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300Ma 4 Pole Rcd - Pme Supply?


jimmybobbob

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Hi all,

 

Here's one to scratch your head at or I'm missing something very serious.......

 

Here you have it:

 

soon to wire up an extension for a builder....nothing to exciting (sockets, LED down lights etc..) and part on the on going job is moving the supply cable numerous times (minds being changed and digging up of driveways grrrrrrrrr)

 

What I'm confused at, although I have my own thoughts, is that the supply comes underground, up the wall, into the meter box and terminates into the usual henleys 3p cut out. Its a 3p 4 wire system and is PME.

 

The previous electrician installed a 3p&n DB with a 300mA 4 pole RCD underneath. The tails came out of the meter box (out of the 4 pole isolation switch obv) up the wall then into the rcd then into the DB main switch. The DB is located in a stable adjacent to the dwelling and is to serve lighting and power in the stable block and obviously supplies the house

.

Now the supply to the house used to be via a 16mm 4 core swa with a separate 16mm cpc buried underground. 2 cores were used to supply the existing house with a view to use the other 2 core to supply a new consumer unit for the extension. Things have been changed around and moved so now I have installed a 25mm 3 core swa to supply both the existing house and the new extension (installing a double decker board)

 

As I previous said I have my own thoughts (unless im missing something serious) but want to put it out there.......what's with the 300mA RCD below the 3p&n DB?A

 

Any comments or answers welcome

 

cheers

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have a think of why a 300mA RCD would be used?

whats protecting the cable?

have a think about the nature of the installation? what type of installation is it?

 

all the answers are in the regs,

are you familiar with working on non domestic properties?

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it may not have been designed to be TNCS...


and if you have your own thoughts, how about sharing that too, instead of suddenly having the same thoughts as someone has posted...

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Yep all in the regs..

 

The SUPPLY might be TNCS, but the install is probably not..

 

If you chose to have a TNCS install, you would find rather onerous requirements for the submain cable that MIGHT have been correctly dealt with by means of the separate 16mm CPC [i think you might find it is/was being used as a bonding conductor] or at least i think that this might have been the original intention.

 

HOWEVER, it sounds to me like the supply is TNCS, but the install is TT and the RCD is there to comply with the regs as regarding the presumably metal DB and the placement of RCD's when using such an arrangement.

 

john...

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What is the earthing method  Jimmmybobobob .  I've never seen a  TPN supply without an earth terminal so I'm wondering too...whats with the 300mA  Arsydee.  Is it a rural area with earthing problems ?

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What is the earthing method  Jimmmybobobob .  I've never seen a  TPN supply without an earth terminal so I'm wondering too...whats with the 300mA  Arsydee.  Is it a rural area with earthing problems ?

 

maybe this is a clue Deke

 

The DB is located in a stable

 

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I missed that the first time...... :shakehead

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Andy tm

 

My thoughts ARE that the rcd is protecting the supply cable going to the house AND the nature of the installation ie stable block so protecting any livestock....rather than coming up with the SAME answers as other posts. Im thinking that he was also covering his own back also from the shoddy electrics that had been previously installed (although am I right in thinking you are responsible ((as a skilled operative)) for what you connect and energies)

 

The rcd isn't type s and there is a PME earth terminal that was/is being used as means of an earth path as apposed to a rod so TT is ruled out I'm affaid.

 

Apprentice87 you are right in thinking that the 16mm earth was used as a bonding conductor as the oil line was buried, as was the water (although a plastic pipe)

 

If it is to be guaranteed that every metallic part of the installation is bonded (even the DB itself obviously) then still the need for the RCD is surely dismissed (although as I,ME,MYSELF already assumed he was thinking of the nature of the installation, ie lifestock and buried cables etc) #Andy tm

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Just because it's not a TT installation now doesn't mean that it hasn't been or shouldn't be...

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The RCD and DB are relatively new about 2 or 3 years old as I understand when the house was renovated and the DNS installed a new 4 wire supply underground getting rid of the old over head supply. As other post's have said my initial thoughts of protecting livestock and the u/g cables was the same thinking as others (but why on a PME) I can see the logic for the old 4 core cable to the house as I wouldn't rely on the swa as an earth to get the correct disconnection time, hence putting the 300ma RCD in.................

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Just because it's not a TT installation now doesn't mean that it hasn't been or shouldn't be...

 

I'd second that,

except I thought that a competent person working on such an install should know whether or not it should be TT or not,

thats why I asked the question as to the OP being familiar with working on non domestic properties.

The RCD and DB are relatively new about 2 or 3 years old as I understand when the house was renovated and the DNS installed a new 4 wire supply underground getting rid of the old over head supply. As other post's have said my initial thoughts of protecting livestock and the u/g cables was the same thinking as others (but why on a PME) I can see the logic for the old 4 core cable to the house as I wouldn't rely on the swa as an earth to get the correct disconnection time, hence putting the 300ma RCD in.................

OK,

so that answers the question,

jimmy, I take it by that post you dont really understand the requirements for earthing farms/stables etc?

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ok......also to protect livestock as stated ^^^^^ after all no one can flog a dead horse (no seriously)

 

maybe the stable side should be TT......but it isn't....its PME,

 

I brought this post to attention originally for that purpose of the 300ma RCD

 

My understanding is that you shouldn't mix TT with TNCS as there may be a PD if a fault occurs ....the DB is metal connected to the PME terminal thus ruling out TT or have I got that totally wrong

 

 

 

should I also mention that when I say 'stable' it was actually a workshop and the horses were kept in the next 2 stables along so contact with the DB was never possible however my first post did say things have changed...it is now going to be a stable for horses so the DB is going to be enclosed behind a cupboard (regardless of this I still appreciate the need of the RCD) I guess im asking am I missing something.....

