Another Earth Rod or Export from MET?

[walec]

I will knock another one in today and let you know.

Put another rod in today, taht makes two, now got a reading of 213ohms, I will talk to my DNO tomorrow and see what they recommend about exporting, I am interested to find out now.

 

[Guest 004]

Why dont you terminate the swa into a plastic enclosure for the garage
TT the garage
Install main bonding to the services in the garage at that pont of entry

The house is Tnc-s so main bond the services where they enter at the house as per regs
The garage sub main cpc connected to the house earthing system

The sevice pipes being common to the garage and the house doesn't make any difference
A serice neutral disconnection will be shared by umpteen parralell earth connections as well as the garage services of your TT system

The 313 ohms is above the minimum recomended but that reason is a possible unstable earth
If it is in wet ground and unlikely to dry out substantially, that would not be unstable and I would be happy to accept that reading.being way below the operating value of the 30 m/amp RCd

Just a bit more of my opinion to add
If you decide to export the Pme the services in the garage would require a 10mm bond back to the Met at the house as well as local to the garage, because the 6mm armouring wont be adequate

Actually you are allowed up to 500 ohms, just make sure you have a main RCD for the garage installation, set at 100 mA. Personally I would put a series of earth rods all connected together until I got a reading below 50 ohms.

If you was to protect your supply with a 30 mA RCD then the max Zs is 1667 ohms, something to think about eh?
Also if you have test equipment, why dont you do an RCD test, and if it trips within 400 ms, you are within the regs, again for me I would be looking at trip times of around 40 ms @ x1, and 20 ms @ x5
http://www.electriciansforums.net/attachment.php?attachmentid=798&stc=1&d=1264478967

 

[Des 56]

Actually you are allowed up to 500 ohms, just make sure you have a main RCD for the garage installation, set at 100 mA. Personally I would put a series of earth rods all connected together until I got a reading below 50 ohms.

If you was to protect your supply with a 30 mA RCD then the max Zs is 1667 ohms, something to think about eh?
Also if you have test equipment, why dont you do an RCD test, and if it trips within 400 ms, you are within the regs, again for me I would be looking at trip times of around 40 ms @ x1, and 20 ms @ x5
http://www.electriciansforums.net/attachment.php?attachmentid=798&stc=1&d=1264478967

Mr enigma

Not quite clear of the advise you are offering the op
Quote
Actually you are allowed up to 500 ohms, just make sure you have a main RCD for the garage installation, set at 100 mA.

The sub main being mechanically protected by use of swa, will more than likely be non rcd protected at the house consumer unit
The figure of 500ohms I believe, you are quoting, the maximum operating resistance required for a 100m/amp rcd

I would be very surprised, if after installing a supply to a garage, that there would be no facility provided for socket outlets, and that a 30 m/amp rcd will be installed

The earth rod will operate that rcd with a reading lower than 1667 ohms,but that figure is the maximum, The advise that I have always been made aware of states that over 200 ohms it may be unstable.
I believe the Niceic recomend to their members a reading under 100 ohms

Are you suggesting that 500 ohms is a satisfactory reading as long as he uses only a 100m/amp rcd ? (which would in my opinion defeat the object of going to the trouble of installing the garage supply)

Even with a 100m/amp rcd the advise should be to try to get the reading below 200ohms

 

[Spudnik]

Indeed i also agree about the less than 200ohms.

If you are deciding to put in another rod, space them at least one rod apart and loop them.

 

[flukey]

did you speak to the dno???????

 

[walec]

I moved the earth rods further apart today, (2off now about 2M apart) I am now getting a Ze of 98ohms which is a vast improvement which I am happy with being below 200ohms, though I do have a third rod which I have not used.
I ran out of time to do any live Zs or RCD testing but will in the morning.
I did speak to the DNO but could not get any sense as the technical guys were all out on jobs...they promised to call me back in the morning, I will let you know what they say.

 

[flukey]

I did speak to the DNO but could not get any sense as the technical guys were all out on jobs...they promised to call me back in the morning, I will let you know what they say.

wouldnt hold my breath lol

 

[Guest 004]

Mr enigma


The sub main being mechanically protected by use of swa, will more than likely be non rcd protected at the house consumer unit


I would be very surprised, if after installing a supply to a garage, that there would be no facility provided for socket outlets, and that a 30 m/amp rcd will be installed

The earth rod will operate that rcd with a reading lower than 1667 ohms,but that figure is the maximum, The advise that I have always been made aware of states that over 200 ohms it may be unstable.
I believe the Niceic recomend to their members a reading under 100 ohms

Are you suggesting that 500 ohms is a satisfactory reading as long as he uses only a 100m/amp rcd ? (which would in my opinion defeat the object of going to the trouble of installing the garage supply)

Even with a 100m/amp rcd the advise should be to try to get the reading below 200ohms

The installation has to HAVE a main RCD, I am assuming that he has a DB in the garage. the regs state that a 100 ma rcd needs 500 ohms to operate, and 30 ma rcd needs 1667 ohms to operate. As I previously said I like to get readings below 50 ohms ( well 5 actually). I am based in Thailand and I deal with these situations all the time, as 95% of installations do not have an earth and are TT, and the ground is very dry. So an earth rod is essential. Please look at the pdf file I posted, done using AMTECH software, at the bottom you will see the figure states 500 ohms. Also the important thing here is the TIME that the RCD takes to trip!, in my opinion the better the earth, the quicker the times

@op

did you install a 3 core swa?

