High PFCs

[ryanryan]

On installation of a 125amp 3P Dist. board I have recorded a PFC of 12kA and PSCC of 8kA (approx 16kA at 400v).

The DB is 20m away from the transformer ( hence the high values) and is supplied via a 100/125A MCCB, with a short-ircuit rating of 36Ka.

My problem is that the mcb's for the DB are rated at 10kA.

Do Square D mcbs go above 10kA?

 

[Darkwood]

Its not always possible to meet the breaking capacity with the individual circuit breaker but you are ok if you fit a device upstream that has a higher breaking capacity than your results e.g. A high rupture current fuse (HRC), this way if the mcb fails or breaks down during a fault because of the high pfc then the upstream device will back it up safely.

 

[ryanryan]

I have gained a little more knowledge today...

0.1s is the instananious diconnetion time, time/current graphs show that a 125a C type mccb will operate in 0.1s at 1250a.....

which (apparently) is <10ka rating of the mbs's..

 

[Darkwood]

I have gained a little more knowledge today...

0.1s is the instananious diconnetion time, time/current graphs show that a 125a C type mccb will operate in 0.1s at 1250a.....

which (apparently) is <10ka rating of the mbs's..

I will stand corrected if im wrong but it would seem your looking at this wrong as if i remember the time/current graph is limited to its lowest rupture time in the sense that 1250 amps will trip it in 0.1 seconds but you have a higher pfc not shown on the graph which YES it will try to operate the device in 0.1 seconds or less but due to the high pfc it could destroy, or cause the device to fail 'closed' as your pfc is above its rating, this is acceptable only if you have a device upstream that will safely operate in that its breaking capacity is greater than the pfc

Additional to you proximity to the supply transformer, you may get nuisance tripping of your rcd's and not find any reason why but if you do it is possible your reletive closeness to the transformer can be a cause and will be a nightmare to get around.

 

[ryanryan]

The device upstream is a 125a mccb with a 36Ka rating, well above the 16Ka PFC

 

[Darkwood]

The device upstream is a 125a mccb with a 36Ka rating, well above the 16Ka PFC

I know you have a mccb upstream i was just explaining you seem to be interpreting the time/current graphs a bit wrong as it extends beyond the page and your pfc is what could go through your mcb but yes you are ok with the higher rated device upstream.

 

[peteludman]

Pfc is mesured in ohms and pscc is ka and 8 ka is fine i think im right never added pfc and pscc because they are ohms and ka tell me if im wrong

 

[Guest123]

Little bit out there Pete.....

PFC - Prospective Fault Current measured in A or KA, is the highest value of the reading of PEFC - Prospective Earth Fault Current measured in A or KA, and PSCC - Prospective Short Circuit Current measured in A or KA.

For a 3P reading the higher of the two values, (PFC) is then doubled to give the actual measurement.

Hope this helps.

 

[peteludman]

I was just being stupid but thanx mate

 

[Risteard]

For a 3P reading the higher of the two values, (PFC) is then doubled to give the actual measurement.

Only the PSCC is doubled. There would be no sense in doubling the PEFC because an Earth fault can't be between phases - only short circuit can.

 

[Guest123]

Sorry it was late....I'm sure you knew what I meant.

 

[Risteard]

Sorry it was late....I'm sure you knew what I meant.

No problem. Wasn't sure what you meant!