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?

 

[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

 

[Engineer54]

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?

You are not ''Exporting'' anything, you are in fact ''Extending'' the equipotential zone when there are extraneous services/metalwork etc!! Which is why you need a minimum 10mm bonding conductor in such circumstances. If no extraneous metalwork then all you require is the standard CPC....

Have you not read the ''EXPORTING PME'' Sticky thread on this forum?? Perhaps you should, it'll answer most if not all your queries on this topic....

 

[Harry Worth]

Gone to read the PME sticky thread. My Ze might be 45 Ohms - Some plonker had done what people had suggested earlier in this thread but I'm not admitting anything.

 

[Engineer54]

Gone to read the PME sticky thread. My Ze might be 45 Ohms - Some plonker had done what people had suggested earlier in this thread but I'm not admitting anything.

Did you use an extendible 5/8'' rod?? If so coupling another rod and going deeper will considerably lower your 45ohm Ra and give your rod the stability all earth rod installations need... Especially if it's now going to be used for that PV system...

 

[Harry Worth]

Hi, No, It's a 3/8 rod. I didn't think 45 Ohms was too bad as compared to the 200 Ohm figure. Grounds sodden though. What figure can I realistically aim for if that's a sensible question bearing in mind how variable I guess 'earth' is.

 

[Engineer54]

It is highly unlikely that you will have any stability with a single 3/8'' 1.2m rod!! Those things should banned full stop, total waste of money and time....

If you want to do things properly, use 2 No 5/8'' rods coupled together to give you a decent depth, which in turn should give you a decent degree of stability. It will also considerably better your final Ra value. At an outside guess based on what your getting now, i wouldn't be surprised at all, if you achieve a substantially sub 20 ohms value, and maybe better, certainly good enough for the PV system you talk about being installed at a later date.

Don't even consider comparing with the 200 ohm figure that's banded about, That's just a meaningless farcical value. Always aim for 10 ohms, you may not achieve it, as that will always depend on ground conditions and how much time and money your willing to invest.

Don't forget to install a protective flush earth pit over any earth electrode position!! This is foremost to protect the connection and as a permanent earth electrode position marker...

 

[Harry Worth]

It is highly unlikely that you will have any stability with a single 3/8'' 1.2m rod!! Those things should banned full stop, total waste of money and time....

If you want to do things properly, use 2 No 5/8'' rods coupled together to give you a decent depth, which in turn should give you a decent degree of stability. It will also considerably better your final Ra value. At an outside guess based on what your getting now, i wouldn't be surprised at all, if you achieve a substantially sub 20 ohms value, and maybe better, certainly good enough for the PV system you talk about being installed at a later date.

Don't even consider comparing with the 200 ohm figure that's banded about, That's just a meaningless farcical value. Always aim for 10 ohms, you may not achieve it, as that will always depend on ground conditions and how much time and money your willing to invest.

Don't forget to install a protective flush earth pit over any earth electrode position!! This is foremost to protect the connection and as a permanent earth electrode position marker...

As always every answer raises another question. I've found the 200 Ohm figure in the regs, OSG and GN3 but I recollect another figure I can't find anywhere which I remember being quoted in a table at college i.e. Ze TNS 0.8, TNCS 0.35 and TT 21 Ohms - I have searched for the source of this TT figure in the regs with no luck. If the other 2 systems were outside the parameters quoted then representations would I guess be made to the DNO yet with the TT system if 21 Ohm can't be met then this figure of 200 Ohms is mentioned. As mentioned, earth is absolutely sodden so that figure I've managed will be best possible case - when we have our next drought (probably in the next few weeks knowing our climate) ground will probably be an excellent insulator. So take your point absolutely.

I probably won't spend too much more time on it as the transformerless inverter being fitted is temporary and will only be there a short time (heard that one before). Also as I understand it, guidelines are being changed shortly so the earth rod won't be required. It's been a sort of learning exercise really - I've learnt a lot already. Trouble is the more I know the more I realise there's far more I don't know!

I bought the earth rod as part of a kit - told I had everything I needed. I've installed a black plastic box along with a warning label over it. Won't be using that kit in anger again!

 

[Engineer54]

As mentioned in my previous post, nothing stopping you from connecting this rod via the garage CU to the MET, That's what i would be doing if it was my house/property...

The 21 ohm figure you mention is the maximum Ra at the DNO's power station/substation, but will generally be much much lower than that 21 ohms....

Like those 3/8'' spikes, those black plastic boxes are also next to useless, they sit above the ground and get battered and destroyed over the years. My best advise to you is that on any future TT systems that you install, is made up of at least 2 X 5/8'' extensible screwed rods via a coupling. And the use of a ''flush'' earth pit!! ...At the very least you will then know, your earth electrode will have a fair degree of stability.

One thing to note when using coupled rods, because the coupling has a slightly larger circumference than the rods, the top rod will take time to reach it's full potential while the soil consolidates back around giving full contact. Some create a pool of water around the rod while it's being driven in and this does help, but only time will give full and complete contact...