- 2072 Magh · 6 marks
MCB types for resistive and inductive loads
Short answer
Resistive loads use Type B MCB (trips at 3–5 In) because switch‑on current is almost normal. Inductive loads need Type C (5–10 In) to withstand motor starting current (5–7 In) and avoid false trips; Type D (10–20 In) suits high‑inrush loads such as large motors and transformers.
Write the types of MCB used in resistive load and inductive loads. Why?
A miniature circuit breaker (MCB) is a small, re-settable automatic switch that trips on overload (thermal bimetal element) and on short circuit (magnetic coil element). MCBs are grouped into tripping curves B, C and D (IEC 60898-1), according to the current at which the instantaneous magnetic trip operates. The choice of curve depends on the inrush (starting) current of the load.
Types of MCB by trip curve
| Type | Magnetic trip range | Typical loads |
|---|---|---|
| B | 3 to 5 × rated current In | Resistive loads: heaters, geysers, incandescent lamps, domestic sockets |
| C | 5 to 10 × In | Inductive loads: small motors, fans, pumps, fluorescent/LED lighting, AC units |
| D | 10 to 20 × In | Highly inductive loads: large motors, transformers, welding sets, X-ray machines |
| K / Z | 8–12 × In / 2–3 × In | K: motors and transformers; Z: semiconductor and electronic circuits |
MCB for resistive loads: Type B
- A resistive load (heater, iron, filament lamp) draws almost its normal current at switch-on; there is no large inrush current.
- So the breaker can be made sensitive. A Type B MCB trips instantly at 3–5 In, which clears faults quickly even on long cables with high loop impedance (small fault current).
- This gives better protection of cables and people without nuisance tripping.
MCB for inductive loads: Type C (or D)
- An induction motor takes 5–7 times full-load current while starting; a transformer takes 8–12 times rated current as magnetising inrush when switched on. Fluorescent lamps with chokes also draw a surge.
- If a Type B MCB were used, this harmless inrush would cross 3–5 In and the MCB would trip falsely every time the load is switched on.
- A Type C MCB (5–10 In) rides through normal motor and lighting inrush but still trips on a true short circuit. For very large inrush (DOL motors, transformers, welders) a Type D MCB (10–20 In) is used.
- The thermal (overload) part works the same in all types, so sustained overloads are still cleared.
Why the difference matters
- Correct type gives selectivity between normal inrush and fault current.
- Too sensitive a type → nuisance tripping; too insensitive a type → slow clearing of low-level faults and possible cable damage.
Example: a 2 kW room heater circuit uses a 10 A Type B MCB, while a 2 hp water pump motor circuit uses a 16 A Type C MCB.
✓ Answer checked by Claude Opus 5.5 (high effort)
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