Jackie Posted August 31, 2004 Share Posted August 31, 2004 Like many properties in France ours has an earth stake somewhere, I have not found it yet, and an earth link on two insulators near the back door. Using a Megger and or a multimeter is there any better test that I can do other than the one shown below? I shove a length of copper pipe into the ground outside and connect a long wire to it. I then connect the Megger/Multimeter to this wire and connect the otherside of the Megger/Meter to the appliance or socket being tested, earth pin of course. On winding the handle I get a low resistance reading. It is difficult to get an accurate figure but looks to be less than 100 ohms. I well understand that this is not putting any significant current through the system....John in 79 Link to comment Share on other sites More sharing options...
Punch Posted August 31, 2004 Share Posted August 31, 2004 [quote]Like many properties in France ours has an earth stake somewhere, I have not found it yet, and an earth link on two insulators near the back door. Using a Megger and or a multimeter is there any bette...[/quote]I would try to beg, steal or borrow (now there's a cue for a song!) a proper earth resistance tester otherwise you will never get a true reading. If you are having a Consuel inspection then it is important that the reading is below 100 ohms as you seem to know already? Depending on the soil conditions, results can vary from day to day due to dry ground etc, so 100 ohms is the absolute maximum you should allow.Punch Link to comment Share on other sites More sharing options...
Jackie Posted August 31, 2004 Author Share Posted August 31, 2004 Thanks for the response. No I have not got an inspection but just checking for my own peace of mind. I take it that an earth resistance tester would put quite a large pulsed alternating current through the earth? I am afraid I don't know anyone who would have one but I have got some meaty PSUs and could arrange to put a controlled few amps through the earth stake and measure the current flowing and voltage across the source to calculate resistance if that is any good, would this do? Mind you it would be d.c. or I do also have some low output voltage, high secondary current rating mains transformers that I could use instead...........John in 79 Link to comment Share on other sites More sharing options...
Nick Trollope Posted September 1, 2004 Share Posted September 1, 2004 [quote]I would try to beg, steal or borrow (now there's a cue for a song!) a proper earth resistance tester otherwise you will never get a true reading. If you are having a Consuel inspection then it is impo...[/quote]Hello PaulWhat is the actual spec for earth resistance? I guess it is 100R @ 1M, but at what "depth" for the test probe? (If the probe is out of the ground, the resistance will be infinite. If you bury an infinitly long probe, then I guess the resistance must be zero....???) There must be a defined spec, therefore... Eh?Yrs, confused.... Link to comment Share on other sites More sharing options...
Jackie Posted September 1, 2004 Author Share Posted September 1, 2004 (If the probe is out of the ground, the resistance will be infinite. If you bury an infinitly long probe, then I guess the resistance must be zero....???) I suspect it is a law of diminishing returns. The further you push the probe in the greater the number of current paths. After a certain distance the resistance change would be negligible I think. This was the case with electric fishing electrode size, something I was involved with once upon a time.........John in 79 Link to comment Share on other sites More sharing options...
Nick Trollope Posted September 1, 2004 Share Posted September 1, 2004 [quote](If the probe is out of the ground, the resistance will be infinite. If you bury an infinitly long probe, then I guess the resistance must be zero....???) I suspect it is a law of diminishing return...[/quote]Indeed so. In the "law of diminishing returns", at infinity the return is zero...... Link to comment Share on other sites More sharing options...
