Workshop supply on a TT

[AA410]

Hi good people i have to run a supply to a workshop from a tt supply can you export the earth to the new fuse board or do you supply its own earth rod the cable to be installed is a 16mm s/w/a 3 core advice please or point me in the right direction many thanks

 

[malcolmsanford]

What is the existing rods Ra?

How far is the workshop from the existing location?

Is there going to be services in the workshop ie gas and water ? Is the workshop metal framed?

As you are going to be fitting 30mA RCD as additional protection in a TT system then quite a few sparks will take it as long as you have a Zs of 1667 then you are covered, and that is correct.

Being of an older school I would see what your Ra is at the original rod, and if it were high over 50 ohm say fit another rod for the workshop. Also if you have services in the workshop then it may also pay you to fit a rod.

Put it this way by fitting another rod your certainly not going to make the installation less safe. Chances are with the RCD protection you will not need to fit one. For me the deciding facto will be the existing rod Ra and wehter the workshop will have services or be metal framed.

 

[AA410]

Thanks for your reply not taken the Ra yet and there is no water supply to the workshop and its a timber frame building looking at the earth rod its been installed not less the 3 year The installation will be as fused isolator at the main 16mm to the workshop with earth and at the consumer unit not connected and then workshop with it own earth rod with 30ma rcd to feed lighting and power

 

[AA410]

Forgot to say the run is approx 80m

 

[malcolmsanford]

If it's 80m away then I would definately be fitting another rod for the workshop.

Also you may have to fit a 100mA S type RCD for fault protection on that sub mains tails mate unless you can get your Zs down low enough just for the Fuse.

 

[AA410]

Thats great advice as far as the sub main i can not see me getting the Zs that low so what is the set on this you say 100ma type s that will not trip if there is a problem from the workshop consumer unit its only for the sub main ?

 

[Engineer54]

S type RCDs will give a time delay before tripping, giving time for the downstream RCD device to operate first, thus leaving the 100mA RCD intact and still operational. ...It's called discrimination!!!

Any problem on the submain it's self will be protected by the 100Ma RCD, and yes with the normal delay in place..

 

[AA410]

So if i install a 100ma s type rcd at the origen of the supply 16mm 3 core s/w/a to the workshop no earth at the the new consumer unit then new earth rod to feed new 4 way consumer with 30ma rcbo for final circuits does that sound right. NOT DONE ONE OF THESE BEFORE

 

[telectrix]

nothing wrong with rodding at the workshop and connecting the earths through. could even give a better Ze at the house end.

 

[AA410]

But will that still give me the discrimination between the 100ma s and the 30ma rcbo-s ?

 

[Engineer54]

So if i install a 100ma s type rcd at the origen of the supply 16mm 3 core s/w/a to the workshop no earth at the the new consumer unit then new earth rod to feed new 4 way consumer with 30ma rcbo for final circuits does that sound right. NOT DONE ONE OF THESE BEFORE

As Tel stated, no need to isolate any earthing on a TT system. Make a good job of the rod installation at the workshop, and connect through to the house earthing system...

 

[telectrix]

yes. because a fault current of c 30mA will trip the workshop RCD but not the 100mA S. irrespective of the earthing arrangement

 

[AA410]

The next question is there such a 100ma s 32a RCBO and lads can i say a great big thanks for your help on this. This forum is a great wealth of knowledge thanks to people like yourselves

 

[telectrix]

personally, i don't know if you can obtain that. i would use a 100mA RCD and a separate 32A MCB, nice and neat in a " garage board "

 

[AA410]

You have just confused me did you mean a 100ma RCD and a separate 32A MCB to feed a garage board. Is the tripping time on a 100ma rcd the same as a 30ma has to be below 40ms a 5x

 

[telectrix]

no i meant fit the RCD and a MCB in a garage board at the house end. by garage board i meant a din rail CU big enough for a RCD and MCB.

 

[AA410]

LOL i think i am loosing it Its been a long day i have been stuck in a loft all day my knees are red raw but again thank for all your help

 

[JD6400]

I may be wrong but if on a stake , you can not use a metal board unless it is shielded in some way around your incoming tails ( not just the point of entry that is , but the enclosure as well ) if the tails were to touch down they would be the wrong side of the earth i think . That is if you have not brought a earth in from the supply side .

 

[johnboy6083]

I may be wrong but if on a stake , you can not use a metal board unless it is shielded in some way around your incoming tails ( not just the point of entry that is , but the enclosure as well ) if the tails were to touch down they would be the wrong side of the earth i think . That is if you have not brought a earth in from the supply side .

the reason for that is as follows. unless your Ra value is very low, then it will be too high to allow your MCB/Fuse to operate within porper disconnection times (0.2 for TT, 0.4 for TN (final circuits below 32A))
what would happen is that in the event of an earth fault, the board itself would remain live.

 

[AA410]

I found a good web site have a look at
Taking electricity outside - DIYWiki