What X/R ratios are UL listed MCBs tested to? And what do IEC standards require? I can't find any X/R listed for 15-800amp molded case circuit breakers.
MV CB per the latest IEEE and IEC Stds are based in X/R ratio same as the HV circuit breakers as follow:
[ul]
[li]X/R = 17 @ 60 Hz[/li]
[li]X/R = 14 @ 50 Hz[/li]
[/ul]
Thanks for sharing the screenshot. Please notice that this standard 551 was issued 14 years ago in 2006 and is not the appropriate standard that dealing with the rating and testing of MV & HV circuit breakers. This is a "Recommended Practice for Calculating AC Short-Circuit Currents in Industrial and Commercial Power Systems".
I do believe that the right document to use is the following IEEE Stds dated most recently in 2018 and is the governed standards for rating MV and HV circuit breakers as follow: [ul]
[li]C37.04-2018 - IEEE Standard for Ratings and Requirements for AC High-Voltage Circuit Breakers with Rated Max Voltage Above 1000 V[/li]
[li]C37.09-2018 - IEEE Standard Test Procedures for AC High-Voltage Circuit Breakers with Rated Max Voltage Above 1000 V[/li]
[/ul]
As a background, both the IEEE and the IEC standard went thought a rigorous homogenization process resulting in adopting the same time constant of decay of the dc component of the short-circuit current of τ=45 ms. This corresponds to X/R=17 for 60 Hz and X/R=14 for 50 Hz.
For similar reference see also thread238-472123. I hope this helps to bring some light to the subject.
Dear Mr. Mbrooke
Q. " What X/R ratios are UL listed MCBs tested to? And what do IEC standards require? I can't find any X/R listed for 15-800amp molded case circuit breakers".
A1. In the IEC world, LV Miniature circuit-breaker (MCB) standard is IEC 60898-1.
A1.1 Reference IEC 60898-1:2003-07 states:
i) Test current 4.5-6kA , power-factor range 0.65-0.70,
ii) Test current 6-10kA , power-factor range 0.45-0.50,
iii) Test current 10-25kA , power-factor range 0.2-0.25.
A2. In the IEC world, LV Moulded-case circuit-breaker (MCCB) standard is IEC 60947-2.
A2.1 Reference IEC 60947-2: 2003 states:
i) Test current 10-20kA , power-factor 0.3,
ii) Test current 20-50kA , power-factor 0.25,
iii) Test current >50kA , power-factor 0.2.
A3. BTW Numerous parts of UL 508 had been harmonized with the IEC Standards IEC 60947, listed as (e.g. UL60947-xyz/IEC 60947-xyz) for about a decade. ALL current tests to UL Standards Shall be tested to UL60947-xyz/IEC60947-xyz. Bye bye to UL508.
Che Kuan Yau (Singapore)
