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Discuss What earthing system is this? in the Electricians' Talk area at ElectriciansForums.net
where is it now?
where is it now?
Err... 42 Helmsome Rd, Leamington Spa, CV92 7DL:wink_smile:
Is that the cover with the sticker saying "earthing terminal PME system"?There appears to be no seal on the cover, so if it was me I would have had a quick peek to see if there is a link between Neutral and Earth
Other than that, testing would prove if similar readings are tested on pefc and pscc
If that is a DNO fix, it leaves a lot to be desired!! You don't mix TN-S and TNC-S. This is especially true in the case of a network supply cable, in the process of being converted to PME!! The TN-S sheath earthing clamp and braid should have been removed and any exposed LEAD sheath insulated with amalgamating tape!!!
Not heard of that before but some else may know.Does this also cure a noisy earth i heard, I might also be talking rubbish.
Well you may be shocked to know this is standard procedure when fitting a new cutout on a paper lead cable.
How are you mixing TN-S and TNC-S? To carry this upgrade out the main must be PME and that was completed on the LV underground network in the 70's in my area so in doing that there was a mix.
**Multiple new supplies to steel framed buildings can have a TN-S service made available from a PME main.
So I'm not sure why you have a problem with mixing systems if carried out correctly.
The LV main is the cable originating in the substation that all supplies will be breached from. it is run as a radial configuration sometimes with the option to be backfed or backfeed other LV mains via link disconnector boxes.
Any joint or repair carried out on a paper lead distribution cable will have neutral and earth connected together in that joint. They will already be connected at the substation and every main will have been PME'd at some point in the past. When a paper lead main or service is repaired wavecon CNE cable or hybrid will be used (3 core and copper neutral/earth 3 phase and 1 core and copper neutral/earth for single phase. On new supplies a rod must be installed every 40m (if there are joints) or before the last 4 properties on that main. This is to reduce the amount of customers affected by a neutral fault.
TN-S can be supplied from a PME'd main as previously mentioned, this is current practice on new installs with multiple supplies to steel framed buildings. This is to stop circulating currents in the frame. Alternatively no earth will be supplied and TT will most likely be used.
All the above is based on my knowledge of the DNO in my area and other areas may work to different practices.
I must admit, that I'm a bit confused here.
With TN-C-S, the supply is TN-C, which is presented at installations as TN-S.
To my mind, whether the TN-C is taken right up to the installation, or only taken to a point some distance away from the installation, It is still TN-C-S.
In either case, PME conditions will apply.
To my mind, the only way that a TN-C-S sytem can become TN-S, is for an isolating transformer to be used.
The TN-C would be fed to the transformer, and all feeds from the transformer would be TN-S.
Earthing the neutral at any point after the transformer, will effectivly re-establish the TN-C-S.
TN-S from CNE cable is achieved either by using split concentric service from main to cutout or CNE cable and a separate earth from the main joint to the cutout.
Neutral faults on the network are always a problem and the results can vary depending on system balance etc regardless of earthing arrangement.
If the earth is derived from the neutral, in either of your examples, then both the neutral and the earth conductor at the cut-out will be at the same potential, and effectively be exactly the same as a service head PME connection. The earth bonded to steel frame is still effectively a neutral connection....
Not quite, Yes neutral and earth will be at the same potential under normal conditions but earth will be separate back to the main. For example take a neutral fault on the service on one supply in the steel framed building, (service and main PME) neutral becomes Open circuit so being PME the current flows through the neutral earth terminal, though the steel frame (bonded) and down the neutral of the other supplies. Not a good situation as overloading of the neutral and bonding conductors can result.
Now take the same idea and the services are TN-S back to the main which is PME'd in the service joint, open circuit fault on one service neutral will mean no current can flow in the steel frame as the only neutral earth connection is in the service joint on the main. Therefore only one service is affected.
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In a new PME cable installation, (for simplicity let's talk about a single phase main cable) the neutral is regularly rodded/grounded along it's length, from the sub-station. There is no common earth in the cable construction. So to give an installation an earth, the earth is derived from the neutral conductor at the service head. Any neutral fault will/should then only affect service heads beyond the fault. That is Not So with a converted TN-S cable, where it's sheath has been included in the N-E connections at joints and/or at the service head cut-outs. Acting as a common conductor, any neutral fault will be transmitted to installations that shouldn't be affected by the fault, the length of the cable in fact. Which is why TN-S sheaths should NOT be connected to a PME cut-out, and underground joints should not include the lead sheath in N-E rodded conversion connections...
TN-S cables have a connection between the neutral core and lead sheath at the substation and at joints along it's length. A neutral fault ie open circuit neutral core (very rare) will result in all neutral current flowing through the lead sheath which is of adequate CSA to carry this current. You would have to lose the lead sheath and neutral core for there to be major problems.
Neutral faults are thankfully rare and are usually caused by alu CNE cable when moisture gets in to the alu neutrals and corrodes them away.
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