Neutral Earth Loop Impedance Testing.

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You have not been paying attention have you?

erm, I have just watched "Olympus has Fallen." brain now dazed and confused. More research in old books tomorrow.


Good night.


Z.

It has slipped back to page three.


 Andy

L-N for volt drop and verify fuse blowing,

and

L-E for what nowadays we'd call ADS, but basically CPC continuity, and Zs


to test N-E requires a small transformer with the mains primary wired L_N,  with a low voltage high current secondary, typically no more than 12V (though if it is a proper early one, it may be about 40V open circuit and 16 A short circuit current, - the  lower voltage versions were needed, and some precision was sacrificed. to work with the VOELCB. ), to inject current up the neutral and get it back down the earth.


Nowadays, this is a test of minimal use if the other two  have passed, except for either identifying N-E faults, or estimating the distance to the substation on a TN-S system, or verifying the NE link on PME.


In the dark ages when houses off earth were perhaps more common, an N-E test was a much safer first test, as a way of verifying  a low impedance earth, either TN-s, or later TNC-s  was actually present before proceeding to the L-E tests - as in the time before RCDs, and also before meters had an internal  microprocessor to provide an automatic 50V lock out and to limit the test duration, to perform  an L-E a test on a faulty installation could liven  up all the metal in the house to full mains voltage for as long as the button was pressed.


EDIT, in the box  there may be an adapter or break-in connection that allows the NE test transformer to double up as a wander lead for R2, and that is where the early 13A plugs with the little access window  exposing the earth pin come in to their own, allowing what we would now call r2 to be measured without unplugging a working load. In effect with the fly lead adapters  the NE range becomes  a mains powered general purpose low resistance meter.

Thanks for that Mike,

                                      that is very useful information. The device does have a facility to break into the N-E and E-E part of the test circuit. There are terminals on the front panel with removable banana plugs. The instructions state that it is possible to test the resistance of a conductor (presumably earth in most cases) with a 16 Amp. 40 Volt supply. Two green leads are supplied with croc. clips on the far ends.


I have serviced the tester and it works well. It is just so well made with 60s/70s electronics and a very heavy transformer. The thermal safety cut-outs are short lengths of 18 s.w.g. 60/40 tin/lead solder, a spare length is supplied inside the case. The innards also contain some very beefy resistors. All inside a solid wooden box.


I could never afford Clare equipment when younger, so it is a privilege to own a piece, albeit elderly, now.


A little chart inside the box lid cover states "Fuse ratings for compliance with I.E.E. Reg E5."


A 60 Amp fuse required a L-E loop impedance of 1.3 Ohms max. (for a 100 Amp. M.C.B. 1.0 Ohms max.)


A 30 Amp fuse required a L-E. loop impedance of 2.6 Ohms max. 


Bye,


Z.

I would not use that instrument on a site - it is similar to the Evershed & Vignoles ones that I had (before I took them apart for the bits inside ?).and they would now be considered to be too dangerous to use.


Mine would inject around 25A for about 30 seconds to do a Live - Earth loop test.  This would blow faulty connections apart and raise the potential on the metal parts of the installation to around 240 volts.  So potentially lethal to any occupants in the building. 


Regards


Geoff Blackwell

Friday afternoon I had a conversation with a advisor in a test equipment manufacturers’ technical department, one of his comments was that I may need to adjust the test time of my no trip loop testers from 20 seconds to 10 seconds to stop Type A RCDs tripping when testing.


The testing regime of the tester you have is the equivalent of hitting the circuit with an electrical sledgehammer, but then that’s what Hager is saying we should do with their 30 mA Type A RCDs If you don’t achieve the required X5 test results by hitting them with 250 mA.


Trying  it on the rusty conduit will result in arcs flying with a crack and a bang.


Andy