Out Building Has No Earth

[Pippyboy1916]

Hi guys,

working on a pir yesterday, 2 buildings, readings in main building fine. 10m away there is an out building getting feed from an isolator "in main building" earth reading 50ohms. Can i supply a new earth via a earth rod for the out building and put a rcd on the main switch at the comsumer unit in the out building? the customer had a spark in a couple of weeks ago, he also stated that the high earth and put a rcd before the main switch "in the main building" the problem that i had with this that i was getting high reading and the disconnection time were not begin meet on the mcb's "out building" comsumer unit. the spark said to the customer that under a 100ohms u can put a rcd on it!

any ideas

 

[johnboy6083]

well the RCD overides the disconnection times for the MCB's as with a Zs of less than 1667 ohms, then an RCD will disconnect within 0.4 secs. I would check the connections of the submain to the outbuilding, it may be a loose terminal or corrosion on a connection ect, and easily sorted.

 

[SimpleSimon]

Hi guys,

working on a pir yesterday, 2 buildings, readings in main building fine. 10m away there is an out building getting feed from an isolator "in main building" earth reading 50ohms. Can i supply a new earth via a earth rod for the out building and put a rcd on the main switch at the comsumer unit in the out building? the customer had a spark in a couple of weeks ago, he also stated that the high earth and put a rcd before the main switch "in the main building" the problem that i had with this that i was getting high reading and the disconnection time were not begin meet on the mcb's "out building" comsumer unit. the spark said to the customer that under a 100ohms u can put a rcd on it!

any ideas

If you were to put an RCD on the outbuilding with an earth rod attached you could than take your max Zs reading as 1667 ohms. Then this would comply with max connexion times. Also to make sure all bonding extraneous parts are bonded in the outbuilding.

 

[Guest123]

Gents, you need to be more specific simply stating 'put an RCD on it and the max Zs can now be 1667 ohms' imo is not correct as there is more than one type of RCD....isn't there.

 

[SimpleSimon]

Gents, you need to be more specific simply stating 'put an RCD on it and the max Zs can now be 1667 ohms' imo is not correct as there is more than one type of RCD....isn't there.

Too shay Lenny (if that's how you can say it.)
To be more specific a 30mA RCD which is used for additional protection.BS 61008 Disconnection time in 0.2s as structure is classed as a TT supply.

If your gona put it in the tails 100mA

 

[johnboy6083]

Gents, you need to be more specific simply stating 'put an RCD on it and the max Zs can now be 1667 ohms' imo is not correct as there is more than one type of RCD....isn't there.

i do aplogise, you are right len, in my first post i was referring to a 30mA.

If there is a 100ma RCD at the front end of the submain, then your max Zs is 500ohms (50/.1= 500, 50 being max touch voltage, 0.1 being fault current)

I still stand by advising you to check all of the connections on the submain.

John

 

[Guest123]

Too shay Lenny (if that's how you can say it)

Touche'

I think a bit more info is needed from the OP to reach a steadfast conclusion i.e what type of earthing system is in the main building (as to me it sounds though it isn't TT), what type of cableing is the distribution circuit to the outbuilding, installation method etc etc.

Why the Zs is then so high at the outbuilding end is slightly confusing??

 

[johnboy6083]

high Zs on a non TT system sounds to me like either, Loose connections, corrosion, or no earth present, and relying on other earth paths, ie bonding/steelwork ect

 

[SimpleSimon]

Touche'

Merci Monseuir.

 

[johnboy6083]

je, ne pas comprende, je ne parle francais, or something like that. Merde!

 

[telectrix]

je, ne pas comprende, je ne parle francais, or something like that. Merde!

Sprechen sie deutch?---- "achtung spitfire"

 

[SimpleSimon]

Nien.

 

[Deleted member 9648]

Whats the R1R2 reading on the distribution circuit ?...(assuming the earth is from the MET).....installing an RCD to a dodgy earth is only half sorting the issue.

 

[Pippyboy1916]

the r1/r2 are fine, its a tncs in the main building, every Zs is the same as the Ze 30 plus ohms at the out building board, is it ok to put a rod in and run it to the out building board for a earth? iv never put one in before. if i do this i would put a rcd on the main switch at the out building board. the problem iv got is why has the spark put an rcd on at the isolator when there really no earth?

 

[malcolmsanford]

What you need to do is establish your Ze at the origin as this is the only place you can have a Ze. The Zdb at the outbuilding is a different animal altogether.

As you know you should with a TNC-S get a Ze below 0.35 ohms, if your not getting this then a call to the DNO will be your next step. In fact it would not be a bad call to contact them anyway to confirm if it is a proper TNC-S. You could also as said previous check all the connections to make sure everything is ok 30ohms would suggest on a TNC-S to either be broken at the link and your getting parallel paths. doing a ZE will give you a better idea.

If the TNC-S as been exported then you have lost it as a building just 10m away should have a pretty good Zdb.

The first call will be to confirm what your Ze is, confirm if it is a TNC-S by the DNO and take it from there.

 

[Pippyboy1916]

Ze in main building 0.09 ohms

 

[malcolmsanford]

Then you have either lost the exported earth, or it as never been exported and is relying on parallel paths. So the next step is to check the continuity of your sub main.

 

[Pippyboy1916]

the earth is lost to the out building, im asking about the earth rod to the out building and putting a rcd on the main switch? a statement was made about the rcd taken over the disconnion times, wel that mean u can have a nail where the mcb's are and i really know thats no kosher!

earth rod yes or no?

 

[malcolmsanford]

Yes you can put an earth rod in and make the out buildings TT.

If you wish you can fit a RCD as a main switch. You will though have to decide now what size RCD as a main switch, are you going to fit a 100mA RCd as that main switch then 30mA for socket circuits and any other circuits that are needing a 30mA. What about fitting a main isolator and RCBO's if the installation is not to big.

Back to original OP, if your circuits are protected by a 30mA RCD then your Zs values will be 1667, so don't get hung up about high values for the MCB, as they are providing overload protection only.

Another plus to a local TT system is that if there are any metal services or metal work frame to the out building then you can bond using the local CU earth bar, rather than running a bonding conductor back to the original MET.

 

[Pippyboy1916]

cheers, so if the board has an rcd on it, up can have Zs of upto 1667 ohms? in the main building it has an rcd before the isolator. just not happy the out building has reallly no earth. the question is that this inspection can be passed as satisfactory?