BREAKING CURRENT - MV CIRCUIT BREAKERS

[reactive]

As I understand it the breaking capacity of an IEC breaker is given by the "rated short circuit breaking" current in kVrms. My queries :
- Must the DC component be considered when determining whether an installed breaker rating is being exceeded, or is the rating a symmetrical rating?
- At what time during a fault must the fault current be considered? Can the IDMT curve of a relay be considered, or should the 'minimum contact parting time + a nominal time for relay operation' be used irrelevant of protection details? What is a reasonable time?
- When using the IEC60909 method with max. C factor the fault current is higher than the ANSI C37 method. Say the IEC current exceeds the breaker rating and the ANSI doesn't what does one do. Any opinions or comments welcome.
- Any comments on the subject would be appreciated.

 

[dpc]

redwing,

This is, unfortunately, a messy topic. ANSI and IEC have different methods for calculating the short circuit current as well as different breaker rating standards.

Since I deal mainly with ANSI equipment, there are probably others who address this more accurately.

The best reference source I can recommend is the book "A Practical Guide to Short Circuit Calculations" by Conrad St. Pierre. It is self-published, but you should be able to find a link to it here:

http://www.easypower.com.

It contains a lengthy section on IEC fault calculations and applications, in addition to ANSI fault calculations.

The question of whether the dc offset must be considered depends mainly on the X/R ratio of your system compared to the X/R ratio or power factor used for the equipment testing. If your X/R ratio is less than the test value, then you can generally work with strictly symmetrical values. But if your actual X/R ratio is greater than the test value, the calculated symmetrical current must be adjusted to account for the possibility of higher asymmetrical currents than were accounted for in the equipment rating.

 

[coyney]

IEC 947-2 covers Circuit-breakers. This has several definitions with which you need to become more familiar.

Icu = ultimate breaking capacity
Isu = rated service capacity
Icw = withstand capacity
Imc = current making capacity.

If you read the definitions and assess them they could be some help.

Regards,