Technical Application Papers No.1: Low voltage selectivity with ABB circuit-breakers

Overview

The paper defines total and partial selectivity per IEC 60947. Total selectivity between a pair of breakers means selectivity up to the lower of the two Icu values; partial selectivity means only up to an ultimate selectivity current Is below that, beyond which the upstream device may also trip. The overload zone runs from In up to about 8-10 x In (thermal / L function) and the short-circuit zone lies above it (magnetic or S, D, I functions). Current through the upstream breaker can exceed the downstream fault current: IA = IB + Iloads for one upstream feeder, IA = (IB + Iloads)/n for n paralleled supplies.

The techniques covered are current, time, energy, time-current and zone (logic-interlock) selectivity. With time selectivity the ultimate limit equals the upstream instantaneous threshold I3 (minus tolerance) or, for an ACB with I3 set OFF, its short-time withstand Icw. For MCCB-MCCB time selectivity, when the upstream S threshold sits above a downstream instantaneous function, set t2A >= 150 ms for I2t=const or >= 100 ms for t=const; for S-over-S with the same curve type, t2A - tolerance >= t2B + tolerance + 50 ms.

Zone selectivity uses lock signals between trip units so only the breaker immediately upstream of the fault trips without long intentional delays; trip times can be below 100 ms but an auxiliary supply is required. Between Emax ACBs (PR122/PR123) it reaches Is = Icw, and between Tmax T4L/T5L/T6L with PR223EF it reaches Is = 100 kA. Directional time and directional zone selectivity for multi-source boards, MV/LV selectivity, residual-current selectivity and a worked LV/LV study are given in appendices.

For MV/LV selectivity the MV relay curve must stay above the transformer inrush curve and below its thermal-withstand point, within the utility's imposed current/time limits. The paper is written around ABB product families, so its settings and thresholds do not transfer directly to other manufacturers.

Key points

1. Total selectivity means selectivity up to the lower Icu of the pair; partial means only up to Is, beyond which the upstream device may also trip.
2. The overload zone runs from In to about 8-10 x In; the short-circuit zone is above that.
3. Upstream current: IA = IB + Iloads for one feeder; (IB + Iloads)/n for n paralleled supplies.
4. With time selectivity the ultimate limit is I3 (minus tolerance) or Icw for an ACB with I3 OFF.
5. MCCB-MCCB time selectivity: t2A >= 150 ms (I2t=const) or >= 100 ms (t=const); S-over-S: t2A - tol >= t2B + tol + 50 ms.
6. Zone selectivity: trip times below 100 ms, auxiliary supply required; Emax ACBs reach Is = Icw, Tmax T4L/T5L/T6L with PR223EF reach 100 kA.
7. For MV/LV the MV relay curve must stay above transformer inrush and below its thermal withstand.

Using it in design

• Model each protective device with Icu, Icw, I1/I2/I3 settings and t2 curve type so the tool can evaluate selectivity pairs.
• Flag partial selectivity where the prospective Ik at the downstream device exceeds the tabulated Is, and distinguish 'total' from 'partial up to X kA' in reports.
• Include parallel load current and number of paralleled sources when computing upstream current for selectivity.
• Offer zone selectivity on main/feeder ACBs with a warning that it needs auxiliary supply and wiring; plot transformer inrush and thermal withstand on the TCC view for MV/LV.

Who should read it

Electrical designers doing protection coordination and selectivity studies in low-voltage distribution.

Related standards

• IEC 60947-1
• IEC 60947-2
• IEC 60364-5-53

Publication details

Authors

ABB

Publisher

ABB

Year

2008

Document number

1SDC007100G0204 (QT1)

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