rcd in a garage

[amlu]

hello, got a garage job coming and a question to ask
do i need a rcd in garage cu?
garage feed (including earth) is coming from a separate breaker in house consumer unit, protected by rcd over there.

 

[GLENNSPARK]

if your garage feed is RCD`ed...then no...
whats the earthing arrangements at the house C/U?
how many spare ways do you have at the house C/U?
is it a `high integrity` C/U?

 

[amlu]

its a tncs with good tested ze, feed is around 5 meters under the house in 2.5mm twin+earth, then around 20 meters of swa thru the garden
cu is a dual rcd unit, there is one spare way, garage is on 20a mcb and sharing rcd with 4 other circuits.
yes i know that half of the house will trip in case of a fault in the garage, anyway even if there is searate rcd in the garage, the house one will trip first. any way to prevent it? (separate rcd in main cu for it?)

adam

 

[Elmer BeFuddled]

At a distance of 20m I would make the garage a TT system.

 

[jeremy]

^^^^why?

 

[Elmer BeFuddled]

To "isolate it" from the house. 40A mcb in house, not rcd protected. SWA earthed at house. Earth pole/rod for garage. But then I do a lot of work out in the sticks.

 

[GLENNSPARK]

for a start...sounds like that armoured is jointed onto the 2.5mmCSA flat twin with probably a metal through box....this is crap.
2 ways here:
either:
1) get the armoured into an IP box mounted on the outside wall of the house (whisca)...and go back entry with the 2.5mm flat or..
2) do away with the 2.5mm flat...use it as a draw wire to pull yourself a new armoured into the C.U...
youv got a joint (inaccessable?) at the mo...2.5mm flat to armoured...thats not good..
your earthing arrangements are TN-C-S....so use an armoured....come of an unprotected way in the house C/U....into the sub board (garage)...RCD it there (30mA)....you can parallel up the armourings with one of the cores (black)..oversleave the black with some G/Y (both ends)...

 

[GLENNSPARK]

your grey core colour on the armoured is oversleaved blue (neutral)..

 

[GLENNSPARK]

use a 4mm CSA armoured of ov a 32 A OPD.....16 A for points in garage....6A for lighting.....do a VD calc...

 

[GLENNSPARK]

any extranious in this garage?

 

[Engineer54]

To "isolate it" from the house. 40A mcb in house, not rcd protected. SWA earthed at house. Earth pole/rod for garage. But then I do a lot of work out in the sticks.

Why would you leave behind a perfectly good earth with around a 0,35ohm Ze, for an earth rod, with 100 to 200 ohm Ra??

Either the OP can mount a small switch fuse, or MCB at the origin and supply the garage from that, providing a typical 2 way garage CU with RCD, or use his existing CU RCD protected MCB, and leave out the RCD at the garage CU. Nothing stopping the OP from also driving a decent sized/length Rod and connecting that in to the EMT, pretty pointless though if it has a high Ra value!!


Amlu

As there would be no discrimination between 2 X 30mA RCD's it would be anyone's guess which RCD device trips first (or both tripping). Ideally you don't want a garage fault to trip out half the house. There is only one way to overcome this, if using 2 X RCD's, that is by the use of a S type (time delayed) RCD upstream....

 

[GLENNSPARK]

Why would you leave behind a perfectly good earth with around a 0,35ohm Ze, for an earth rod, with 100 to 200 ohm Ra??

Either the OP can mount a small switch fuse, or MCB at the origin and supply the garage from that, providing a typical 2 way garage CU with RCD, or use his existing CU RCD protected MCB, and leave out the RCD at the garage CU. Nothing stopping the OP from also driving a decent sized/length Rod and connecting that in to the EMT, pretty pointless though if it has a high Ra value!!


Amlu

As there would be no discrimination between 2 X 30mA RCD's it would be anyone's guess which RCD device trips first (or both tripping). Ideally you don't want a garage fault to trip out half the house. There is only one way to overcome this, if using 2 X RCD's, that is by the use of a S type (time delayed) RCD upstream....

but its TN eng.....so:
if he does away with that crappy bit of 2.5 flat twin...and uses an armoured (full length)...he`s killin several birds...
1)its a Tn system....so no RCD at origin
2)no hidden joints...and you know what i think about them!!

now...the point here is this:
do you
a) RCD it at the house C/U end...and risk taking out half the house (as eng has quite rightly pointed out) or..
b) accept the extra work involved (replacing that 2.5 flat)...and run an armoured full length...thus allowing you to RCD the garage on its own...thereby not taking out circuits within the house (lighting on stairs maybe)....a better way...

