Exporting TN-C-S

[Suffolksparky]

H, I know this has probably been covered a thousand times, but just interested to here what people wouldn't of on this set up feeding a brick garage with steel frame And sprinkler system that will need bonding


TN-C-S (200amp) 70mm tri-rated tails (50mm Earth)(25mm bonds)DB1 Out off DB1 with 16mm 3core via 45amp mcb to garage DB

now should this garage be put on its own TTsystem with the earth wire isolated?

or should I drag 25mm bonds from the steel and water to the main building MET

if anyone can point me to a reg or info on this it would be much appreciate

 

[Paul.M]

Thread approved and moved into the commercial/ industrial section.

 

[davesparks]

If running a 25mm bond to the garage is feasable then do that,

 

[Sintra]

H, I know this has probably been covered a thousand times, but just interested to here what people wouldn't of on this set up feeding a brick garage with steel frame And sprinkler system that will need bonding


TN-C-S (200amp) 70mm tri-rated tails (50mm Earth)(25mm bonds)DB1 Out off DB1 with 16mm 3core via 45amp mcb to garage DB

now should this garage be put on its own TTsystem with the earth wire isolated?

or should I drag 25mm bonds from the steel and water to the main building MET

if anyone can point me to a reg or info on this it would be much appreciate

What type of cable is the 16mm 3 core?

 

[Suffolksparky]

The 16mm 3core is swa

 

[Leesparkykent]

Keeping the TN-C-S earth and bonding the extraneous conductive parts back to the MET would be my preferred choice. If it isn't feasible to do so then making the the outbuilding installation a TT system , isolating the TN-C-S earth at outbuilding and bonding the extraneous conductive parts at the outbuilding back to the sub board will be fine.

 

[Richard Burns]

If this is a new install then just run 25mm² 3 core SWA out there, bond the metalwork in the garage back to the garage DB as an EMT and use the 25mm² core as combined earthing and bonding conductor back to DB1.
Depending on distance and so cost, you might be able to run it as 4 core 16mm² SWA and use two cores as the combined conductor in parallel.


As far as practicable you want to maintain the best earthing arrangement you can.
Guidance note 8 provides some useful information regarding maintaining earthing and bonding on multiple buildings.

 

[Jonspark]

H, I know this has probably been covered a thousand times, but just interested to here what people wouldn't of on this set up feeding a brick garage with steel frame And sprinkler system that will need bonding


TN-C-S (200amp) 70mm tri-rated tails (50mm Earth)(25mm bonds)DB1 Out off DB1 with 16mm 3core via 45amp mcb to garage DB

now should this garage be put on its own TTsystem with the earth wire isolated?

or should I drag 25mm bonds from the steel and water to the main building MET

if anyone can point me to a reg or info on this it would be much appreciate

Why the hell would anyone want to ditch a perfectly good TNCS system for a slapdash TT lashup is beyond me!!! lol!!!
Your not exporting anything!!! lol!!! It's called extending!!!!lol!!
(you can thank me later Eng!)

 

[andyb]

Why the hell would anyone want to ditch a perfectly good TNCS system for a slapdash TT lashup is beyond me!!! lol!!!

Well for one it may be a lot easier & less expensive. Especially if the armoured is already in place.
Also it's not a lash up, it's just another method, perfectly acceptable.

 

[davesparks]

Why the hell would anyone want to ditch a perfectly good TNCS system for a slapdash TT lashup is beyond me!!! lol!!!
!)

How do you know that the OP would not install the TT system properly?

 

[Jonspark]

How do you know that the OP would not install the TT system properly?

Calm down dear, just a bit of sarcasm.

 

[Deleted member 26818]

Running a 25mm cable for bonding is well over the top to my mind.
TT it.

 

[Engineer54]

From what i can make out from the OP of this thread, this doesn't sound like your typical domestic garage install but rather a commercial or industrial set up especially as it has it's own sprinkler system. Though we as usual get very little information to base anything on.....

So far from being over the top, it makes far better sense to maintain the TN earthing system throughout this installation....

 

[HandySparks]

Calm down dear, just a bit of sarcasm.

I spotted the sarcasm, but you should have use the word 'buckshee' instead of 'slapdash' for true authenticity. :biggrin:

 

[Suffolksparky]

Having thought about this abit more the sprinkler system feed is in plastic the goes to steel in the building so no need to bond, so I only have the structure to bond,
the 16mm swa is already in place so can't go for 25mm swa which to be honest is a bit of an overkill for a what is a glorified garage, and a duct is in place to the mains any way so will be easy to pull in a25mm bond

thinking out out of the box here, wouldn't the 16mm Earth core plus the swa armouring combined be equivalent to 25mm??

 

[Engineer54]

thinking out out of the box here, wouldn't the 16mm Earth core plus the swa armouring combined be equivalent to 25mm??

In a single word ....NO!!

One or the other needs to be capable of carrying the full expected fault current, you can't combine two different metals with two vastly different conductivity values for obvious reasons....

Oh and if as you say this garage is steel framed and the sprinkler system has been fixed to the steel anywhere, it will still need main bonding. Always test all metal pipework services whether they enter a building in plastic or not....

So what is this installation all about then?? How about giving us a bit more information??

 

[Leesparkykent]

If the SWA cable to be used as a combined CPC and main bonding conductor it must meet the requirements of both 544.1.1 and 543.1.1, In addition with a PME system there may be network circulating currents flowing normally in the protective conductors which may raise the temperature of the SWA cable. In these circumstances the cable will need to be de-rated to accommodate this temperature rise. The resistivity of copper at 20C is 17.2 x10-9. The resistivity of steel varies with carbon content from 100 to 1000 x 10-9 that's why GN8 suggests using a factor of 8 for the ratio of the resistivity of copper to steel.

To put it another away your 3core 16mm swa won't cut the mustard to attempt it this way.

It's simple pull a 25mm bond in or TT the outbuilding, that's the only two options you have.