FARM INSTALLATION- First Time

[Piratepete]

I'm doing my first farm building installation and would like to get it right.

The Main Barn has a 3ph 100A TN-C-S supply to a 3ph Meter. I'm ripping out the the rodent eaten ancient switchfuses, domestic consumer units etc and starting again.

My plan is to feed a new local 3ph board. From this, Apart from local 13 Amp sockets and lighting, I will be feeding 1ph sub-boards (using SWA) in 3 other buildings. The customer is only looking for light and limited power (for trace heating) in all buildings.

1. I was thinking of keeping the TN-C-S in the Main Barn and TTying the sub boards - Ok?
2. My understanding is that every circuit has to be a minimum 300mA RCD protected If so, is the way to do this to use a 300mA RCD main switch on the Main Dis board? Then fit 30mA Single Pole RCBOs in the respective boards for any 13Amp socket circuit?

Cheers
Pete

 

[Deleted member 9648]

Section 705 is essential reading if you are doing this work for the first time,it'll answer all your questions.

 

[Richard Burns]

The main 300mA RCD would need to be time delayed to provide adequate discrimination. Might be cheaper to go for a 100mA time delay.
Make sure everything is IP44.

Read section 705!

 

[Piratepete]

The main 300mA RCD would need to be time delayed to provide adequate discrimination. Might be cheaper to go for a 100mA time delay.

Read section 705!

Thanks.
I have read 705. There's the problem of putting regs into practice which is why we have an On-Site Guide for domestic work. Is there an equivalent On-Site Guide for Agricultural Installations?

Yea.I was wondering about RCD discrimination. Wylex only do a 100mA S type 3ph RCD for NH boards. This is over £100 and I'm dealing with a farmer! Then 30mA RCBOs would be required in the sub-boards.
Would it be acceptable to fit a standard 3 pole (4 not required for TN-C-S?) incomer in the Main Board and feed the sub-boards with single pole 30mA RCBOs. No discrimination needed, no rcds in the sub-boards though any circuit on the sub-board will trip the RCBO in the Main Board.
Does the fact that the sub-boards are seperately earthed make this cheaper solution unworkable?
Do the 1ph sub-boards need protecting by double pole RCBOs? They would of course have dp main switches for isolation.
Cheers
Pete

 

[malcolmsanford]

Do you know if these buildings have metal grids laid in the floor??

 

[Chr!s]

Thanks.
I have read 705. There's the problem of putting regs into practice which is why we have an On-Site Guide for domestic work. Is there an equivalent On-Site Guide for Agricultural Installations?

Not that i am am aware of, its just general installation as per 7671 with the added section of 705.

Yea.I was wondering about RCD discrimination. Wylex only do a 100mA S type 3ph RCD for NH boards. This is over £100 and I'm dealing with a farmer! Then 30mA RCBOs would be required in the sub-boards.
Would it be acceptable to fit a standard 3 pole (4 not required for TN-C-S?) incomer in the Main Board and feed the sub-boards with single pole 30mA RCBOs. No discrimination needed, no rcds in the sub-boards though any circuit on the sub-board will trip the RCBO in the Main Board.
Does the fact that the sub-boards are seperately earthed make this cheaper solution unworkable?
Do the 1ph sub-boards need protecting by double pole RCBOs? They would of course have dp main switches for isolation.

Well you need to decide on which circuits need discrimination, then you need to consider the types of loads, these can upset some rcd's, so you may need an rcd which is specific to the loads. The front end rcd need to be sized carefully, on a large install cable capacitance alone can be quite high, coupled with leakage and appliance filters etc 100 mA can soon be past. Id be inclined to go for a mccb up front with an adjustable 300 Ma add on. Down stream you can then scale down to 100 mA td and then to 30 mA. I would ensure if discrimination is required to go for double pole, the neutral needs switching. Isolation on tt need's to be double pole so that allows isolation of individual circuits without having to shut down a group.

The sub boards may be individually earthed, but the front end rcds will see the total leakage.
Cheers
Pete

Regards Chris

 

[Engineer54]

Thanks.
I have read 705. There's the problem of putting regs into practice which is why we have an On-Site Guide for domestic work. Is there an equivalent On-Site Guide for Agricultural Installations?

