Advice on TT system

[Skys]

When carrying out Ze with main earth disconnect I get a reading of 77.25 ohms. All good. When main earth is reconnected and all parallel paths in place reading comes down to 0.60 ohms. I have 3 lighting circuits not covered by an rcd all Zs readings are well in permitted Zs. Would any body apply a code to this when carrying out a EICR. Never had this scenario that the Zs falls so low on a TT due to parallel paths . Thanks

 

[mattg4321]

Do you know where this parallel path is? A metallic service pipe maybe running close to a pme earth outside the building?

 

[pc1966]

That sounds like service pipes that are bonded and shared with nearby properties on TN systems, that can easily give < 1 ohm earthing. However, they can also vanish one day if said pipes are replaced with plastic.

 

[pc1966]

I have 3 lighting circuits not covered by an rcd

So is this a TT system without any RCD incomer?

I would say it is C2 as you cannot depend on service pipe earths, and without it those circuits have no effective means of earthing to meet disconnection times in any way (Ra/ Ze is well above MCB Zs requirement).

 

[Skys]

Do you know where this parallel path is? A metallic service pipe maybe running close to a pme earth outside the building?

No don’t know where the parallel path is. Bonding in place for water and gas but not sure there runs when leaving the building in relation to the earth spike

 

[Strima]

Edit: I've just reread the post.

C2 for lack of RCD protection as the Ra is not sufficiently low enough for the OCPD to operate under earth fault conditions.

It's very common for parallel paths to bring down the Zs in TT systems. One building I regularly work on has an Ra of around 180 ohms but the water bond drags this down to a Zs of around 0.20 on most circuits.

 

[Skys]

So is this a TT system without any RCD incomer?

I would say it is C2 as you cannot depend on service pipe earths, and without it those circuits have no effective means of earthing to meet disconnection times in any way (Ra/ Ze is well above MCB Zs requirement).

No rcd incomer. plastic board with tail gland to hold tails in place. One side rcd protected. The 3 lighting circuits not rcd protected. Obviously want to push to change mcbs to rcbos. But want to make sure I am in the right with the Zs been acceptable as it stands

 

[pc1966]

Changing to RCBOs would be the best solution in my mind.

Regulation 542.2.6 is the one saying you cannot use service pipes as the earth. They may be beneficial to the normal Zs (as clearly the case here) but

• They can be changed by outside parties, rendering the system inadequately protected, and
• Under fault conditions they are a hazard to anyone working on them elsewhere if the fault cannot disconnect independently (i.e with them opened for a joint to be changed, etc.)

 

[loz2754]

411.5.2 One or more of the following types of protective device shall be used, the former being preferred:
(i)An RCD
(ii)An overcurrent protective device.
NOTE 1: An appropriate overcurrent protective device may be used for fault protection provided a suitably low value of Zs is permanently and reliably assured.
NOTE 2: Where an RCD is used for fault protection the circuit should also incorporate an overcurrent protective device in accordance with Chapter 43.

 

[stevethesparks]

As it stands a C2.
Obviously any circuits without rcd/rcbo protection needs to be remedied on a TT system.
As far as the RA is concerned, according to BS7671 and indeed the NICEIC recommendation of below 100 ohms, this meets requirements.
Main protective bonding is in place and so that meets requirements.
So once the rcd protection is installed, we have a system that should comply but doesn't really as we shouldn't reply on other services as the main earth, but do so by default.
Apart from trying to improve the RA by installing additional rods, its still unlikely to get to a level that will be below the main protective bonding readings.
As far as I can see, all you can do is make sure everything is as should be (as above). Unless the householder wants to have plastic inserts installed at the income of the other services.
Edit: far too long typing that others have replied first