Zs at the FCU’s – advice needed, please!

[Goody]

I have been testing a ring circuit with two FCU’s, one of which feeds a cooker hood & another a oven light and gas igniters. Found R1+rR2 and R1+Rn on all the sockets including the FCU’s and Zs on all the sockets using Fluke with 3 pin cable set.

As the cables to the hood & the gas cooker are concealed under tiles and their ends are not accessible (going into sealed connectors), will R1+R2 readings at the FCU’s suffice? What am I going to write on the EIC for these?

Again, for Zs at the FCU’s, can I use hypothetical values {Zs=Ze+(R1+R2)} or should I actually measure them at these points? Also how do I NULL (zero) the 3 pin cable set prior to measuring Zs at the sockets?

Thanks in advance!

 

[telectrix]

you should measure the Zs's, and you don't null the leads for Zs. only null for R1+R2 ( continuity ). measured at the FCu's is fine IMO.

 

[andekoch]

Depends what you are testing for. Normally the FSU results will be where you end, same question on an immersion heater in another post. The EIC will record the values at the FSU. As long as you note that you have calculated Zs on the EIC it is ok IMO. since generally the measures Zs will be lower than the calculated Ze + (R1 + R2) due to parallel earths.

 

[tomma]

If the Zs is going to be lower, with the parallel paths, then the fault current will higher, so attention needs to be on whether the MCB is capable of handling the breaking capacity. A direct measurement is always best.

 

[IQ Electrical]

If the Zs is going to be lower, with the parallel paths, then the fault current will higher, so attention needs to be on whether the MCB is capable of handling the breaking capacity. A direct measurement is always best.

No, that's why PFC is measured at the point at which the protective device is required to break the fault current, not at the extremity of a circuit where Zs should be measured.

 

[tomma]

Sorry my mistake.

 

[andekoch]

The regs say you need to measure R1+R2 612.2.1 and that Zs needs to be measured 612.8.1 in order to proove compliance with 411, TN requirements. So there you have the regs view.

 

[Goody]

Thank you all, guys for the prompt replies!

Cheers!

 

[Deleted member 26818]

From GN3 page 95:
"*12 Earth fault loop impedance Zs
This may be determined either by direct measurement at the furthest point of a live circuit or by adding (R1 + R2) column 6 to Ze. Ze is determined by measurement at the origin of the installation or preferably the value declared by the supply company used.
Zs = Ze + (R1 + R2). Zs should be less than the values given in appendix 2 of the On-Site Guide or Appendix 2 of GN3."

 

[Jurasic Spark]

The regs say you need to measure R1+R2 612.2.1 and that Zs needs to be measured 612.8.1 in order to proove compliance with 411, TN requirements. So there you have the regs view.

Doesn't say that in my copy of BS7671.

 

[IQ Electrical]

The regs say you need to measure R1+R2 612.2.1 and that Zs needs to be measured 612.8.1 in order to proove compliance with 411, TN requirements. So there you have the regs view.

Absolutely not, there is no requirement in BS7671:2008 to measure R1+R2, the requirement is to prove continuity of the protective conductor-this can be done with a bell/buzzer set as long as it meets the requirements of 612.2.1 regarding output voltage and short circuit current.

 

[andekoch]

Strictly speaking the regs say.
612.2.1 "a continuity test shall be made..."
and 612.8 a) "measurement of earth fault loop impedance (see 612.9)
and 612.9 "...the relevant impedances shall be measured, or determined by an alternative method."

Ok so my paraphrasing not quite what some were expecting.
The key things being proof of conductor (include cpc) continuity and a verification that Zs is less than or equal to the allowed value, be this by measurement or "an alternative method".

I hope this keeps all happier.