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Breaking capacity

Breaking capacity

Breaking capacity

(OP)
A switchboard got this data: Ik(peak): 100kA
Ik(rms): 40kA
Which one of these values should be used for calculating the minimum breaking capacity for a breaker in a cabinet placed X ft. from the switchboard?

AntonX

RE: Breaking capacity

The switchboard rating (nameplate) may be much different than the calculated values. You should calculate the available SCC at the point of interest.

The breaker duty shall be checked for both symmetrical rms and asymmetrical peak values. Compare the calculated values to its rating. Asymmetrical peak depends on the X/R ratio too.  

Rafiq Bulsara
http://www.srengineersct.com

RE: Breaking capacity

In other words (word)... both.


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RE: Breaking capacity

(OP)
Thanks.

These values are allready calculated for this switchboard.

By calculating new SCC (for the cabinet placement) based on the cable length and dimension and switcboard data I get:
Ik(peak): 25kA
Ik(rms): 15kA
These are the values for the new breaker in the cabinet.

The supplier of the breaker asked for only one value - Ik.
Then I'm not sure if we can use a 20 kA breaker :)

AntonX

RE: Breaking capacity

Asymmetric peak rating for breakers and/or cabinets are a certain multiplier of the symmetrical ratings depending up on the voltage class and which standard (UL 891 vs UL 1558 for example or whether it is panelboard, switchboard or panelboard) and also the breaker type.

You need to confirm that with the vendor. The standards assume a certain X/R ratio, you can adjust the rated asymmetrical rating based on the actual X/R ratios.

Also look up momentary duty and interrupting duties.

Rafiq Bulsara
http://www.srengineersct.com

RE: Breaking capacity

(OP)
The table tells that the x(peak factor) is 1.6
The tables are prepared by the switchboard maker.

RE: Breaking capacity

Quote:

The table tells that the x(peak factor) is 1.6
The tables are prepared by the switchboard maker.
These are the values that the panel maker assumed. The actual value will depend on the X/R ratio of the supply transformer and the impedance of the conductors feeding the panel.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Breaking capacity

Quote:

Then I'm not sure if we can use a 20 kA breaker :)

It depends on the type of breaker. For MCCBs 1/2 cycle momentary duty applies, that is the asymmetric peak. Its interrupting capacity must be equal to or greater than asymm peak.

Rafiq Bulsara
http://www.srengineersct.com

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