Farm TT quandary

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lyledunn: 
 

Agree with option 2 but I think I would have at least 2 RCDs in series!

Yes, the belt and braces approach is a good one.

When younger I once carried out an experiment.

I wired up a 3 Amp fuse in series with an earth rod in the back garden. There was no R.C.C.B. protection of any kind. I livened up the 3 Amp fuse at 240 Volts. The fuse promptly blew.

The earth rod may have been close to a P.M.E. transformer and earthing rod, perhaps 15 meters away, I can not clearly remember. But the earth return was good and the fuse blew instantly.

So a lighting circuit protected by  say a 6 Amp M.C.B. may well be safe and faults cleared quickly in case of a L to E fault.

Perhaps the flood lights could be mounted on electrically insulating blocks of nylon to prevent a light to barn metal connection.

Z.

 

 

We discussed in a recent thread, the fact that RCDs are wholly unable to provide overcurrent protection (and unsuitable for that purpose) …. this scenario confirms that also. 

Quote. “We discussed in a recent thread, the fact that RCDs are wholly unable to provide overcurrent protection (and unsuitable for that purpose) …. ”

Yes I knew that.

Z.

  Adding in 0.1Ω for the L side gives a loop impednace of 4.6Ω. For Uo of 230V that's then an Earth Fault current of 230V/4.6Ω = 50

So with a 50 Amp fault current, assuming a fault of negligible impedance the disconnection time of  a B6 M.C.B. will be 0.01 s or less.

Z.

An electric fencer can produce about 8kV. But at a very low current, say 120mA typically. It does not harm cattle that come into contact with it.

So, just what energy will a cow be subject to if it touches a live barn support, (one of 22) whilst the fault clears very quickly via the B6 M.C.B. and or R.C.D.(s)?

Will the energy be at a safe level so as not to harm the animal?

Z.

So, the big question is:

How electrically conductive are cattle hooves? They do not have fleshy feet like us. Are they naturally insulated from Earth by the construction of their hooves? Cattle can have very wet noses that must be conductive, but their feet may be more insulating.

Z.

I suspect that cow's hooves are similar to horses - hard hoof material around the sides but more fleshy in down the middle. Combined with the wet ‘organic material’ they tend to be stood in in and around farm buildings, I wouldn't have thought we could rely on much in the way of insulating properties.

There are well documented cases of cattle refusing to enter farm buildings or milking stalls due to a small voltage difference (usually PME related) - they're certainly more sensitive than humans to that sort of situation. How long they can withstand a shock current before risking fatality however is quite a different question. I don't know, but being larger animals I might suspect they might be no worse than humans on that count.

   - Andy.

Thank you for all your comments, option 2 seems to be the favourite.  I will see if the farmer will splash out on two RCDs. The explanation of voltages on earthed metal, series/parallel resistances, is obvious when someone points it out?, my old aching remaining grey matter takes a long time to get up to speed these days. It looks like I will also be struggling with SWA at height and getting the BBB out for TT earth conductor sizes and working out whether I can use the armour as an earth conductor and the CPC  or will I need three cores? Given the small cost difference it will probably be three but I have not priced SWA in the last few months?.

On all the other issues raised: 

I did TT a different metal barn on the same farm a few years ago using the posts as earth, no livestock, and the Ze at the barn was less than 20 Ohm on 8 bonded posts.

It was a dairy farm for many years but the farmer gave up on dairy a few years ago as it was not profitable and he was effectively paying to produce milk. Also the company who bought the milk would reject a days production with made up reasons very now and again. He now takes holidays as he is no longer tied to the farm 365 days a year.

Interestingly there was no grid in the floor of the milking parlour, the supply was PME  and I did not see any Green or even G/Y bonding to the stall metalwork anywhere. The concrete was probably laid on the sandstone with no DPC and would always be wet form the washing down twice a day.

The livestock may have insulating hooves when dry. But the bedding becomes wet and soiled within a day of being laid so for part of the time they are on wet bedding and have wet hooves. With the salts etc in urine I suspect they will be very conducting. 

The farmer is a tenant and the landlord is responsible for the electrics in the rented buildings but the farmer is responsible for the buildings he has constructed and any additions he requires. The farm electrics had an unsatisfactory EICR last year. I am told the inspector was happy with the bits I had installed?.  The farmer  has not heard about what work or when any rectification work will be done . 

Kevin

kfh: 
 

Thank you for all your comments, option 2 seems to be the favourite.  I will see if the farmer will splash out on two RCDs. The explanation of voltages on earthed metal, series/parallel resistances, is obvious when someone points it out?, my old aching remaining grey matter takes a long time to get up to speed these days. It looks like I will also be struggling with SWA at height and getting the BBB out for TT earth conductor sizes and working out whether I can use the armour as an earth conductor and the CPC  or will I need three cores? Given the small cost difference it will probably be three but I have not priced SWA in the last few months?.

On all the other issues raised: 

I did TT a different metal barn on the same farm a few years ago using the posts as earth, no livestock, and the Ze at the barn was less than 20 Ohm on 8 bonded posts.

It was a dairy farm for many years but the farmer gave up on dairy a few years ago as it was not profitable and he was effectively paying to produce milk. Also the company who bought the milk would reject a days production with made up reasons very now and again. He now takes holidays as he is no longer tied to the farm 365 days a year.

Interestingly there was no grid in the floor of the milking parlour, the supply was PME  and I did not see any Green or even G/Y bonding to the stall metalwork anywhere. The concrete was probably laid on the sandstone with no DPC and would always be wet form the washing down twice a day.

The livestock may have insulating hooves when dry. But the bedding becomes wet and soiled within a day of being laid so for part of the time they are on wet bedding and have wet hooves. With the salts etc in urine I suspect they will be very conducting. 

The farmer is a tenant and the landlord is responsible for the electrics in the rented buildings but the farmer is responsible for the buildings he has constructed and any additions he requires. The farm electrics had an unsatisfactory EICR last year. I am told the inspector was happy with the bits I had installed?.  The farmer  has not heard about what work or when any rectification work will be done . 

Kevin

Thanks for the update Kevin. Don't forget to strap yourself to the cage when working at high level.

Z.

Zoomup: 
Thanks for the update Kevin. Don't forget to strap yourself to the cage when working at high level.

That is with a full harness, lanyard, and hard hat; and not forgetting that powered access ticket. Frankly a cage on a telehandler sounds a bit dodgy, especially if you don't have control from the cage.

Chris Pearson: 
 

Zoomup: 
Thanks for the update Kevin. Don't forget to strap yourself to the cage when working at high level.

That is with a full harness, lanyard, and hard hat; and not forgetting that powered access ticket. Frankly a cage on a telehandler sounds a bit dodgy, especially if you don't have control from the cage.

This sort of thing Chris?