TT help and info please

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bigsyd

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i have never worked on a TT system, but the job i am on now will be using TT,

Its a large house with a 3phase pme supply and a large detached garage and also detached stable block. i will be feeding the garage with a 3core 16mm swa, then feeding the stable block with a 3core 10mm swa from the garage D/B, now am i right in thinking i have to use an insulated D/B enclosure , or can i use a metal D/B and take the swa straight in to the D/B.

will i have to use a rod at each D/B board or just use the cpc in the 10mm swa ( i think rod at each )

i presume on the garage D/B i will use a 100ma time delay rcd to protect the stable block swa , and at the stable block D/B a 100ma rcd (to protect the D/B ) and 30ma to protect everything else

next question, i understand the Ze at the garage D/B should be less than 200 ohms (nic 100), so just say i got 90ohms, now that will make the Zs way high, i understand about The tabulated values for max Zs for fuses and circuit breakers are for TN systems only not TT systems. For TT the maximum Zs is 50/Ideltan.

when filling out the certs what do you put in the column for max Zs permitted by bs7671( would it be N/A ) and do you put the measured figure you actually got in the max earth fault loop imp column

sorry its a bit long winded but as i said..i have never worked on a tt system b4... am i on the right track , any input is very welcome

 
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Hello

Bigsyd.

If using a metal enclosure cable will need to be put through nylon gland armouring and cable earth wiill need isolating. Depending on ground usually earth would be around 20-40 ohms which is easily adequate. You could use a 5/8 threaded earth rod as then if earth is not adequate you could possibly knock another earth rod in. I personally would put earth rod at each location. I don't no if there are any changes in the 17th editin regarding agricultural installations.

batty

 

steptoe

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only issue I see(well the first main one anyway)

is why are you using 3core SWA?

if you are taking all 3ph to garage etc then surely wouldnt you also require a N,? hence 4 core.

and as you are not allowed to export the Earth, (see other long forgotten post) if you are only using single ph 2core SWA would be adequate.

you are correct in thinking the values for Zs can be disregarded.

shouldnt all supplies outside be protected by 30mA if its for fault protection?

100mA&300/500mA are I would be of the opinion for indirect contact or fire prevention only. (wait for the corrections quoting the regs!)

HTH

:)

 

bigsyd

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thanks for the replies lads, i was using a 3 core swa as it was just habit, and forgot that i was making it a tt system, i was going to use a 100ma incomer at the garage as the swa feeding the stables was going to be protected by a 32A MCB non rcd protected and all rcbo's at the stable block

 

steptoe

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that sounds OK buddy,

just wondering about distances tho.? 10mm SWA buried on a 32A...

and do you mean the 32A to the stables will or wont be protected by the 100mA RCD.?

(imho you should protect it)

as should all ring finals(outside the zone or potentially) be protected by 30mA, tho remember that for 17th ANY buried/hidden cable(<50mm) without mech protection requires 30mA protection as well.

 

bigsyd

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that sounds OK buddy,just wondering about distances tho.? 10mm SWA buried on a 32A...

and do you mean the 32A to the stables will or wont be protected by the 100mA RCD.?

(imho you should protect it)

as should all ring finals(outside the zone or potentially) be protected by 30mA, tho remember that for 17th ANY buried/hidden cable(<50mm) without mech protection requires 30mA protection as well.
the 10mm will be on the 100ma side of the dis board , cheers steptoe for youer help :)

 

steptoe

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I thank you bigsydd,

and I think you're going swimmingly well.

dont worry too much buddy, if you do everything as you would pme and get your R1 R2 OK then disregard your TT potential (as long as below 200ish).

and I think your protection will be fine.

as you know its hard to judge from afar, but just use your professional judgement and some common sense, you wont go far wrong.

glad to be of help.

:) :)

Albert.

 
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