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 t_min

Ik” – Initial current

f – Frequency

t_o = Opening time of the breaker

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

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

Device current at t_min

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.

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)

More Information

	www.easypower.com EasyPower, LLC 7730 SW Mohawk St. Tualatin, OR 97062 (503)655-5059	techsupport@easypower.com feedback@easypower.com EasyPower knowledge base EasyPower Version 10.0 Copyright © 2018 EasyPower, LLC. All Rights Reserved. Help was last updated on 5/22/2018