Greenhouse Electrics

[JohnW2]

The impedance ( rod to ground ) of most ground rods won't allow that much current to flow so the whole CPC goes high voltage.

Exactly. No matter how many tens or hundreds of amps might flow to earth through a very low impedance path (if there were one) in the presence of a TN-C-S 'lost neutral', the current which can actually flow to earth through the average earth rod is very limited - as you say, the result of that will be a rise in potential of the earth rod and every CPC connected to it. My point was that the sort of currents that most earth rods would allow should not really present any fire risk, unless some of the CPCs or bonding conductors were ludicrously undersized.

Something such as the signal lead between a PC earthed to the CPC and other equipment earthed directly to ground would take a proportion of the net current, that cable might burn out.

That, of course, is what bonding and equipotential zones is all about. If that is satisfactorily in place, the scenario you describe should not be able to arise.

Kind Regards, John.

 

[bernardgreen]

That, of course, is what bonding and equipotential zones is all about. If that is satisfactorily in place, the scenario you describe should not be able to arise.

It is the data cable going out of one equi-potential area and into another equi-potential area that is at at risk if CPC to one of them is affected by a fault. Hence the use of galvanic separation in such leads being highly recommended. Most times the tracks in the PC ( or other equipment ) melt and disconnect the connection. But the interface chips are invariable blown as well. Dead PC but the cable survives. But if one or more of the wires in the cable is connected directly to the CPC earthed chassis at both ends then it is at risk.

 

[ericmark]

It does not really matter if one is talking about a metal soil pipe or a metal framed green house should the neutral be lost there is a risk of electrocution with a TN-C-S system.

Should cars be banned because they can kill or do we look at how often and balance the quality of life with cars to the risk that these machines could kill us.

If you look at how often a neutral is lost on a TN-C-S supply it is very rare. What is far more likely is water ingress causing a structure to become live. So we bond and use RCD protection.

Personally in 40+ years I have come across it once. OK losing neutral many times but on a TN-C-S supply losing combined neutral and earth only once.

As I have said it's all down to Risk assessment.

 

[JohnW2]

It is the data cable going out of one equi-potential area and into another equi-potential area that is at at risk if CPC to one of them is affected by a fault.

That's true, but the existance of such a connection is a violation of both of the alegedly equipotential zones in question. If steps are taken to ensure that the equipotential zones really are equipotential by bonding or isolation as appropriate), then such things as you describe can't happen.

Kind Regards, John.

 

[JohnW2]

It does not really matter if one is talking about a metal soil pipe or a metal framed green house should the neutral be lost there is a risk of electrocution with a TN-C-S system.

Indeed - but, ironically, in both those cases, it would seem that the risk of electrocution due to supply-side faults only really exists if the metallic parts is question 'benefit from' (albeit well-intentioned) bonding!

Should cars be banned because they can kill or do we look at how often and balance the quality of life with cars to the risk that these machines could kill us.

You're preaching to the converted, since I've spent much of my life working in areas where a high proportion of the decisions taken every day are based on risk-benefit balances. However, in the current context, if it became known that car manufacturers were adopting a practice which unnecessarily increased the risk of death, solely to save money, there might be some critics in this day and age! .. or am I missing some advantage of TN-C-S other than money?

If you look at how often a neutral is lost on a TN-C-S supply it is very rare.

I'm sure that's true - but it's the issue which is so often wheeled out whenever issues of earthing/bonding in TN-C-S supplies are being discussed.

Are you prepared to answer the question from your personal viewpoint (i.e. make a risk assessment for a situation you can identify with)? If you had a TN-C-S supply to your house and had a metal-framed greenhouse a reasonable distance away from the house, and knowing how you thought the electricity supply to the greenhouse would be used, what arrangement of the electricity supply to the greenhouse would make you feel most comfortable when you and yours were working in that greenhouse? This is in no way a trick question - I'm just interested to know the answer (from you and/or anyone else who cares to answer).

Kind Regards, John.

 

[ericmark]

In my house because the garden is surrounded by houses all with PME supplies all I did was bolt the metal clad socket to the green house which in turn bonded whole green house to house PME earth.

However if the greenhouse was at the bottom of a long garden with only fields beyond then I would sink an earth rod and attach it to the frame and use a TT supply.

In my case if I did have a long garden with fields at the bottom I would likely also have a radio shack and I would not want RF from the radio to upset neighbours so having my own independent earth would be favourite to reduce EMC.

If I could afford it I would use an isolation transformer and so the shack and greenhouse would in fact be on a TN-S supply.

Unfortunately I don't have a large garden and can't even run a half size G5RV so I use VHF only from home QTH. In the main I only use my radios now for RAYNET and even my HB9CV and rotator has been removed from mast which now only has TV aerials on it.

Money does modify how we act. I would if funds permitted installed an isolation transformer on my sons narrow boat. But because I did not have spare cash instead used a diode block in the earth. The whole idea of putting diodes into the earth connection to shore supply seems wrong to me. Yet it is a standard method and very common. As to what would have happened had he transmitted from the boat I hate to think. Even rusty bolts in a tower can act as diodes and produce some really bad EMC. I am sure if he had transmitted then he would have needed to use an isolation transformer.

73 Eric

 

[JohnW2]

In my house because the garden is surrounded by houses all with PME supplies all I did was bolt the metal clad socket to the green house which in turn bonded whole green house to house PME earth.
However if the greenhouse was at the bottom of a long garden with only fields beyond then I would sink an earth rod and attach it to the frame and use a TT supply.

