House earth test

[JohnRoss]

Always been a bit suspicious of the quality of the electrical earth in this house. Live and neutral single phase mains feed with a local earth. No sign of an earth rod but I suspect some sort of earth mat somewhere yet to be found. As an experiment I connected a 12 dc PSU between the house earth and neutral and measured the current flowing and the voltage across neutral and earth. This gave by calculation a resistance of 209 ohms and 213 ohms with the polarity reversed. I then hammered in one of these copper earthing stakes near the house and connected it to the house earth circuit, i.e. in parallel with the assumed earth mat. The current was significantly increased and the resistance calculated at 86.7 ohms and 93 ohms with the polarity reversed. I understand that the earth loop impedance should be less than 100 ohms so I am tempted to install the earthing stake properly and permanently connected to the house earth. Now I have an earth link near the back door and the top end is wired to the earth bar in the consumer unit and the bottom end has a wire that I assume goes to the earth mat. I also assume that the neutral is earthed at the sub-station and that only a small percentage of the calculated resistance is due to the resistance of the neutral conductor between the house and the sub-station.

 

The questions are has my experiment any validity? If I do install the earth stake what size wire should I use? What type of connector should I use to connect to the brass clamp on the stake and what connector type should I use to connect this wire to the bottom end of the earth link, spade? I could repeat the experiment using a low voltage ac supply from a transformer but I would have thought doing it with a dc supply and reversing the connections should be almost as good and I would be suprised if the reactive elements, line series inductance and shunt capacity, would make a great deal of difference if I used ac but it must be said that I am a bit out of my comfort zone here. What do you think folks?.........JR

[Théière]

JohnRoss, have you posted this on the other forum? There were lots of posts on the subject and I gather (no real knowledge) that the test is carried out under typical French attention to detail. 3 seperate earth pins are put into the ground at specific distances to your own earth and then tested and you can get certificate. It does sound like you do need that extra earth from your tests but remember some time back there can be problems with multiple earths, you need just one good one.

 

[La Guerriere]

If you have a new-ish house, the earth may be taken to the bottom of the foundation trench, with a wire laid along the bottom of the trench rather than a rod. It's a good system if you have it, but it's obviously not a good wheeze for renovations.

[Gluestick]

John:

Useful article here: and a URL for Meggar's site which allows free download of a comprehensive file.

See here:

EDIT:

Megger's site seems to require a log-in to download:

However here it is anyway! See here:

[JohnRoss]

Thanks for the replies folks and I have looked at the other forum to see what has been said about this subject. The rod, if used and I think I will install it as the tests showed considerable improvement, would be connected to the same point as the existing earth so I don't anticipate that would cause a problem. I will trace the lead running from the bottom of the borne link thingy and see if I can determine what I have got as an earth at the moment. Thanks for the link to the two articles as they do make interesting reading......................Cheers........................JR

[JohnRoss]

Carried out one more test with interesting results. I wired a light load, did try a light bulb but the switch on surge tripped the 500mA differential trip as you might expect cold resistance of the bulb being very low,  using a 25W soldering iron as the load between live and the house earth. I measured the potential between the house earth and neutral and the current flowing through the soldering iron. Calculation of resistance gave almost the same figures as my previous experiment using the d.c. power supply so the house earth is just not good enough. The neutral was only a volt above earth before the experiment so I think any error will be quite small and I took this into account in the calculation of the earth resistance.Will now look for the wire coming out of the house on the borne earth link to connect the earth stake.........JR

[JohnRoss]

Carried out the test as suggested on page 14 of the article suggested by Gluestick, the one with the three electrodes and using a transformer having a 30V secondary as the voltage source. With the earth link in place the current and voltage readings give an earth impedance of 166 ohms. With the earth link removed the impedance increased to 266 ohms. Neither of these are low enough so yes there needs to be an improvement in the house earth. However the difference in these readings gave me cause for concern so I connected the transformer to the top of the earth link unit, i.e. the earth wire coming from the consumer unit. The other side of the transformer secondary was still connected to the furthest electrode. The current flowing indicates to me that somewhere in the house wiring there is another connection to earth. This would explain why the impedance of the house earth rose when I disconnected the link. We clearly have the proper earth in parallel with another earth connection in the house.

