Earth rod and Zs values in farm. Help needed | Page 2 | on ElectriciansForums

Discuss Earth rod and Zs values in farm. Help needed in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

I am rather hesitant to say yes !?
I am assuming that the OP was talking about a 40 A circuit and not one supplying a socket outlet of 32 A or less .

JD, were talking about providing protection on a farm TT system, so final circuit protection should be 30mA or less.
 
Fair enough. Used the wrong terminology. Obviously not a Zs value between L and N but a line impedance value which is used to derive the PSSC.

On a three core SWA the CPC core and the armouring are connected at the 'supply' side of the circuit but not at the 'load' end. Therefore they are deemed 'earthed'. At the 'load' end there is a 16mm2 CPC to an earth rod. Most Zs results at the CU at the 'load' end of the circuit have been around 12 - 14ohms with a PFC of about 14A.
Although under earth fault conditions the 14A would be enough to operate the 100mA RCD, it would not operate the disconnection device at the 'supply' end, ie, 40A MCB in time / at all.
However, the measured line impedance of say 0.39ohms with a PSSC of 0.594kA would.
The question I was trying to ask in a very roundabout way is that although I can see in theory that this would all stand up to scrutiny (I thought), is it correct in your opinions?

Could I ask anyone (perhaps you E54) why someone would do this? If it is ok to extend a PME then why has someone got a TNCS but has not connected the 3rd core of the SWA at load end to utilise it (and get a much lower Zs), rather they have a TT with a higher fault loop impedance?
 
lol!! I would if i could, but alas it's after 8 pm here i'm at home and the BGB is at the office. So how about enlightening me as to what 705.411.1 is saying??

OK:

705 Agricultural and Horticutural Premises
705.411 Protective measure: ADS
705.411.1 General

In circuits, whatever the type of earthing system, the following disconnection devices shall be provided:

(i) In final circuits supplying socket-outlets with a rated current not exceeding 32A, an RCD having the characteristics specified in Regulation 415.1.1 [30mA].

(ii) In final circuits supplying socket-outlets with a rated current more than 32A, an RCD with a rated residual operating current not exceeding 100mA.

(ii) In all other circuits, RCDs with a rated residual operating current not exceeding 300mA.
 
Could I ask anyone (perhaps you E54) why someone would do this? If it is ok to extend a PME then why has someone got a TNCS but has not connected the 3rd core of the SWA at load end to utilise it (and get a much lower Zs), rather they have a TT with a higher fault loop impedance?

The short answer, is that it goes against the regs:

Agricultural and Horticultural Premises
705.411.4
A TN-C system shall not be used....


I'm sure others can go into the reasons regarding voltage on the neutral under fault conditions, reduced resistance to current in persons and animals in contact with the ground, susceptibility of livestock to electric current, etc.
 
Hi Happyhippydad , just take a look at section 705 in the regs and it will become a lot clearer .
If you want , you can PM me and I will give you a few pointers out of sight of the preying public !:49:
 
Hi Happyhippydad , just take a look at section 705 in the regs and it will become a lot clearer .
If you want , you can PM me and I will give you a few pointers out of sight of the preying public !:49:

Thats very kind of you JD but it really was just asked out of interest. It would be out of my comfort zone and not something I would undertake. I will have a read up on section 705 though just to get the answer in more detail :smile5:
 
Cattle and other 4 legged livestock are highly susceptible to step voltages which can especially develop in the vicinity of an earth rod. Because of this cattle shed are treated as a special location in the regs.
 
Cattle and other 4 legged livestock are highly susceptible to step voltages which can especially develop in the vicinity of an earth rod. Because of this cattle shed are treated as a special location in the regs.

Great when the freezer needs stocking up. So long as the animal is bled straight away the meat is fine.

I’ve attended a few incidents where MV lines have come down and cattle and sheep have been downed.
 
OK:

705 Agricultural and Horticutural Premises
705.411 Protective measure: ADS
705.411.1 General

In circuits, whatever the type of earthing system, the following disconnection devices shall be provided:

(i) In final circuits supplying socket-outlets with a rated current not exceeding 32A, an RCD having the characteristics specified in Regulation 415.1.1 [30mA].

(ii) In final circuits supplying socket-outlets with a rated current more than 32A, an RCD with a rated residual operating current not exceeding 100mA.

(ii) In all other circuits, RCDs with a rated residual operating current not exceeding 300mA.

Well anyone that is happy to protect persons against earth faults with 100/300mA RCD's on ''final circuits'' of a TT earthing system is a far more confident man than me!! lol!! Fine using these sort of RCD tripping values on distributing sub-main branch cables.... But NOT on final branch circuits...

I think i'll stick with my way of doing things and disregard that pile of crap to file 13 with the rest of the crap i've found in BS7671!! lol!!
 
Well anyone that is happy to protect persons against earth faults with 100/300mA RCD's on ''final circuits'' of a TT earthing system is a far more confident man than me!! lol!! Fine using these sort of RCD tripping values on distributing sub-main branch cables.... But NOT on final branch circuits...

I think i'll stick with my way of doing things and disregard that pile of crap to file 13 with the rest of the crap i've found in BS7671!! lol!!

I would not have put it quite as strongly as that , but I have to admit that I always prefer to see a 30mA on final branch circuits .
I would go as far as to admit that we sometimes trade off segregation of supply over having just one 30mA main switch on sub boards , depending on the situation and make of equipment used .


Edit ; It was more I thought that I had been missing something , when you mentioned the need for the 30mA , than anything else .
 
I would not have put it quite as strongly as that , but I have to admit that I always prefer to see a 30mA on final branch circuits .
I would go as far as to admit that we sometimes trade off segregation of supply over having just one 30mA main switch on sub boards , depending on the situation and make of equipment used .


Edit ; It was more I thought that I had been missing something , when you mentioned the need for the 30mA , than anything else .

Crap is crap whatever way you want to look at it, ...or come to that, smell it!! lol!!
 

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