No Power to a Circuit: MCB Tripped, RCBO Fault or Broken Ring - Step-by-Step Diagnosis

Quick Answer: When a single circuit loses power, check first whether an MCB or RCBO has tripped at the consumer unit (90% of cases). A tripped MCB typically indicates an overload (too many appliances) or short circuit; a tripped RCBO indicates an earth leakage fault. Reset and observe — if it trips again immediately, there's a fault on the circuit. A broken ring final circuit (one leg of the ring open) leaves outlets working but with overloaded conductors and overheating risk — diagnosed by an inspection and testing visit using insulation resistance and continuity tests under BS 7671:2018+A2:2022. All circuit-side investigation must be by a competent person under Part P; resetting tripped switches at the consumer unit is the only homeowner-safe diagnostic.

Summary

Single-circuit power loss is one of the most-called electrical faults in UK domestic property because it's specific (one room or set of outlets stops working) but not usually an emergency (the rest of the house is fine). The diagnostic pattern is consistent: check the consumer unit first, identify which MCB or RCBO has tripped, attempt reset, observe whether it trips again, then trace the fault.

The trade pricing pattern: most circuit-loss calls are £80–£180 for diagnosis and reset where the cause is a faulty appliance, plus the cost of any specific repair. £180–£400 for a broken ring or loose terminal that needs trace-and-repair work. £400–£900 if multiple components need replacing or rewiring is involved.

The safety-critical case is the broken ring final circuit — a ring circuit where one leg has been broken (often by a DIY drilling into wall, or rodent damage) but the other leg still carries current. The outlets still work but the cable is now operating at twice its design current, and the conductors heat up under load. This is invisible without testing and is a fire hazard. EICR inspection picks it up; routine reset doesn't.

Key Facts

Quick Reference Table — Circuit Loss Diagnosis

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Symptom Most likely cause
MCB trips immediately on reset Short circuit on circuit
MCB trips after a few seconds Overload (too many appliances)
MCB trips intermittently Loose terminal, intermittent short, sparing fault
RCBO trips immediately on reset Earth fault on appliance or circuit
RCBO trips intermittently Damp accumulation, deteriorating insulation
MCB resets and stays on Reset successful — observe pattern
Outlets work but overheating Broken ring — testing required
One outlet dead, others on circuit work Local outlet fault (loose terminal, failed back box)

Detailed Guidance

Systematic Diagnosis Flow

NO POWER TO CIRCUIT

Step 1: Identify which circuit
├── List affected outlets/lights
├── Match to circuit chart on consumer unit door
└── Identify the MCB or RCBO protecting that circuit

Step 2: At the consumer unit
├── Is the relevant MCB/RCBO tripped (down/off)?
│   ├── YES → Continue to Step 3
│   └── NO  → Check connections inside CU (ELECTRICIAN ONLY)
│
Step 3: Reset attempt
├── Unplug all appliances on the circuit
├── Switch the MCB/RCBO back ON
│   ├── Stays ON → Plug appliances in one at a time to find faulty one
│   ├── Trips immediately → Short or earth fault on circuit
│   └── Trips after seconds → Overload or developing short
│
Step 4: If MCB trips immediately
├── Likely a short circuit
├── Disconnect circuit from CU at this point — DO NOT reset further
└── Electrician needed for testing
│
Step 5: If RCBO trips immediately
├── Likely an earth fault
├── Unplug everything and reset
│   ├── Holds with nothing plugged in → Fault is in an appliance
│   │   └── Plug back in one at a time to identify
│   └── Still trips with nothing plugged in → Fault on fixed wiring
│       └── Electrician needed for insulation resistance test
│
Step 6: If MCB stays on after reset
├── Identify which appliance caused trip
├── Inspect appliance flex and plug
├── Replace appliance, fuse in plug, or have appliance repaired
└── Monitor for re-trip

Common scenario — overload trip

Most-common cause: customer plugs an extra heater or appliance into a circuit already running near capacity. MCB trips after a few seconds.