 

[Engineer54]

but its TN eng.....so:
if he does away with that crappy bit of 2.5 flat twin...and uses an armoured (full length)...he`s killin several birds...
1)its a Tn system....so no RCD at origin
2)no hidden joints...and you know what i think about them!!

now...the point here is this:
do you
a) RCD it at the house C/U end...and risk taking out half the house (as eng has quite rightly pointed out) or..
b) accept the extra work involved (replacing that 2.5 flat)...and run an armoured full length...thus allowing you to RCD the garage on its own...thereby not taking out circuits within the house (lighting on stairs maybe)....a better way...

I know it's a TN system, which is why i'm really suggesting he protects the origin with a fuse or unprotected MCB etc..., but if the OP just wants to install straight off an already protected way, then best not include an RCD at the garage.

Fully agree with your suggestion of SWA from the CU, and maybe converting his existing dual RCD CU to having an unprotected way. ...But as you say, is he prepared to go along with the bit more extra work involved?? lol!!

 

[GLENNSPARK]

I know it's a TN system, which is why i'm really suggesting he protects the origin with a fuse or unprotected MCB etc..., but if the OP just wants to install straight off an already protected way, then best not include an RCD at the garage.

Fully agree with your suggestion of SWA from the CU, and maybe converting his existing dual RCD CU to having an unprotected way. ...But as you say, is he prepared to go along with the bit more extra work involved?? lol!!

see, he`s got the costs of a 100mA s type to consider here as well....yes i know that cost always comes secondary to safety & compliance.....but it will probably be cheaper to just get a new length of armourd pulled in or if the existing armoured is suitable (no damage and CSA)...then a scotchcast is another option (if the existing armoured is buried in the garden out to the garage..under grass maybe).....better than jointed on a throughbox...onto a stupid bit of flat twin..lol...

 

[Elmer BeFuddled]

So, shoot me down in flames (if needed). I'm always willing to learn a bit more, or even the correct way. I've been lead to believe that extending TN-S is not good. TNC-S (PME) def frowned on. So supplying the out building at that distance is to make it TT.

Now if I've totally ballsed up my understanding of the situation, correction will be gratefully and gracefully received.

Mind you, when the garage is a 5 car + helicopter garage at the bottom of your "yard", I'd want it totally isolated from t'house.

 

[GLENNSPARK]

So, shoot me down in flames (if needed). I'm always willing to learn a bit more, or even the correct way. I've been lead to believe that extending TN-S is not good. TNC-S (PME) def frowned on. So supplying the out building at that distance is to make it TT.

Now if I've totally ballsed up my understanding of the situation, correction will be gratefully and gracefully received.

Mind you, when the garage is a 5 car + helicopter garage at the bottom of your "yard", I'd want it totally isolated from t'house.

eh?....for them that don`t know maybe...lol...
stick around....you may get the insite on how to
extend the equepotential zone on a PME....lol....

 

[Engineer54]

So, shoot me down in flames (if needed). I'm always willing to learn a bit more, or even the correct way. I've been lead to believe that extending TN-S is not good. TNC-S (PME) def frowned on. So supplying the out building at that distance is to make it TT.

Now if I've totally ballsed up my understanding of the situation, correction will be gratefully and gracefully received.

Mind you, when the garage is a 5 car + helicopter garage at the bottom of your "yard", I'd want it totally isolated from t'house.

This is all too often banded about by the die hard scaremongers, normally those that can't get their heads around, or past TN-S and TT systems. It's been around now in the UK for around 60 years or so, ...and a hell of a lot longer in Europe. How many deaths or injuries have you heard of or know about due to lost/broken neutrals??

Truth is, this is a very rare occurrence, and one that most people including electricians, will never witness in their entire lives. But you always get the guy(s) that will swear they have seen many instances, when in reality they are just conveying and thus perpetuating what other scaremongers have told them!! And so the merry-go-round continues ...lol!!

 

[La Poste]

RCBO the garage supply that way only that one circuit will trip and no others.

 

[Octopus]

RCBO the garage supply that way only that one circuit will trip and no others.

Tricky if its a dual RCD board!

 

[La Poste]

Stand alone plastic enclosure next to the fuse board.