Does this make sense ? What do you think.

[Pat Tester]

I have a job to connect up a shed and greenhouse (socket in each) from a detached garage. House is TT and garage is fed via swa up garden.
I am unsure of the correct procedure re bonding in the greenhouse or not. I spent some time looking and could only find conflicting advice
and nothing specific within the regs.

I have already upgraded c/u with rcbo's on all circuits during loft conversion work.

So as I pay a considerable sum per annum to one of those scheme providers (no names but I like their biscuits) I contacted tech dept and asked should I seperate earth from house, put a rod in for the garage with a small c/u and 30ma rcd and then bond greenhouse, only 5M away from garage ?

I was told they would like to see greenhouse treated as a seperate zone, with its own rod and c/u and rcd within the greenhouse ! leaving the shed and garge with the house earth.

Has anyone put a consumer unit within an 10' x 6' aluminium greenhouse ?

I would appreciate some input from respected members who will know a great deal more than I..

 

[Engineer54]

Why would you WANT to make a TT installation a separate entity from a TT system?? If anything you would want the exact opposite and bond/connect ALL earthing rods together. Your not achieving Anything by splitting a TT system, certainly not in this case anyway.


That NIC guy was talking out of his arse suggesting such a set-up, as he advised to Pat Tester. Why on earth he wanted a separate Zone for the greenhouse is totally beyond me!! ...lol!!!

 

[ackbarthestar]

Why would you WANT to make a TT installation a separate entity from TT system?? If anything you would want the exact opposite and bond/connect ALL earthing rods together. Your not achieving Anything by splitting a TT system, certainly not in this case anyway.


That NIC guy was talking out of his arse suggesting such a set-up, as he advised to Pat Tester. Why on earth he wanted a separate Zone for the greenhouse is totally beyond me!! ...lol!!!

Read my previous post.
There are several ways of creating a safe electrical system, some can become more complicated than others.

Having read some of your previous posts regarding TT systems, I'm having problems grasping your aborance with this type of system.
It is very common in rural areas of this country and one that does work. It is recommended by more or less all electrical bodies as a safe way of providing an independent earthing system where there is any doubt to the possible breakdown in the integrity of the supplying earthing system.

As you have stated already, if you don't trust TT earthing systems, why would you then want to export it to another installation?

 

[the pict]

I would never put a CU in a greenhouse, would you put one in a sauna? proper box for outdoor use and feed in from out side direct to appliances via outdoor switches timrers etc, no sockets, no light switches in the greenhouse, my opinion only from using common sense
, and spike the thing but not the frame thats just daft,
Pict

 

[ackbarthestar]

I would never put a CU in a greenhouse, would you put one in a sauna? proper box for outdoor use and feed in from out side direct to appliances via outdoor switches timrers etc, no sockets, no light switches in the greenhouse, my opinion only from using common sense
, and spike the thing but not the frame thats just daft,
Pict

I work with them all the time, not the smaller domestic ones found at the bottom of people's gardens. I also see alot of M/C DBs where I have to do remedials on.

IP ratings of all the accessories, switchgear and fittings are probably the most important aspect. I would agree, where possible locate the DB in a dry position and export the supply to the final circuits. But that is not always possible, particularly where you have to supply power, pumps, sprinkler systems, lighting, heating, time-switches.

 

[Engineer54]

Read my previous post.
There are several ways of creating a safe electrical system, some can become more complicated than others.

Having read some of your previous posts regarding TT systems, I'm having problems grasping your aborance with this type of system.
It is very common in rural areas of this country and one that does work. It is recommended by more or less all electrical bodies as a safe way of providing an independent earthing system where there is any doubt to the possible breakdown in the integrity of the supplying earthing system.

I know there are several ways of creating a safe electrical system for this installation, that wasn't my point at all. My point is why would you separate a TT system from a TT system, as in this case it's a totally pointless exercise!!

I think you have totally misread my opinions on TT systems, i certainly don't have any abhorrence for them whatsoever. What i don't like, is what is being ''called'' a TT system in the UK these day's, where electricians think that by bunging a short thin rod in the ground they have made a TT system. When the truth is, ...they Haven't!!! There is nothing wrong with a well planned and constructed TT system, it can, if conditions are good, rival a DNO TN system earth. But until electricians take placement of Rods seriously, drive them to decent depths, and become aware of the various proven methods of ground and rod enhancement, then the chances are, you'll end up with a sub-standard earthing arrangement. That and the fact that virtually all TT installation installed in the UK these days, will be ''totally'' relying on a RCD device for it's earth fault protection the TT system literary providing no protection (as in protection device disconnection) at all, save maybe, just maybe a 50 V touch voltage protection.


As you have stated already, if you don't trust TT earthing systems, why would you then want to export it to another installation

Soooo, your right in a way, i don't trust UK TT earthing systems as a whole, but i'd certainly trust one that was installed correctly, like those we have just installed here at Ra levels less than 0.5 ohms... Again, why would anyone want to export a TT system, when by adding extra rods and connecting them together with the original system, you will be improving the Ra for both the original system and the new installations?? Which was my Original point!!!

 

[ackbarthestar]

I know there are several ways of creating a safe electrical system for this installation, that wasn't my point at all. My point is why would you separate a TT system from a TT system, as in this case it's a totally pointless exercise!!

