Coordinating MCB rated current and selectivity in a board

What do rated current and selectivity mean?

The rated current In is the continuous current an MCB can carry without tripping. It is matched to the conductor cross-section and its permissible loading, not to the load alone. Selectivity (discrimination) means that on a fault only the breaker immediately upstream of the fault trips, while the device above it stays closed.

In a graded distribution board several protective devices sit in series: the main switch or upstream fuse, then group or sub-distribution boards, and at the bottom the final circuit. Without coordinated grading a short circuit in one socket could shut down the entire board.

Which tripping curve fits the load?

The tripping curve defines at what multiple of the rated current the instantaneous magnetic release operates. To IEC/EN 60898‑1 the usual types are B, C and D. The right choice prevents nuisance tripping on energising while still ensuring fast clearance on a short circuit.

• Type B: lowest inrush margin, trips early - ideal for resistive loads and shock protection on long runs.
• Type C: the standard for mixed loads with moderate inrush currents.
• Type D: for loads with high momentary starting currents so the breaker does not trip needlessly.
• Always verify that the disconnection condition to IEC 60364‑4‑41 is still met at the end of the line.

How do you grade rated currents selectively?

In the overload region a common rule of thumb is a step ratio of about 1.6:1 between the upstream and downstream rated current - for example 63 A over 40 A or 40 A over 25 A. This keeps the upstream breaker thermally stable while the downstream one reacts first.

In the short-circuit region rated-current grading alone is often not enough. Here you rely on current discrimination (the upstream breaker does not reach its magnetic threshold at the fault current present) or on time discrimination using selective main circuit breakers or moulded-case breakers with an adjustable short-time delay.

• The breaking capacity Icn of the upstream device must cover the maximum prospective short-circuit current at the location (typically 6 kA residential, 10 kA and above commercial).
• Downstream breakers must not exceed their back-up protection limit provided by the upstream fuse.
• Always check selectivity pairwise against the manufacturer table, never assume it globally.

Frequently asked questions