Maximum zs on TT

[NEBITOMI]

HELLO
Im learner yet!
i would like to know something about the TT system!
if i would have an outbuilding installation, like sumer house!the main earthing sytem is tncs! so the bs 7671 recomend ONLY DO NOT export the earth from the house !what about if i still use the tncs earth to summerhouse! just i would not relay on the ground resistance! do i think wrong?
Also for example :if the TT ze :6ohm in the summer house fuseboard ,then if i put 32a circuit braker for the ring circuit .then would r1+r2: 0.20ohm and i need to add ze:6 ohm =zs would be :6.20 !but in this case the braker will not trip in required times as the regulation show me!Becasue that braker need maximum zs:1.44 ohm!
so please can you explain sortly what i thinking wrongly?!Or can you make me understand more of it!?
Thank you!

 

[James]

where does it recommend DO NOT EXPORT EATH FROM HOUSE??

 

[NEBITOMI]

where does it recommend DO NOT EXPORT EATH FROM HOUSE??

sorry!bs7671 it says recomend to put earth rod !
i wrote maybe wrong and you got confused!
so it is no mandatory! am i wrong?

 

[James]

what page are you looking at?

 

[Baddegg]

Depends on extraneous conductive parts boss.....even then you could bond back with relevant size bonding, 10mm for tncs...

 

[telectrix]

HELLO
Im learner yet!
i would like to know something about the TT system!
if i would have an outbuilding installation, like sumer house!the main earthing sytem is tncs! so the bs 7671 recomend ONLY DO NOT export the earth from the house !what about if i still use the tncs earth to summerhouse! just i would not relay on the ground resistance! do i think wrong?
Also for example :if the TT ze :6ohm in the summer house fuseboard ,then if i put 32a circuit braker for the ring circuit .then would r1+r2: 0.20ohm and i need to add ze:6 ohm =zs would be :6.20 !but in this case the braker will not trip in required times as the regulation show me!Becasue that braker need maximum zs:1.44 ohm!
so please can you explain sortly what i thinking wrongly?!Or can you make me understand more of it!?
Thank you!

exactly. so you then use a RCD for fault protection as with a TT earthing system it's very difficult to achieve a ZE (Ra) low enough to allow the requisite fault current to trip fuses/breakers.

 

[pc1966]

The regulations only give the disconnection times as a requirement and do not say how you achieve them. In practice with TT you have to rely on RCD to disconnect on a fault as it is very difficult to get Ra low enough for even a 6A B-curve MCB used for lighting, etc, let alone 32A ring finals, etc.

Traditionally that meant an incomer RCD, typically 100mA delay, and then possibly 30mA instant RCD (or RCBO) for additional protection, though if all circuits are on RCD then you can often drop the RCD incomer and use a switch.

But the high Zs at the board also means any fault before the RCD will not clear on the supply OCPD, so you have to take great care that cannot happen (as you should be doing any way!) so a proper gland to support the tails, ensuring the terminals are properly torqued, busbar cannot accidentally touch anything, etc.

If going from TN-C-S (in home) to TT (in outbuilding) you would typically earth the feed cable armour to the TN-C-S supply earth using a proper SWA gland, etc, so any fault in the cable (e.g. someone putting a garden spade or pick-axe in to it, etc) will take out the supply OCPD. Then at the out building isolate the earths, with the SWA supported by a plastic gland to only provide L & N, and the local earth rod being the means of earthing within the outbuilding installation.

 

[NEBITOMI]

The regulations only give the disconnection times as a requirement and do not say how you achieve them. In practice with TT you have to rely on RCD to disconnect on a fault as it is very difficult to get Ra low enough for even a 6A B-curve MCB used for lighting, etc, let alone 32A ring finals, etc.

Traditionally that meant an incomer RCD, typically 100mA delay, and then possibly 30mA instant RCD (or RCBO) for additional protection, though if all circuits are on RCD then you can often drop the RCD incomer and use a switch.

But the high Zs at the board also means any fault before the RCD will not clear on the supply OCPD, so you have to take great care that cannot happen (as you should be doing any way!) so a proper gland to support the tails, ensuring the terminals are properly torqued, busbar cannot accidentally touch anything, etc.

If going from TN-C-S (in home) to TT (in outbuilding) you would typically earth the feed cable armour to the TN-C-S supply earth using a proper SWA gland, etc, so any fault in the cable (e.g. someone putting a garden spade or pick-axe in to it, etc) will take out the supply OCPD. Then at the out building isolate the earths, with the SWA supported by a plastic gland to only provide L & N, and the local earth rod being the means of earthing within the outbuilding installation.

thank you ! but better to use pme earth then, TT spike rod! ? no?
so actualy i still can use pme at outbuilding wood house? !
what do you think if the outbuilding summer house next to it an hot tube! but thete is not extraneous conductiv part! i would still use the pme earth ,is it breach the regulation?

 

[telectrix]

use the existing pme earthing and if necessary add a rod at the outbuilding to negate lost N fault.

 

[NEBITOMI]

use the existing pme earthing and if necessary add a rod at the outbuilding to negate lost N fault.

when is necessary if the ze on tncs is very good? and can i just connect the earth rod earth wire to the consumer unit also with pme -earth together? if the N fault occour then the tt earth can protect life? if yes how? can you sort explain please!thx