Lowest Zs reading and highest PFC i've seen. Answers on a postcard please.....

DaveyLe_Spark · Sep 16, 2015

I did an EICR for a client over a year ago and as part i took the Ze reading at the incoming supply. All normal for a TNC-S it was about 0.14 ohms. I installed a socket in his workshop recently and as his tenant was in i could not remove the main earth so i took a Zs at the same position. The Zs reading came out at 0.01 ohms (the property is right next to the sub station) My issue now is that with a Zs so low it is giving me a PFC of over 14.5kA. I looked into replacing the MCB's for D-Type. Most of the values of circuit Zs are ok but a couple are higher than the stated regs. Although these are covered by a 30mA RCD, there are a few RCBO's in there which are not available as a typr D to withstand that potential in rush in the case of a fault. I also cannot find any literature on the fault rating of the main switch and RCD's in the board too. The make is M2 (CEF) I have called my registered body and they cannot suggest anything, i thought about putting some sort of resistor in line with the structural steel bond (which is the one causing this headache i believe) but i am not sure on the regs for doing that. It just seems all wrong! Has anyone come across this before and does anyone else test both Ze and Zs at the main board? I was only ever taught to take a Ze as required by the certs but i will take both from now on!

Thanks in advance.

Dave.

cliffed · Sep 16, 2015

As long as the main intake fuse can stand the Huge Pfc ,then all is fine.Usually a 1361 type 2, thinking it's rated at 16ka is ok.the DB would also have that rating too.

Richard Burns · Sep 16, 2015

Type D breakers are only designed to cope with a normal overload current that is briefly high not a fault current that is high.
You would need to select breakers with a higher breaking capacity to do what you are doing.
I.e. the Icn should be sufficiently large to cope with the fault current without failing.
However most domestic breakers are 6kA or perhaps 10kA.
However as cliffed has said is you have a consumer unit and a main fuse as a BS1361 then it is considered sufficiently safe up to 16kA.

With a low Zs it may be an idea to measure as stated in this thread to ensure you have a correct value.

Electric Magic · Sep 16, 2015

DaveyLe_Spark said:
I did an EICR for a client over a year ago and as part i took the Ze reading at the incoming supply. All normal for a TNC-S it was about 0.14 ohms. I installed a socket in his workshop recently and as his tenant was in i could not remove the main earth so i took a Zs at the same position. The Zs reading came out at 0.01 ohms (the property is right next to the sub station) My issue now is that with a Zs so low it is giving me a PFC of over 14.5kA. I looked into replacing the MCB's for D-Type. Most of the values of circuit Zs are ok but a couple are higher than the stated regs. Although these are covered by a 30mA RCD, there are a few RCBO's in there which are not available as a typr D to withstand that potential in rush in the case of a fault. I also cannot find any literature on the fault rating of the main switch and RCD's in the board too. The make is M2 (CEF) I have called my registered body and they cannot suggest anything, i thought about putting some sort of resistor in line with the structural steel bond (which is the one causing this headache i believe) but i am not sure on the regs for doing that. It just seems all wrong! Has anyone come across this before and does anyone else test both Ze and Zs at the main board? I was only ever taught to take a Ze as required by the certs but i will take both from now on!

Thanks in advance.

Dave.

This may also have an implication for protective conductor sizes (see 434.5.2 of BYB and table B7 of OSG, although the later only includes up to 6kA).

Marvo · Sep 16, 2015

As Richard Burns suggests, use the alternative method for measuring Ze using a known resistance. With really low impedance readings your instrument inaccuracies increase and the alternative method will give you a more realistic result.

UKMeterman · Sep 16, 2015

The other scary thing to consider is the magnetic force on the conductors under fault conditions, depending how close together they are you could get upto 1/2 ton force per meter or more.. http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/wirfor.html

DaveyLe_Spark · Sep 17, 2015

Thanks for your response. I have looked further into this and i contacted the manufacturer (Proteus) Up until late last year the braking capacity of the main switch was 2kA and the Rcd's still is 4.5kA. Late last year when the new regs amendment was imminent they all now have to be manufactured to a different BS, ensuring that all breakers and the main switches have a 16kA capacity. I will source some B type from this new batch and hey presto, problem solved.