 

[Des 56]

The installation has to HAVE a main RCD, I am assuming that he has a DB in the garage. the regs state that a 100 ma rcd needs 500 ohms to operate, and 30 ma rcd needs 1667 ohms to operate. As I previously said I like to get readings below 50 ohms ( well 5 actually). I am based in Thailand and I deal with these situations all the time, as 95% of installations do not have an earth and are TT, and the ground is very dry. So an earth rod is essential. Please look at the pdf file I posted, done using AMTECH software, at the bottom you will see the figure states 500 ohms. Also the important thing here is the TIME that the RCD takes to trip!, in my opinion the better the earth, the quicker the times

@op

did you install a 3 core swa?

TT installations are very common in Wales as well

The times that the trip take to operate are characteristics of the RCd and are not affected by good or bad earth readings, unless they are too excessive for them to operate at all (this would be very unlikely)

Trip times vary only when loads with some existing earth leakage are left in circuit and tend to distort the correct tripping time by allowing a false reading in these circumstances



If he decides on a TT installation,then he has no choice other than to protect the TT system with a 30m/amp rcd,being a requirement of Bs 7671

We all would prefer to have readings below 50 ohms but in practise that is considered an exceptionaly good reading
The 1667 ohm operating resistance for the 30m/amp rcd will not be adequate,because as stated previously,anything above 200 ohms is considered unstable

 

[walec]

I spoke to the DNO today, they said as the garage is outside the fabric building of the house they would recommend earth rod or rods are installed.

The installation:
6mm SWA earthed in house at MET
2 earth rods with a Ze of 98ohms
Plasitc CU in garage with a 63amp 30ma RCD main switch.
RCD Testing:
I 17ms
I x 180 39.4ms
5 15ms
5 x 180 38.9ms

I do have another question though.
when filling in the certificate I put the electrode resistance down as 98ohms
But in the Zs column I put 1667ohms as per table 41.5
Is that right?

thanks guys

 

[Des 56]

Agreed
The Ze of the rod 98 ohms is permitted to be used as the rod resistance in that box and the max permitted Zs for the 30 m/amp is 1667 ohms

 

[Davey101]

Rather than start a new thread, I'll jump on this one, if the op doesn't mind.

My installation- 3 core SWA , fed from non rcd switch fuse-feeding detached brick built garage 2 metres from house. Supply to house is TNC-S. RCD board in garage. No water or gas in garage. Metal railings between house and garage.

Do I need to install an earth rod?
Do I need to isolate the SWA and CPC at garage end.
Can I just wire the swa and cpc's as if it were the same building?


Installation 2- As above, but garage is 20 meters from house, no railings, fed with 2.5 swa, 16 amp mcb from an RCD protected board (single rcd, supply used to be TT). Same questions as above.

I've heard and read LOTS of different opinions on this, would appreciate this forums thoughts.
My thoughts are that install 1 can be regarded as the same zone, due to the proximity, therefore use house earth, this would be the prefered option, as I dont like the idea of earth rods. Install 2 is further away, so may be different due to being outside of equipotential zone.

Looking for actual opinions,as well as regs please.

 

[Des 56]

I'll just throw a personal opinionn then on deciding which system to use

When the pme is exported in close proximity
I look at the chances of coming into contact with conductive parts of the pme and a true earth at the same time
Even if I used the pme and had doubts I could always add a rod to the pme

With a remote building 20 meters or so,services or not I would go with a tt install everytime
Its safe and presents none of the pme potential unsafe problems with possible raised neutral potentials and
the bonus being the dno have little say in the matter either

 

[walec]

Davey101, my garage is only 2M from the house and I do have gas and water in it, but when I spoke to the DNO yesterday I did not tell them that as I wanted their opinion without that knowledege because I thought if they knew they would say TT, they said TT anyway, as the garage was outside the fabric of the main building, the guy I spoke to was the supervisor in charge of Installation teams, he seemed to know his stuff, he even referenced Faradays cage...so from now on I would always install an earth rod and treat the garage (near or far) as a seperate building with RCD main switch as per regs, earthing the SWA at the house end, everythiing else earthed in the garage. At least thats the conclusion I have come to over the last week.

 

[Davey101]

Thanks for the replies. I just have trouble getting my head around the fact that you are being advised to do away with the lovely, low reading earth that the supplier has given you, and fit an earth rod, which you may struggle to get below 100ohms What about exporting the supply earth, AND using a rod? Is this better or worse (presuming worse) than just exporting the supply earth?

Edit- just re-read Des's post, I like the idea of installing a rod to the PME.

Cheers, Dave.