The Darkwood is Stumped

[Darkwood]

Not doing domestic too often ive come across a situation which has got me thinking, basically i have one long cottage building split into three (1/2 as main house, 1/4 as small dwelling and the final 1/4 as store building) now remembering its all one building the large house has its own tncs supply, the mid small house has its own tncs supply but the end store building is to be fed from a supply from the customers side in the first main house.
Now the question is- is this outside the equipotential zone and to be treated as a outbuilding with seperate earth rod and earth isolation from it supply cable or as its one continuous building can the earth be effectively run through from the main house and be classed as still in zone as there is a building with seperate supply in the middle???

 

[scotsparky]

sound like the storage building could be classed as a sub main D.B and as such is in the same building and therefore part of the same equipotential zone.

 

[Jason Smith]

Ok, want to learn from this as well, I am stumped too, but a difficult problem is easier when broken down, firstly what is an equpoten. zone. Its where all extran etc parts are bonded together, if you step into another zone, out into the garden forexampl etc, you can contact the general mass of earth which of course is not earthed, so in fault conditions etc, you will die. Right I am gonna guess you will be in zone and protected by whatever board you are coming off of, as you are in the building, all earth faults will be detected by the board that feeds it.

 

[scotsparky]

Hi Jason

The equipotential zone is esentuially the "footprint" of the installation or the building.
Out side the "footprint" of the installation is classed as out of zone.
The problems lie when you use items plugged ito an outlet in the zone oputside. hence the reason RCDs are fitted on sockets that can be used outside.
Theb reason for this is that as you have taken it out of "zone " you are relying on the earth of the flex onlyand not any other supplimental bonding to protect you in case of a fault. As you are standing on the mass of "earth" the potential difference is 230/400v depending on equipment so you have to provide a means of disconnection that will provide ADS in the event of a fault. As the earth path will be high unlike equipment in zone RCDs are used as they will disconnect after a fault as long as the Ful current to earth is over 30ma which means a resistance of 1667 ohms or less.

any clearer?

 

[Darkwood]

Sorry to add what might be a crucial detail their is no gas water etc in the store so effectively the equipotencial zone may stop short of the store and with an independant zone inbetween it just got me thinking?

 

[bugsy]

rod !!

 

[Deleted member 9648]

There is no reason at all why an outbuilding cannot be on a tncs earth as long as it is not a construction site,temp installation,marina,exhibition stand ,swimming pool etc . If there are any other services in the outbuilding they would need to be main bonded back to the MET but otherwise no reason why the tncs cant be exported.....it amazes me how often this keeps coming up on here.

 

[Darkwood]

There is no reason at all why an outbuilding cannot be on a tncs earth as long as it is not a construction site,temp installation,marina,exhibition stand ,swimming pool etc . If there are any other services in the outbuilding they would need to be main bonded back to the MET but otherwise no reason why the tncs cant be exported.....it amazes me how often this keeps coming up on here.

Its probabbly because the theory of the regulation is contradicted by different circumstance, why is a garage 5m away from a house with TNCS not within its equip' zone and thus needs earth rod yet a example above is the same in every sense except its linked together by a continuous outside wall, the gas and water stop in the previous premises so this stops the equipotential zone in my mind, you are several meters away from the tncs supply just like the example garage and their is no gas/water bonded to carry the zone across.
This situe above then gives rise to possible p.d. between supplied earth and earth mass.

 

[Deleted member 9648]

Where precisely does it say a garage 5m away from the E/Z cant be on a TNCS ??

Unless it is one of the tncs restricted installations I listed above the need to tt an ordinary garage/remote building is just a long standing myth.

 

[Darkwood]

Where precisely does it say a garage 5m away from the E/Z cant be on a TNCS ??

Unless it is one of the tncs restricted installations I listed above the need to tt an ordinary garage/remote building is just a long standing myth.

Ive queried Clark of Electrical works and this is probably where my confusion is arising from, a previous job was required to make the garage a tt with rod but hes expressed it was on the energy suppliers request due to unusual circumstances, its a type of job i rarely do as i build machine controls but recall tncs systems under some circumstances require garages, outbuildings etc to be converted to tt with rod but ive been strungling to locate regs to back this up.

 

[Sintra]

Reg 542.1.8

 

[Jurasic Spark]

See if this helps. It was published in 2005 (pre 17th) but still applies.

http://www.------.org/publishing/wiring-regulations/mag/2005/16-elect-inst-outdoors.cfm?type=pdf

 

[Deleted member 9648]

Reg 542.1.8 does not state that TNCS cannot be used for an outbuilding,it merely states that all earthing conductors where there are separate earthing arrangements shall be capable of carrying the maximum fault current likely to flow in them...etc
The article posted by Jurasic spark states that TT is an option and that if TNCS is used a main bond is required back to the MET for any extraneous parts...which I stated in my first post.
As I said....this is just a long standing myth.

 

[Darkwood]

Sorry Wirepuller should have thanked you, i read the regs and it cleared it up for me like you had suggested earlier but thanked sintra for finding the relevant regulation.
Its just that ive been off building sites domestic etc for a while i do have a very good sense of regs but try focusing on engineering for 2 yrs and the regs get fraid in your head and just need a reminder sometimes.