Earth Loop Impedance Testing
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Maybe not set out explicitly, but if you don't test it, how can you be sure that it meets the 25-ohm requirement?
Equivalent in principle of operation, although the RCD's employed in British homes these days most often have a sensitivity of 30mA whereas the North American GFCI used in homes is typically 5mA.
Yes, the standard American arrangement would be classified as TN-C-S.
First letter:
T = Supply neutral is connected to earth/ground at source (terre).
Second letter:
N = Installation's protective earthing system is connected to the supply neutral.
T = Installation is earthed solely by a local earth electrode.
Subsequent letters, if present:
C = Protective earth path is combined with the neutral throughout.
S = Protective earth path is entirely separate.
C-S = Path is combined with the neutral some of the way, separate for the rest.
There are several different arrangements found in Britain.
TN-S is as you describe, with separate neutral and earth all the way back to the transformer. It can be found in rural and semi-rural areas on overhead lines, but is most common in older urban areas where the supply is by way of underground lead-sheathed cables, the lead armor being used as the protective earth/ground. There is no requirement to have additional earth electrodes at each house, although there is nothing to prevent it either, and naturally there are bound to be effective earth electrodes by way of bonded water piping etc.
TN-C-S is essentially the same as the American system, with no separate earth conductor and each house grounded to the incoming neutral. In the U.K. this bond is made at the service cutout ahead of the meter, never at the neutral busbar of the main distribution panel as is common in North America. The other major difference is that in the U.K. there is no requirement to have a local earth electrode at each house, although there is nothing to prevent one being installed if desired. Most commonly these days, the supply arrangement uses a concentric cable where the outer is the neutral.
TT is where the house relies solely upon a local earth rod and there is no direct connection between the installation's protective earthing system and any incoming conductor from the supply. This is where the loop impedance will be very much higher due to the fact that there is no solid metallic path back to the transformer.
Note that it's not unusual to find a mixture of arrangements even among homes all in one street, e.g. some houses might be wired for TN-C-S whereas others on the same supply cable in the street are arranged as TT.
Although every house in the U.S. is required to have a local earth electrode, is it not a TT arrangement because the installation's protective grounding system is connected directly to the incoming neutral and thus the local rod is not providing the sole fault-current path. The fault path is via separate grounding conductors within the home (the older 3-wire range & dryer arrangements excepted) then combined with the neutral from the main panel back to the transformer, hence TN-C-S.
You don't need a separate earth/ground conductor for the test though. It's still performed in the U.K. where TN-S is not found.
Relative to the neutral, the waveforms on each of the two "hot" conductors are 180 degrees out of phase. The transformer secondary is just a single winding with a center tap which is grounded and run to the house as the neutral (or two single windings connected in series to form the equivalent).
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In the UK we had a huge project which we call the national grid and this has resulted in a reasonable standard system. We learnt from USA mistakes and took a proven design and improved it. USA did the same when they copied our passenger jet technology.
In the USA there are many different power supply methods even in the major cities they still used DC for many years after us as it would simply cost too much to change. They still in parts use delta secondary step down transformers in some areas with the mid point of one winding being tied to earth. This includes different voltages as a result in the main it is 120 - 0 - 120 but not always. They also use different words in the main these can be translated i.e. hot = line but not always.
I would say far safer to ask on a USA forum as just too easy for you to have one of the unique USA systems.
In the USA there are many different power supply methods even in the major cities they still used DC for many years after us as it would simply cost too much to change. They still in parts use delta secondary step down transformers in some areas with the mid point of one winding being tied to earth. This includes different voltages as a result in the main it is 120 - 0 - 120 but not always. They also use different words in the main these can be translated i.e. hot = line but not always.
I would say far safer to ask on a USA forum as just too easy for you to have one of the unique USA systems.
If we are using TN_ CS here you would expect to have a fourth insulated wire run all the way back to the xtrm. Consisting of two live wire , neutral and earth. In the UK you would have only have 3 wires to accomplish this system
Live, neutral, earth. Correct?& Earth rod at the xtrm?
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"Glad we still have our rockets to get us to the moon and back"
Seriously, I think we should continue this international dialogue as we can all learn something by sharing information. Doctors communicate medical information world wide why not electricians. The point of my post was to try to determine from our external wiring if in fact I could do a Ze test from the meter back to utility xtrm. I think the confusion might be that our internal house wiring uses TN-C-S. For example a 240 volt electric oven employs two lives wire a neutral and a ground wire. The older system use to be only 3 wires and the regs changed it to four wire .Our 120volt circuits have live, neutral, and earth wire.
