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!