Understanding MCB trip curves: Type B, C and D

Helpful video reference. We use eFIXX's UK tutorial "Type B MCBs and RCDs, Clearing Up the Confusion" as the video reference here. eFIXX covers the difference between trip curve types clearly and correctly, including why Type B is standard for domestic circuits and when a different curve is genuinely needed.

1. Open the consumer unit cover

Consumer units have an outer cover held by one or two screws or clips. With the main switch still in the on position, open or remove this cover carefully. You are only reading labels, not touching anything live. Most boards have a circuit schedule on the inside of the cover showing which MCB protects which circuit - useful if the MCBs themselves are not individually labelled.

2. Find the trip curve letter on each MCB

Look at the front face of each MCB. You will see a marking such as B32, C16 or B6. The letter is the trip curve. The number is the rated current in amps - the maximum continuous current the breaker will carry without tripping on overload.

Most domestic boards contain almost entirely Type B MCBs. If you see a C or D, that is intentional and worth understanding.

3. Understand what Type B means

A Type B MCB has two separate tripping mechanisms:

• Thermal trip (overload): a bimetallic strip bends slowly when the current stays above the rated value for a sustained period. This protects the cable from overheating.
• Magnetic trip (fault current): a solenoid trips the breaker almost instantly when current reaches 3 to 5 times the rated value. This clears short circuits before the cable can be damaged.

Type B is correct for lighting circuits, socket ring finals, cooker circuits, immersion heaters and most other resistive loads in a UK home. Under BS 7671 Table 41.3, the magnetic trip is fast enough to satisfy the automatic disconnection requirement on a typical domestic cable run.

4. Understand what Type C means

Type C trips magnetically at 5 to 10 times the rated current. That higher threshold exists because some loads draw a large inrush at switch-on - far higher than their running current - before settling down.

EV chargers with internal DC components, air source heat pumps, compressor-based air conditioning units and certain types of motor-driven equipment all fit this description. Fitting a Type B on these circuits sometimes causes the breaker to trip every time the load starts, even though nothing is actually wrong. A Type C (or an RCBO with a Type C characteristic) solves the nuisance tripping while still providing proper overcurrent protection.

5. Understand what Type D means

Type D trips magnetically at 10 to 20 times rated current. This is rarely needed in domestic properties. It suits welding equipment, large transformer loads and industrial motors. If you see a Type D in a domestic consumer unit, it may have been fitted by a previous owner for a specific piece of equipment, or it may simply be wrong. Either way, it is worth having an electrician check the circuit schedule and confirm the installation is correct.

6. Match the type to the load

The quick guide:

• Lighting, sockets, immersion heaters, cookers, showers: Type B is correct.
• EV chargers, heat pumps, compressors, some motors: Type C is usually correct. Check the charger or equipment manufacturer's data sheet for the recommendation.
• Transformers, welding sets, large motors: Type D in specialist cases only.

If the wrong type has been fitted - for example a Type C on an ordinary socket ring final where a B is specified - the protection still works, but the fault current clearance time is slower than designed. On a long cable run, that can mean the cable heats up more before the breaker clears the fault.

When to call us

If you have an EV charger or heat pump installed and your MCB keeps tripping on startup, it is almost certainly a trip curve mismatch. Richard can check the circuit, confirm the correct MCB type and replace it in a single visit. Fuse box inspections are also part of every EICR we carry out.