Thanks for those insights. Just to be clear, in the latter case, would you use only the TT earth rod (which is what I think I'd do) or are you talking, as I think you were earlier, of having both that and the PME earth? I would be interested in understand the thinking which brings you to those different approaches in the two situations - is it just because you feel an earth rod near to the houses would be within the resistance area of nearby PME earths, or what.

My house is closer to the 'fields beyond' situation. My greenhouse is around 35m from my house, and at least 50m, probably more, from any other house. As I think I've said, as things are I'm relying just on the house's (TT) earth, which is bonded to the greenhouse frame. I'd probably be slightly more comfortable with a local TT setup (isolated from the house's) - although I'm not entirely sure why! ... any comments?

The rest of what you write is all rather nostalgic. You write almost as if you know that I am, at least on paper, a radio amateur - although I don't think I've ever written anything here to reveal that! I say 'on paper', because it's literally decades since I was active, although I still have the licence. I got it in 1963 and (after the inevitable baptism on 'top band') my period of intensive activity ('misspent youth' - we had nerds even then!) was from then until the early 70s, nearly all on VHF/UHF (apart from HF contests). For here, I'll abreviate my callsign to G3S**, but I'd be delighted to fill the gaps in privately, if you so wished (can't really think why you would!). What you say about EMC makes sense, although it was called TVI in my day - and I remember having great problems which were eventually traced to diode action in rusty joints in guttering, just like you mention!

73, John.

 

[ericmark]

In the open field yes I would go total TT. But main point is I can test the rod. For a DIY guy would can't test rod then both house earth and rod is way to go.

In general I am nervous about telling people what they can do as likely there is something they have not told you. An open field for example except for the main transformer for whole district.

There seems there is always something. And lack of test equipment means really they should not be doing the job. Hence belt and braces approach.

73 GW7MGW Ex VP8BKM and VR2ZEP

 

[bernardgreen]

There seems there is always something.

Most situations are unique and need to be treated as such.

And lack of test equipment means really they should not be doing the job.

Or lack of means and knowledge to test the means using improvised but effective tests.

73 GW7MGW Ex VP8BKM and VR2ZEP

Cheers
Bernard
Ex M2~~~~~ oops can't mention that one.

 

[bernardgreen]

Deleted a double post

 

[JohnW2]

In the open field yes I would go total TT. But main point is I can test the rod.

Again, I am going to try to probe you further! Is that because you agree with the 'list in order of decreasing safety' I proposed, and therefore regard total (separate) TT as always being the safest, but think it would be inappropriate/unhelpful if the rod was within the resistance area of PME earths? - or what?

For a DIY guy would can't test rod then both house earth and rod is way to go.

OK, so you'd be thinking of that rod as being part of the PME (if supply was PME) and not get caught up in worrying that it represented an extraneous-conductive-part (needing adequate MPB) as far as the main installation was concerned?

In general I am nervous about telling people what they can do as likely there is something they have not told you. An open field for example except for the main transformer for whole district.

I agree totally, and that's why I thought it would be easier to ask you what you'd do in your garden. Giving advice as to what people can/should do on any topic is iffy enough via this medium (because there are so often 'things we haven't been told') - but with an issue as complicated as this one, I don't think anyone can sensibly give such advice 'from afar'.

73 GW7MGW Ex VP8BKM and VR2ZEP

A well-travelled man!

Kind Regards, John.

 

[JohnW2]

And lack of test equipment means really they should not be doing the job.

Or lack of means and knowledge to test the means using improvised but effective tests.

Indeed. As I said, I've used the 'transformer method' in my time - and it's very simple and safe if one knows what one is doing. However, I really don't see the point these days. At least for my purposes, I'm more than happy to just measure what EFLI/Zs the rod provides.

Kind Regards, John.

 

[JohnW2]

I recently wrote:

In general I am nervous about telling people what they can do as likely there is something they have not told you. An open field for example except for the main transformer for whole district.

I agree totally, and that's why I thought it would be easier to ask you what you'd do in your garden. Giving advice as to what people can/should do on any topic is iffy enough via this medium (because there are so often 'things we haven't been told') - but with an issue as complicated as this one, I don't think anyone can sensibly give such advice 'from afar'.

It's interesting that, although I have treid to start this thread twice, very few people have felt inclined/able to offer an opinion. I wonder if that is a consequence of the comments above - i.e. that it is a difficult area (often with no 'ideal' solution), with differing opinions and countless factors which need to be taken into account, such that most people, like ericmark, feel somewhat too nervous to offer what might appear to be advice as to what should be done without being conversant with everything to do with the situation in question?

Kind Regard, John.

 

[bernardgreen]

However, I really don't see the point these days.

An old hand many years ago use to put a few amps into the earth wire when measuring the resistance of the earthing. His reasoning was that a short length of thin ( corroded ) wire could still return a low impedance reading but melt if it had to carry a substantial fault current until the fuse wire melted.

With RCDs protecting the system the earth wire should never have to carry more than 30 milli-amps so proving the earth wire can carry the fusing current is no longer necessary. Or is it ?

 

[bernardgreen]

I wonder if that is a consequence of the comments above - i.e. that it is a difficult area (often with no 'ideal' solution), with differing opinions and countless factors which need to be taken into account

Too many variables in the equation means there is not, cannot be, one fit all solution to the question.

And then there are too many different way to interpret the often conflicting regulations.