Yea.I was wondering about RCD discrimination. Wylex only do a 100mA S type 3ph RCD for NH boards. This is over £100 and I'm dealing with a farmer! Then 30mA RCBOs would be required in the sub-boards.
Would it be acceptable to fit a standard 3 pole (4 not required for TN-C-S?) incomer in the Main Board and feed the sub-boards with single pole 30mA RCBOs. No discrimination needed, no rcds in the sub-boards though any circuit on the sub-board will trip the RCBO in the Main Board.
Does the fact that the sub-boards are seperately earthed make this cheaper solution unworkable?
Do the 1ph sub-boards need protecting by double pole RCBOs? They would of course have dp main switches for isolation.
Cheers
Pete

No, it isn't acceptable to supply a 3 phase DB with a 3 pole disconnection means, it will always require a 4 pole. You need to disconnect ALL incoming conductors at the DB's Main Switch...

Won't comment further, as Malcolm has asked you an important question that you need to answer, before progressing further

 

[Piratepete]

No, it isn't acceptable to supply a 3 phase DB with a 3 pole disconnection means, it will always require a 4 pole. You need to disconnect ALL incoming conductors at the DB's Main Switch...

Hi Mr Engineer54

Doesn't 537.2.1.1 permit a 3 pole isolator? The supply is TN-C-S. Maybe I've misunderstood this clause.

Pete

 

[Piratepete]

Do you know if these buildings have metal grids laid in the floor??

Hi Malcolm
There are no floor grids. The Main Barn has concrete on hardcore, the other buildings have hardcore. Plenty of steelwork to bond though.

These folks only want a socket in each building for trace heating and some lighting so I'm trying to put in a basic inexpensive installation that complies with the Regs.

Can you see any flaws in the simple solution I proposed in my 2nd post?

Regards
Pete

 

[Guest123]

No, it isn't acceptable to supply a 3 phase DB with a 3 pole disconnection means

Yes it is, for TN-S & TN-C-S systems.

A TT system would require a 4 pole isolator.

 

[Engineer54]

That's acceptable on a MDB according to BS7671?? Oh my God!!! Think i'll stick to the higher standards of Europe on that one then. You've actually shocked me a little here, i can maybe understand for sub-DB's with balanced loading, but no-way for a MDB, i would want to know that everything has been isolated downstream, and no nasty surprises!!!


So why do they call for 4 pole isolation on 3 Phase TT systems earthing then??

 

[Guest123]

That's acceptable on a MDB according to BS7671?? Oh my God!!! Think i'll stick to the higher standards of Europe on that one then. You've actually shocked me a little here, i can maybe understand for sub-DB's with balanced loading, but no-way for a MDB, i would want to know that everything has been isolated downstream, and no nasty surprises!!!


So why do they call for 4 pole isolation on 3 Phase TT systems earthing then??

The reg reads....


537.2.1.1

"Every circuit shall be capable of being isolated from each of the live supply conductors. In a TN-S or TN-C-S system, it is not neccessary to isolate the neutral conductor where it is regarded as being reliably connected to earth by a suitably low impedance".


WRT to a TT system, the neutral cannot be said to be 'reliably' connected to earth by a suitably low impedance.

Single pole RCBO's are also a no no on a TT system.

 

[Engineer54]

Just trying to get my head around that one Lenny. My mind isn't in top gear so to speak, but what difference is there in a TN-S neutral and a TT neutral?? They will both be solidly bolted to earth Only at the supplying TX...

 

[Guest123]

They are, yes. But, the impedance of the earth path on a TT system from the installation end back to the supply Tx, can be extremely high as it is via the general mass of earth.

Also, in a TT system, the impedance between the MET and the general mass of earth (via the MEC & earth electrode(s)) and consequently the star point of the supply transformer can be very high (20ohms). In TN-S and TN-C-S this impedance is much lower (0.8 & 0.35ohms respectively) because the MET is directly connected to the star point of the transformer via the supplied earth or neutral conductors.

