I made reference to this the other day on another thread and it (probably rightly) got lost in the replies...
Is 20A testing for conduit used as cpc still a thing? I can find no obvious mention of it in 18th GN3 or the relevant parts of s5. I also get the general impression that most modern MFT's won't produce a 20A test function (happy to be schooled on that!).
Reason for asking is that it came up in conversation a while back with someone who's a C&G assessor who was claiming it is, and it's bugged me enough to now go and look, yet I can't find anything BS7671:18 related - unless I'm being blind - that upholds that.
Found this on the IET Discussion web page vaguely remember the 25A test
"The answer, as always ... it depends what you are testing for, why, and what the associated risks are.
Yes, BS 7671 used to incorporate test at 25 A AC or DC. We used to call this the 'conduit ohmmeter' test ... whilst conduit ohmmeters used 4-wire measuring instruments, the resolution was not down in the micro-ohms or 0.1 microohms range that we see on ductor testers with kelvin measurement methods today, but instead down to perhaps 5 or 10 milliohms on a 200 milliohms full-scale deflection. The simple reason for this (and the 0.1 ohm limit on bonding resistance and appliance protective bonding circuit tests, e.g. in PAT testing) is that it's not much use going below this resolution unless you use Kelvin measurement methods because contact resistance starts to dominate.
The reason this was stopped for 'dead test' continuity testing, was precisely because some high resistance conduit joints were observed to be glowing during tests ... which is a greater concern. With certain exceptions, such tests would not be recommended today on installations unless you analyzed the risks.
25 A tests are still used for some appliances in production line tests and portable appliance tests, where you can have control over test areas (and any resulting overheating). However, with some equipment, if you accidentally pass the current between the wrong two points (where fault current would not usually flow) you might destroy PCB tracks.
BS 7671 (and BS EN 50310 for initial verification of telecomms bonding networks) doesn't prohibit test currents above 0.2 A (although instruments to BS EN 61557-series are stated).
Machinery may need a bonding test according to BS EN 60204-1, and may require currents in excess of 0.2 A, but again typically have resolutions in milliohms not microohms.
Ductor tests are used on higher current switchgear and busbar testing to the relevant standards (not BS 7671)."