 

My understanding is that you shouldn't mix TT with TNCS as there may be a PD if a fault occurs ....the DB is metal connected to the PME terminal thus ruling out TT or have I got that totally wrong

 

or do I just need to go back to school

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I asked whether the 300mA rcd was an S-type because you will certainly have 30mA rcds downstream of it for your new extension. There wlll be no discrimination between these rcds, either could trip on a fault. 

So if the block containing the main intake becomes a stable, and the 300mA rcd trips, the whole installation goes off and the owner would have to go to the stable, past dobbin, to reset the rcd, possibly in the dark.

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ok......also to protect livestock as stated ^^^^^ after all no one can flog a dead horse (no seriously)

 

maybe the stable side should be TT......but it isn't....its PME,

 

I brought this post to attention originally for that purpose of the 300ma RCD

 

My understanding is that you shouldn't mix TT with TNCS as there may be a PD if a fault occurs ....the DB is metal connected to the PME terminal thus ruling out TT or have I got that totally wrong

 

 

 

should I also mention that when I say 'stable' it was actually a workshop and the horses were kept in the next 2 stables along so contact with the DB was never possible however my first post did say things have changed...it is now going to be a stable for horses so the DB is going to be enclosed behind a cupboard (regardless of this I still appreciate the need of the RCD) I guess im asking am I missing something.....

 

My understanding is that you shouldn't mix TT with TNCS as there may be a PD if a fault occurs ....the DB is metal connected to the PME terminal thus ruling out TT or have I got that totally wrong

 

or do I just need to go back to school

 

yes,

either that or read the regs,

it really does seem you have absolutely NO understanding of agricultural premises and their requirements for earthing

it does NOT matter what is actually there,

it matters what is supposed to be there, and what NEEDS to be there for it to be safe

BTW, please make your mind up, is it TNCS or is it PME,?

no, its not the same,

but either way, you need to understand what you are allowed and what you are not allowed on these type of installs.

I asked whether the 300mA rcd was an S-type because you will certainly have 30mA rcds downstream of it for your new extension. There wlll be no discrimination between these rcds, either could trip on a fault. 

So if the block containing the main intake becomes a stable, and the 300mA rcd trips, the whole installation goes off and the owner would have to go to the stable, past dobbin, to reset the rcd, possibly in the dark.

tbh,

thats pretty much the least of his worries the way things stand,

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Im being thick...ill read the regs im missing a lot and eating humble pie....tbc when I know what im dealing with.

 

I have done past work on farms but they almost always had TT systems with either 100mA RCD's with 30mA downstream or 30mA straight of....hence a lot of fault finding when they have no power at all (not installed by me I should add before I get ripped apart)

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So


What is the purpose of the 100ma RCD,?

What do you think the 300ma is doing that is different? 

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This thread, rightly or wrongly, (and it's no reflection on Jim) highlights how little knowledge is gained from doing a course, for what is taught or learned only remains if used regularly, I'm sure at some point most of us have learned about agricultural/farm installs but without utilising that knowledge it is easily forgotten?

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This thread, rightly or wrongly, (and it's no reflection on Jim) highlights how little knowledge is gained from doing a course, for what is taught or learned only remains if used regularly, I'm sure at some point most of us have learned about agricultural/farm installs but without utilising that knowledge it is easily forgotten?

Never a truer word said.

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sharpend and essex1, thank you on your understanding rather than jumping down my neck like others......

 

Ive now read 705 in the regs which is backed up by your previous post......if your not using it everyday you cant implement it and fully understand it as much as one would like. I guess this makes me a useless spark does it and I should put the tools down!

 

The purpose of the 300ma is to allow discrimination of rcd's downstream,or have i got that wrong aswell?

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I'm afraid that is wrong, only a time delay RCD allows discrimination, 

Think about the location,

This is NOT a domestic install, 

Forget about any other RCD that may, or may not be fitted, what is the purpose of the 300mA RCD,?

And think about earthing type on agricultural premises. 

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sharpend and essex1, thank you on your understanding rather than jumping down my neck like others......

 

Ive now read 705 in the regs which is backed up by your previous post......if your not using it everyday you cant implement it and fully understand it as much as one would like. I guess this makes me a useless spark does it and I should put the tools down!

 

The purpose of the 300ma is to allow discrimination of rcd's downstream,or have i got that wrong aswell?

 

not knowing every regulation doesnt make you useless, however you should already know the basics of what most of it is, then look up relevant section as and when required.

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Fair comment I would agree with that Andy

Could say ahem it's horses for courses!!

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You get used to it on forums don't worry and don't take it personal. People get brave behind a keyboard

I was always taught. A good electrician should be able to find the answer. A good college will teach you how to navigate the book not learn it. The more you do in a particular area the more second nature it becomes.

Forums for me are a great place to see how others do. There is no shame in not knowing. I am not afraid to look stupid as Sharpend will confirm.

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LOL that's why the regs are there,,, for you to refer to when needed.. None of us know all the regs inside and out.

When thinking about why the RCD is there don't think about it's value too much,,, think about what it's doing

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