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Short note: Protective devices and sizing

Short answer

Protective devices include fuses, MCBs, MCCBs, RCCBs/ELCBs and surge protectors; they disconnect faulty circuits automatically. Size them so the device rating In lies between the design current Ib and cable ampacity Iz, with breaking capacity above the prospective short-circuit current and correct voltage/curve; e.g. a 3 kW geyser at 230 V uses a 16 A MCB, 4 mm² cable and 30 mA RCCB.

Write a short note on Protective Device and their sizing.

Protective devices disconnect a faulty circuit automatically to prevent damage to cables, appliances and persons. Their sizing must match the load and the cable.

Common devices:

  1. Fuse: wire that melts at overload. Rated in amperes.
  2. MCB: thermal-magnetic breaker for overload and short circuit; ratings 6, 10, 16, 20, 32, 40, 63 A.
  3. MCCB: moulded-case breaker for higher currents (up to a few hundred amperes).
  4. RCCB/ELCB: detects earth leakage (residual current, typically 30 mA for people) and trips quickly.
  5. Isolators and surge protectors for switching and voltage surges.

Sizing rules:

  • Rated current of the device InI_n must be at least the design (load) current IbI_b, and not more than the current-carrying capacity of the cable IzI_z:
Ib≤In≤IzI_b\le I_n\le I_z
  • The breaking capacity must be greater than the prospective short-circuit current at the point of installation (e.g. 6 kA for a house).
  • The rated voltage must match the system (230/400 V).
  • Select the curve type: B for resistive loads, C for mixed/inductive, D for motors with high inrush current.
  • For a motor, the fuse/breaker is chosen for about 1.5-2.5 times the full-load current to allow for starting.

Example: a geyser of 3 kW at 230 V draws 13 A, so a 16 A MCB with a 4 mm² cable is chosen, and a 30 mA RCCB is added.

✓ Answer checked by Claude Opus 5.5 (high effort)

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