EICR - RFC higher than calculated end to end on cpc (code advice required please)

[freedomrun]

TN-C-S domestic, T&E (2.5mm L&N 1.5mm CPC), RFC, Zs within limits and RCD protected to 30mA anyway.

L-L (0.11 ohms)

N-N (0.11 ohms)

cpc-cpc (0.48 ohms)

Testing this RFC as part of an EICR - I'm thinking that i have a loose cpc connection somewhere. Taken all accessable s/o off as I did'nt like what i saw behind a couple chosen at random. Tightend all cpc connections on these without any joy in bringing that cpc reading down. Thinking i must have a JB in the install with a loose termination maybe.

What code should i put on the EICR and what should i stse on the report about this? Thanks in advance for any advice.

 

[sb_89]

Are you kidding me ?! Maybe it's because the cpc is 1.5 and the live conductors are 2.5 ? Did you expect them to have the same reading ?!

 

[alpha spark]

If its twin and earth 0.48 is way too high the earth is 1.67 times smaller than the l & n reading he should be around 0.18 sb

 

[Zeespark]

How old is the 6242Y roughly?

AndyGuinness

 

[alpha spark]

Are cpc s doubled over I've had it before where there about to snap this at a few sockets could dramatically increase the resistance probably a long shot

 

[freedomrun]

CU dates from 1991, cable looks about the same age.

 

[Ash]

you would really code that?

I'm sure if you worked out everything and did the adiabatic it would be fine.

 

[SLIPSHOD & SLAPDASH]

Thats a very short ring ? Are the cpcs connected in the same terminal in the sockets, or the back box's & are they separately sleeved

 

[Adrian]

I reckon they are 1mm cpc looking at the reading

 

[NozSpark]

I reckon they are 1mm cpc looking at the reading

Still doesn't add up though.

 

[freedomrun]

Most of the cpc's are twisted together and in single sleeve, connected to the front plate cpc terminal. As it's an EICR i'm not spending too much time over this, just want some advice on what to put on the report really. The L-L & N-N readings were 0.29 ohms with a cpc-cpc of 0.69 ohms before i tightend all of them up as i inspected each s/o.

 

[NozSpark]

Those first readings are much more like it,,, especially if 2.5/1.0 T&E

 

[Ash]

Decided to work this out as a challenge.

based on 2.5 twin, the length of you ring is around 16.5m long

That would make your CPC ring to be 0.59mm theoretically by your measured value

Adiabatic on a 60898 requires the CPC to be 0.44mm

(correct me if I have that wrong)

This would make me say it need not be coded, esp with RCDs and most likely a decent Zs reading

 

[freedomrun]

That's some good working out there - much appreciated !! I took a live Zs reading of 0.54 ohms but as said RFC cct is 30mA RCD protected anyway.

 

[Doc Hudson]

How many rooms does this "ring" supply? 15m as Slipshod said is a very short ring. (0.11ohms/7.41)x1000 = 14.8m not 16.5m. What was your Ze? For a 15m ring either the Zs is high or your Ze may be high for TNCS (0.54-0.35)=0.19, calculated R1+R2=(0.11+0.48)/4=0.147. But I suspect either this is not a full ring or some significant errors in test readings or calculations here. I was always under the impression that these basic assessments and calculations would be carried out whilst on site whenever anomalies arise in your readings. Is this a college test rig question or a real job? I am genuinely concerned if this a real job.

Doc H.

 

[Ash]

Sorry Doc, but

(1.724x10-8*16.5)/2.5x10-6 = 0.114ohms

 

[Doc Hudson]

Table 11 of the onsite guide suggest that 2.5mm2 CSA conductor has a resistance of 7.41mohms/m. As such (16.5m x 7.41mohms/m) /1000 would be a resistance value of 0.122ohms. whereas the OPs 0.11ohms divided by 7.41mohms * 1000 is 14.84m of 2.5mm copper conductor.@ 20oC ambient temp. Even so 15m or 16m is still a very short ring compared to a 26m or 40m ring implied by the CPC calculations. But the CPC length would appear to be a more realistic ring length! I am inclined to think I would have investigated this further whilst on site to establish more facts before asking on the internet how to code what should be a relatively simple PIR evaluation for a competent person as required by guidance note 3. As I previously asked Is this a college test rig question or a real job? should it be moved to the student area? as I am genuinely concerned if this a real job.

Doc H.

 

[freedomrun]

The Ze is measured at 0.21 ohms. It's a real job and i repeated the end to end test twice at another socket outlet on the circuit with the same results. From the calculated length of the ring that has been stated, something has got to be wrong as the circuit supplies s/o in two rooms with an estimated distance much greater. Not sure what to do now.

 

[batty]

Did you zero your leads before testing?

 

[freedomrun]

Yes, i made very sure of that. Thanks.