reset it, with nothing else changed. There is often a discernible pop, burn mark, or heat damage at the fault point.

Faulty applianceA failing motor winding, compromised heating element, or deteriorated flex can cause either overcurrent (tripping the MCB) or earth leakage (tripping the RCBO). The appliance may appear functional between faults. Any device that reliably reproduces the trip when plugged in should be taken out of service until it has been properly checked.

Damaged cable insulationCable driven through by a nail, trapped under a joist, or nicked during building work can allow live conductors to contact each other or earth. This creates a persistent fault on the fixed wiring — the MCB trips every time you restore power, regardless of whether any appliances are connected. Location requires insulation resistance testing and systematic circuit sectioning.

Water ingressMoisture in a fitting, outdoor socket, bathroom accessory, or cable run can degrade insulation and create leakage to earth that trips the RCBO — sometimes only when conditions are damp. Garden circuits, bathroom zone fittings, and circuits on external walls or in areas below bathrooms are most commonly affected.

Neutral-to-earth faultWhere the neutral conductor makes unintended contact with the circuit protective conductor (CPC) at any point on a circuit — inside a socket, junction box, or fitting — the RCD detects the current imbalance and trips, even with no faulty appliance. This can be difficult to locate without insulation resistance testing across all conductor pair combinations: L-N, L-E and N-E.

An Important Point

Why Repeatedly Resetting Is Not a Solution

A circuit breaker tripping is protection in action. The fault that caused it to trip is still there after you reset it. Resetting re-energises the fault — and on a circuit with damaged cable insulation or a connection that is arcing, repeated re-energising accelerates the damage and can start a fire at the fault point before the protection operates again.

On properties that still have older rewirable fuse boards, there is a dangerous shortcut of re-wiring the fuse with heavier wire to stop it blowing. This removes protection from the circuit entirely. The circuit cable was rated to the fuse — uprating the fuse means a genuine fault can now escalate to a point where the cable overheats and ignites surrounding material before anything trips.

When to Stop Resetting and Call Us

The MCB or RCBO trips again immediately with nothing plugged in
Any burning smell at any point before or after the trip
The breaker trips during normal low-load use, not just under heavy load
The trips are becoming more frequent over days or weeks
Several circuits have tripped simultaneously on an older board
There was a bang, flash or spark before the trip occurred

Our Process

How Entigen Diagnoses a Repeatedly Tripping Breaker

Identify the Protective Device

We confirm whether it's an MCB, RCD or RCBO, and which circuits are affected. A modern all-RCBO board with a single circuit down is very different from an older split-load board with multiple circuits off together.

Appliance Elimination

De-energise the circuit, remove all appliances. Restore power. If the breaker holds, reconnect appliances one at a time under controlled conditions. The causative appliance will reproduce the trip. If it trips with nothing connected, the fault is in the fixed wiring.

Insulation Resistance Testing

With accessories disconnected, we apply 500V DC across L-N, L-E and N-E. Readings below 1MΩ confirm compromised insulation. This test identifies whether the fault is in the cable run, a specific accessory, or both.

Circuit Sectioning

For wiring faults without an obvious single location, we disconnect at intermediate access points and retest each section. This narrows the fault to a specific cable run without opening every access point on the circuit.

Repair, Retest and Certificate

Fault repaired, full circuit retested, breaker verified under load before re-energising. Minor works certificate issued on completion. No surprises on the invoice — cost is confirmed before we start.

We cover Kettering and the surrounding Northamptonshire area — Barton Seagrave, Burton Latimer, Corby, Wellingborough, Northampton, Earls Barton, Rushden, Desborough, Rothwell and the wider area.

Related Faults & Services

Common Questions

MCB / Fuse Tripping FAQs

Why does my MCB keep tripping?+

An MCB trips when the circuit draws more current than its rated capacity — either from overload or a short circuit. A repeated trip that doesn't resolve with appliance removal usually means the fault is in the fixed wiring. That requires insulation resistance testing to locate — resetting the breaker repeatedly will not clear it, and can accelerate damage.

What is the difference between an MCB trip and an RCD trip?+

An MCB trips on overcurrent — too much current due to overload or short circuit. An RCD trips on earth fault current — current is leaking to earth somewhere on the circuit. Knowing which type of device has tripped determines what type of fault to look for. An RCBO combines both functions and confines any trip to a single circuit, which makes diagnosis significantly more straightforward.

Is it safe to keep resetting a tripping breaker?+

One reset with all appliances unplugged is safe and informative. If the breaker holds with nothing connected and trips only when a specific appliance is plugged in, that device is the likely cause. If the breaker trips immediately with nothing connected, there is a wiring fault. Repeated resetting with the fault present re-energises the fault each time, which can accelerate cable damage and create a fire risk at the fault point.

Can a neutral-earth fault cause repeated RCD trips?+

Yes. Where a neutral conductor has made unintended contact with the circuit protective conductor somewhere on the circuit, the RCD detects a current imbalance and trips — even with no faulty appliance connected. This won't resolve on its own and isn't detected by a plug-in socket tester. It requires insulation resistance testing across all conductor pair combinations (L-N, L-E and N-E) and systematic circuit tracing to locate the contact point.

Fault Support

If your issue sounds similar, these pages may help you understand the fault before getting in touch.

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