Punch Posted September 1, 2004 Share Posted September 1, 2004 [quote]Hello Paul What is the actual spec for earth resistance? I guess it is 100R @ 1M, but at what "depth" for the test probe? (If the probe is out of the ground, the resistance will be infinite. If you ...[/quote]Nick, I don't have any calculations for depth in respect of earth resistance, in fact as soil resistance is an variable unknown factor due to the many diffrent soil types, I doubt if it could be calculated.Howver every earth electrode has a definite electrical resistance to earth.Current flowing from the electrode to the general mass of earth has to traverse concentric layers of soil immediately surrounding the electrode. Since the soil is a relatively poor conductor of electricity and as the cross sectional areas of the layers of soil nearest to the electrode are small, the result is of a "graded" resistance concentrated mainly in the soil surrounding the electrode. In fact the suface of the soil surrounding the electrode can become "live" under fault conditions and this is shown in some agicultural incidents where livestock especially cows have been killed by voltages not much more than 25 volts. 50 volts is supposed to be the limit for humans! This is how the regs arrive at 100 ohms maximum, as this would limit the fault voltage to 50V with a inter differentielle (RCD) of 500 milliamp trip current. John - Testing the earth rod resistance. Proprietary testers are much easier than the method below which comes from my apprenticeship days ;An alternating current of a steady value is passed between your earth electrode under test (lets' call it T) and an auxilliary electrode (T1) placed at such a distance from (T) that the resistance areas of the two electrodes do not overlap. I would recommend 6 metres minimum for this. A second auxilliary electrode (T2) which can be a metal spike driven into the ground, is inserted halfway between (T) and (T1), and the voltage drop between (T) and (T2) measured. The resistance of the earth electrodes is then the voltage between (T) and (T2) divided by the current flowing between (T) and (T1), providing THERE IS NO OVERLAP OF THE RESISTANCE.To check that the value of the earth electodes resistance is a true value,two readings are taken with the second auxilliary electrode (T2) 6m further from, and 6m nearer to (T) respectively. If the three results are substantially in agreement, the avearge of the three readings is taken as the resitsnce of the earth electode under test (T). If there is no such aggreement then the tests are repeated with the diatnce between (T) and (T1) increased.The test is made with either with current at normal power frequency, in which case the resistance of the voltmeter used must be high ( in the order of 200 ohms per volt), OR with alternating current from an earth tester which produces ac current via a rectifier, and has a ditrect reading ohmeter (such as a megga). The source of the AC current if using the first method must be isolated from the mains supply by the use of a double wound transformer. And please make sure that the actual elecrode under test is isolated from the rest of the house earthing, by undoing the barrette de mesure.Phew, now I'm off to get some real "Tea" Punch Link to comment Share on other sites More sharing options...
SC Posted September 1, 2004 Share Posted September 1, 2004 I may be a bit adrift here, but wouldn't the EDF test the resistance difference between the supplied neutral and the installation's earth conection? Surely that's what the test is all about.I think (IMHO) that a good connection to ground is important, especially these days for protecting electronic equipment from lightning strikes, but from a personal safety point of view, less important than making sure that you have a working disjoncteur differential fitted.sc Link to comment Share on other sites More sharing options...
Punch Posted September 2, 2004 Share Posted September 2, 2004 [quote]I may be a bit adrift here, but wouldn't the EDF test the resistance difference between the supplied neutral and the installation's earth conection? Surely that's what the test is all about. I thin...[/quote]In a Domestic situation, the EDF have nothing whatsover to do with the earth connection and therefore carry out no tests, unless you engage them privately and pay for an independant test. The responsibility is on the homeowner or their electrician to provide an earth point. The Consuel inspector will carry out the test if a certificate is required.regards Punch Link to comment Share on other sites More sharing options...
Jackie Posted September 2, 2004 Author Share Posted September 2, 2004 Many thanks for the comprehensive reply Paul. I will have a go later and let you know how I get on. As my old boss used to say "Most scientific".............John in 79 Link to comment Share on other sites More sharing options...
Ptostevin Posted September 2, 2004 Share Posted September 2, 2004 Hi - I have been recently reading up on le prise de terre to see what I need for an earth and generally 100 ohms is quoted as the maxium resistance but I just found a document from Promotelec (that I must have downloaded ages ago) that states 'avec un disjoncteur differential de 500mA --> 100 ohms' but 'avec un disjoncteur differential de 650mA --> 77 ohms' - this is referring to EDFs disjoncteur and the 650mA seems to be the older model. I'll have to check when I'm next in France what we have as its all pretty old! Does anyone know under what circumstances EDF change the meter, disjonteur etc free of charge?? Paul Link to comment Share on other sites More sharing options...
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