I removed all plugs in the house including the water heater but no change. I suppose there could be an earth neutral short somewhere or a deliberate connection between earth and a water pipe but if so I have not seen it and but it could be buried in a wall somewhere I guess. Can anyone suggest anywhere else this apparent second earth could be. I would hate to have to lift every earth wire that comes onto the consumer unit earth bus bar to identify the circuit....................................JR  

[andysparks]

Sounds like you are getting a bit bogged down. You really need a friendly electrician with a loop tester to make certain the earth is good, Over here in blighty up to 200ohms is deemed to be acceptable by some, but NICEIC like it to be below 100.

[JohnRoss]

I am not so worried about the quality of the house earth as I know I can improve it following some of the suggestions received here and in another place but I am a little concerned about this second apparent earth somewhere in the house.

So on two fronts then. 1/ I need to improve the house earth and 2/ I feel it would be advisable to find why the top of the earth link is connected to earth somewhere else with the link removed.

Another issue may be that water pipes in the house are all in copper but the incoming feed pipe is plastic. Do French regs say that the copper water pipes should be bonded to earth? If so would it be sufficient to connect the nearest pipe to the consumer unit (tableau électrique) earth bus bar? Thanks for the help folks.............................JR

[Chancer]

JR

You are probably picking up a resistance to earth caused by the liason equipotentielle, do you have an incoming water pipe in lead by any chance?

The test borne that you have found is used to check the resistance of the piquet de terre, I dont suppose that anyone has ever thought or bothered to take an upstream measurement, as you rightly say it can only add to your security and you are right to try and improve your main earth to the figure required by Consuel.

Fro your information it should be:

Less than 100 ohms for 500ma disjoncteur de branchement

...............167...............300ma...........................................

...............500...............100ma...........................................

[JohnRoss]

I assume from what you say you refer to the practice of connecting all pipes, gas,water etc, to the earth bus bar in the consumer unit. We have no gas and the mains water pipe coming into the house is plastic. I have seen most of the pipework in the house, under the bath, in the cellar and round the chauffe-eau, all the places you might expect to find earth wires connected, and found none. The pipe runs between the walls might have a connection I suppose but I doubt that would be the case as I would have thought such bonding should be visible for inspection purposes.

 I will remove the earth link again and inject current into the top end, the end that is wired to the consumer unit, and measure current flow again but with the 500mA differential trip, between the incoming mains and the consumer unit, first on and then in the off position. I will have to use d.c. as the disjoncteur I note is 2 pole so I assume it will also disconnect the neutral and with it off I will have no volts for the transformer. If the injected current still flows then I think that would indicate that somewhere there is a connection to earth in the house and not to neutral. If the current stops or is significantly reduced then I would assume that there is an earth neutral short somewhere which would concern me! What do you think?............................JR

[Chancer]

I think my brain hurts [:)]

I will read it again later when I can digect what you say or perhaps tomorrow morning (I have problems reading at the moment my eyesight is better first thing when the medications ahve worn off, you can probbaly tell from my terrible spelling that I can barely read what I type.

If I can read and get my head around what you have written I will comment.

[andysparks]

Have just wired my house & fortunately for me what plumbing I have is all plastic, so for your installation I would say, yes you should make sure you have continuity between all the copper pipework & the earth bar. That copper is most probably whats giving you that earth you are looking for.

