Proving an earth is a good one. | District Dave's London Underground Site

Proving an earth is a good one. Apr 12, 2011 14:00:54 GMT

Post by paterson00 on Apr 12, 2011 14:00:54 GMT

Hello fellow railway people,

I recently failed my 3C assessment on a few minor things, silly mistakes. One of which was during my cable testing. I didnt show the assessor that I had proved that the earth I was using to prove insulation from core to earth was a good earth. He asked how I would do this and my answer was that I would put a loop on from a known good core to earth at one end of the cable and at my end I would have one leg of the meter to the core of the cable and the other leg of the meter to the earth point I wanted to use, have the meter on ohms and look for a low resistance. He was satisfied with the answer but with the other couple of bits I did wrong it was too much to pass me.

I spoke with my TIC about the assessment and he asked me how else I could prove the earth point was a good one and I was suprised to find that I was stumped and couldn't think of an answer before he gave me one of his own. His response was that I could again have the meter on ohms and hold one leg to the earth I wanted to use and have the other leg to the earth pot (since I was in an REB). I took this as a good answer BUT while typing it out here it has just occured to me that it still doesnt prove a good earth, all that does is prove the earth I want to use is connected to the Earth pot which is presumably a good earth. He advised my to speak to the 3B's I work with and ask them how they would prove it.

I am going to ask the same question on a similar forum dedicated to london underground since that is where I am working at the moment and they earth everything to a "Star plate" and the earth plate goes to a rail or the air main and Im not sure how that works.

For those of you who are wondering what else I failed on, I forgot to remove the links etc when belling internal wiring to the busbar so of course I didnt prove that the wire was the correct one or that it went to the correct place on the busbar, only that it does in fact go to somewhere on the busbar, (silly mistake) and also this point which is under contention from a few of my collegues. When we arrived at the REB the circuit that we used to carry out the cable testing was powered up and although I removed the links from either end of the cable, isolating it, I didnt request the circuit to be powered down from the busbar and as a result I had a feed of 110VAC on the internal side of the MCB. The assessor wasnt happy about this saying that the Mod5 could have come to harm or I could have caused damage if the Mod5 ended up firing 1000v into the busbar by accident by placing the meter at the wrong point ie the internal side of the MCB.

Keen to hear your opinions and advise on this one since I have a re-assessment in a months time.

Thanks in advance..

Paul

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Proving an earth is a good one. Apr 13, 2011 3:06:58 GMT

Post by railtechnician on Apr 13, 2011 3:06:58 GMT

TIC....... training instructor? REB ......... breaker? Not LUL abbreviations !

What is a good earth? One has to assume a good earth is the continuous running rail where this exists, the air main where it exists, tunnel segments in a tube tunnel lined with CI rings etc but one has to remember that not all earths are equal. The best earth must certainly be the one that the circuit under test is deemed to be referenced to, i.e. the earth busbar at the supply or distribution point as found in a relay room or trackside kiosk for example.
Earthing can be a very tricky subject, however, logically speaking one has to trust the circuit reference earth and any other earth used to test a circuit should be tested and found to have no resistance when an insulation test between the two earthing points is carried out.
The method of using a 'known good core' as you describe is a suitable way of checking the integrity of the earth to be used to test a circuit. However, to prove the main earth is a good one, one method is to read the voltage from phase to that earth, one would expect to see the full phase voltage potential.

The LUL air main, continuous rail and CI tunnel rings are connected to earth busbars in equipment rooms but one must be careful because there are areas where isolations exist to separate zones, a good example is Ealing Broadway where the whole area to Hanger Lane Junction District and Central lines is electrically separated to limit any NR traction faults to earth which will or may be dissipated via LUL infrastructure.

As for proving wiring there is only one correct way to prove that a wire does run from A to B and that is to have it disconnected at both ends and prove it's continuity. It is perhaps the most basic of signal installation testing and backed up on paper with wire count schedules against which every wire must tally as a basic integrity check against the design.

As for 'working live' I know that it is frowned upon nowadays but finding faults in signalling circuitry is much easier with the power on. However, you have not stated the purpose of the test or indeed said what kind of meter you were using though I assume that you were given a 'megger' and asked to prove the circuit wiring. In that case you must isolate completely in order to prove continuity and to be absolutely sure where the two ends of any given wire terminate.