Neutral Earth Loop Impedance Testing.

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.

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.