Since the Utilitiy does not come under the NEC regs they run 3 wires to the meter
Two lives, one insulated neutral . we refer to it as the grounded neutral conductor since it connects to xo at the transformer.
The Utilitiy side must be a TT ?
Can't use earth as the sole return path here
To perform a Ze test?Can I test from one of the utility live wires and the neutral or would this pointless?
I think we should go to an English pub a discuss it someore
Seriously, I think we should continue this international dialogue as we can all learn something by sharing information. Doctors communicate medical information world wide why not electricians. The point of my post was to try to determine from our external wiring if in fact I could do a Ze test from the meter back to utility xtrm. I think the confusion might be that our internal house wiring uses TN-C-S. For example a 240 volt electric oven employs two lives wire a neutral and a ground wire. The older system use to be only 3 wires and the regs changed it to four wire .Our 120volt circuits have live, neutral, and earth wire.
Since the Utilitiy does not come under the NEC regs they run 3 wires to the meter
Two lives, one insulated neutral . we refer to it as the grounded neutral conductor since it connects to xo at the transformer.
The Utilitiy side must be a TT ?
Can't use earth as the sole return path here
To perform a Ze test?Can I test from one of the utility live wires and the neutral or would this pointless?
I think we should go to an English pub a discuss it someore
Has this discussion at College Park Maryland in 2002, The rockets were designed by a German
That would be needed to make the arrangement TN-S, not TN-C-S. It would also be necessary to keep the neutral and ground completely separate at each installation, the neutral busbar at the main distribution panel being floating as in a sub-panel rather than being bonded to the panel casing etc.
Yes, you could do it. It's measured in the U.K. all the time on supply systems which don't have a completely separate earth conductor all the way back to the transformer.
Not always insulated - The neutral on many overhead service drops is bare.
You can't look at either the utility or the customer side alone, since the designations refer to the arrangement as a whole. In the TT designation the first T means that the supply is grounded at source, while the second T means that the installation at the premises is grounded solely by way of a local earth rod. As you state, the NEC does not permit the earth to be used as the sole fault path, so TT is not used in the U.S.
Not pointless at all. It would give you an indication of the maximum possible fault current, just as in the U.K.
Thanks for all of this information.
I'm good with the bare neutral wire. Nowadays theyare usingore insulated
You might find an article called single - earth retune on Wikipedia it almost like what the utility around here provides us.Except they tie the other side of the HV coil to the secondary side
Neutral. A little weird
I'm good with the bare neutral wire. Nowadays theyare usingore insulated
You might find an article called single - earth retune on Wikipedia it almost like what the utility around here provides us.Except they tie the other side of the HV coil to the secondary side
Neutral. A little weird
Single wire earth return? It's been deployed quite widely in some countries (e.g. Australia) but is rare in the United States.
It's more likely a combined HV/LV neutral, in which the neutral is still run back to the HV source. That arrangement is common across the country (although not in this part of northern California, since HV distribution here is without neutral, as in Britain).
On the single phase supply xtrm
In this case has the one HV
Wire feeding the top then looking at the LV side there are three flags posts two outer L1& L2 , middle flag has the neutral and wire comes off hits the can and all of these Wires are connected to feed the LV side triplex which feed the home. The LV triplex also strings between the poles and other single phase transformers in the net work eventually the neutral must ties back at a 3 phase xtrm . Does this sound correct ?as looking up it looks like a spaghetti junction hard to tell??? That's my guess??this sound Correct from what you described in the previous thread that sounds right. On the other side of the single phase xtrm there is another sole connection. This must the other side of the HV coil that connects to the LV triplex neutral as well. When I first looked at it I thought it was a single wire earth return but then relized this extra tap connected to the LV side neutral configuration . Utilities do this to save on wire and HV equipment. How are they saving by not running the HV neutral with the HV top set ? They still run the triplex lower set all the way back ?It must be that HV equipment cost considerably more?
In this case has the one HV
Wire feeding the top then looking at the LV side there are three flags posts two outer L1& L2 , middle flag has the neutral and wire comes off hits the can and all of these Wires are connected to feed the LV side triplex which feed the home. The LV triplex also strings between the poles and other single phase transformers in the net work eventually the neutral must ties back at a 3 phase xtrm . Does this sound correct ?as looking up it looks like a spaghetti junction hard to tell??? That's my guess??this sound Correct from what you described in the previous thread that sounds right. On the other side of the single phase xtrm there is another sole connection. This must the other side of the HV coil that connects to the LV triplex neutral as well. When I first looked at it I thought it was a single wire earth return but then relized this extra tap connected to the LV side neutral configuration . Utilities do this to save on wire and HV equipment. How are they saving by not running the HV neutral with the HV top set ? They still run the triplex lower set all the way back ?It must be that HV equipment cost considerably more?
