MCB or fuse? Choosing circuit protection
Basic differences between miniature circuit breakers and fuses, MCB trip curves, and what to specify when ordering. · Updated
The difference
A fuse protects a circuit by melting a calibrated element when current is too high, and must be replaced after it operates. A miniature circuit breaker (MCB) trips mechanically and can be reset. MCBs are convenient for final distribution circuits; fuses such as HRC cartridge fuses are widely used where very high breaking capacity or specific co-ordination is needed.
MCB trip curves (IEC 60898)
- Type B: trips at 3 to 5 times rated current; for resistive loads and lighting.
- Type C: trips at 5 to 10 times rated current; for general and small inductive loads such as motors.
- Type D: trips at 10 to 20 times rated current; for high inrush loads such as transformers.
What to specify
- Rated current (amps) and number of poles
- Trip curve for MCBs, or fuse type and size
- Breaking capacity required
- Supply voltage and mounting
Frequently asked questions
What is the difference between B and C curve MCBs?
A B-curve MCB trips at 3 to 5 times its rated current and suits lighting and resistive loads. A C-curve MCB trips at 5 to 10 times rated current and tolerates the inrush of motors and other inductive loads.
Should I replace fuses with MCBs?
It depends on the installation's fault level, co-ordination and applicable rules. Have a qualified electrician check the breaking capacity and protection requirements before replacing one with the other.