TN-C-S to Outhouse - Don't Quite Understand

[Hawkmoon]

TN-C-S supply with submain to outhouse:

Scenario 1 -
10mm 2 core SWA to insulated board in outhouse - SWA not banjo linked to local ET and earth rod installed (TT) - nothing extraneous in building.

Scenario 2 -
10mm 3 core SWA to metalclad board - SWA banjo linked to local ET and 10mm bond taken to extraneous incoming pipework (eg plumbing for sink)

Considering that bonding extraneous pipework is little different than installing an earth rod...


WHY do we have to keep the supply earth separate in scenario 1?


In scenario 2 aren’t you effectively TTing by main bonding?

Tin hat on

 

[Farmelectrics]

i thought the idea of having an earth rod supplying the out house was in case you lost your neutral on your pme sysyem hence having no earth

 

[Engineer54]

i thought the idea of having an earth rod supplying the out house was in case you lost your neutral on your pme sysyem hence having no earth

It is, but then if the neutral was lost, it would be on the DNO side of things and would include the house as well.... The chances of that scenario is however remote to say the least, and far better to extend the earthing and/or equipotential zone of the PME installation than to rely on what will almost certainly be a totally crap TT system. ....0.35ohms is always going to be better than 200 ohms in anybodies language!! lol!!

 

[Hawkmoon]

It is, but then if the neutral was lost, it would be on the DNO side of things and would include the house as well.... The chances of that scenario is however remote to say the least, and far better to extend the earthing and/or equipotential zone of the PME installation than to rely on what will almost certainly be a totally crap TT system. ....0.35ohms is always going to be better than 200 ohms in anybodies language!! lol!!

Totally agree - was more questioning the separation of the SWA earth (TT) and what it actually achieves.

 

[Zebra Stripe]

Eternal champ.......separation of DNO earth from outhouse in scenario 1 IMO is pointless.

If the outhouse has extraneous conductive parts, and a suitable protective bonding conductor is not available or inpractical to install, then separate the installations with insulated inserts etc.....and bond outhouse ECPs to outhouses MET.

 

[Deleted member 26818]

It's all down to cost
Which is cheaper, running a 10mm earth to the out house, or installing an earth rod.

 

[Hawkmoon]

From the answers given I'm guessing people have misread post 1, perhaps I should have worded it differently...

My confusion lies in why we have to keep separate the SWA earth when TTing an external building? What does this achieve/prevent?

 

[Zebra Stripe]

Think about where the MET will be located and if ECPs are present. As far as im aware there is no need to separate a TT earth in the instance you state in scenario 1, unless ECPs are present and you want to bond to a local MET and make outhouse and house two separate installations with their own equipotential zones.
Apart from negating the need to run out a protective bonding conductor to house MET I cant see the point.

 

[Hawkmoon]

Think about where the MET will be located and if ECPs are present. As far as im aware there is no need to separate a TT earth in the instance you state in scenario 1, unless ECPs are present and you want to bond to a local MET and make outhouse and house two separate installations with their own equipotential zones.

See what you're saying - I always assumed, from IET literature, that you had to do this in all instances.

 

[Zebra Stripe]

Well, as far as Im aware I dont think that is the case. As I said, apart from allowing the MET to become local to the outhouse I cant see what separation would gain…in other words use the DNO earth facility cos its better than most rods

 

[Hawkmoon]

Well, as far as Im aware I dont think that is the case. As I said, apart from allowing the MET to become local to the outhouse I cant see what separation would gain…

That's the part I don't get because bonding something extraneous in both scenarios is no different (in effect) to installing earth rods - why does one require separation of SWA earth and the other not? How can any type of hazard be present in one and not the other? (if you get what I'm saying lol)

 

[Zebra Stripe]

It is different cos if TT is separated from DNO supply earth then the house and outhouse become two different installations each with there own METs…if not separated then they will be one installation with one MET

 

[wozz]

I think the point you are getting at is both has extraneous (as such) parts and you have introduced one of them with installing the rod. Usually the pipe work would be accessible, where are a properly installed earth rod will have no accessible conductive parts therefore wouldn't pose the same risk.

Is that what you was getting at?

 

[Hawkmoon]

In a TT setup why do you need to ensure the SWA earth is kept isolated from the outbuilding? What danger will connecting it present?

Bear in mind if you bond to pipework originating from the main building you are already 'connecting' both earthing systems.

Even with an earth rod of low impedance you are effectively 'connecting' to the main building MET through its own bonding etc.

It is the need to separate the submain earth that I don't understand.

 

[wozz]

Ahhhhi think I've got it, you are talking about a shared metallic service between both structures?

 

[Hawkmoon]

Yes and also low impedance through general mass of earth from bonding, rodding whatever. What's so important about SWA CPC that it has to be separated?

I always extend the zone (3C 10mm sub) with TN-C-S - so my question is more to do with an area of bonding/earthing I don't understand rather than an issue I have in a practical sense.

 

[wozz]

Firstly anything other than industrial I'd say this situation is rare as rocking horse with the mass use of mdpe and other non metallic pipes.

It depends if you decide you need two zones if you do isolate the pipe work with plastic or make it not accessible some how. Basically remove the need to bond it. That's how I would go about it anyway.

If not two zones you are essentially bonding it in two places to a local earth reference.

 

[Hawkmoon]

I agree there are methods that can be employed to remove the need to bond - but if you do have to bond then the reason you need to disconnect the submain CPC is.........???

 

[wozz]

If you feel the need to bond it you are saying you are going with one earthing zones so you will be using the earth in the cable. As you won't be able to separate the two systems.

 

[Hawkmoon]

As soon as you start bonding and rodding you can't guarantee separating the two systems anyway so why not connect CPC to improve disconnection times etc. What does it matter if the two earthing systems 'overlap'? Everything in the outbuilding will still be equalised.