R1 R2 values fluctuate over time

[Gn96]

Bare with me people as I try to explain this. Currently working on a site, been responsible for testing under floor power tracks that are fed in 3 core 10mm swa on 32a mcb. Tested a bunch of them maybe 4-5 weeks ago. Got the results all happy days. In work this week I was informed a separate company is employed by the main contractor to “witness” me testing circuits I have already tested to “confirm the same or similar readings” which the main contractor wants as confidence the testing is being done correctly. So example 1. 1no of power track tested weeks ago Recorded R1 R2- 0.27. These have not been powered up yet so no live testing done. I’ve then tested the same track with the “witness” company and the R1 R2 value is now 0.10 less.= 0.17.

Example 2.
6no of fan coil units supplied in 2.5mm singles in 25mm galv conduit. End of line reading R1 R2 - 0.65. Tested last week. On to this week re testing the same circuit with the “witness” the current R1 R2 value is 0.55.

So got a few funny looks from the two men who work for the witness company and questioned why it was lower like I was in a police interview. I always test correctly as it’s my name signing the test sheets and I’m liable so never blag test readings as some people do. The only explanation I could possibly give them is 1. Other circuits have now been connected running in the same containment and fan coils being connected up, which are bolted to the concrete giving numerous parallel paths and as more circuits are connected the earth reading would come down surely? It wouldnt be practical to go around every circuit on the job to disconnect every earth and eliminate parallel earths and then reconnect them again. This explanation didn’t seem to satisfy them and a few huffs and puffs and they left me too it. I’m yet to hear back from their report. Anyone got advice if I’m in the right? I was always told R1-R2s can fluctuate in time. And to be fair the first time these circuits were tested it was -2 on site. Currently it’s 10c, I’ve heard temperature can effect resistance in cable but always took that with a pinch of salt. Let us know what you think lads and ladies

 

[binky]

Test equipment can vary too, especially if batteries are getting low.

 

[Sharpend]

I would say test equipment and batteries don’t like -2 temps.

 

[Murdoch]

Test equipment, old test leads and temperature …..

 

[UNG]

I have serious doubts about the "witness" company and their operatives, I would like to assume that they were competent in testing electrical installations but the fact they are questioning the variations in the results suggests differently

As others have suggested the number of variables that can affect test results is numerous how do you achieve the exact same environmental and climatic conditions, when a tester is calibrated properly it is left to stablise in a climate controlled enviroment for 24 hours before it is calibrated so it follows that when using the meter onsite over the course of the day as the meter acclimatises to the ambient conditions the readings will change slightly, similarly the installation will also change during the day as the building is heated or cooled

As there is no requirement to record the temperature, humidity, atmospheric pressure etc and recreating the same parameters would be neigh on impossible to replicate the test under what may be considered lab conditions

One example of fluctuating test results, how often do you find when testing a socket circuit that the simple action of operating the switch a few times results in a change of the test result

 

[roys]

I remember once I got wildly different results either side of a lunch break, left my meter on top of a panel went for lunch, retested when I came back from lunch and it took a minute or two to figure out that the panel I had laid my meter on was at about 35deg C, it totally threw my readings out until my meter had cooled down.

 

[UNG]

I remember once I got wildly different results either side of a lunch break, left my meter on top of a panel went for lunch, retested when I came back from lunch and it took a minute or two to figure out that the panel I had laid my meter on was at about 35deg C, it totally threw my readings out until my meter had cooled down.

And if the leads were laying on the panel they will have been affected by the heat as well