Selective Coordination (Cooper Bussmann Electrical Protection Handbook section)

Overview

Three study methods are described: published fuse selectivity ratios, computer time-current analysis (to be interpreted against available fault current), or hand overlays; some breaker makers publish tested coordination tables. Time-current curves cannot prove coordination below 0.01 s: in the example, 100 A and 400 A time-delay fuses show coordination on curves only up to about 11,000 A, and above that both operate in under 0.01 s so ratios must be used.

Fuse selectivity requires the load-side total clearing I2t to be less than the line-side melting I2t; Low-Peak fuse families need a 2:1 amp-rating ratio, while other combinations need up to 8:1.

Once a breaker unlatches it will open; instantaneous-trip breakers typically unlatch in less than 0.01 s, so overlap of curves in the instantaneous region means lack of coordination. Breakers with short-time delay can hold on to faults for up to 30 cycles so downstream devices clear first; MCCBs/ICCBs with STD still have an instantaneous override, and STD-rated breakers may carry lower interrupting ratings than the same frame with instantaneous trip.

The section also discusses mandatory coordination for emergency and critical systems and common objections.

Key points

1. Three methods: published fuse selectivity ratios, computer time-current analysis, or hand overlays; some breaker makers publish tested tables.
2. Example: 100 A and 400 A time-delay fuses coordinate on curves only up to ~11,000 A; above that both operate in under 0.01 s and ratios are needed.
3. Fuse selectivity: load-side total clearing I2t below line-side melting I2t; Low-Peak families need 2:1 amp ratio, other combinations up to 8:1.
4. A breaker that unlatches will open; instantaneous-trip breakers typically unlatch in under 0.01 s, so curve overlap in the instantaneous region means lack of coordination.
5. STD breakers can hold faults up to 30 cycles; MCCBs/ICCBs with STD keep an instantaneous override, and may have lower interrupting ratings than the same frame with instantaneous trip.

Using it in design

• In the coordination checker store device type (fuse class, breaker with or without instantaneous, STD band) and use ratio tables or tested pair tables rather than curves when fault current puts both devices below 0.01 s.
• Flag breaker pairs whose instantaneous regions overlap at the calculated fault current.
• For designs claiming full selectivity (emergency or critical branches, Tier IV) require documented tested pairs.
• Show a trade-off warning when STD increases arc-flash energy.

Who should read it

Electrical designers doing protection coordination and selectivity studies.

Related standards

• NEC 700.32
• NEC 701.32
• NEC 708.54
• NEC 517.26
• IEC 60947-2 Annex A

Publication details

Authors

Eaton

Publisher

Eaton

Year

2008

Document number

Bussmann SPD 2008

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation