When testing for earth contiuity is a tester that only delivers up to 200mA OK in every situation or should an instrument that has a 10A or 25A be used with certain loads or in certain types of bonding? Is this stated somewhere in the regulations?

Question

I'm looking to ensure that some power supply units that are used are adequately earthed. The units vary in load from <1kW to 15kW and have type 1 insulation class. The manufacturer recommends a bonding conuctor rated to carry 80A for the larger units. These PSUs are disconnected and moved from time ot time and I'd like to establish a guidance note for checking earth continuity when re-connected.

AJJewsbury · Accepted Answer

HIgh current (e.g. ductor) tests used to be popular when testing steel conduit etc., but that seems to have fallen out of fashion of late - I suspect there's a school of thought that high current tests aren't without their risks and really only proved that something wasn't just hanging on by a fine strand - whereas a low current test plus visual inspection (e.g. all strands go into each terminal) can produce near enough equivalent results with less hazard during the test itself. I don't recall anything demanding high current tests in BS 7671 these days. That's for fixed wiring etc though - when it comes to individual items of equipment or assemblies that come under other standards, you'd have to check the particular standards concerned ... and of course any manufacturer's instructions. 80A seems quite a lot really, and might make quite a pop if something did fail (which presumably is a possibility, otherwise it wouldn't need testing). I suspect others here have better knowledge of other standards than me.... 
 - Andy.

gkenyon · Answer

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).

When testing for earth contiuity is a tester that only delivers up to 200mA OK in every situation or should an instrument that has a 10A or 25A be used with certain loads or in certain types of bonding? Is this stated somewhere in the regulations?

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