 

[Piratepete]

Just trying to get my head around that one Lenny. My mind isn't in top gear so to speak, but what difference is there in a TN-S neutral and a TT neutral?? They will both be solidly bolted to earth Only at the supplying TX...

Hi
If you Google the question about 3 or 4 pole isolation you'll find a link to a very long discussion on the IET Forum which does explain this in some detail.

NOT REQUIRED - Just seen Lenny's latest reply!
Cheers
Pete

 

[Engineer54]

They are, yes. But, the impedance of the earth path on a TT system from the installation end back to the supply Tx, can be extremely high as it is via the general mass of earth.

Also, in a TT system, the impedance between the MET and the general mass of earth (via the MEC & earth electrode(s)) and consequently the star point of the supply transformer can be very high (20ohms). In TN-S and TN-C-S this impedance is much lower (0.8 & 0.35ohms respectively) because the MET is directly connected to the star point of the transformer via the supplied earth or neutral conductors.

Now, ...Why didn't that come to mind to me earlier?? lol!! Yes i can see the point of the requirement now though, i still think it's advisable for all systems regardless. I can see far more benefits to having 4 pole isolation than 3 pole and a tied neutral, ....One obvious benifit is the total ease of testing.

You would also Need a 4 pole RCD device, if it were to be incorporated as forming part of the main switch/isolator or MCCB with earth fault facility!!

As i said before, i'll be sticking with the higher European requirement of a 4 pole means of disconnection to TP+N DB's... lol!!

 

[hasel5]

Single pole rcbos ?

 

[Piratepete]

Hi Hasel5

Single pole rcbos ?

What about them?

The Main DB is on a TN-C-S supply so sp rcbos protecting sockets is OK.

So the question is - is a single pole (only the live is switched but both poles sensed) rcbo acceptable for protecting a sub board which is TT?

Cheers
Pete

 

[kenny7askew]

They are, yes. But, the impedance of the earth path on a TT system from the installation end back to the supply Tx, can be extremely high as it is via the general mass of earth.

Also, in a TT system, the impedance between the MET and the general mass of earth (via the MEC & earth electrode(s)) and consequently the star point of the supply transformer can be very high (20ohms). In TN-S and TN-C-S this impedance is much lower (0.8 & 0.35ohms respectively) because the MET is directly connected to the star point of the transformer via the supplied earth or neutral conductors.

thanks for your interesting and informative post lenny, i do understand much of what you are saying but could you provide a practical scenario so i may undersrtand the reasons why

 

[Guest123]

thanks for your interesting and informative post lenny, i do understand much of what you are saying but could you provide a practical scenario so i may undersrtand the reasons why

These fault loop diagrams should explain it in a bit more detail.

It's all to do with the path the fault current has to take from the point of fault, through the installation, along to the supply Tx and back to the installation.

 

[Piratepete]

Hi Lenny
May I clarify that the posts 19 and 20 relate to the earlier discussion about the use of 3 pole or 4 pole main isolators and are not an answer to my question on Single Pole RCBOs in post 20!
Pete

 

[Specialist]

They are, yes. But, the impedance of the earth path on a TT system from the installation end back to the supply Tx, can be extremely high as it is via the general mass of earth.

Also, in a TT system, the impedance between the MET and the general mass of earth (via the MEC & earth electrode(s)) and consequently the star point of the supply transformer can be very high (20ohms). In TN-S and TN-C-S this impedance is much lower (0.8 & 0.35ohms respectively) because the MET is directly connected to the star point of the transformer via the supplied earth or neutral conductors.

:clap:Lenny I've gotta be fair to you mate, the way you answer peoples questions Rocks. :icon4:

 

[Chr!s]

Hi Hasel5



What about them?

The Main DB is on a TN-C-S supply so sp rcbos protecting sockets is OK.

So the question is - is a single pole (only the live is switched but both poles sensed) rcbo acceptable for protecting a sub board which is TT?

Cheers
Pete

Yes, its acceptable to use a Rcbo for protecting a TT circuit in some instances.

Regards Chris