[JohnRoss]

Got some interesting results as a result of the test I described above. Removed the earth link and checked for volts across the link terminals, no reading. Checked resistance between the copper water pipes and the top of the earth link with the link removed. This gave a reading of 0.3 ohms between the pipe and the top of the link, i.e. the earth bus bar in the consumer unit. So somewhere there is a good connection to electrical earth for the water system in the house. Not the chauffe-eau as I unplugged everything before doing the test. I then connected my jump starter 12V pack across the link terminals, with the link still removed, and noted a current flowing. I tripped the 500mA disjoncteur differential, 2 pole so I assume it chops neutral as well as live, to cut off the ac feed to the consumer unit and there was no change in the current observed flowing from the 12V pack. So no short between earth and neutral then. A cold resistance check between earth and neutral agrees with this which is good news.

I think I will bond an earth connection between the water pipes and the earth bus bar in the consumer unit just to be certain all is proper. The pipes in the house are copper but the water main feed from the road is plastic. I will also install an earth rod somewhere. One is tempted to put it in the ground in my damp cellar and connect it to a water pipe down there but I suppose that might be against the rules, pity if true. If I have to put it outside can I run a cable from it to the bottom of the earth link in buried gaine of the ICTA grey type or should it be red?.................................JR

PS Thanks for your advice and interest folks, every little helps!

[powerdesal]

[quote user="JohnRoss"]

 If I have to put it outside can I run a cable from it to the bottom of the earth link in buried gaine of the ICTA grey type or should it be red?.................................JR

PS Thanks for your advice and interest folks, every little helps!

[/quote]

I must admit that I did mine in grey gaine, Its a while ago and I probably checked the regs (because I usually do) so IMHO grey gaine will be OK.

[JohnRoss]

Ok and thanks for the info. Some very helpful person in another place has said that bolting the water pipes to an earth stake is a no no so an outside earth it is then. It was also suggested that a 25mm squared earthing wire could connect the stake to the bottom of the earth link and that wire could be uninsulated and buried to further improve earth contact. From this I conclude that corrosion is not considered a problem as far as running a cable to an earthing stake is concerned. Would this be a 6mm diameter wire I wonder and would that be multistrand or single? Don't remember seeing any at Brico places but will have another look this weekend.

The one piece of test equipment I lack is an earth impedance meter though I would have thought the three electrode test described before should be o.k. I have a digital multimeter, Avo 8, 'scope and a E&V 500V Megger. What other kit would a proper electrician have? 

It has been suggested that the chauffe-eau should be wired in and not, as we have, on a plug and socket. Ours is on it's own circuit with a 20A socket. It was there when we moved in and when we had the chauffe-eau replaced the bloke made no comment about it. Is that a problem or is it something that has come in of late and may not apply to older installations.

Anyway thanks for the help and support folks for which I am grateful...........JR

[andysparks]

I dont know how close you are to me, but when I actually manage to get to france, my place is about 20 mins south of La Roche, I am hoping to be there in June, & can bring a Loop tester along.

[La Guerriere]

Does one get a different earth reading in dry and wet conditions ? We have a spring right under the house and it's permenantly damp down there ....

[JohnRoss]

I would have thought that a damp soil provides better contact with the earth electrodes as suggested by the table for soil resistivity on this site  http://www.electrical-installation.org/wiki/Installation_and_measurements_of_earth_electrodes which I found this morning. I imagine that the pH of the soil is another factor in determining contact resistance and/soil conductivity.................................JR

PS Thanks andysparks for your kind offer and will see how we get on and get back to you if necessary, cheers.

 

[andysparks]

Damp ground is definately better, my place has granite/stone below it, the only way to get a good earth was to connect it to the steel grid when the slab went down

[Chancer]

If you like me have gone native and pee outside the piquet de terre is the best place to do it, especially just before a Consuel inspection [;-)]

I didnt have a tester yet the Consuel application form asks for a reading, I just wrote down 59 ohms but made sure that I had all the material to hand to fit a second piquet in parrallel if necessary, he did the proper test, it was higher than the figure that I had written but still low enough.

Interestingly he told me to use the same piquet for the remaining 6 logements.

[LEO]

Hi  Chancer,

almost midnight and your talking about ohms etc.

surely there is more to life !

Kind and thoughtful regards from,

Leo

[Chancer]

Not in these parts sadly Leo [:(]