High Ze - Low Zs

[Big_Spark]

Chris, the SWA of any electricv cable MUST be connected to Earth. It is a requirement of the Regs for Installations and it a legal requirement under the ESQR 2002 that generators/distributors connect all metalwork to Earth.

The responsibilty rests on the DNO for a sheath of a supply cable, but it MUST LEGALLY BE CONNECTED EARTH..this is a FACT.

 

[Big_Spark]

Have now downloaded the ESQCR 2002 doc and trawled through it. I can't find anything about responsibilities towards TT earthing connections. This morning, I rang the ENA.

They told me that the DNO does not have any responsibility towards the provision or upkeep of an earthing terminal on a TT supply.

They don't even have any responsibility to provide one on other supplies, assuming there was not one already provided. They only have a responsibility to maintain one already in service.

They will, where possible, upgrade supplies from TT to PME, but reserve the right to charge for doing so.

Simon, I have had a chat with an Engineer I know who works for EDF and he said that the supplier does have a legal responsibilty under ESQCR 2002 for the provision and maintenance of an Earth connection on all types of electricity supply. If no Earth connection exists then they do not have this responsibility untilthe make a change to the supply, then they must provide one.If the Installation is a TT and has an Earth Rod, then it is the responsibility of the user, HOWEVER, if the cable that supplies the TT has a conductive armour, whether SWA or sheath, this must be connected to Earth at the supply head regardless of whether it is exposed or not, and this has always been the case. In this instance the provision of Earth then belongs to the supplier and would be their responsibility. Such installations are rare as most SWA/Sheath armoured TT installations were upgraded many years ago and the cable replaced before the ESQCR 2002 came into force, however he is aware that some still do exist, usually on polyphase supplies to rural installations, but he knows of a handful in the EDF London region. They are earmarked for replacement with PME services.

He said that if someone has such an installation in their home and they are concerned about it, they should contact their supplier who has a legal enforcable responsibility under ESQCR 2002 to inspect, maintain and in required, upgrade the said connection to Earth, or install it if missing from an exposed conductive armour.

On the 2.5mm² question that started all this thread..he laughed..rather loudly. He restated the Regs that require the main Earth connection to be no less than 50% the size of the supply phase conductors and if the rod is a supplementary bond then it must be sized as per BS7671 for such a conductor, but not less than 6mm².

 

[securespark]

Can he point me to the relevant bits of the doc? I have a hard copy of it.

 

[Chris.J]

From his EDF contact : --
If the Installation is a TT and has an Earth Rod, then it is the responsibility of the user, HOWEVER, if the cable that supplies the TT has a conductive armour, whether SWA or sheath, this must be connected to Earth at the supply head regardless of whether it is exposed or not, and this has always been the case. In this instance the provision of Earth then belongs to the supplier and would be their responsibility.

Therefore, in this case, as the earth is the DNO's responsibliity, they must provide a "system" to prevent the voltage on the protective conductors of the property rising to dangerous levels. If the DNO cannot provide PME, then the only alternative is TT, and I would have thought that they would have to install an RCD as part of their equipment?

I note from ESQR 2002 para 3a) that the outer of CONCENTRIC supply cables must be connected to earth, this infers that the requirement does not apply to non concentric?
A live/sheath fault on a concentric cable is highly unlikley, as a live/neutral short will occur first, blowing the upstream fuse, so, connecting the sheath of a concentric supply cable to a TT rod is not likley to prove hazardous.
However, I still maintain that connecting it on a non concentric cable, particularily an old one, is hazardous.

 

[Big_Spark]

Chris..think about what makes a TT supply a TT supply.

Now think about what has been discussed..

By it's very nature a TT supply consists of either a 3 phase and Neutral or a single phase and neutral feed, with no Earth provision.

Now lets assume, due to logistics at installation, part of the cable from the Tx was buried underground...well even under the old supply Act this cable had to be protected by an armoured, non-magnetic sheeth or sleeve. Generally this would mean one of the several types of SWA cable.

Now this sheeth/armouring is NOT connected physically to the distributors network, and if not connected to Earth could remain live in the event of a fault between phase and Earth (the armouring). Nothing would automayically operate to disconnect the service, and some poor shlob would get one hell of a jolt sometime when they earthed the cable through contact.

This is why it MUST be Earthed...it is the whole point of ensuring that EXPOSED conductive parts are earthed.

If it were earthed at the suppliers end it would be a TN-S service!!