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Some meters may compensate for voltage, but I noted with my meter it did not matter what the actual voltage was if I used 240 volt the prospective short circuit current and the loop impedance always matched up. As to if meter actually measured PSCC and converter to loop impedance or other way around I don't know I have never stripped a meter to find how they work.
To my mind the loop impedance measured is an indication not an accurate measurement. Measure twice you always get a different reading as to if this is due to resistor warming up or actual different reading is at best guess work.
With any split phase we need to know the transformer type. I have used multi-voltage generators which can be set single or three phase however in single phase mode the windings are arranged zig zag so if used split phase the loop impedance for one side of split is different to other side of split so same supply could have genuine three loop impedance readings.
Even in the UK which one considers as reasonably standard throughout the country Ulster uses 10kV and rest uses 11kV so transformers vary. I understand some English speaker in France asking on a UK forum but in the USA they nearly speak English and this should really be asked on a USA forum. I have used this forum to find out how UK and USA systems compare and I would suggest you use a USA forum for USA questions it is just too easy for us in UK to miss something and give wrong answers.
To my mind the loop impedance measured is an indication not an accurate measurement. Measure twice you always get a different reading as to if this is due to resistor warming up or actual different reading is at best guess work.
With any split phase we need to know the transformer type. I have used multi-voltage generators which can be set single or three phase however in single phase mode the windings are arranged zig zag so if used split phase the loop impedance for one side of split is different to other side of split so same supply could have genuine three loop impedance readings.
Even in the UK which one considers as reasonably standard throughout the country Ulster uses 10kV and rest uses 11kV so transformers vary. I understand some English speaker in France asking on a UK forum but in the USA they nearly speak English and this should really be asked on a USA forum. I have used this forum to find out how UK and USA systems compare and I would suggest you use a USA forum for USA questions it is just too easy for us in UK to miss something and give wrong answers.
I find it hard to believe the high voltage feed (11kV) neutral at the pole mounted transformer would be in any way connected to the Neutral that went to domestic properties. An open circuit fault in the high voltage feed "neutral" could result in the neutrals of domestic 120 volt supplies being pulled up toward 11kV.
But then our colonial cousins do some odd things.
On the outskirts of Nashville there was three phase HV ( 11kV ) on one cross bar of the pole and the transformers hung on poles along the street were each connected to two of these in sequence along the street. Above the HV was another cross bar with a 3 phase supply which, going by the relative sizes of the insulators was a lot more than 11kV
In Salem Mass. ( IIRC ) one pole top transformer was steaming in the rain.
But then our colonial cousins do some odd things.
On the outskirts of Nashville there was three phase HV ( 11kV ) on one cross bar of the pole and the transformers hung on poles along the street were each connected to two of these in sequence along the street. Above the HV was another cross bar with a 3 phase supply which, going by the relative sizes of the insulators was a lot more than 11kV
In Salem Mass. ( IIRC ) one pole top transformer was steaming in the rain.
It cannot be very accurate given the very short duration of the measurement.
I have found over the years that a reasonable accurate result for loop impedanced can be calculated by adding a known load such as a kettle and measuring the drop in voltage when the kettle is on compared to when it is off. It takes a few minutes to average out the variations caused by other loads on the network being switched on and off. The reading I got that way was with 5% of the reading the service installation engineer provided.
The real problem that I see is when qualified/professional people don't ask questions, this applies to trades as much as to engineers. Many of them pretend to know what they are doing. The longer that this goes on for the harder it is to admit that they don't know something and as a consequence chances of an accident increases. I would far rather that people like this look for assitance ask questions and learn. I have also found that by answering questions I have learnt from it too.
"When I was an apprentice (over 20 years ago) when many apprentices did "day release". One day a week they were sent to college. Frequently instructors did not even turn up, and of the ones that did some were of little use. I remember an entire class failed thier exams, shortly afterwards day release was abandoned for good. Remember at the time an electrician could fail the exams and still qualify. Many of these electricians are still around today and some of them may never have been involved in testing."
I found the above posted on an Irish forum Where I once severed my time as an apprentice and was one of these unfortunate candidates. I speak mostly Irish( English) left there many years ago.
I' m now a dual of both countries.
I find it is interesting to talk to the Brits about there electrics. It's difficult to talk to an American if they aren' firmiliar with the UK wiring. So I think this gives me the right to ask questions here and I expect some may not have a complete answer but at least I can draw my own conclusions from these discussions.
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