garage power help from the pros

[alpha spark]

So does this rule also apply to a sub mains in a commercial or industrial nature or is this purely a domestic issue only

 

[Zeespark]

Yes they do, if supplied on a TNCS system, commercial is no different in its use than in it is in domestic, just a bigger supply.

AndyGuinness

 

[alpha spark]

I've carried out many of installations over the last 10 years adding submains to commercial industrial installations and have never made them tt at the new submain end dnos have had to check the work and test sheets prior to powering the supply up and not once has any 1 said anything is this a new rule or is it something that should be done but nobody does

 

[Zeespark]

If I am correct it comes under the Electrical Safety, Quality and Continuity Regulations (DNO's bible)

AndyGuinness

 

[Andy™]

no-one said anything about submains needing to be made TT

 

[alpha spark]

It's the same principle isn't it if that sub main is connected to a tncs supply and at the other end say the gas water steel is bonded then surely if the neutral at the origin fails then so the earth which could make water gas pipes live am I Misunderstanding something or is what I am saying correct cheers

 

[Andy™]

were on about exporting earths. installing a submain is not always exporting an earth

either way, even if there was no submain and a neutral failure on DNO side, then neutral & earth will become live

 

[ProDave]

....either way' date=' even if there was no submain and a neutral failure on DNO side, then neutral & earth will become live[/quote']

On the one occasion I have witnessed that happening, local earth did indeed become "live" (my neon screwdriver illuminated when touched to the MET!!!) but there was enough return path via bonded water and gas pipes to allow enough current to pass to dimly illuminate a couple of lights in the house.

So the bonding of gas and water limited just how much the MET floated upwards, which is the whole point of the "multiple" bit of PME I wish now I had tried to measure just how much "earth" had floated upwards, but as soon as I realised it was enough to light my neon screwdriver, I switched off and called the DNO.

You have to ask which is more dangerous: everything floating upwards (pipes bonded) or "earth" floating upwards but local pipework not (no bonding in place)

But that's a separate topic. The DNO tell us neutral failure in PME is rare, but as I say, I've witnessed it once. Think about this. A split outer sheath of a burried concentric will allow water in to the outer cores, and slowly allow them to corrode. So logic says a ruptured outer core of a concentric must be many many more times more likely than a ruptured inner core.

 

[alpha spark]

Now I'm completely lost exporting earth?

 

[steptoe]

first up alpha, do you understand the different principles between TNCS and PME ?

submains do not always have to be TT'd as you do not always 'export' the earth,

in this context 'exporting' is regarded as taking the EARTH outside of the 'equipotential zone' created within the building of origin,

it only becomes an issue when you take a supplied earth to a point away from that origin that may also be affected by the 'general mass' of earth, that may in effect be at a different potential than the DNO supplied earth.

the first step in grasping this may be found here

 

[chen77]

so, is it PME or simply TNCS? yes, it makes a massive difference.

Sorry can you explain what you mean because in 7671 and OSG they state TNCS and then in brackets (PME)

Im still learning so any knowledge would be a help.

And just to put evrryones mind at rest. As I stated on my first post im not going to actualy be doing the job I just wanted to test myself to see how my studying was going.

Ive still got a hell of alot to learn. Hence why I find this forum very interesting

 

[steptoe]

have a look at the link on my previous post,

yes, BS7671 does clasify it as the same, and the DNOs are under pressure to ensure they comply,

but, 7671 is only a guide and you 'would' be the one signing to say it is safe,

 

[alpha spark]

No I can honestly say I thought and was taught that they were the same your link doesn't do any think

 

[haggis31]

If you do export the earth from a pme to the garage can you add an additional TT earth rod and connect to the pme earth in the garage db?

I added power to my garage the only conductive part in the garage is the db housing, so this could become live if the neutral is lost from the cu in the house?

 

[a1]

your link doesn't do any think

http://www.talk.electricianforum.co.uk/student-learning-zone-c-g-2330-levels-2-3-c-g-2381-c-g-2391-other-co/15480-simplified-earthing-arrangements.html

 

[ianmacd]

If you do export the earth from a pme to the garage can you add an additional TT earth rod and connect to the pme earth in the garage db?

And what earthing arrangement would that be? As Andy mentioned above, isolate the incoming earth at the cu.

I added power to my garage the only conductive part in the garage is the db housing, so this could become live if the neutral is lost from the cu in the house?

If you exported tn-c-s then yes but then again, why did you not fit a plastic one?

 

[alpha spark]

So u can fit a swa to a garage on a tncs system providing no bonding is needed in there and a PVC cu is fitted otherwise if bonding is required then it needs making tt at the garage end in simple terms is this acceptable. Y is it we are having to make are life harder just because of the dnos chances that a neutral will fail in my opinion they should take actions so that never fails

 

[Zeespark]

You can fit an SWA (2 core or a third core in cable but floating/disconnected) to an outbuilding with minimal issues Alpha, you would just need to make sure that in all cases, the armour is connected at the supply end only. If you are going from TNCS to TT, then earth the outbuilding section locally with a TT earthing rod.

AndyGuinness

 

[alpha spark]

Ok am clear now thanks again Andy

 

[ianmacd]

So u can fit a swa to a garage on a tncs system providing no bonding is needed in there and a PVC cu is fitted otherwise if bonding is required then it needs making tt at the garage end in simple terms is this acceptable. Y is it we are having to make are life harder just because of the dnos chances that a neutral will fail in my opinion they should take actions so that never fails

Who's Earth are you trying to export?