High PFC

SKY · Aug 30, 2012

Following on from another thread, I thought I would post up the results of my ZE and PFC.
I think the PFC results are high:

TNS
ZE is 0.7ohms
PFC (L to N) is 6.5ka and (L + E) is 4.4ka.

Would you fit 10ka breakers?

Richard Burns · Aug 30, 2012

With those results you must be getting an amazing earth from your incoming service because your Ze would need to be about 20 times lower to get those values of PFC. 10m from the substation could cause those values though.
However if your PFC is >6kA then 10kA breakers would be required if the exemption for domestic consumer units protected by a BS1361 (i think) fuse being considered to have a breaking capacity of 16kA is not applicable.

Taylortwocities · Aug 30, 2012

For domestic you'll be fine.
As a bit of homework have a read of 434.5.1.

This permits devices to be protected against fault current by an upstream device; with limited A2s let through energy.
This is the case with circuit breakers and BS 1361 fuses where the fault current does not exceed 16kA.
Conditional ratings apply to the distribution board and its main switch and again usually relate to the protection being provided by a BS 1361 device and fault current upto 16kA

You must be close to the sub station then?

plugsandsparks · Aug 30, 2012

Be interested in Zs at a nearby cooker cct or similar radial - just to confirm...

Chr!s · Aug 30, 2012

Ze 0.7. L-E 4.4 Ka

It doesn't add up.

Read up on Annex Za

MarkieSparkie · Aug 30, 2012

Your measurements are suspicious, it would appear either your Ze, or PEFC and PSSC are adrift by a factor of 10 high relative to each other. You would need some very good parallel earth paths (eg.structural steel) to get this sort of disparity in measurements.

SKY · Aug 30, 2012

What can be done?
Really It should be approx: 328amps using the formula 230/0.07.

I'm about 3 houses away from the supply transformer "with the Danger of Death" signs on it!

I'm gonna change the megger batteries tomorrow as well just in case its a erroneous reading