Surprise at symptoms due to earth loss!

[ericmark]

Call from daughter-in-law got a shock getting into the caravan so picked up meters and went up to their house.

Plugged in earth loop impedance tester which recorded 140 volt line - earth and lit the red do not test lamp which seems it indicate line - neutral reversal.

It took some time to find where the fault was not helped by tamper proof screws but transpired lost earth on an extension reel being used for the supply.

I fully expected the RCD to trip once reconnected but it did not. Tested RCD 18ms at 5 times and 32ms at 30ma so RCD OK. I was not daft enough to grab caravan myself but told like touching an electric fence.

Clearly something in caravan must be leaking but not enough to take out the trip either fridge or water heater but did not look further once main fault found my son can do that emergency was over.

However was surprised that such a belt was experienced due to loss of earth wire yet not enough to trip RCD once earth reconnected. Whole house TT. Two RCD's in series both 30ma but second one not tested as first one trips first.

This weekend going to install the dedicated supply to caravan with it's own earth rod ready for when house is made TN-C-S.

If my son had been nearer I would have kicked him. His comment was "Oh did my dad actually find a fault then?" good job he was other end of the phone.

 

[FDJ]

Possibly an appliance with a "delta suppressor" with capacitors L-N, L-E and N-E.
Would just have been a few uA leakage.
Frank.

 

[JohnW2]

.... transpired lost earth on an extension reel being used for the supply. I fully expected the RCD to trip once reconnected but it did not. Tested RCD 18ms at 5 times and 32ms at 30ma so RCD OK. I was not daft enough to grab caravan myself but told like touching an electric fence.

Did you measure the potential of the caravan (relative to earth) before and/or after you mended the extension?

However was surprised that such a belt was experienced due to loss of earth wire yet not enough to trip RCD once earth reconnected. Whole house TT. Two RCD's in series both 30ma but second one not tested as first one trips first.

Do I take it that you are fairly sure that it was not a static shock which she experienced?

Kind Regards, John

 

[ericmark]

The loop impedance tester measured 140 volt line - earth before repair and 240 volt after so one would assume 100 volt caravan body to earth.

I am certain not static by the fact a real fault was found and when she switched of fridge and water heater so no longer got a shock. She though it was a master switch in fact it only switched central heating, water heating and fridge the socket was still live.

What was the surprise was a significant shock did not trip either of the RCD's and does point out that RCD's have only a limited protection to personnel.

At first I thought it was due to distance between house and caravan after it had sank overnight the wheels and steady jacks where placed on concrete slabs which isolated the caravan body from the ground. Until this no problems but a combination of heavy rain and the slabs resulted in the caravan step also metal being a different voltage to caravan.

It was because I had not expected this to happen I thought it worth reporting. It seems daft in a way that the original ELCB-v would have tripped but the ELCB-c failed to trip as although 100 volt the current flow was too low.

 

[JohnW2]

The loop impedance tester measured 140 volt line - earth before repair and 240 volt after so one would assume 100 volt caravan body to earth.

When you say 140V "line to earth", do you mean from line to the earthing system/CPCs on the caravan side of the fault in the extension lead? If so, that "earth" (and the caravan) was surely just 'floating', so that the 140V measurement would be pretty meaningless, at least in terms of ability to supply any appreciable current? If it were floating, then about half of the supply voltage (as measured with a high-Z voltmeter) wouldn't be surprising, would it? It would have been the voltage to 'true earth' that would have really mattered.

I am certain not static by the fact a real fault was found and when she switched of fridge and water heater so no longer got a shock. She though it was a master switch in fact it only switched central heating, water heating and fridge the socket was still live.

I'm not sure I understand all that. If by 'real fault', you mean the 'earth' fault in the extension lead, then that could facilitate a static shock (or, rather, an intact CPC would help to avoid one). Are you perhaps saying that you have also identified some other fault - and, if not, what do you think could have resulted in a 'real' (relatively low impedance source) high voltage appearing on the caravan? If your daughter-in-law experienced a 'real' shock, it must have come from 'somewhere' - an 'earth fault', on its own, obviously would not result in such a 'real' voltage (as measurable with a low-Z meter), or a shock.

What was the surprise was a significant shock did not trip either of the RCD's and does point out that RCD's have only a limited protection to personnel.

I suspect that a lot of people don't realise how nasty a 30mA shock is (or, at least, can be). Much lower shock currents than that, far too low to trip a 30mA RCD, can be extremely unpleasant and frightening, and can sometimes cause injury.