 

[philrosenberg]

Hi everyone
I've been following this thread quite closely because I'm on a TT supply and my supply cable comes from underground on a lead sheathed cable so this stuff may apply to me.
Should the lead sheathing be earthed (it isn't at the moment) or is the sheathing earthed through its own contact with the ground where it is buried?
If it should be earthed, then presumably it should not be connected to my earthing rod because of the reasons above (DNO's responsibility and possibility of earth fault failing to blow DNO's fuse and raising the voltage on my rod to unsafe levels).

Sorry if I'm highjacking this thread, but I thought that this question may be very relevant to the topic

 

[Spark123]

This is why it MUST be Earthed...it is the whole point of ensuring that EXPOSED conductive parts are earthed.

But still, in the event of a fault would enough current flow to disconnect the supply (DNO service fuse at sub station) if it were only earthed to the consumers TT electrode?

 

[Big_Spark]

Spark, the substation fuse is irrelevent, connected to the individual line, as I said in a previous post, is a Line Isolator/fuse link. This is rated at anything from 60 to 150A for single phase lines and 80 to 600A for 3 phase.

It is this fuse link that will blow and disconnect the service. The only way the links will operate is if the metalic sheath/armour is earthed..that is if it has a metalic sheath/armour.

 

[Spark123]

Where in the circuit is the line Isolator / fuse link?
In the event of a fault would enough current flow to disconnect the supply (line isolator / 60A - 150A fuse link) if the cables outer metallic sheath was only earthed to the consumers TT electrode?

 

[philrosenberg]

Spark, the substation fuse is irrelevent, connected to the individual line, as I said in a previous post, is a Line Isolator/fuse link. This is rated at anything from 60 to 150A for single phase lines and 80 to 600A for 3 phase.

It is this fuse link that will blow and disconnect the service. The only way the links will operate is if the metalic sheath/armour is earthed..that is if it has a metalic sheath/armour.

But as Plugwash said, if the domestic TT supply earth rod is used with a resistance of 10 ohms we only get a current of 24A so the fuse still won't opperate, it will just make every extraneous metal part in your house live. You'd need a Ze of around 1 ohm to blow one of these fuses.
If my DNO came round and linked their cable to my earth rod and then said "by the way, your Ze is too high to earth our cables you'll have to upgrade it" then I'd tell them where to get off

 

[Big_Spark]

Phil, your forgetting about secondary earthing, such as main equipotential bonding pulling this down to below 1 Ohm, and further, as HAS BEEN STATED AT LEAST 5 TIMES in this thread so far..once the sheath is earthed to the service rod, this is then the property and RESPONSIBILITY of the DNO, and they have a legal duty to ensure that it complies, and thus guarantees disconnection..which means either changing the rod so that the resistance is lower than 1 ohm, or they convert the supply to PME, thereby achieving the same result.

 

[Spark123]

Would it pull it down below 1 ohm though? Isn't there a reg which says you must not use the watermain or gasmain as an earth electrode?

 

[securespark]

I'm sorry, Jim, I'm not being argumentative, however, everyone I have spoke to says the DNO's have no responsibility for earthing on a TT system, including a senior engineer with UU I spoke to today. Nobody can point me to the relevant regs from the aforementioned document.

Given this, I can only disagree with you. If your EDF friend can point me to a regulation, that would be really helpful.

 

[owls]

I know the discussion has moved on a little from the original point....but if anyone is interested the distributor has informed me that it is supposed to be a TT system (which didn't have a earth electrode). However they have now offered to convert it to a TNC-S system.

They have confirmed that in their opinion the earthing to the supply sheath was a DIY effort.

 

[Big_Spark]

Simon..who are UU??

Remember that EDF generate, distribute and supply electricity all over Southern England, they may operate differently to other DNO's as a result. Adam has been in meetings all day so I have not had a chance to pick his brains further on this topic..and I am certainly not phoning him at home for this...

However I think that your misinterpreting the ESQCR and what I have said.

At no point have I stated that the DNO is responsible for providing an Earth connection on an existing TT service, neither are they responsible for any Earth Rods, Matts or grids that may be installed at an installation.

However they are responsible for that connection if THEY have a supply with a metalic sheath/armour on the cable at the service end that requires earth connection and it is connected to Earth. Further, if they install a TT system and they use armoured cable, then they MUST supply an Earth connection. This last point is clearly covered by the ESQCR 2002.