The trip ranges behind Type B, C and D miniature circuit breakers, how to check discrimination against inrush, and why 6 kA is not always enough.
Three MCBs can carry the same 16 A and still behave completely differently the day a lighting circuit energises with cold filaments, or a motor starts. The trip curve decides whether the breaker rides through that and stays closed — or trips and stops the job.
An MCB combines two sensing elements. The thermal element is a bimetal strip that responds to sustained overload: it follows an inverse-time curve, so a 30 % overload takes minutes to trip while a 100 % overload takes seconds. The magnetic element is a solenoid that responds to a large fault current almost instantaneously. The curve letter defines the band in which the magnetic element operates, expressed as a multiple of the breaker's rated current.
| Type | Magnetic trip band | Typical use |
|---|---|---|
| B | 3–5 × In | Resistive loads, lighting, socket circuits in domestic installs with low inrush |
| C | 5–10 × In | General mixed loads, motor circuits, small transformers, most commercial panels |
| D | 10–20 × In | High inrush: large motors, welding sets, big transformers, large LED drivers, X-ray equipment |
| K, Z | Wider / narrower bands | K for engineering / inductive loads, Z for electronics and semiconductor protection (not to IEC 60898) |
The curve controls when the breaker opens; the breaking capacity (Icn, the "kA" figure) says how much fault current it can interrupt without being destroyed. 6 kA covers most domestic and light commercial installations. Near a large transformer, or in an industrial panel with a high prospective fault level, specify 10 kA or higher — otherwise the breaker may trip but vent, weaken or weld internally, and be unreliable from that day on.
In a board, the upstream device should not trip when a downstream circuit faults. Check the manufacturer's discrimination tables rather than assuming that a bigger breaker upstream is automatically selective: with two devices of the same type and similar rating, selectivity at high fault levels is often only guaranteed by current-limiting behaviour or by a large rating ratio. If the installation must keep the rest of the board live during a local fault, that check belongs in the design.
ELEHPD manufactures MCBs in Type B, C and D, 6 kA and 10 kA, single and multi-pole, together with distribution boards, RCDs and residual-current breakers.
It is the magnetic (instantaneous) trip threshold as a multiple of rated current: Type B trips at 3-5x, Type C at 5-10x and Type D at 10-20x. Type B suits resistive and electronic loads, Type C is the general-purpose choice for lighting and sockets, and Type D is for high-inrush loads such as small motors and transformers.
Match it to the prospective fault current at the installation point: 6 kA is typical for final distribution in homes, while 10 kA or 15 kA is usual for commercial and industrial boards fed from larger transformers. Always confirm with the supplying utility or the system designer - an underspecified breaking capacity is a safety issue, not a preference.
Yes, provided the motor starting inrush stays below the magnetic trip threshold (under 5-10x rated current); for high-inrush loads that would nuisance-trip a Type C, choose Type D. Remember an MCB protects the cable, so size the rated current to the conductor first and check the curve second.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.