IEC Equipment Duty Calculations

This topic describes the equipment duty calculations for breakers and fuses when your system is set to use the IEC standard.

HV Breakers

Initial Duty

  • The bus X/R ratio and the maximum symmetrical current (device current) is considered for calculating the peak current.
  • The device current can either be the branch current through the device or the difference of bus current and branch current.
  • Duty amps is calculated by the formula:

Breaking Duty

  • If fault X/R is greater than test X/R, then the duty amps are:

    • Where,

    Ib – Breaking current at tmin

    Ik” – Initial current

    f – Frequency

    to = Opening time of the breaker

    tmin = Breaking time determined by the opening time of the breaker

Breaking Duty

t0(s)

tmin(s)

0 < t0 <0.05

0.02

0.05 <= t0 < 0.1

0.05

0.1 <= t0 <=0.1

0.1

0.25 <= t0

0.25
  • If fault X/R is less than test X/R, then the duty amps are:

    • Device current at tmin

LV Breakers

  • The fault X/R is compared with a standard test X/R taken from the table as per the standard 60947-2.
  • If the fault X/R is smaller, the device current (symmetrical) becomes the duty amps.
  • If the fault X/R is larger, then the device current (peak) is divided by a factor.
  • The standard X/R table based on the breaking capacity of the breaker and the division factor for each category is given in the table below.
LV Breakers

Breaking Capacity, Isym (kA)

Test X/R

Division Factor
4.5 <= Isym <= 6 0.943 1.5
6 < Isym <= 10 1.782 1.7
10 < Isym <= 20 3.294 2.0
20 < Isym <= 50 4.023 2.1
Isym > 50 4.966 2.2

HV and LV Fuses

  • If fault X/R is greater than test X/R, then duty amps are:

  • If fault X/R is less than test X/R, then duty amps are:

    • Device current (symmetrical)

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