RCD Discrimination, TT systems and Ze

[Harry Worth]

Again WHY TT the garage, when you have a perfectly working TN system at the house?? One things for sure, banging in a single thin short rod ain't gonna give you an Ra of 0.61 ohms, not even in the very best conditions!!!

. I'm always being told you can't export an earth with a PME system so I concluded I needed a TT system. There's going to be a PV array on the roof that needs an earth rod as the inverter is transformerless (DTI guidelines). The Cu in the garage is a steel unit but I disconnected the incoming rod earth when I read Ze. The Ze of 0.61 Ohms really took me by surprise - I did check it a couple of times as I felt the value was unlikely but this TT is a first for me - I'll have another look at it today. Might move the rod elsewhere and see if it changes (and possibly introduce a garden feature water fountain from what everyone is saying). Ze for the house was 0.28 ohms. Hope you can all clear this up for me. I notice the regs say that 'the earth electrode is electrically independent from the source electrode.' to me that reads you don't in my case, carry the CPC from the house through to the garage. I can't see why and felt I was misinterpreting it in someway. I'm sort of guessing all I've done is add another 'multiple earth' to an already PME system.

 

[Octopus]

Going back to the house CU, is it a dual RCD unit? If so you 'll have to remove the RCD in the garage.

 

[Harry Worth]

Single RCD - I'll swap it out.

 

[Engineer54]

. I'm always being told you can't export an earth with a PME system so I concluded I needed a TT system. There's going to be a PV array on the roof that needs an earth rod as the inverter is transformerless (DTI guidelines). The Cu in the garage is a steel unit but I disconnected the incoming rod earth when I read Ze. The Ze of 0.61 Ohms really took me by surprise - I did check it a couple of times as I felt the value was unlikely but this TT is a first for me - I'll have another look at it today. Might move the rod elsewhere and see if it changes (and possibly introduce a garden feature water fountain from what everyone is saying). Ze for the house was 0.28 ohms. Hope you can all clear this up for me. I notice the regs say that 'the earth electrode is electrically independent from the source electrode.' to me that reads you don't in my case, carry the CPC from the house through to the garage. I can't see why and felt I was misinterpreting it in someway. I'm sort of guessing all I've done is add another 'multiple earth' to an already PME system.

As i said before Harry, some of these collage lecturers really need to go back to collage themselves!!! No Wonder, why we get so many daft, basic, and idiotic questions being asked on these forums...

There is absolutely NO reason NOT to take the PME into the garage. If there is any water/gas service in the garage (or other Extraneous metalwork etc) take a 10mm bonding conductor with you, if the cable is T&E or 2 core SWA. If the SWA cable is 3 core 10mm or greater, use the 3rd core as your bonding conductor and the SWA as your CPC... Hang on a moment, ...Your lecturer doesn't also say you ''Can't'' use the armouring of a SWA cable as your CPC as well does he??...lol!!!

You can leave the rod you have provided, and connect it to the garage EMT, as you say it then becomes another multiple earth rod on the PME supply. In fact most European supply companies insist that a ground rod is present at the premises being supplied... Or, as you stated, leave it in place for the PV installation....

I can almost guarantee you will not achieve an Ra of 0.61 ohms with a single I.2m 3/8'' / 5/8'' earth rod, you certainly won't achieve any stability....

 

[Octopus]

Single RCD - I'll swap it out.

Doing what? Protecting a single circuit or multiple circuits?

 

[Harry Worth]

Doing what? Protecting a single circuit or multiple circuits?

Single circuit.

 

[drew35]

Wouldn't it be cheaper to just use an RCBO at the house and do away with the RCD in the garage? Then everything is protected, and you dont need to worry about discrimination. And use the PME.

 

[Deleted member 26818]

No it wouldn't.

 

[fancypantsyeah]

yeah ...strange one tncs is a excellent earthing system no need for messing around with adding an earthing rod??? no need for 2 rcd's either, mcb in the ccu in the house , 3 CORE OR T&E swa sub main feeding a garage ccu with rcd . ... job done.. don't complicate things is my motto.....lifes complicated enough .......

 

[SPARTYKUS]

Tncs- ahh yes, the earthing system that became favourite suddenly when the laws of physics changed in the 70's.


Nothing to do with cost.


;-)

 

[Engineer54]

Tncs- ahh yes, the earthing system that became favourite suddenly when the laws of physics changed in the 70's.


Nothing to do with cost.


;-)

They started PMEing supplies long before the 70's, more like late 40's early 50's. Mainly on the overhead supplies to give customers an earthing terminal. Even though TT systems were a dammed sight better in the UK in those day's, than they are today, few rural and semi rural premises had a supply company earth terminal... No cure-all RCDs in those day's either..lol!!

Seems a shame after all that work and expense of converting overhead and now cable installations, there are more than some going round TTing them all again!! lol!!!

 

[Harry Worth]

Just found this paragraph in the DTI guidelines?

The PME earth cannot be taken outside the equipotential zone. This is to prevent the potential shock hazard should the supply neutral ever be lost.

Is the source of this the 17th edition somewhere - anybody?

 

[SPARTYKUS]

Just found this paragraph in the DTI guidelines?

The PME earth cannot be taken outside the equipotential zone. This is to prevent the potential shock hazard should the supply neutral ever be lost.

Is the source of this the 17th edition somewhere - anybody?

Hi
Harry


Someone will be more eloquent than me, but I think that is why you would need to run a separate bonding conductor to the offending oil/gas/water/other service pipe in the said garage.

 

[Harry Worth]

Thanks for that - I think the penny's dropped!

 

[SPARTYKUS]

Theres alot to learn. Don't know about you but my brains full, i have to defrag & compress old files before anything new goes in