FARM INSTALLATION- First Time

[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