Three phase pfc too high!

[shaun gold]

I have a three phase dis/board with a measured PFC of 10 kA across each phase (20 kA total) and a Ze of 0.03. The origin is 100 amp BS 1361
type IIb x 3 and the tails are 1.5m long. The MCB's are type B and have a short ciruit rating of 6 kA, should these be changed for 10 kA or higher.
The dis/board is to BS EN 60439-3 and i have checked my results a few times with other (nulled) testers.
The DNO state the three phase supply at 25 kA but this exceeds the permitted 16 kA listed in regulation 434.5.1.
Not really sure what to record on the ECR to tell the client without being 100%.

 

[Engineer54]

I have a three phase dis/board with a measured PFC of 10 kA across each phase (20 kA total) and a Ze of 0.03. The origin is 100 amp BS 1361
type IIb x 3 and the tails are 1.5m long. The MCB's are type B and have a short ciruit rating of 6 kA, should these be changed for 10 kA or higher.
The dis/board is to BS EN 60439-3 and i have checked my results a few times with other (nulled) testers.
The DNO state the three phase supply at 25 kA but this exceeds the permitted 16 kA listed in regulation 434.5.1.
Not really sure what to record on the ECR to tell the client without being 100%.

One thing you need to be aware of here, is that your standard MFT or Separate ELI tester will not have a low enough resolution to accurately measure down to those sort of Ze values. There is a sticky here, that can explain a simple remedy/method that can give you a better accuracy using your existing test equipment.

Check this Sticky thead out by IQ!! http://www.electriciansforums.net/i...38-higher-accuracy-low-ze-test-procedure.html

Exactly where is this DB situated, very close to the supplying TX?? If the DNO are certain that the PFC at your location is 25KA, then the DB is obviously not up to the Job.

 

[Guest55]

as the earth loop is as low as 0.03 im assuming the dno TX is within 100mtrs of the supply head , so your pfc will be high.
and yes , your 6ka mcbs arent going to cut it im afraid.
10ka will be better.

 

[plugsandsparks]

If you are confident in the Ze and the board is next to the incoming, then yep 6K breakers are under spec for that install. If you have to go for 25KA stuff your client will get a shock when you price it

 

[Engineer54]

Personally i don't actually believe the DNO's stated KA level. 25KA is around what you would expect available at the LV terminals of a 1000KVA distribution TX. Every impedance from those terminals (bolted connections, cables CSA, cable distances, Breakers etc ) would then effect and determine the actual fault current available at the point in question...

Bit more to it than that obviously, but it's the general idea of how KA levels will fall, the further away from the TX you get...