[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

A sub-distribution board is fed by a 160 A MCCB in the main switchboard. The board has C-curve MCBs from 16 A to 32 A for final circuits. The prospective short-circuit current at the sub-board busbar is about 6 kA.

• Is it enough that 160 A is much larger than 32 A?
• How do I check selectivity in the overload zone and in the short-circuit zone?
• What do I write on the single-line diagram so the contractor cannot substitute devices that break selectivity?

[MEP Center Engineering]

A large ratio of rated currents gives overload selectivity, not short-circuit selectivity. Above the MCCB’s instantaneous pickup both devices can trip, so total selectivity up to 6 kA can only be confirmed from the manufacturer’s tested selectivity tables (IEC 60947-2 Annex A) for that exact pair.

Step 1: Overload zone

Plot both time-current curves on the same sheet. With 160 A upstream and 32 A downstream the thermal curves are far apart, so overloads are selective. For gG fuses a 1.6 : 1 rating ratio is the classic rule. For breakers, compare curves with their tolerance bands; a ratio alone is not enough.

Step 2: Short-circuit zone

• C32 MCB magnetic trip: 5 to 10 × I_n = 160 to 320 A.
• MCCB magnetic set at 10 × I_n = 1 600 A, with a typical ±20 % tolerance, so it may trip from about 1 280 A.
• Without tested data, the selectivity limit is therefore only about 1.28 kA. That is far below the 6 kA available, so selectivity is partial.

Manufacturer tables often show a higher limit, because a current-limiting MCB clears the fault before the MCCB unlatches (energy selectivity). Such values are only valid for the tested combination. Different brands, or a different frame or trip unit, invalidate them.

Step 3: Ways to reach total selectivity

1. Choose an MCCB with an electronic trip unit and use a short-time delay, with the instantaneous threshold I_i set above the fault level at the sub-board. Then check the cable’s I²t withstand and the fault-loop disconnection time with the new setting.
2. Use a pair the manufacturer lists as “T” (total) up to the fault level at the sub-board.
3. Reduce the prospective fault at the sub-board, for example with a longer or smaller feeder, if the voltage drop allows.
4. Zone-selective interlocking is an option between ACBs and electronic MCCBs, not between an MCCB and an MCB.

Worked check (illustrative)

Fault level at the sub-board: 6 kA. MCCB 160 A, electronic, I_sd set to 8 × I_n = 1 280 A, short-time delay 0.1 s, instantaneous I_i above 6 kA. The C32 clears any fault above 320 A in less than 10 ms, well inside the MCCB’s 0.1 s delay. The pair is selective up to the instantaneous threshold, subject to the manufacturer’s table. Then confirm the feeder cable withstands the I²t passed in 0.1 s.

What to put on the drawings

• A note: “Selectivity between the upstream and downstream devices shall be total up to the prospective short-circuit current shown. Contractor to submit manufacturer selectivity tables for the proposed devices.”
• The fault level at each board, the trip settings (I_r, I_sd, t_sd, I_i) and the curve type.

Codes

IEC 60364-5-53 asks for selectivity where safety or continuity requires it, and for safety services it is essential. NEC 700.32 and 701.32 require selective coordination for emergency and legally required standby systems, and 620.62 for elevators. See also 10 Common Mistakes on Single-Line Diagrams.

For deeper study: Electrical Power Systems Design course.

Check the edition of the code that applies to your project.

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