MCB Types B, C and D Explained
The type letter on an MCB tells you how much current it takes to trip instantly — and that choice affects both nuisance tripping and the maximum earth loop impedance you can allow.
Two MCBs with the same amp rating can behave completely differently on a fault. The type letter — B, C or D — sets the multiple of rated current at which the breaker’s magnetic element trips instantly, so a Type B and a Type D of the same rating disconnect at very different fault currents.
This guide explains the two mechanisms inside an MCB, sets out the instantaneous trip ranges for each type with their typical uses, and shows why the type you pick directly affects the maximum permitted Zs and how to choose the right one for inrush loads.
Key takeaways
- The type letter sets the instantaneous magnetic trip threshold as a multiple of rated current (In).
- Type B trips at 3–5×In, Type C at 5–10×In and Type D at 10–20×In.
- Every MCB has two mechanisms: a thermal element for overloads and a magnetic element for short-circuits.
- A higher type needs more fault current to trip instantly, so it demands a lower maximum Zs.
- Choose Type B for general and resistive loads, and Type C or D where inrush would nuisance-trip a Type B.
Thermal and magnetic tripping
Every MCB contains two independent trip mechanisms. The thermal element is a bimetallic strip that bends as it heats up under sustained overload — it gives a deliberately slow trip, so a small, brief overload rides through but a persistent one eventually disconnects. This part is the same regardless of type letter.
The magnetic element is an electromagnet that trips the breaker almost instantly once current reaches a set multiple of the rating — this is what clears a short-circuit or earth fault quickly. The type letter defines the threshold of this magnetic trip, and it is the only thing the type letter changes.
The trip ranges for B, C and D
Type B trips instantly at 3 to 5 times its rated current, Type C at 5 to 10 times, and Type D at 10 to 20 times. So a 32 A Type B trips magnetically somewhere between 96 A and 160 A, while a 32 A Type D needs between 320 A and 640 A to trip in the same instant. The thermal overload behaviour is unchanged — only the fast magnetic threshold shifts.
The higher the type, the more inrush it tolerates before nuisance-tripping, but the more fault current it needs to disconnect quickly. That trade-off is the whole story of choosing a type.
| Type | Instantaneous trip range | Typical use |
|---|---|---|
| Type B | 3–5 × In | General lighting and socket circuits, resistive loads |
| Type C | 5–10 × In | Small motors, fluorescent banks, moderate inrush |
| Type D | 10–20 × In | Transformers, welders, X-ray and high-inrush equipment |
Why type affects maximum Zs
To disconnect within the required time on an earth fault, an MCB relies on the fault current reaching its instantaneous magnetic threshold. The fault current equals the supply voltage divided by Zs, so the higher the trip threshold, the more fault current you need, and the lower Zs has to be to deliver it.
That is why the maximum Zs tables give a smaller figure for a Type D than a Type C, and a smaller figure for a Type C than a Type B, at the same rating. A circuit that passes Zs on a Type B may fail if someone swaps in a Type C or D of the same amperage — the breaker now needs far more current to trip in time.
Check the type, not just the rating
TradePlanr’s max Zs calculator asks for the device type as well as the rating, because a 32 A Type B and a 32 A Type D have very different maximum Zs values — using the wrong one is an easy way to pass a circuit that should fail.
Choosing a type for inrush loads
Many loads draw a large surge of current for a fraction of a second when switched on — motors, transformers, LED drivers and banks of fluorescent fittings all do this. If that inrush exceeds a Type B’s magnetic threshold, the breaker trips every time the load starts, even though nothing is wrong. Moving up to a Type C, or a Type D for the highest inrush, lets the surge pass while still protecting against genuine faults.
The catch is the higher Zs demand that comes with it. Choose the lowest type that reliably tolerates the inrush: Type B for general and resistive circuits, Type C for moderate inrush, and Type D only where the load genuinely needs it — then confirm the circuit’s Zs still meets the tighter maximum that type requires.
Lowest type that copes
Don’t reach for a Type C or D just to stop occasional nuisance trips — first confirm the inrush really is the cause, because every step up in type makes the earth loop impedance requirement harder to meet.
Frequently asked questions
What is the difference between a Type B and Type C MCB?
The instantaneous magnetic trip threshold. A Type B trips at 3–5 times its rated current and a Type C at 5–10 times. The Type C tolerates more inrush without nuisance-tripping, but needs more fault current to disconnect quickly, so it demands a lower maximum Zs than a Type B of the same rating.
Why does the MCB type change the maximum Zs?
Disconnection depends on the fault current reaching the breaker’s magnetic trip threshold. A higher type has a higher threshold, so it needs more fault current, which means a lower earth loop impedance. That is why the maximum Zs tables list smaller figures as you move from Type B to C to D.
When should I use a Type D MCB?
Only for loads with very high switch-on inrush — transformers, welders, some motors and X-ray equipment — where a Type B or C would nuisance-trip. Because a Type D needs 10–20 times rated current to trip instantly, always confirm the circuit’s Zs meets the much lower maximum a Type D requires.
From guidance to action
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RCBO vs RCD vs MCB: What’s the Difference?
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RCD Types: AC, A, F and B Explained
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Max Zs Values Table (BS 7671 18th Edition)
Full maximum Zs tables for Type B/C/D MCBs and RCBOs, the Cmin formula and the 80% rule.