I think you have totally misread my opinions on TT systems, i certainly don't have any abhorrence for them whatsoever. What i don't like, is what is being ''called'' a TT system in the UK these day's, where electricians think that by bunging a short thin rod in the ground they have made a TT system. When the truth is, ...they Haven't!!! There is nothing wrong with a well planned and constructed TT system, it can, if conditions are good, rival a DNO TN system earth. But until electricians take placement of Rods seriously, drive them to decent depths, and become aware of the various proven methods of ground and rod enhancement, then the chances are, you'll end up with a sub-standard earthing arrangement. That and the fact that virtually all TT installation installed in the UK these days, will be ''totally'' relying on a RCD device for it's earth fault protection the TT system literary providing no protection (as in protection device disconnection) at all, save maybe, just maybe a 50 V touch voltage protection.


As you have stated already, if you don't trust TT earthing systems, why would you then want to export it to another installation

Soooo, your right in a way, i don't trust UK TT earthing systems as a whole, but i'd certainly trust one that was installed correctly, like those we have just installed here at Ra levels less than 0.5 ohms... Again, why would anyone want to export a TT system, when by adding extra rods and connecting them together with the original system, you will be improving the Ra for both the original system and the new installations?? Which was my Original point!!!

I'll work backwards with this one

Firstly there is no guarantee that you will achieve an Ra of 0.5 Ohms, nice idea but DNOs can only give a guaranteed maximum value of 21 Ohms, as quoted from SEB earthing manual circa 1984, add that to your measured soil resistivity, the contact resistance of your electrode/s and earthing conductor values. You'd be lucky, by probability, in a domestic environment to get less than 10 Ohms. ( I'll leave the 'mathematicians' to work that one out

Secondly, Its interesting that you think by using a front end RCD you have 'literally no protection'. BS7671; 411.5.2 explicitly states Either OPDs or RCDs can be used for earth fault protection in TT systems although RCDs are preferred.
Now why would BS7671 state the preferred method of protection should be via a RCD?

Thirdly, we are in agreement about correctly evaluating and installing earth electrodes. If more Tech colleges were based in the countryside then more students would be taught the necessary fundamentals of the TT earthing system.

 

[Engineer54]

I'll work backwards with this one

Firstly there is no guarantee that you will achieve an Ra of 0.5 Ohms, nice idea but DNOs can only give a guaranteed maximum value of 21 Ohms, as quoted from SEB earthing manual circa 1984, add that to your measured soil resistivity, the contact resistance of your electrode/s and earthing conductor values. You'd be lucky, by probability, in a domestic environment to get less than 10 Ohms. ( I'll leave the 'mathematicians' to work that one out

Did i say that you would be guaranteed to get a 0.5 ohm Ra on a domestic, no i didn't think so either!! Have you ever seen a 21 ohm Ze at a DNO substation, because i haven't!! You might see that sort of level at a small remote pole mounted TX but that's about all!! As for domestic TT systems, i can assure you that there are quite a few areas in Essex that are sub 1 ohm using one 3m 3/4'' rod. But sure, it depends very much on the soil type your dealing with... As for a Ra of 10ohms, That will give you a degree of protection if/or when that RCD fails. The protective devices might not operate in the specified times, but it will operate, unlike a 200 ohm Ra, which is what your great BRB calls for!! To be honest, from a safety aspect, all UK TT systems should be on a CU with all RCBO protective devices not on these split CUs. At least then, if one RCBOs RCD side of things fail, it's limiting the problem to one circuit, and not many!!


Secondly, Its interesting that you think by using a front end RCD you have 'literally no protection'. BS7671; 411.5.2 explicitly states Either OPDs or RCDs can be used for earth fault protection in TT systems although RCDs are preferred.
Now why would BS7671 state the preferred method of protection should be via a RCD?


I don't believe i mentioned anything of the kind in the above posts, i just stated that you are more often than not, relying ''totally'' on a RCD for your earth fault protection. Implying that RCDs haven't a particually good reliability record!! Why does BS7671 state the preferred method of protection be by RCD, .... Because the section on TT systems is a crock of crap, that's based around a max Ra of 200 ohms. RCDs are an absolute must in any UK based TT system, that has anything like a 200 ohm Ra. As stated above, the problem with that is, the RCD device itself and it's reliability... RCDs are prudent on TT systems, but shouldn't negate the need of at least a working (to some degree of protection) ADS system. They shouldn't, but more often are be regarded as the ''sole'' means of protection.

Thirdly, we are in agreement about correctly evaluating and installing earth electrodes. If more Tech colleges were based in the countryside then more students would be taught the necessary fundamentals of the TT earthing system.

Well, ....at least we agree on one thing then!! ...lol!!

And what's all this, got to do with splitting a domestic TT system?? ....Nothing as far as i can see!!!

 

[Chr!s]

Why on earth would you bond a Greenhouse ?

 

[Engineer54]

Why on earth would you bond a Greenhouse ?

Exactly!!

 

[topquark]

Why on earth would you bond a Greenhouse ?

Didn't you know plants were extraneous Chr!s

 

[ackbarthestar]

Someone said that it was metal framed attached to the general mass of earth with electrical equipment installed inside - didn't they?

 

[Chr!s]

Someone said that it was metal framed attached to the general mass of earth with electrical equipment installed inside - didn't they?

And what difference will that make?

 

[ackbarthestar]

And what difference will that make?

As long as an earth fault clears if the frame becomes energized then not much difference at all.

 

[Chr!s]

As long as an earth fault clears if the frame becomes energized then not much difference at all.

And what if the earth fault doesn't clear?

 

[ackbarthestar]

And what if the earth fault doesn't clear?

Then the person holding onto his flower pots will have a problem. - As you well know its related to creating a reliable earth return path which will disconnect the RCD within 40ms.

I agree, that the fault would probably clear pdq. However, you would need to verify this. (Basic protection)