Reset procedure:

  1. Identify and unplug the high-current appliance
  2. Reset MCB
  3. Run on lighter load
  4. Long-term: dedicated circuit for the high-current appliance, or accept the load profile

A 32A ring circuit (typical UK socket circuit) handles about 7,360W continuous (32A × 230V). A kettle at 3,000W + microwave at 1,200W + toaster at 1,000W = 5,200W = OK, but adding a 2,500W heater pushes it over. Trip is the safety system working correctly.

Common scenario — short circuit (MCB instant trip)

Live and neutral conductors making direct contact, or a faulty appliance with internal short. Symptoms: MCB trips the moment it's switched on.

Diagnostic:

  1. Unplug all appliances
  2. Try MCB reset
    • Stays on → fault in unplugged appliance; reintroduce one by one
    • Trips even with nothing plugged in → fault in fixed wiring (cable damage, faulty back box, water ingress)

If the fault is in fixed wiring, an electrician investigates by disconnection at the CU and live-side testing. Common fixed-wiring causes:

Repair cost: £140–£500 depending on access and extent.

Common scenario — earth fault (RCBO trip)

Earth leakage exceeds 30mA (RCD trip threshold). Causes:

Diagnostic:

  1. Unplug everything on the circuit
  2. Reset RCBO
    • Holds → fault is in an appliance (plug back in one at a time)
    • Doesn't hold → fault in fixed wiring
  3. For fixed wiring: insulation resistance test (Megger) at 500V — should read >1MΩ; failure indicates earth leakage point

Specialist scenario — broken ring final circuit

A ring final circuit has two legs running from the consumer unit, around the property, and back. If one leg is broken (cable cut, terminal disconnected, rodent damage), outlets still work because the remaining leg supplies them — but at twice the design current.

Symptoms:

Detection: continuity test of conductors at both ends of ring. Both legs should give similar readings. A broken leg shows infinite resistance on one side.

Repair: identify break point, restore connection. Cost £180–£500 depending on access (cable in floor void, behind plaster, etc.).

EICR inspection picks this up — homeowners with no symptoms but a broken ring won't know without periodic testing.

Specialist scenario — failed MCB or RCBO

Less common, but MCBs and RCBOs do fail:

Diagnosed by competent person; replace device (£15–£40 supply, £80–£180 fitted including isolation and testing).

A consumer unit older than ~25 years may have multiple devices nearing end of life — full unit replacement (£600–£1,200 typical) often more economic.

When to call out an electrician immediately

Resetting safely as a homeowner

The only homeowner-safe diagnostic is resetting an MCB or RCBO at the consumer unit:

If the device won't reset, or trips again immediately, stop and call an electrician. Repeated reset attempts on a fault circuit risk fire and personal injury.

Frequently Asked Questions

Why does my MCB keep tripping?

Three possible causes: overload (too many high-current appliances), short circuit (live touching neutral), or faulty MCB. Unplug everything, reset, then plug in one at a time to identify a faulty appliance. If it trips with nothing plugged in, you need an electrician.

What's the difference between an MCB and an RCBO?

MCB protects against over-current (overload, short circuit). RCBO does both that and earth leakage protection. RCBOs are now the standard in modern consumer units, replacing the older MCB+RCD split-load arrangement.

Can I replace a tripping MCB myself?

No. Work inside the consumer unit is notifiable under Part P (England and Wales) and must be done by a competent person who can self-certify. DIY replacement risks injury and invalidates insurance.

Why do RCBO trips happen at random times?

Often cumulative leakage from multiple appliances, or a marginal fault that develops under specific conditions (temperature, humidity). Pattern-tracking helps: note when trips occur (time, weather, what was running). Sometimes a single appliance is the cause but only at end of cycle.

How much does an electrician charge to diagnose a tripping circuit?

£80–£180 typical for a diagnostic visit. Repair cost extra: £40–£100 for accessory replacement, £180–£500 for fixed-wiring repair, £600–£1,200 for consumer unit replacement.

Regulations & Standards