Earthing Systems

[ccevolution]

Hi I am a new member of this forum and this is my first post. Having worked as enthusiastic DIY electrician doing simple things and relying on others for more complex projects I wanted to train as a domestic installer. Having recently completed 17th edition and part P courses and I am committed to more courses in the year to learn more. In the meantime I am looking to learn from people who are more experienced and have a question regarding an TNC-S earthing system I have come across , which in addition to the suppliers earth connection has a separate earth spike connection running to the MET, is this normal where the supply is fed by overhead cable or is this a legacy of the properties previous use as an agricultural small holding. On the basis that this is hopefully not a stupid question could anyone throw any light on it ?

 

[Lurch]

It sounds like it was a TT which was converted to TN-C-S. The rod should have been removed technically as you can't have both types of earthing system on one installation, although I wouldn't be too keen on leaving a 2 wire overhead supply as TN-C-S so I can see why the original electrician would leave the rod in if this was the case.

What is the overhead cable type?

 

[Tony S]

How far away is the transformer? The rod could be there for a good reason.

Don’t disconnect it before getting professional advice.

 

[apprentice87]

You cannot have exposed conductive parts in an installation connected to different earthing systems, as they would now be "extraneous parts" [read the definition] and there could be a potential difference between them. HOWEVER, no matter what earthing system, you could bang in as many earth rods as you liked and in the case of TNCS it would make it PME [sort of] You would be better off, as in the event of a DNO neutral fault, all your rods would tend to drag down the voltage that now appears on ALL your earthed metalwork.

john...

 

[Lurch]

HOWEVER, no matter what earthing system, you could bang in as many earth rods as you liked and in the case of TNCS it would make it PME [sort of]

Sort of, as in not PME at all as multiple rods on the customer side do not change the suppliers earthing method.

You would be better off, as in the event of a DNO neutral fault, all your rods would tend to drag down the voltage that now appears on ALL your earthed metalwork.

I can't disagree with this, but as we are not allowed to do it we have to live life on the edge and hope for the best if we do it to the regs.

 

[Tony S]

How far away is the transformer? The rod could be there for a good reason.

Don’t disconnect it before getting professional advice.

There’s a reason for me asking that,

From a friend in the wilds a Scotland. The DNO’s rod is at the house.

Just be careful telling someone “it shouldn’t be there”.

 

[apprentice87]

Hi Tony, It appears the DNO left their cable a bit short there!!!

john..

 

[Tony S]

Apparently it’s quite common in the area. A mixture of poor ground conditions and the close proximity of the transformer.

 

[Lurch]

It appears the DNO left their cable a bit short there!!!

Nope, some DNO's use a bit of chipboard in the bottom of the meter box to fit the cutout on the angle as is seen there. You can see the last bit of it still just about there, the rest seems to have disintegrated with the damp conditions.

 

[apprentice87]

Hi Lurch.. Yes you are right, i remember i saw one once supplied with a meter box i fitted but did not use it. Forgot about them!!!

john,,

 

[kerching]

Nope, some DNO's use a bit of chipboard in the bottom of the meter box to fit the cutout on the angle as is seen there. You can see the last bit of it still just about there, the rest seems to have disintegrated with the damp conditions.

VERY common around here

First time I saw one it was a case of WTF?

Got used to,them now....along with a DNO, fitted REC2 only breaking the Line side

Just observing

 

[apprentice87]

That is interesting that they fit isolators that only break line. The regs state that an isolator for a domestic installation has to break all live conductors...

john...

 

[kerching]

That is interesting that they fit isolators that only break line. The regs state that an isolator for a domestic installation has to break all live conductors...

john...

That is what i thought.

Stupidly never got a photo...cannot remember where it was now

 

[Lurch]

More likely a clueless fitter than a DNO standard surely?

 

[Tony S]

That is interesting that they fit isolators that only break line. The regs state that an isolator for a domestic installation has to break all live conductors...

john...

Which regulations?

 

[apprentice87]

Bs 7671 ??

john..

 

[apprentice87]

537.1.4 A main linked switch or linked circuit-breaker shall be provided as near as practicable to the origin of every installation as a means of switching the supply on load & as a means of isolation.
A mainswitch intended for operation by ordinary persons, e.g. of a household or similar installation, shall interrupt both live conductors of a single-phase supply

john...

 

[Lurch]

That regulation is covered by the main switch in the CU.

 

[apprentice87]

If the DNO fit an isolator in the meter box of a house it is obviously intended for use by ordinary persons so it MUST break all live conductors, Makes no difference if the main switch in the customers CU does as well, If the DNO fit a switch, and it is intended for use by ordinary persons, then the reg applies..

john..

 

[HandySparks]

If the DNO fit an isolator in the meter box of a house it is obviously intended for use by ordinary persons so it MUST break all live conductors, Makes no difference if the main switch in the customers CU does as well, If the DNO fit a switch, and it is intended for use by ordinary persons, then the reg applies..

john..

Surely it's intended for the electrician doing the next consumer unit change or similar work?