Main circuit breaker (low-voltage air circuit breaker as the incomer of a main switchboard)

01

Short-circuit ratings at the switchboard voltage

An incomer is rated four ways for faults: ultimate breaking capacity Icu, service breaking capacity Ics, short-time withstand Icw for a stated time, and making capacity Icm. Himel's HDW3 data sheet for a 6300AF frame, 4,000 A, 3-pole draw-out breaker lists Icu of 120 kA at 400 V and 85 kA at 690 V, Ics of 100 kA at 400 V and 75 kA at 690 V, and Icw of 85 kA at 400 V and 75 kA at 690 V, both for 1 s, with a maximum tripping time of 25 ms. [2] WEG's ABW range brochure gives Ics as 100% of Icu across the range, Icw for 1 s from 42 to 100 kA, for 3 s from 25 to 85 kA, and rated currents from 800 A to 6,300 A, to IEC 60947-2. [1]

Editorial calculation from Himel's figures: Ics is 100 ÷ 120 ≈ 83% of Icu at 400 V and 75 ÷ 85 ≈ 88% at 690 V, so on that breaker the service rating, not the ultimate one, is the figure to compare with a fault level the breaker must clear and keep in service. [2]

WEG notes that above 2,000 m altitude voltage and current derating factors apply, and that above 40 °C ambient the maximum rated current values must be checked. [1]

Give the maker the transformer or generator rating and impedance, the prospective fault current at the board, the board voltage and earthing system, the time the incomer must ride through a downstream fault, and the ambient temperature and altitude.

02

Selectivity: category B, Icw and zone interlocking

WEG lists its ABW breakers as utilization category B with Icw ratings for 1, 2 and 3 s, and its trip units offer LSIG protection and "Protection coordinated by the ZSI (Zone Selective Interlocking)"; maximum opening time is 40 ms and closing 80 ms. [1]

ABB's Emax overview table shows the trade-off. In its column marked L, Icu at 415 V is 130 kA, but Icw for 1 s is 10 kA, the utilisation category (to BS EN 60947-2) is A rather than B, and the break time above Icw is 12 ms instead of 30 ms. The table's column headings are garbled in the PDF, so confirm the frame and version against the current ABB catalogue. [3]

For a main incomer that must hold while feeders clear their faults, ask for category B with an Icw and duration that cover the slowest downstream clearing time, and state whether ZSI wiring to the feeder breakers is wanted.

03

Draw-out cradle positions, padlocking and interlocks between incomers

WEG recommends the withdrawable version "for applications in which the occasional replacement or maintenance should be done as quickly as possible" and describes three positions in the rack: inserted, with power and control circuits connected; test, "position in which only the control circuits are connected"; and withdrawn, with both disconnected. The rack position can be locked by padlock, and cell switches signal each position. [1]

For boards with two incomers or an incomer and a bus coupler, WEG offers a mechanical interlock that "enables two or three circuit breakers to be mechanically interlocked with cables", with 1.6 m cables. [1] Himel's 4,000 A draw-out breaker weighs 233 kg and measures 441.5 mm high, 815 mm wide and 508 mm deep, figures to check against the board's cubicle and lifting arrangements. [2]

04

Main circuit breaker order details

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Supply: [transformer/generator kVA; impedance; voltage; earthing]
Current: [In; ambient; altitude]
Faults: [prospective kA; Icu; Ics; Icw and duration; Icm]
Selectivity: [category B; ZSI; downstream breakers]
Trip unit: [LSIG; metering; communication]
Mounting: [fixed or draw-out; poles; neutral]
Controls: [motor; closing and shunt coils; undervoltage]
Interlocks: [incomers; bus coupler; padlocks]

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