Farm land installation

[The apprentice]

Hi there just have a few questions regarding an agricultural installation.

1. Due to the existing installation being a TN-S earthing supply I understand that regulation 705.415.2.1 (note) that unless a metal grid is laid in the floor (which there is not) the use of a PME is not recommended ? This 3 phase supply also supply’s a house so could we keep the house on the TN-S and there for make the outgoing to the farm TT ?

2. Also 300mA main switch s type there for outgoing circuits 30mA unless it’s rated more than 32A 100mA protection?

3. When bonding/equipotential banding of the 3 phase consumer unit was to be turned into a TT there for when the earth rode is inserted into the ground we would then run all bonding cables to that with it being a TT system has all circuits to trip within 0.2
Thanks in advance

 

[pc1966]

Hi there just have a few questions regarding an agricultural installation.

1. Due to the existing installation being a TN-S earthing supply I understand that regulation 705.415.2.1 (note) that unless a metal grid is laid in the floor (which there is not) the use of a PME is not recommended ? This 3 phase supply also supply’s a house so could we keep the house on the TN-S and there for make the outgoing to the farm TT ?

Yes, if makes sense to assume that TN-S could become TN-C-S at some point in the future, so keep it away from actual farm animal areas.

Retaining the low Ze of the TN earth for the house or similar makes a lot of sense as there is no reason to give up OCPD disconnection unless you absolutely have to.

2. Also 300mA main switch s type there for outgoing circuits 30mA unless it’s rated more than 32A 100mA protection?

I'm not sure I completely understand you, but I think you are asking about selectivity for such a system?

With a 300mA delay (S-type) RCD up front as a fire precaution you should plan that every down-stream circuit is on an RCD or RCBO (preferably double-pole RCBOs though rarer) so a final circuit fault does not take out the lot.

For most circuits (lights and sockets to 32A, etc) you would be looking at 30mA RCDs. That is usually OK unless there is a lot of leakage, for example a room full of IT equipment. In most cases the best option is to divide up big leaky circuits to reduce the risk of nuisance trips.

For anything you expect is more leaky then yes, a 100mA "instant" trip RCD would still achieve selectivity with a 300mA delay RCD.

3. When bonding/equipotential banding of the 3 phase consumer unit was to be turned into a TT there for when the earth rode is inserted into the ground we would then run all bonding cables to that with it being a TT system has all circuits to trip within 0.2

Again, not 100% sure but I think what you mean is keep all of the TT earth/CPC together with multiple earth rods so you are as low a Ra/Ze as practical on the TT side.

With a 300mA up-front RCD you need below 167 ohms worst case, but it used to be (16th edition?) farms has a lower voltage threshold (25V instead of 50V) and that is a good goal - that your Ra ought to be below 25V/300mA = 83 ohms.

You usual delay RCD should trip in around 300ms though for the supply/sub-main you have 1s for TT (5s for TN), but for sure the final 30mA/100mA "instant" RCD would meet the TT disconnection times.

 

[The apprentice]

Yes, if makes sense to assume that TN-S could become TN-C-S at some point in the future, so keep it away from actual farm animal areas.

Retaining the low Ze of the TN earth for the house or similar makes a lot of sense as there is no reason to give up OCPD disconnection unless you absolutely have to.


I'm not sure I completely understand you, but I think you are asking about selectivity for such a system?

With a 300mA delay (S-type) RCD up front as a fire precaution you should plan that every down-stream circuit is on an RCD or RCBO (preferably double-pole RCBOs though rarer) so a final circuit fault does not take out the lot.

For most circuits (lights and sockets to 32A, etc) you would be looking at 30mA RCDs. That is usually OK unless there is a lot of leakage, for example a room full of IT equipment. In most cases the best option is to divide up big leaky circuits to reduce the risk of nuisance trips.

For anything you expect is more leaky then yes, a 100mA "instant" trip RCD would still achieve selectivity with a 300mA delay RCD.



Again, not 100% sure but I think what you mean is keep all of the TT earth/CPC together with multiple earth rods so you are as low a Ra/Ze as practical on the TT side.

With a 300mA up-front RCD you need below 167 ohms worst case, but it used to be (16th edition?) farms has a lower voltage threshold (25V instead of 50V) and that is a good goal - that your Ra ought to be below 25V/300mA = 83 ohms.

You usual delay RCD should trip in around 300ms though for the supply/sub-main you have 1s for TT (5s for TN), but for sure the final 30mA/100mA "instant" RCD would meet the TT disconnection times.

So if it’s turned into a TT system and rcd main switch, and there for 30 mA outgoing rcbos with it being a TT system what is the desired trip time for the rcbo outgoings I thought for TT systems everything had to trip within 0.2 of a second thanks mate

 

[pc1966]

So if it’s turned into a TT system and rcd main switch, and there for 30 mA outgoing rcbos with it being a TT system what is the desired trip time for the rcbo outgoings I thought for TT systems everything had to trip within 0.2 of a second thanks mate

Yes, for TT with Uo = 230V then it is less than 0.2s. You will probably find that ALL of the RCBO meet that.

There is practically no demand for a delay-RCBO as it would only be of use in feeding a sub-main, and then your MCB/MCB cascade gives poor selectivity anyway on high current faults.

For TT systems I would always go with both a delay-RCD as main switch (100mA or 300mA) as well as an all-RCBO board, even though the regs don't demand it. The main reason being I don't like the idea of a RCD (part of RCBO) being a single point of failure that could cause ADS to fail, with an up-front RCD you don't meet the 0.2s in such a case, but getting < 0.5s is a hell of a lot better than no disconnection at all!

For single phase then the like of Wylex, Crabtree and Fusebox have compact RCBO that are double-pole switching so if a final circuit trips on a N-E fault it isolates it and you don't get the incoming tripping as well. Off hand I don't know if Fusebox do a delay-RCD incomer though, but I know Wylex do.

For TPN boards you don't often see neutral-switching RCBOs, so there is a minor risk of N-E faults not being selective with the incomer.