L&N reverse (experiments)

[Andy™]

NOTICE: THIS IS SOMEWHAT DANGEROUS IF YOUR NOT FULLY AWARE OF WHAT YOUR DOING. DONT TRY IT AT HOME (even i went to my workshop to try this)

following KME's thread, here's some experiments replicating his problem / results etc

the 4 way extension is fed from a TT suppply (L&N taken from TNS supply, earth connected directly to a rod, 203 ohms).

the multimeter is connected between the earth rod & the TNS earth so you can see exactly what voltage the rod is at

here's the setup as normal. neons are not as clear in the pic as i thought they would be. in this, all 3 lights are lit (polarity correct). megger also says polarity correct

KME said there was a 13 ohm fault L-E. my substitue is a 2KW heater (around 30 ohm) wired between live & earth (grey plug) not quite 13 ohm, but its close enough, and i couldnt be bothered altering the wiring in another heater to get it closer to 13 ohm.

also, fluke T-140 did buzz to warn of live conductor when it touched either live or earth.

dont own a neon tester so couldnt check that (sorry steps!)

not too clear, but plug in tester is saying L&N reversed. although the middle neon was very dimly lit.

the TT 'earth' is now over 200V compared to the TNS earth

and finally

Andy,It has a path trust me!

IT may be you, it may be something else, but it has a path!

You need to think about Op Amps and the gain that can be achieved in analogue electronics with say a Darlington pair?...

looks like you were right

fluke T-140 was hung in the air by a live conductor. on its own, it didnt warn of anything live.

but as soon as any part of the tester (body, flex, other probe (not tip) came into contact with anything else (me, me with insulated gloves, concrete floor, flex of live conductor it was hanging from etc), it did warn it was live.

 

[Apache]

I like experiments!

 

[kme]

I like it Andy; but here is my (slight) concern with these experiments v the actual scenario.

When the faulty circuit was energised, the earthing all through the house rose up to that potential! Effectively, we`re in a form of "Faraday cage", unless we find a lump of steelwork embedded in the concrete floor - that would get painful in a hurry .

Obviously, that is a condition that isn`t easily replicated in anyone`s shed (with any degree of safety) ; And MY T-140 equivalent (the Di-log) didn`t see earth as an issue, unless it was referenced to neutral (which it also would if there WAS a RP situation.

For info - my test equip actually used is:

Fluke 1652

Di-log 6780 (combivolt 1) voltage tester

Martindale E-Z check socket tester / loop tester (Xtra Non-Trip Earth Loop Impedance Indicator)

I have some other meters (including the kewtech KT64 - but I`m pretty sure it would have agreed with the others, TBH!)

Watching with interest......

KME

 

[Andy™]

i can see the earthing in the house would rise to a high voltage - thats why i added the multimeter between TT earth and my TNS earth to show what it would be at

and as for your T-140 'equivilent', i also have got a di-log i got when i 'broke' my t-140. it didnt work on single probe test. so i ended up taking apart my T-140 and fixing it (cable coming out of bottom broke. turns out to be a fairly easy fix, and an even easier fix to stop it happening again) and the di-log is now a spare

 

[Manator]

THIS EXPERIMENT WAS CARRIED OUT BY A COMPETENT PERSON AND SHOULD NOT BE REPLICATED WITHOUT FULL KNOWLEDGE OF THE DANGERS.

Just thought I would emphasise the warning.

 

[steptoe]

THIS EXPERIMENT WAS CARRIED OUT BY A COMPETENT PERSON AND SHOULD NOT BE REPLICATED WITHOUT FULL KNOWLEDGE OF THE DANGERS.Just thought I would emphasise the warning.

correct,

me and Deke were only observing,

 

[ProDave]

How hot did the heater get?

If you still have it set up, put the multimeter to current, and tell us how much current was flowing down the TT earth.

I'll bet if you tested it with a neon, you would still have found the L was the only one that light the neon, not the neutral. Can you not spend a couple of quid on one, just to complete the experiment (and IMO prove the usefulness, when used correctly, of such a tool)

I'll also bet the neon would light up on the TT earth, now at an elevated potential.

 

[Andy™]

not set up any more, but it wont take long to get it set up again.

heater didnt get hot. the fan didnt even do anything.

no idea about current to earth. didnt think to get the clamp meter out at the time

ill get hold of a neon tester and try that.

any other ideas you lot want tested?

 

[pickleye]

Aahhhh.... learned something new. I read the original thread with interest.

Thanks for taking the time do the experiment. I don

 

[Phoenix]

Some further ideas to muse over for you all...

I'm assuming that andy's earth rod was high at 203 ohms because it was a screwdriver pushed into the ground or something, but to put some real numbers into a scenero, lets say you had a house in the sticks and your measured Ze was 85 ohms, and lets say this was made up of 70ohms between the local electrode and the physical mass of the earth and 15 ohms for the DNO owned one at the pole pig transformer, and the live conductor between house and transformer obviously negiable at this scale.... You might wish to draw this out.

A fault of 15 ohms occurs between live and cpc and the vo-elcb is not working.... so thats 15+85 = 100 ohms, the current that will flow will be 230/100 = 2.3A which won't blow any fuses, but will get the worms comming up out the ground . A 'step potential' will form around both earth electrodes, you ought to take small steps around them... and animals which a large distance between feet such as horses probably ought to stay well away!pr

Now lets look at the rod at the transformer pole, its 15ohms and there is a current of 2.3A flowing, so thats a voltage of 15*2= 34.5v between the pysical earth (which all the houses are picking up via their electrodes) and the stair point of the transformer (which all the houses get via the neutral conductor), which you would be exposed to if you were working on one of these properties and only isolated the live to work.... and safe isolation procedure would not always pick it up, becuase fault in house down road might come on after you had tested!. Now if this property you were working in (you were only there to hcange pendant set and you have finished ) happened to have a neutral to earth fault, the RCD might not trip straight away on a TT, but it would when there is 34.5v between earth and neutral, so if the fault in the original house was on a strapper of the stairway 2 way switching, we could have a situation where switching off the stair lights trips the RCD in a house a few doors down

The voltage on the earth is comming from the earth being 'dragged' in potential through the fault towards the potenial on the live, now if we assume that the house you were called to put the new pendant up is on a different phase to the first house, its a different phase the earth is being dragged to, so you might for example see something along the lines of (and I've lost enthisum for doing the numbers fully!) l-n = 235v (normal), n-e = 32v, l-e = 250v

 

[steptoe]

andy's earth rod was high at 203 ohms

thats not really that high for an earth rod TBH,

its pretty good for something that he has just stuck in the ground for an experiment without (probably) much thought into its positioning or anything.

 

[Andy™]

Some further ideas to muse over for you all...I'm assuming that andy's earth rod was high at 203 ohms because it was a screwdriver pushed into the ground or something,

it was 203 ohms because in KME's situation Ze was 200 and i was trying to match that. it was a real earth rod used, but it wasnt fully in the ground, depth & moisture (OK, i poured water on it) was altered to get it as close to 200 as possible

i also have TP at my unit so for the next experiment i may use different phases for each 'property'

also some interesting theory there. may have to include some of that next time

 

[steptoe]

[quote name='Andy

 

[Andy™]

may aswell do it properly. no point in bodging it and getting different results

if the Ra was much lower, then the voltage would be lower, and the 'L&N reversed' may not happen. no sure what the 'cut off' point is, but thats another thing to be tested

 

[steptoe]

:Salute