Farm TT quandary

Is the supply absolutely definitely PME? I'm just thinking that PNB is common in remote locations and would carry a lot less risk than real PME (ignoring the DNO's usual request to treat PNB as if it were PME in case they change their minds at a later date)

   - Andy.

Almost certainly PME. Supplied from a substation supplying the rest of the village, everything is PME and some still have the little white plastic PME label. 

2 seems to be a good option. S.W.A. cable in the barns and the barn metalwork as a good earth electrode can't be beaten. Those numerous earth electrodes supporting the building will sink quite a fault large current and dissipate any fault Voltage effectively. The R.C.D. will trip quickly enough not to cause any danger.

Z.

Depending on distances from anything connected to PME, I'd agree that 2 could be a good option. RCDs are required in any case (Regulation 705.411.1) so that should not be an impediment to the use of TT if it is safe to provide it (i.e. separation from PME).

In locations intended for livestock, BS 7671 requires re-bar in the concrete floor to be main bonded though, regardless of earthing arrangement, see 705.415.2.1, although doesn't tell you what to do if there are none … except that PME should not be used unless there is a metal grid in the floor.

 

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

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.

3- TT the barn lighting circuit with a separate earth. It will very very difficult  to get a Ra lower than the barn supports due to the sandstone around most of the farm so potential for step voltages again and problem of finding an accessible place away from animals;

In any pure TT system (i.e. not one that's been made TN-like by bonding to shared extraneous-conductive-parts that are bonded to a TN Earth elsewhere), exposed (earthed/bonded) metalwork will always be pretty close to 230V during a fault - and no (practical) amount of electrodes will make much difference to that. What you have is a potential divider with a L conductor and the traditional “fault of negligible impedance” on one side, and the means of Earthing on the other. It doesn't really matter if the earthing gives 500 Ohms, 50 Ohms or 5 Ohms - it's going to do very little against a fault of negligible impedance to pull the touch voltage down. (The best I can imagine was if the fault was on the end of a long thin line, so R1 was perhaps as high as 0.5 Ohms and Ra was really low - say 5 Ohms - in which case you might shave perhaps 20V off the touch voltage - but 200V+ is still almost as lethal as 230V)

The point is that in TT systems protection against shock is provided by disconnection time rather than limiting touch (or step) voltages. 0.2s for 230V provides a similar level of protection as 0.4s for 115V (i.e. the typical TN situation) - and even a S-type RCD should easily open within 0.15s for an earth fault - so there should still be a quite reasonable level of protection for both humans and animals.

For sure, low Ra is good - especially for dealing with protective conductor currents (earth leakage) that's just too low to trip the RCD  - and burying the tops of rods (e.g. at the bottom of a chamber) can much reduce step voltages at the surface of the ground.  But I wouldn't loose too much sleep over the step voltages during a fault scenario. Farms have been TT'd for years (along with fully bonded steel framed barns) and the piles of electrocuted cattle are pretty conspicuous by their absence.

   - Andy.

AJJewsbury: 
 

3- TT the barn lighting circuit with a separate earth. It will very very difficult  to get a Ra lower than the barn supports due to the sandstone around most of the farm so potential for step voltages again and problem of finding an accessible place away from animals;

In any pure TT system (i.e. not one that's been made TN-like by bonding to shared extraneous-conductive-parts that are bonded to a TN Earth elsewhere), exposed (earthed/bonded) metalwork will always be pretty close to 230V during a fault - and no (practical) amount of electrodes will make much difference to that.

Is that really true?

So the numerous steel barn upright supports buried in concrete in soil will not hold down the barn's metal structure potential in the event that a flood light tries to liven it up due to a fault?

Isn't that like a mouse trying to lift a car?

I would have thought that the many supports would each dissipate a proportion of the fault Voltage between  themselves so that the situation is safer than presuming that the whole metalwork will reach 240 Volts.

Isn't that the reason that in a P.M.E. situation there are many earth electrodes connected to the C.N.E. conductor along its route? 

The barn has 22 steel support posts, so if a high level flood light became faulty wouldn't the 240 Volts to earth be divided approximately by 22 at each post.

Z.

Zoomup: The barn has 22 steel support posts, so if a high level flood light became faulty wouldn't the 240 Volts to earth be divided approximately by 22 at each post.

Z.

 

No, the current will be divided through the 22 posts, NOT the voltage.

The voltage rise to Earth will depend on the total resistance to Earth Ra, that is, the individual resistance of all posts in parallel, PLUS the resistance of the supplier's earth Rb … Rb can be anything up to about 20 ohms. However, local bonding will hopefully make the local touch voltage nearer zero … for the cattle, that is the reason for the grid in the floor because they are much more sensitive to the effects of touch voltage than humans.

The barn has 22 steel support posts, so if a high level flood light became faulty wouldn't the 240 Volts to earth be divided approximately by 22 at each post.

The current would be divided rather than the voltage.  To divide the voltage the posts would have to be in series rather than in parallel. Check my maths, but as I see it:

Let's say each post has a resistance to Earth of 100Ω, and the supply has an impedance of 0.1Ω on the L side.

22 lots of 100Ω in parallel (1/R = 22x 1/100Ω) gives a total resistance to Earth of about 4.5Ω (and presume the bonding conductors connecting the posts together have a negligible impedance).  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 = 50A.

If all the posts have the same resistance then the the current is split equally between all 22 posts, so 50A/22 = 2.27A through each post.

Thus each post, by Ohm's Law, will develop a voltage of 2.27A x 100Ω = 227V. Less than 230V … but not by much.

Rework with other arbitary numbers if you like (e.g. 20Ω at each post gives 207V for me). The big issue is the low impedance on the L side - it's really difficult to match that using electrodes in soil, even if you're using loads of them in parallel.

   - Andy.