Exporting PME to garage

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peteelecs

Please canyou help I am a bit confused about the earthing to a shed if the house is PME can I just run an earth from the MET to the shed or can I only do this if house is TNS.How do i get earth to shed if its PME hope this doesnt sound to confusing any suggestions would be appreciated
 
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No difference between TN-S and TN-C-S (PME) once it's in the house in that respect. Separate neutral and earth once you're past the service head, whatever the supply may be.

Simon.
 
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PME earthing should only be exported to buildings that are suitable as a seperate equipotential zone, if the building is suitable then an equipotential bonding conductor must be run equal to or larger csa to the one in the house and in any case a minimum of 10mmsq

Risks associated with sheds/garages are wet concrete floors and perhaps external metal cladding, for these reasons it is usual to create a segregated TT installation in the shed/garage,

PME can be used for earthing the sub main which is then segregated from the TT installation either by cutting back the armour or terminating in an insulated box, if you use this method the CCU in the garage must be all insulated
 
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Yep as said TT it ! didnt think the DNO would allow PME to be exported anyway because if they drop a neutral its a bigger risk so TT is the best option
 
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For the Zillionth trillionth time there are no regulations preventing the TNCS earthing system being used for an outbuilding.If there are extraneous conductive parts in the outbuilding requiring main bonding then you must run that bonding back to the MET,which means a TT is more sense....if there are no parts requiring main bonding then use the TNCS earth as for any other install....or if you prefer waste your time and money providing a totally unecessary separated earth.
 
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But everyone isnt as key'd up on the regs such as joe blogs and doesnt know about bonding do they ? just a thought if you TT then their cover'd either way just in case thay add something to the structure !
 
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kung said:
if the garage is attached id keep it PME if detached i always TT its just something i always do buddy
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Thats your choice........but there is nothing in the BRB preventing the use of a TNCS for an ordinary outbuilding,that should be made clear.
 
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wirepuller said:
For the Zillionth trillionth time there are no regulations preventing the TNCS earthing system being used for an outbuilding.If there are extraneous conductive parts in the outbuilding requiring main bonding then you must run that bonding back to the MET,which means a TT is more sense....if there are no parts requiring main bonding then use the TNCS earth as for any other install....or if you prefer waste your time and money providing a totally unecessary separated earth.
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Unusual to get the correct 'exporting pme' answer in just 9 posts :)
 
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kung said:
But everyone isnt as key'd up on the regs such as joe blogs and doesnt know about bonding do they ? just a thought if you TT then their cover'd either way just in case thay add something to the structure !
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cant see the relevance.....the 'added' extraneous conductive part would need bonding even if it was TT....just the method of bonding would be different.
 
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wirepuller said:
For the Zillionth trillionth time there are no regulations preventing the TNCS earthing system being used for an outbuilding.If there are extraneous conductive parts in the outbuilding requiring main bonding then you must run that bonding back to the MET,which means a TT is more sense.
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I know theres no requirements of TT in the brb its what i always do as said its personal pref only although the DNO might have a different view !
 
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A garage will prob have a concrete floor then an issue is that the garage may be damp, have incoming services etc.
In which case you would not export a PME/TN-C-S house earth from the house, you would make the garage a TT install with earth rod
 
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Is this a garage or a shed?
Are there other services in there?
What is the construction of the garage/shed?

The answers to these questions are rather important here.
 
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wirepuller said:
For the Zillionth trillionth time there are no regulations preventing the TNCS earthing system being used for an outbuilding.If there are extraneous conductive parts in the outbuilding requiring main bonding then you must run that bonding back to the MET,which means a TT is more sense....if there are no parts requiring main bonding then use the TNCS earth as for any other install....or if you prefer waste your time and money providing a totally unecessary separated earth.
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It's late and I'm tired. Wirepuller could you explain to me how you mean by saying you can use the TNCS install? Does this mean you. Would use the main MET?
Sorry for being dense.
 
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I think with all of these things you want to do what complies with BS7671 and is the most economical. If you have a tn-c-s earthing arrangement at the main DB you can most economically just export the earth to an outbuilding. However if , for whatever reason, there is main bonding required in the outbuilding, then you will have to run a 10mm cable back to the MET. If the outbuilding is any distance from the main DB then it rapidly becomes more economical to bang in an earth rod (£10ish) to save the long run of the main protective bond (£1.50+/meter).
 
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Exactly as i said wirepuller, if it can be bonded safely bond it and use pme, if it doesnt need bonding use pme , it does not matter whether its attached or not bearing in mind that megas of new properties have the tn-c-s service and main bonding in the garage anyway

I would suggest that anyone wanting to beef up on this subject and earthing in general buys a copy of Gudance Notes 8 from the IET.
 
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SimpleSimon said:
It's late and I'm tired. Wirepuller could you explain to me how you mean by saying you can use the TNCS install? Does this mean you. Would use the main MET?
Sorry for being dense.
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Yes,as long as there is nothing in the outbuilding requiring main bonding it is acceptable and more economical to use the earthing system from the MET.
 
