High Ze - Low Zs

[owls]

I know this is a DIY site and so this might be a little beyond what should be posted on here, however i'd really like some opinions on this!

Went to look at a system today, earthing arrangement appeared to be TN-S however the earth connection wasn't sweated onto the cable but connected via a earth clamp on the main incoming cable (Its the old lead/paper style).
I know these don't give a great connection to earth but i've also been told a million times that you aren't supposed to reapply them as you'll crush the cable contained within it.

The supply is via overhead lines though (which i assume is either TT or converted TNC-S?).

Anyway, tested Ze and got a reading of 1.54 ohms (obviously way too high for a TN-S).

This was a bit of a puzzler as i'd already tested Zs on the circuit i was working on before i'd started and it was well within 7671 limits. I re tested the entire DB circuit by circuit for Zs and all were well within tolerance.

I know parallel earth paths could be pulling the Zs readings down to within acceptable limits but its the overall Ze reading thats troubling me.

My question is.... Do i have a TN-S with a way too high Ze that the distributor needs to recitfy or should i convert it to TT (or is it already a TT with no electrode?) and install a electrode?

Hope someone can give me some useful advice

 

[securespark]

The Zs reading could well be pulled down by the MEB's. On a TT system I inspected recently, the Ze was 49.7, the Zs from S/O was 0.88, once MEB's had been reconnected.

 

[Big_Spark]

I agree with Simon here, it is likely that the Main Equipotential bonding conductors are pulling the Zs down to acceptable levels.

The main earthing arrangments at the supply are the LEGAL responsibility of the DNO, so do not touch it as you could open yourself up to legal action or worse. I would suggest you contact the DNO and ascertain from them the type of supply it is supposed to be, and then if it is not a TT system, which it doesn't sound like, they need to upgrade the main Earth

Alternatively, it could be a true TT which has simply been fed with a SWA cable (or similar) and they have simply tapped off the installation Earth to ensure it is earthed as it is exposed..Have you looked for a Rod?

 

[securespark]

But, to avoid confusion, earthing arrangements on TT are the responsibility of the consumer, not the DNO.

 

[Big_Spark]

But, to avoid confusion, earthing arrangements on TT are the responsibility of the consumer, not the DNO.

NOPE, the law was changed Simon, all main earthing arrangements are now the sole responsibility of the DNO, that is the main reason many TT's get changed to PME.

It was all changed when they changed the Electricity Supply Act to the new one (name escapes me off the top of head)

 

[plugwash]

Alternatively, it could be a true TT which has simply been fed with a SWA cable (or similar) and they have simply tapped off the installation Earth to ensure it is earthed as it is exposed.

it sounds rather dangerous to me to connect the sheath of a non-rcd protected cable (the incoming cable) to a TT earth.

 

[Big_Spark]

Plug, why is it dangerous?

If it is a TT, then I agree it should have an RCD installed, but connecting exposed or extraneous conductive metalwork to earth for equal potential is perfectly safe and the whole point of main equipotetial bonding conductors and supplementary bonding conductors.

 

[plugwash]

Plug, why is it dangerous?

because if a live wire from before the RCD touches your TT earth you will get a fault current flow that may well never be disconnected. This fault current flow will establish a dangerous potential between your earthing system and real earth.

 

[Big_Spark]

Plug, why is it dangerous?

because if a live wire from before the RCD touches your TT earth you will get a fault current flow that may well never be disconnected. This fault current flow will establish a dangerous potential between your earthing system and real earth.

What have you been drinking?

By this your stating that the incoming cable should not be earthed if it has a sheath made of a conductive material, this would be a breach of health and safety, and the new supply Regs.

Nothing faulty on the incoming side of a RCD of any installation will cause it to operate so that is a flawed argument.

By connecting the Sheath to the system Earth you are removing any potential fault should one occur as it would blow the Main fuse, disconnecting the supply, or it would operate the RCD, were it installed, as this should be of the Type S variety that would detect the fault current on the Earthing system.

 

[securespark]

But, to avoid confusion, earthing arrangements on TT are the responsibility of the consumer, not the DNO.

NOPE, the law was changed Simon, all main earthing arrangements are now the sole responsibility of the DNO, that is the main reason many TT's get changed to PME.

It was all changed when they changed the Electricity Supply Act to the new one (name escapes me off the top of head)

You mean the ESQCR 2002?


I shall check that with the ENA tomorrow...!

 

[Spark123]

You mean the ESQCR 2002?

SI available here:
http://www.opsi.gov.uk/si/si2002/20022665.htm

 

[Spark123]

By this your stating that the incoming cable should not be earthed if it has a sheath made of a conductive material, this would be a breach of health and safety, and the new supply Regs.

It needs to be earthed, but that is the DNOs responsibility, not the consumers. Whilst they are at it they may as well provide the customer with a TN supply.

 

[Big_Spark]

Spark, I know whose responsibility it is, I was simply commenting on Plugs assertion that earthing the damned thing would be dangerous..

 

[Big_Spark]

Simon, from Section II Protection and Earthing of ESQR 2002

(3) A generator or distributor shall, in respect of any low voltage network which he owns or operates, ensure that -



(a) the outer conductor of any electric line which has concentric conductors is connected with earth;

(b) every supply neutral conductor is connected with earth at, or as near as is reasonably practicable to, the source of voltage except that where there is only one point in a network at which consumer's installations are connected to a single source of voltage, that connection may be made at that point, or at another point nearer to the source of voltage; and

(c) no impedance is inserted in any connection with earth of a low voltage network other than that required for the operation of switching devices or of instruments or equipment for control, telemetry or metering.


And from Part VII SUPPLIES TO INSTALLATIONS AND TO OTHER NETWORKS

(4) Unless he can reasonably conclude that it is inappropriate for reasons of safety, a distributor shall, when providing a new connection at low voltage, make available his supply neutral conductor or, if appropriate, the protective conductor of his network for connection to the protective conductor of the consumer's installation.

(5) In this regulation the expression "new connection" means the first electric line, or the replacement of an existing electric line, to one or more consumer's installations.


Note part 5, whilst not saying so, it has been widely accepted that the replacement of the head of an installation, or upgrading of the service head also requires the installation, if none present, of a connection to earth.

 

[Spark123]

I am probably reading what was said above differently to you, what I think Plug is saying is that with a DNO supply which is designated TT and a supply cable which is not protected against earth fault, if your electrode was connected to the DNO cable as its only earth bond and a fault occured in the cable it will drag the voltage of the electrode (hence all eq. bonding) up to mains voltage. The upstream overcurrent device may not operate owing to the increased efli seen in a TT system.