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Hi just a quick question I used to know this but now can't remember if the old PILC cable joined to a PVC covered cable as a replacement for a faulted section how do they keep the earth impedance down? Especially if the PILC length is now a lot less than it was supply's in properties along the road are a mix of TNS and CNE Thanks
depends what you mean by the earth impedance -
The electrode impedance between the cpc and the terra-firma is raised by the loss of exposed lead surface, but usually earth rods or tapes are buried in the pit beside the joint as you would on a PME type set-up
If you meant the Zs loop impedance back to the transformer, the jacket of the PILC is interrupted but as per the previous answers bonded via the neutral on both sides of the new section Mike
Correct Andy , I was posting with my phone and it's awkward!
They make it all TNS by bonding the neutral to the PILC sheath.
Slight typo from David there I suspect - I think it should have read: They make it all TN-C-S by bonding the neutral to the PILC sheath.
- Andy.
Not quite ... the issue is it's tricky and you'd need to check with the DNO.
Some arrangements where it's not separate neutral and earth (SNE) all the way back to the transformer are still classified as TN-S (confusingly), and PME conditions do not apply.
Where the system is "full TN-C-S" and PME conditions apply, the consumer is supposed to be notified by the DNO.
I know this goes against our strict definitions of "TN-S", but it's important to note that "TN-C-S" is not always PME (as implied, but not implicitly stated, by BS 7671), and there are some "strictly TN-S by the logic and book" earthing arrangements, such as some PNB arrangements, where PME conditions apply. This situation all comes out of ESQCR and G12/4.
In a lot of DNO literature they sidestep the loaded 'is it really proper PME ?' question by referring to CNE 'Combined Neutal and Earth' or SNE 'separated Neutral and earth'.
Avoids saying things like
'my supply is TNS-C-S-C-s '
which is actually quite representative of the situation you may have in an older street with a few repairs and new streetlights etc, as at every new joint or new service tap the N and E will be bonded, but the older properties will still present a TN-S service head.Hence the general advice that if we are not certain to assume PME-like conditions apply, as these are the most onerous.
We may not be allowed more than one NE bond on a genset, or indeed any on the consumer side of a mains supply, but the networks, who have more control over their cables and routes, and most importantly know better the current rating of their earthing arrangements, are more than happy to have many.
Mike
We may not be allowed more than one NE bond on a genset, or indeed any on the consumer side of a mains supply, but the networks, who have more control over their cables and routes, and most importantly know better the current rating of their earthing arrangements, are more than happy to have many.
Agreed - particularly as consumers receiving their supply at LV are not permitted to combine neutral and earth in the consumer's installation by Regulation 8(4) of the Electricity Safety, Quality and Continuity Regulations.
Kelly
As others above have explained PILC cables are repaired with a CNE cable.
Down here in UKPN land this is known as a PILC transition joint. About 2m of the PILC is cut out to expose 2 ends of the PILC cable. The lead each side is cut back to expose the 4 copper live conductors. The lead sheaths are cleaned up each side and a flat copper braid is held in place with a constant pressure spring on the lead sheath and covered with self amalgamating tape. The 3 copper line conductors are joined to the 3 Wavcon aluminium cores with a compression joint with a snap off torque bolt clamp then insulated. The neutral PILC cores are joined to the Wavecon copper combined neutral earth with the braids from the lead sheath. A separate earth is also brought out of the joint. The joint is surrounded top and bottom with a torpedo plastic cover and the joint filled with a 2 part epoxy resin. The earth brought out of the joint is connected to a normal 1.2m copper clad earth rod laid next to the joint. The cable jointers are not allowed to dive the rod in due to historic mishaps. I have seen this done live with the jointer sitting on rubber mats in 1000V wellies and gloves. Only one cable core is exposed at a time with sticky wraps around the other cores. Also done on single phase service cables from the 3 phase main the same way but for some reason no earth rod on that joint.
This is an every day event in UKPN land with the decaying PILC network decades old. A few years ago UKPN declared where they have given you a service with an earth there whole network is PME as they have done so many PME joints. So my 1935 supply from PILC with a sweated earthing conductor from the lead sheath is PME despite its appearance. My house is in a group of houses in 2 roads which is supplied in a radial from a transformer in the grounds of the nearby girls school . It has gone bang in various locations over the years and is slowly being replaced in 2m lengths. A bit like triggers broom. When we go all electric for cars and heating no doubt we will be on first name terms with the cable jointers during their frequent visits.
JP
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