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kung said:
A garage will prob have a concrete floor then an issue is that the garage may be damp, have incoming services etc.
In which case you would not export a PME/TN-C-S house earth from the house, you would make the garage a TT install with earth rod
Click to expand...

I presume then that every external light/socket etc which is derived from a TNCS source and is subject to damp you install a separate rod for then?
 
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wirepuller said:
Yes,as long as there is nothing in the outbuilding requiring main bonding it is acceptable and more economical to use the earthing system from the MET.
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I get ya. And if bonding is required best install a separate rod ane MET in the shed and use an earth from swa cable to provide an earth return path to the main MET?
 
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wirepuller said:
I presume then that every external light/socket etc which is derived from a TNCS source and is subject to damp you install a separate rod for then?
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Good point ! my last post was a copy that i forgot to edit the you and replace with i ! what i am saying is if its a detached garage from the house with a tncs/pme earthing i always tt it as it is easier and cheap and just as safe ! i did also acknowledge the fact that there is no requirements in bs7671 that doesnt permit a pme being used ! as also said its personal pref ONLY

Houses usually (although not always) have insulating floors etc which mean that main bonding can establish a reasonably equipotential zone, so even if the electrical system's earth is at some higher voltage (e.g. PME with a broken CNE) it would be difficult to get a shock while inside the house. Go outside, or in an outbuilding with a damp concrete floor (no damp proof membrane) and voltage differences can rocket!
 
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SimpleSimon said:
I get ya. And if bonding is required best install a separate rod ane MET in the shed and use an earth from swa cable to provide an earth return path to the main MET?
Click to expand...

If the outbuilding is very close though it may still be more economical and/or convenient to just run your bonding back to the MET :)
 
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dunno about exporting earths. my missus says if i spend all day on this forum, she will export me and the PC to the garage!
 
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kung said:
A garage will prob have a concrete floor then an issue is that the garage may be damp, have incoming services etc.
In which case you would not export a PME/TN-C-S house earth from the house, you would make the garage a TT install with earth rod
Click to expand...

You are right to point out the damp floor situation which was known as an area of increased risk in my days as an installations inspector, what has not been mentioned so far on this long discussion is that the electrician has now been duckshoved the legal responsibility of using pme safely since the demise of the inspector, I say legal as it is the ESQCR who impose the pme regs and lets not forget that this is statutory document which every contractor should be aware of and possibly one that could be used against him/her as a duty holder
 
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trebor said:
You are right to point out the damp floor situation which was known as an area of increased risk in my days as an installations inspector, what has not been mentioned so far on this long discussion is that the electrician has now been duckshoved the legal responsibility of using pme safely since the demise of the inspector, I say legal as it is the ESQCR who impose the pme regs and lets not forget that this is statutory document which every contractor should be aware of and possibly one that could be used against him/her as a duty holder
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In the kind of domestic situation discussed here the key document is bs 7671.....as long as the installing electrician complies with that he has met his responsibilities.
 
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hi everybody can i clear a few points up a friend has asked me to wire up his garage/workshop . i have had a look over the job which i was going to install a metal clad db . reason being so i can gland the armoured in and metal clad sockets for mechincal protection and the feed with a 3core swa cable from the db in the house which is not rcd protected but i can fit a rcbbo to hager db to protect the cable and all the circuits the garage construction is breeze block and gurder its 10meters from the house its more like a workshop to be honest is my method wrong ? oh yes the house earthing sytem is pme


thanks james
 
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jamesb1 said:
hi everybody can i clear a few points up a friend has asked me to wire up his garage/workshop . i have had a look over the job which i was going to install a metal clad db . reason being so i can gland the armoured in and metal clad sockets for mechincal protection and the feed with a 3core swa cable from the db in the house which is not rcd protected but i can fit a rcbbo to hager db to protect the cable and all the circuits the garage construction is breeze block and gurder its 10meters from the house its more like a workshop to be honest is my method wrong ? oh yes the house earthing sytem is pme

thanks james
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You don't need to protect the distribution cable to the garage with an rcd as it is already has mechanical protection
, just use a submain with an rcd in.
 
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ok thanks for that i just thought it would be easier to protect to whole thing from the house as i need a breaker for it and the just fit a dp mian switch at the garage db, and mcbs but can i use a metal clad db and sockets with a pme earthing system and the breeze block building with gurder type construction ?????????????????????
 
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no probs there mate does that need to be a seperate earth back to the main earth terminal 10m/2 or can a spare core be used within the armoured if say a 4 core was used what about pme being used to earth a out building
 
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jamesb1 said:
no probs there mate does that need to be a seperate earth back to the main earth terminal 10m/2 or can a spare core be used within the armoured if say a 4 core was used what about pme being used to earth a out building
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If you use the suppliers earth, your distribution circuit will have to be sized as a combined CPC and main protective bonding conductor and sometimes this is a better reason for making an outbuilding a TT system rather than worrying about this 'exporting PME' stuff.

The minimum CSA will be 10mm from table 54.8
 
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