Understand why the percentage of the aperiodic component does not appear on LV ACB 2

[98mlo]

In the event of a short circuit, the circuit breaker shall be capable of breaking all short circuit currents such that:
- the periodic component is less than or equal to the rated value
- the percentage of the aperiodic component is less than or equal to the rated value

My question is: why is this second condition only specified in HV equipment (like circuit breaker data sheets)?
I have never seen this data for LV breakers (the ability to interrupt the aperiodic component of the short circuit current for an ACB), yet we have the same phenomenon independent of voltage (fault near a generator exposed to a strong DC component).

PS: I work in an IEC environment.

Thank you.
Sincerely

 

[dpc]

I can't speak specifically about IEC ratings, but for NEMA/UL rated breakers in the US: It is true that the breaker has a short circuit rating based on symmetrical rms current. But this can be directly applied only if the X/R ratio of the system at the fault is equal to or less than the X/R value that the breaker was tested at. This is due to the asymmetrical current you mentioned. Low voltage power circuit breakers in the US are tested at an X/R ratio of 6.6 as an example. As long as the actual X/R at the fault is 6.6 or less, then the symmetrical rating can be compared against the calculated symmetrical fault current. But if the X/R is greater than 6.6, the breaker must be de-rated. Some analysis software will make this adjustment automatically based on the calculated X/R ratio.

So the asymmetrical current is a factor that must be considered, even though the breakers are rated on a symmetrical current basis.

Hope that helps a little.

Dave

 

[che12345]

Dear Mr. 98mlo (Electrical)(OP)25 Jan 23 18:25
"#1.....In the event of a short circuit, the circuit breaker shall be capable of breaking all short circuit currents such that:
- the periodic component is less than or equal to the rated value
- the percentage of the aperiodic component is less than or equal to the rated value..."
1. I refer to the IEC see below 2 :
(a) For 50Hz, i[sub]p[/sub]) = 2.5 times I[sub]k[/sub].
(b) For 60Hz, i[sub]p[/sub]) = 2.6 times I[sub]k[/sub].
where I[sub]p[/sub] rated peck withstand current
I[sub]k[/sub] rated short-time withstand current
note: the value > 2.5 or 2.6 may be required according to the characteristic of the system.

"....#2. I work in an IEC environment".
2. IEC 60694 Common specification for HV switchgear and controlgear standards
Che Kuan Yau (Singapore)

 

[98mlo]

Thank you for your answers
@che12345
Ok I understand that you are basing yourself on the IEC 60694 standard (replaced by IEC 62271-1:Common specifications for alternating current switchgear and controlgear )

Do you have a data sheet that specifies the percentage of asymmetrical component that a given brand of low voltage circuit breaker can break?
The NEMA/UL approach of our colleague(DPC) has its merit: an X/R value that would downgrade the circuit breaker (or other device) if the X/R value is exceeded.

In the end, it may be a data that is not directly accessible in the datasheets and should be specifically requested from the manufacturer depending on the range.
Thanks again

 

[che12345]

Dear Mr 98mlo (Electrical)(OP)26 Jan 23 12:18
"....Do you have a data sheet that specifies the percentage of asymmetrical component that a given brand of low voltage circuit breaker can break? ..."
1. In the IEC world, for LV ; IEC 60947-2 " Low-voltage switchgear and controlgear , Pt2. Circuit-breaker"
2. Let A = Short-circuit making (I[sub]cm[/sub]) max prospective peak current,
B = short-circuit breaking capacity (I ) [kA[sub]rms[/sub]]
where A/B = n
With B= 20-50kA, n=2.1, pf=0.25
B= 50kA, n=2.2, pf=0.2
Note: for special requirements >n factor subjected to agreement between manufacturer and user.
Che Kuan Yau (Singapore)

 

[che12345]

Dear Mr 98mlo (Electrical)(OP)26 Jan 23 12:18
"....Do you have a data sheet that specifies the percentage of asymmetrical component that a given brand of low voltage circuit breaker ca n break?...."
Further to my post earlier dated 27 Jan 23 04:33; I wish to submit the following for your information.
In the IEC world:
1. Refer also to the IEC 60909-0 " Short-circuit currents in three-phase AC systems"
This document shows the SC envelope; i[sub]p[/sub] peak SC current, and i[sub]k[/sub] steady state SC current. The i[sub]p[/sub] is always > i[sub]k[/sub] by a factor "n" dependent on the power factor of the system.
2. IEC rated CB are tested with different " n " factors depending on LV (up to 1kV), MV (up to 52kV) , HV (>52kV)
Che Kuan Yau (Singapore)