RCD Discrimination, TT systems and Ze

[Harry Worth]

Just installed a 5 way consumer unit in a garage/workshop. It's fed from a garage unit in the house. I've started testing and on the garage 5 way the RCD test is OK but the RCD unit in the house trips at the same time (apart from one random event). I'm not happy about this and a bit puzzled as I did something similar last week with no such issues. In fact I was worried that time as to which RCD would trip but only the one tested did ( I was impressed at how clever RCDs were). I didn't give it a second thought on this installation until I started testing! I've installed an earth rod for the garage so it's a TT system (first for me). The Ze on this surprised me - I was expecting to struggle to get a decent value yet I have a figure of 0.61 Ohms (yes I disconnected the CPC so no parallel paths etc when I tested Ze). All my test figures seem OK.

My question - How do I get the RCDs to discriminate? i.e. What have I done wrong?

 

[telectrix]

what cable feeds the garage? if it's SWA then ditch the RCD at the house end.

 

[Harry Worth]

what cable feeds the garage? if it's SWA then ditch the RCD at the house end.

Yes - SWA. 2nd mcb is unused at present but may get used in the future. OK to run the SWA without an rcd through a garden, routed at 12" above ground clipped to a wall so it's clearly visible!

 

[telectrix]

as SWA has it's own earthed metallic sheath (the armour) no need to protect with RCD.

 

[The Ranger]

use a type S

 

[PC Electrics]

as SWA has it's own earthed metallic sheath (the armour) no need to protect with RCD.

Eerr! apparently this is a TT !

Yet another one who doesn't know how to discriminate RCDs. Don't they teach this stuff anymore?? This question seems to be becoming a 'regular'.

OP, you need a time delay unit (S Type) upstream to provide discrimination.

 

[Harry Worth]

Eerr! apparently this is a TT !

Yet another one who doesn't know how to discriminate RCDs. Don't they teach this stuff anymore?? This question seems to be becoming a 'regular'.

OP, you need a time delay unit (S Type) upstream to provide discrimination.

Yes - We were told about time delay rcds (OK - I'd forgotten but I'm coming clean) but again re the replace the rcd with an mcb. Replacing with a time delay device will again surely present an issue in the future if the second mcb is utilised for a circuit within the house - Or do I worry too much about future proofing and go with the present? The circuits in the garage are protected by RCD as the system here is TT.

 

[Deleted member 26818]

You need to tell us what the earthing sytem for the house is.
If it is TT, then the SWA cable will have to be protected by at the least a 100mA RCD preferably time delayed, to allow for discrimination with the 30mA RCD protection in the garage/workshop.
If however it is TN, then no there is no requirement to protect the cable by means of an RCD.

 

[Harry Worth]

You need to tell us what the earthing sytem for the house is.
If it is TT, then the SWA cable will have to be protected by at the least a 100mA RCD preferably time delayed, to allow for discrimination with the 30mA RCD protection in the garage/workshop.
If however it is TN, then no there is no requirement to protect the cable by means of an RCD.

Hi spinlondon - Thanks for that - House is TNCS.

 

[jase]

if house is TNCS then why TT the garage?

 

[Deleted member 26818]

Then you need to remove one of the RCDs.
If you remove the one protecting the SWA, then you will minimise inconvenience.
If you remove the one in the garage/workshop, it will be more inconvenient as someone will have to traipse from the garage/workshop to the house to reset the RCD in the event of a fault.

 

[cbw]

And a Ze of .61 is a bit strange.

Are you sure they have not terminated the SWA into a metal C.U ?

 

[Engineer54]

Hi spinlondon - Thanks for that - House is TNCS.

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!!!

 

[Jimmyb]

did u null the test leads ?

are you testing L - PE not L - N ?

Did you install the rod near any pipe work or cables ?

 

[Deleted member 9648]

if house is TNCS then why TT the garage?

Looks like another 'you cant export the PME' disciple!......unless of course there's bonding in that there garage!

 

[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?