DP Isolation and PME (fault)

bernardgreen · 2 Apr 2009

What happens if a PME system neutral fails ?

This, in theory, leaves the earth provided to the property floating if the neutral conductor is open circuited.

Severe phase un-balance in the local area can force the neutral and hence the "protective" earth of a PME system many volts above true ground potential.

This could present a danger of shock to someone standing on the ground while holding an electrical item that is earthed to the house protective earth.

One incident was a water fight in a garden where the water tap bonded to protective earth was noticably "live" relative to the wet ground. This was due to a significant phase un-balance on the local 230 volt distribution.

Is there a case to isolate LIVE NEUTRAL and IMPORTED EARTH when the protective earth ( derived from PME neutral ) rises more than say 25 volts above true ground ?

Lectrician · 2 Apr 2009

PME can be a danger.

If you get a high impedance or open circuit neutral then depending on your loading (even a tiny load) your exposed conductive parts will rise in potential, potentially upto full phase volts. This is deemed not too much of an issue due to bonding, but is far from ideal!

This is why caravan parks are not allowed to supply PME to pitch supplies. Imagine the chassis of a caravan being at 230v as you hold onto the door with your feet on the earthy soil.

There is always the issue of "percieved shock" which is where people using a building and touch exposed conductive parts recieve tingly shocks. These are due to circulating network currents and volt drop in the neutral conductor causing the voltage between the neutral and therefore your exposed conductive parts being at a different potential to mother earth. This is why commercial shower rooms and swimming pools etc require special consideration when only PME is available (rather than TN-s).

Another example of "percieved shock" is documented by many distributors, but maybe more so by DNO due to the amount of surfers......

It is common for a surfer to walk up fro the beach with his board, soaking wet. He goes into his back garden standing bar foot on the ground and turns on the garden tap to rinse the board and wetsuit. The tap is connected to the PME earth and the voltage is floating higher than the mother earth he is standing on. He recieves a "percieved shock".

bernardgreen · 2 Apr 2009

"Perceived shock" Is that a euphamism for a shock that was felt and then talked about by the person. As apposed to the shock that the person did not talk about afterwards.

I guess that PME in a fault free world is better than having to depend on an earth rod being able to sink enough energy to trip a safety device when the mother earth is bone dry.

Where a 100% reliable earth rod was available then a TT supply would be my idea of the safest method. With all at risk metal that was not an electrical conducter being connected to mother earth via the ground rod.

NOTE this is a discussion and NOT advice to DIYers.

plugwash · 2 Apr 2009

As well as the issue of getting a good connection from your earth rod the other problem with TT is you have to rely on a RCD for earth fault protection. Afaict RCDs are a lot less reliable than overcurrent protective devices.

TN-S is probablly the safest system IMO but the extra cost means it's rarely used for new domestic supplies theese days.

bernardgreen · 2 Apr 2009

plugwash said:
As well as the issue of getting a good connection from your earth rod the other problem with TT is you have to rely on a RCD for earth fault protection. Afaict RCDs are a lot less reliable than overcurrent protective devices.

The current operated RCD operates on the difference in the currents in the Live and Neutral through the sensing element. If this difference is above the threshold then the device trips. It will not trip on an overload if the Live and Neutral currents are equal.

It would ( should ) trip if there is any path to any ground for current flow above the trip threshold and current flows along that path and not back along the neutral.

TN-S is probablly the safest system IMO but the extra cost means it's rarely used for new domestic supplies theese days.

Cost versus best practise. I feel a compromise has happened.

ricicle · 2 Apr 2009

bernardgreen said:
It will not trip on an overload if the Live and Neutral currents are equal.

I hope it would ! (or do you mean earth fault ?)

bongos · 2 Apr 2009

plugwash said:
TN-S is probablly the safest system IMO but the extra cost means it's rarely used for new domestic supplies theese days.

I assume the perceived risk must be low enough for PME to be the more favourable way of installing? All the houses round here have TN-S but it seem the newbuilds on estates are all TN-C-S.

