Industrial agricultural installlation

[The apprentice]

Hi guys me and my boss will be doing a complete re new of a farm installation we will be removing the bus bar system and installing a a 3 phase distribution board to supply sub main boards to other locations on the farm with my understanding a would not put a 300mA main switch incase it took out the whole of the installation there for would I put a normal main switch inside the 3 phase distribution board and all 3 phase sub boards have 300mA main switch and outgoing circuits at 30mA rcbos and for single phase boards all to be rcbos or split rcd boards ? Or is my understanding wrong to this?

 

[Lucien Nunes]

What will be the Ze at the main board at the origin? Will it be low enough to protect the board without the aid of the single upfront 300mA RCD?

BTW, please call them RCDs. You can't have a '300mA main switch' although you can use a 300mA RCD as one.

 

[The apprentice]

What will be the Ze at the main board at the origin? Will it be low enough to protect the board without the aid of the single upfront 300mA RCD?

BTW, please call them RCDs. You can't have a '300mA main switch' although you can use a 300mA RCD as one.

What will be the Ze at the main board at the origin? Will it be low enough to protect the board without the aid of the single upfront 300mA RCD?

BTW, please call them RCDs. You can't have a '300mA main switch' although you can use a 300mA RCD as one.

My boss’s has walked over the job I haven’t went to the job personally yet thing is I’ve never done a farm before as iam a 3rd year apprentice but more knowledge from you guys the better hence it’s quarter past 9 on a Wednesday night and here iam let’s say it’s a tns with a ze of 0.22ohms evan if it is low enough regulation could it evan be a 100 mA there for I need the theory behind your answer thanks mate

 

[JD6400]

Is it just an arable farm only or is there livestock about or likely to be in the future.
But what ever the earthing arrangements , you will need an s type RCD to a maximum value of 300ma up front.

 

[The apprentice]

Is it just an arable farm only or is there livestock about or likely to be in the future.
But what ever the earthing arrangements , you will need an s type RCD to a maximum value of 300ma up front.

Thanks mate! So 300mA upfront my worry was incase it tripped that it would take out every sub board? There is live stock in a cow shed

 

[JD6400]

If you have livestock , you can pretty much guarantee you will need to stake the place.
The 300mA is for fire protection .
Have a good read of section 705 .

 

[The apprentice]

If you have livestock , you can pretty much guarantee you will need to stake the place.
The 300mA is for fire protection .
Have a good read of section 705 .

Will have a good read of it mate thanks a lot for this as in stake it you mean earth electrode then earth all the conductive parts that the live stock touch

 

[JD6400]

If there is livestock present it is almost impossible to satisfy the requirements not to TT the place .
As for segregation ( or what ever it is now referred to in the 18th ) the s type upfront will let you discriminate locally through the installation if design correctly

 

[JD6400]

Also I prefer to not only stake at the supply incomer and locally throughout the installation , but also tie them all together albeit through the armoured only if not a core as well to give you you what I suppose you could call an earth nest.
As of yet I've not been shown why this isn't a good idea but I will put my tin hat on now !

 

[The apprentice]

Also I prefer to not only stake at the supply incomer and locally throughout the installation , but also tie them all together albeit through the armoured only if not a core as well to give you you what I suppose you could call an earth nest.
As of yet I've not been shown why this isn't a good idea but I will put my tin hat on now !

Yeah I get what you mean mate thank-you for the update mate I appreciate it greatly

 

[loz2754]

Am I the only one wondering why it seems to be left to an apprentice to design such a safety critical installation? No offense intended to the "apprentice", at least he seems to be conscientiously trying to do the right thing.

 

[The apprentice]

Am I the only one wondering why it seems to be left to an apprentice to design such a safety critical installation? No offense intended to the "apprentice", at least he seems to be conscientiously trying to do the right thing.

No your absolutely right on this topic mate I study a lot out of hours to get things right and correct I love to learn my journey min is out his depth by far it is a worry you would need to be very competent for this kind of install

 

[pc1966]

Also I prefer to not only stake at the supply incomer and locally throughout the installation , but also tie them all together albeit through the armoured only if not a core as well to give you you what I suppose you could call an earth nest.
As of yet I've not been shown why this isn't a good idea but I will put my tin hat on now !

I would agree you want as many earth rods as practical, and spread out over the area so there is never a big difference in potential between the CPC system and the local Earth.

There was another thread where I mentioned a 25V limit for agriculture (i.e. Ra = 25/In = 83 ohms for 300mA RCD) but seems that was 16th edition, that has now gone and it is now the usual 50V in the regs for reasons unknown as cows are just as conductive.

But remember that this is the maximum Ra and you really want as good a local earth as you can get with reasonably effort. So if it an 8 foot rod at each board plus bonded pipe work you might see a 1/10 of that.

 

[pc1966]

Thanks mate! So 300mA upfront my worry was incase it tripped that it would take out every sub board? There is live stock in a cow shed

If you have an up-front delay RCD then you need all down stream circuits to have faster and lower current double-pole RCD protection to totally avoid that problem (i.e. complete selectivity).

The requirement for double-pole isolation is a bit odd, but basically if you have a N-E fault and the RCBO trips but the N-E connection remains and any significant current flow then can trip the RCD incomer.

Some of the popular single phase consumer boards have 30mA RCBOs that are DP-switching, such as the compact Fusebox, Wylex, and Crabtree ranges, but I don't know of many for 3-phase boards.

If it is a big install, then you can get fancy adjustable MCCB style RCDs and that could be set to 500mA and a longer delay (say 0.5s) for a basic distribution board, and the sub-boards could have 300mA normal delay as incomers and then single-pole RCBOs for the final circuits. That will be mostly OK, but you still have the risk of a N-E fault tripping that local incomer (so less of a problem, but not eliminated).

Again, if going TT then you would need a lower Ra here.

However, you might have a case where the main house / supply stays on TN-S but the out-buildings are TT'd. Then your sub-boards would need the 300mA incomers and you would absolutely have to use SWA to feed them as not RCD's up front, and robust enough not to be a fire hazard themselves (assuming end of sub-main Zs for feed OCPD is met). Each board would need a couple of good rods for adequately low Ra, even under drought or frost conditions.

Even then getting total selectivity is hard. You don't get it from MCB feeding MCB/RCBO to any useful degree, you are better to look at switch-fuses (or a Ryefield board) for the incoming distribution. Or if it has to be friendlier to a non-electrician to reset, then spend the (not inconsiderable) extra on a MCCB incoming board for distribution so you can get the 'short delay' on the MCCB over-current to allow a down-stream MCB/RCBO time to clear a fault.

 

[pc1966]

Finally, check with the farmer if there is a need for backup power to keep any "intensive farming" animals alive (i.e. to keep air cooling and water supplied running if mains fails).

You really want to plan that before the new system is designed and installed!