MCB Types B, C and D: Trip Characteristics Explained

A Miniature Circuit Breaker (MCB) is an automatically operated electrical switch designed to protect a circuit against damage caused by overcurrent — both overloads and short circuits. MCBs are manufactured to BS EN 60898 and are the standard protective device in UK consumer units and distribution boards, having replaced rewireable fuses (BS 3036) and cartridge fuses (BS 1361) in all new installations.

Unlike a fuse, which melts and must be replaced after operating, an MCB can be reset by flipping the toggle back to the ON position. This makes MCBs far more practical — a tripped MCB can be restored in seconds without needing replacement components. However, an MCB that trips repeatedly indicates a fault that must be investigated, not simply reset.

MCBs come in three main types — B, C, and D — defined by their magnetic trip characteristics. The type determines how much instantaneous fault current is needed to trip the MCB within its fastest operating time. Choosing the correct type for the load is essential: too sensitive and the MCB will nuisance-trip on normal inrush currents; too insensitive and the MCB may not disconnect fast enough during a fault, compromising the earth fault loop impedance compliance of the circuit.

Every MCB contains two independent protection mechanisms that work together to protect the circuit against different types of overcurrent.

The thermal and magnetic mechanisms are completely independent. A sustained 50A overload on a 32A MCB will trip the thermal element without involving the magnetic element. A 500A short circuit fault will trip the magnetic element in milliseconds without waiting for the thermal element to respond.

Type B is the standard MCB type for domestic and light commercial installations. The magnetic trip mechanism operates between 3 and 5 times the rated current. For a B32 MCB, this means the magnetic trip activates between 96A and 160A. At currents below 96A, only the thermal mechanism provides protection (slow trip). At currents above 160A, the magnetic mechanism guarantees instantaneous disconnection.

Type B should be the default choice unless there is a specific technical reason to use Type C or D. Using Type B wherever possible maximises the maximum permitted Zs for each circuit, giving the best chance of compliance during testing.

Type C MCBs have a higher magnetic trip threshold — between 5 and 10 times the rated current. For a C32 MCB, the magnetic trip activates between 160A and 320A. This higher threshold allows the MCB to withstand the larger inrush currents produced by inductive loads without nuisance tripping.

The trade-off with Type C is a lower maximum Zs. A C32 has a maximum Zs of 0.68 ohms compared to 1.37 ohms for a B32 — half the value. On installations with higher Ze values (TN-S systems, long supply cables), achieving a Zs below 0.54 ohms (corrected) on a Type C circuit can be challenging. Always verify that the loop impedance will be within limits before specifying Type C.

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Type D MCBs have the highest magnetic trip threshold — between 10 and 20 times the rated current. For a D32 MCB, the magnetic trip activates between 320A and 640A. This extreme threshold is designed for loads with very high inrush currents that would trip even a Type C device.

Type D MCBs are rarely appropriate for domestic installations. The maximum Zs for a D32 at 0.4-second disconnection is just 0.34 ohms — achieving this requires an extremely low Ze and very short circuit runs with large conductors. In most domestic installations, particularly those with TN-S earthing, this Zs is impossible to achieve. If you find a Type D MCB in a domestic consumer unit during an EICR, it is almost certainly the wrong type and should be noted as an observation.

The selection process is straightforward: identify the inrush current characteristic of the load and choose the MCB type that can tolerate it without nuisance tripping, while maintaining the lowest possible magnetic trip threshold to maximise the available Zs margin.

A common rule of thumb: if you are not sure whether the load needs Type C, try Type B first. If the MCB trips on inrush when the load is switched on, then consider upgrading to Type C. But never jump to Type C (or Type D) "just in case" — the lower Zs margin makes compliance harder and reduces the safety of the installation.

The MCB type has a direct and significant impact on the maximum permitted earth fault loop impedance. This is one of the most important practical considerations when selecting MCBs for an installation.

The Max Zs (x 0.8) column is the on-site acceptance limit per BS 7671 Appendix 3 and GN3. Conductors are measured cold at ambient temperature during testing, but under fault conditions they heat up and their resistance rises. Multiplying the tabulated Zs by 0.8 gives a conservative site limit that accounts for this difference — your measured Zs must be below the corrected value to satisfy Regulation 411.4.4. This is the figure you compare against your loop tester reading.

The difference is stark. A B32 gives you 1.10 ohms of headroom (corrected). A C32 cuts that in half to 0.54 ohms. A D32 reduces it further to just 0.27 ohms. On a TN-S installation where Ze might be 0.60 ohms, a B32 circuit with R1+R2 of 0.50 ohms would give a Zs of 1.10 ohms — comfortably within the B32 limit. But the same circuit with a C32 would fail (1.10 ohms exceeds the 0.54 ohm limit). With a D32, it would fail dramatically.

This is why MCB type selection is not just an academic exercise — it directly determines whether circuits will pass or fail during EICR testing. Specifying the correct (lowest appropriate) MCB type at the design stage avoids compliance problems at the testing stage.

The tabulated Zs values for MCBs to BS EN 60898 appear in two places in BS 7671: Table 41.3 lists values for the standard disconnection times used in most final-circuit verification, and Table 41.6 lists alternative values. Both tables apply to BS EN 60898 devices (domestic and light-commercial MCBs). Where a manufacturer supplies specific Zs data for a device, that manufacturer's data may be used instead — the tabulated values are described in BS 7671 as more onerous and may in some cases require a larger CPC to achieve compliance.

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70+ calculators including Zs by MCB type

Elec-Mate has every BS 7671 maximum Zs value built in. Select the MCB type (B, C, or D) and rating…

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In practice, MCB type selection errors are common. Knowing what to look for during inspection work helps you identify defects and recommend the right remedial action.

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Voice test entry records MCB details hands-free

As you inspect the consumer unit, speak the MCB details — 'Circuit 1, Type B, 32 amp, ring final' — and Elec-Mate fills in the schedule.

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