Have any of you sparks on here come across high impedance/broken circuit neutrals on a PME system? Is it a common enough occurence to consider TT instead?

flameport · 2 Apr 2009

With an underground supply, a broken neutral is very unlilkely. There is only one cable, so for the neutral to be disconnected the entire outer circumference of the cable would have to be removed, yet leaving the centre conductor intact.

Overhead supplies are a different matter. Here the line and neutral are likely to be separate wires (typically 2 or 4 wires on poles along the road, with concentric cables from the poles to each property).
One of the wires breaking is certainly possible - such as a tree branch falling onto the wires.

TNS supplies were good when they were installed - however over time, the outer metal sheath of the cable rots away, breaking the earth connection. Withoug regular testing, there is no way to know if this has happened.

TNCS is the standard now, as it is cheaper to install (2 wires instead of 3) and lead sheathed cables would be far more expensive to manufacture than plastic, and would take longer to join/install.

TT is only for specific situations where TNCS would be considered high risk (such as caravan sites or swimming pools). For a normal installation, there is no reason to consider TT as an alternative to TNCS.

sm1thson · 2 Apr 2009

Playing devils advocate here, but isnt this:

flameport said:
With an underground supply, a broken neutral is very unlilkely. There is only one cable, so for the neutral to be disconnected the entire outer circumference of the cable would have to be removed, yet leaving the centre conductor intact.

and

flameport said:
TNS supplies were good when they were installed - however over time, the outer metal sheath of the cable rots away, breaking the earth connection. Withoug regular testing, there is no way to know if this has happened.

essentially the same chance of occurring? but on the TNS (second scenario)you loose your earth and on the PME (first scenario) your earth goes live.

Lectrician · 2 Apr 2009

I have seen several high impedance/open circuit neutrals.

I posted a pic here a while ago of a melted terminal block inside an accessory. It turned out that the earth of the lighting cicuits was common to the supply of the house next door. The house next door lost their neutral and the lighting circuits CPC carried the current of the house. Many of the lighting cables where damaged, and terminals overheated.

I was changing some consumer units once, and decided to start with the off peak board and sort the bonding, leaving the main CU live to power tools etc. As I cut through the bonding wire while removing the off peak board there was a huge arc. Again, a neutral was a drift and the two properties adjacent shared a common lead water pipe.

A house complained of the RCD tripping, turned out to be an old ELCB and a TT system (not pme). The neutral had lifted on this supply, and due to a neutral to earth supply on the socket circuit the load from the house was travelling through the imedance of the earth rod. Lights where dimmer than usual etc. The ELCB did keep operating intermitantly, but the customer could reset it time and again.

Similar one on a site with loads of storage heaters. Customer said they had a burning smell most nights. This was also a TT with no RCD. Again, neutral to earth fault giving a return path for the neutral current - this time a nice low impedance return path due to parallel paths to next doors PME which was intact. The burning smell was the storage heaters load travelling through the neutral to earth fault and associated wiring of the immersion heater circuit.

Yup, I have seen many. These are only a few. all where overheads, but I have heard of underground too. Although the cables maybe of concentric construction, you still have a single point where the neutral is terminated.

Guitarguy · 2 Apr 2009

flameport said:
With an underground supply, a broken neutral is very unlilkely. There is only one cable, so for the neutral to be disconnected the entire outer circumference of the cable would have to be removed, yet leaving the centre conductor intact.

Overhead supplies are a different matter. Here the line and neutral are likely to be separate wires (typically 2 or 4 wires on poles along the road, with concentric cables from the poles to each property).
One of the wires breaking is certainly possible - such as a tree branch falling onto the wires.

TNS supplies were good when they were installed - however over time, the outer metal sheath of the cable rots away, breaking the earth connection. Withoug regular testing, there is no way to know if this has happened.

TNCS is the standard now, as it is cheaper to install (2 wires instead of 3) and lead sheathed cables would be far more expensive to manufacture than plastic, and would take longer to join/install.

TT is only for specific situations where TNCS would be considered high risk (such as caravan sites or swimming pools). For a normal installation, there is no reason to consider TT as an alternative to TNCS.