Kind Regards, John

 

[mfarrow]

Our old consumer unit was split load with each side including an isolator.

The Sangamo timer for the downstairs lights took it's live from the non-RCD side (on the down lights MCB), but the neutral was on the RCD busbar.

I found this out when modifying sockets with RCD side all off, and touching neutrals. Not nice.

 

[westie101]

Ericmark

I'm sure that similar symptoms have been reported on that other forum about caravans we arre both members of (I'm under a different name)
With the fault being the same, one possibility I think discussed was the earth being in contact with the live in the affected plug

 

[JohnW2]

With the fault being the same, one possibility I think discussed was the earth being in contact with the live in the affected plug

That certainly sounds more credible. Loss of 'earth' alone obviously cannot result in 'electrified caravans' or shocks.

Kind Regards, John

 

[JohnW2]

Duplicate deleted

 

[westie101]

Yes, found one

http://www.caravantalk.co.uk/community/topic/72338-caravan-chassis-live-when-on-ehu/?hl=shock


Back to the original point, bear in mind that the human body can feel and react to currents far below 30mA.
Given that most caravans in full operation have a number of heating elements leakage from these would be enough to be detected by the body, but still below the 30ma threshold

 

[ericmark]

I think the steady legs were enough to earth the caravan until it was placed on concrete blocks that was where the problem started.

I has intended to use the ohm meter but on arrival found the leads damaged so just grabbed any meter to see what is said although not pressing the button on the loop impedance tester it did show a fault so since easy to plug in I slowly moved from point to point to find where that fault was.

The house is being re-wired which is why they are living in the caravan just 4 sockets at the consumer unit the rest of house is dead so extension leads galore. It also means there is very little bonded as yet and that was my first thought could the earth rod be too far away.

Clearly something in the caravan is leaking current but that's down to my son to find and if required correct not a clue as to ohms per volt of meter so working out the current is impossible.

The voltage could have been caused by the meter it's self it was just a surprise that she got a belt just through lack of earth does show how important the earth is.

 

[bernardgreen]

The "earth" wire ( aka the CPC ) of the caravan when not connected to the ground ( or the neutral ) will float to a voltage somewhere between the potentials on the live and neutral in the caravan. The actual voltage will depend on the ratio of capacitive couplings Live to CPC and Neutral to CPC

So if the caravan frame is bonded to the caravan's "earth wire" the frame also be at that floating voltage. Enough coupled energy to create a perceivable ( and maybe painful ) shock when a person on ground touches the caravan.

 

[ericmark]

I do agree with you also it's dependent on how well they touch it. I do remember in my younger days when more foolish holding the leads of a megger tightly while the apprentice turned the handle and because held tight the voltage could not build up so my shock was minimal. Then get the apprentice to put leads on wire while I turn handle but he is apprehensive so just lightly held as a result he got a good belt.

So grabbing the door frame to pull ones self in would lightly give one less of a belt than just brushing against the frame. Once on gets a belt one would only dab at any contact so again more voltage than it grabbed hold of it firmly.

Washing machine in the house was also on the un-earthed supply and it had been raining so ground rather wet. As I said I am not daft enough to touch to see if I get a shock so no real idea how bad it was.

 

[JohnW2]

So if the caravan frame is bonded to the caravan's "earth wire" the frame also be at that floating voltage. Enough coupled energy to create a perceivable ( and maybe painful ) shock when a person on ground touches the caravan.

Indeed - I made that point earlier. 'Perceivable', perhaps, but I would personally doubt that the reactance of the capacitive coupling would be low enough for the shock to be remotely painful.

Even if, rather than capacitive coupling, there were (as I think eric has suggested), a moderate impedance L-CPC fault in one of the caravan appliances, it would (given non-operation of a tested 30mA RCD after the CPC was reconnected) be difficult to get a high enough shock current to result in much of a shock. If the Zs (after reconnection of CPC) of the circuit (TT installation) were about 100&#937;, and the fault were producing <30mA through that path, then (before restoration of the CPC), if the path to earth through a person were, say, about 2k&#937;, then that same fault would result in a current through the person of <1.5mA, which is probably on the borderline of perceivability.

Kind Regards, John

 

[securespark]

Do I take it that you are fairly sure that it was not a static shock which she experienced?

Is it a static caravan?