It is very unlikely, but when the main in the street is a Consac cable which is CNE, and because of the way the cable is made up (3 paper insulated solid aluminium cores, one for each phase, and a solid aluminium sheath which serves as the neutral/earth. This has a PVC oversheath and If this gets damaged water will get in and turn the aluminium sheath to white powder which does not conduct electricity so the neutral/ earth gets lost. I work for the electric board and this is one of the main reasons we get underground mains faults. They are trying to overlay as much consac as they can to stop this problem.

Lectrician · 2 Apr 2009

flameport said:
With an underground supply, a broken neutral is very unlilkely. There is only one cable, so for the neutral to be disconnected the entire outer circumference of the cable would have to be removed, yet leaving the centre conductor intact.
Except for the point where the neutrals are jointed. they do run a bare hard drawn cable in treches for a few meters and connect this to the neutral to bond it down. Think earth rod/tape.

Overhead supplies are a different matter. Here the line and neutral are likely to be separate wires (typically 2 or 4 wires on poles along the road, with concentric cables from the poles to each property).
One of the wires breaking is certainly possible - such as a tree branch falling onto the wires.
The neutral is the bottom conductor for this reason. A tree falling should hit the phases first. Again, the neutral is bonded down at intermediate points, generally every other pole if not all. The most common reason for a lost or high impeadance neutral is a corroded crimped joint up a pole.

TNS supplies were good when they were installed - however over time, the outer metal sheath of the cable rots away, breaking the earth connection. Withoug regular testing, there is no way to know if this has happened.

TNCS is the standard now, as it is cheaper to install (2 wires instead of 3) and lead sheathed cables would be far more expensive to manufacture than plastic, and would take longer to join/install.
TN-S can be provided by the use of a 4 core wave form cable rather than a 3 core wave form. A SPLIT concentric would then be used as the service cable to the head. This is quite common to see, although not so much these days on standard installs. TN-S comes in many guises, Lead sheathed PILC cables being the older type of TN-S.

TT is only for specific situations where TNCS would be considered high risk (such as caravan sites or swimming pools). For a normal installation, there is no reason to consider TT as an alternative to TNCS.
TN-S is provided where ever possible to locations with swimming pools and caravan parks etc. Where TN-S cannot be provided it is common for a PME terminal to be provided, but it is up to the designer to decide if they use this for some, done or all of the install. TN-S cannot be supplied from a shared transformer or sub station if that transformer or substation serves other properties with TN-C-S. The DNO's will not mix earthing types. (although they do have the good old hybrid PME/TN-S setup where PILC cables fail!).

bernardgreen · 2 Apr 2009

flameport said:
With an underground supply, a broken neutral is very unlilkely. There is only one cable, so for the neutral to be disconnected the entire outer circumference of the cable would have to be removed, yet leaving the centre conductor intact.

Situation here last year was an underground cable faulted phase to phase. For a rapid restoration the DNO connected the two shorted phase conductors to a single phase in the sub-station. So it was two phases and neutral leading with twice as many properties on one phase as on the other. The neutral voltage was far from ground and voltage fluctuations were large.

Lectrician · 2 Apr 2009

With two phases distributed, the neutral would have had to take a little more current than usual due to the imbalance. This extra current would have caused a greater voltdrop in the neutral, and hence the reason why it floated noticeably apart from mother earth.

All overhead and UG networks do have the CNE conductor 'staked' down regularly to try to keep the neutral voltage as steady and as close to earth as possible.

Guitarguy · 2 Apr 2009

Lectrician said:
With two phases distributed, the neutral would have had to take a little more current than usual due to the imbalance. This extra current would have caused a greater voltdrop in the neutral, and hence the reason why it floated noticeably apart from mother earth.

All overhead and UG networks do have the CNE conductor 'staked' down regularly to try to keep the neutral voltage as steady and as close to earth as possible.

yes that is what the 'multiple' in protective multiple earthing (PME) stands for. You have an earth rod at the end of the cable, the LV earthing at the substation and depending how long the run of that cable is, maybe another one out of a straight joint.

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DP Isolation and PME (fault)

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