I can't remember the location, but something in the back of my mind is saying that connecting the generator earth and the DNO earth is not permitted.
I'll try and check it
I'll try and check it
Generator N/E link
- Thread starter ban-all-sheds
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That's an interesting possibility.
Not allowed to put the earth through the C/O switch either, so not being allowed to connect to the DNO earth means either:
1) Not earthing the genny, which I guess makes it an IT supply.
2) Earthing the genny but not using the earth in the installation it's supplying, ditto.
3) Earthing the genny and using the earth but disconnecting the main earth and main bonding connections to the DNO earth.
None of those seem satisfactory to me....
Not allowed to put the earth through the C/O switch either, so not being allowed to connect to the DNO earth means either:
1) Not earthing the genny, which I guess makes it an IT supply.
2) Earthing the genny but not using the earth in the installation it's supplying, ditto.
3) Earthing the genny and using the earth but disconnecting the main earth and main bonding connections to the DNO earth.
None of those seem satisfactory to me....
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For a single standby generator - that is only used as an alternative to the DNO supply - the main areas of concern for the DNO are the safety of their systems and staff, and limiting or removing any liability.
They have made various statements over the years concerning the use of their earth terminal – ranging from requiring a signed letter of indemnity, to do what you like but don’t rely on the DNO earth
.
In general for TN-C-S they require that you do fit your own electrode as a means of referencing the system to earth, as they reserve the right to disconnect their neutral, and thus their earth without notice – especially during fault location.
The recommend earth electrode resistance given in BS 7430 is 20 ohms and this might be difficult to achieve unless you have enough space to install multiple electrodes. However, this figure is general and you might wish to argue that, if a front end RCD is employed, a higher value could be considered.
Giving any advice on this subject is always problematic as it really all depends on intended use and the balance of risks.
You need to balance the cost and complexity of a fixed permanent installation against a less formal arrangement where equipment is supplied as needed – possibly via extension leads. Remember power outages are relatively rare in urban areas at the moment – but who knows what the future will bring.
Small cheap petrol generators: are very noisy; produce dangerous fumes: and are not very reliable.
They have made various statements over the years concerning the use of their earth terminal – ranging from requiring a signed letter of indemnity, to do what you like but don’t rely on the DNO earth
.In general for TN-C-S they require that you do fit your own electrode as a means of referencing the system to earth, as they reserve the right to disconnect their neutral, and thus their earth without notice – especially during fault location.
The recommend earth electrode resistance given in BS 7430 is 20 ohms and this might be difficult to achieve unless you have enough space to install multiple electrodes. However, this figure is general and you might wish to argue that, if a front end RCD is employed, a higher value could be considered.
Giving any advice on this subject is always problematic as it really all depends on intended use and the balance of risks.
You need to balance the cost and complexity of a fixed permanent installation against a less formal arrangement where equipment is supplied as needed – possibly via extension leads. Remember power outages are relatively rare in urban areas at the moment – but who knows what the future will bring
.Small cheap petrol generators: are very noisy; produce dangerous fumes: and are not very reliable.
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Interesting that it shows a local earth rod, but doesn't show the generator frame being earthed, or if/where N is tied to earth.
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Where does it say your not allowed to connect to DNO earth? You can't rely on their earth so have to have alternative methods of earthing but as far as I am aware there is no requirement to disconnect their earth.
It would cause many problems to have to disconnect the DNO earth and I think only the name changes, there is no disconnection of any item able to provide an earth.
An "Earth electrode" becomes an "Extraneous-conductive-part" when supplied from a DNO TN supply but does not need disconnecting it just changes name.
The IT system is used in many homes with units complying with BS EN 61558-2-5 and is a standard safety method used with shavers in a bathroom. Although not strictly complying with regulations using single items from a generator does not cause a danger where there is no earth stake in the same way. It is with multi items where the problems start and in order for a RCD to work there must be a return earth path. However other than using items outside the house in real terms there is not a need for an earth rod there is enough bonded items within the house to allow the RCD to operate under fault conditions.
So the earth rod is just to satisfy the rules and has very little to do with the safety of whole system.
As BAS has clearly identified it is the connection between neutral and earth which is far more important.
As long as there is a connection between neutral and earth and the connection is before the change over switch so when running from the DNO supply it does not connect the DNO neutral to earth exactly where the connection is made is not really important.
If we consider a BT building and the 100kva generator on a concrete plinth once fitted very little is going to change so exactly where does not matter. However where a 3kva generator is used these tend to be no where near as well made and there is a good chance that some time in the future it will be removed and re-fitted either in order to use it else where or because it has failed so we have a problem.
1) If we link at the generator it may be changed so removing the link.
2) If we link else where it may cause a short if linked other than neutral to earth at the generator.
With large generators used in the gas terminal at point of air there was a huge resistor connecting the earth so if the DNO earth was not the same voltage as the terminals 100's of earth rods there would be a limit to what current could flow.
So using a resistor between neutral and earth would allow the RCD to work but not cause a short if a generator with a tapping of output winding goes to earth rather than the neutral.
However we can try to stop people doing things wrong after we leave and try to make it fool proof as much as we like and bitter experience shows use we will just get more inventive fools.
So I would say put the link where you want and document it. Should some one in the future change the generator and as a result either form two links or no links that's their problem not yours.
There is a limit to how much cotton wool we can wrap people in and we must assume anyone working on the system will be competent.
And if the supply uses cables with a PVC outer sheath and the service cable is cut under fault conditions, where is your earth now?
As I said I need to check but it is in an Electricity Supply Industry document that is referred to when approval is sought by a customer to connect a generator.
As I said I need to check but it is in an Electricity Supply Industry document that is referred to when approval is sought by a customer to connect a generator.
You have to make provision for the fact that a DNO earth might be missing in the event of a powerloss but there is no requirement for generator earths and DNO earths to be kept seperate.
acually diynot has been giving me probs for a while but been getting loads of dns errors so think it might be a prob with my router or isp
ps
apologies to b.a.s for going off topic
Matt
ps
apologies to b.a.s for going off topic
Matt
agreed and theres no reason why they should
hi westie, approval is only sought when connecting generator sets that also syncronise and export to the network
BAS I'm not trying to be funny, but has it really taken you three years to wire up a genny??
//www.diynot.com/forums/viewtopic.php?t=115488
//www.diynot.com/forums/viewtopic.php?t=115488
My supply is TN-S. But I expect that another right they reserve is to re-designate it TN-C-S when the connection back to the substation earth completely rots away.
Planned local earth for the generator is about 10m of 25x3mm tape.
There will be a front end RCD.
My thinking exactly.Remember power outages are relatively rare in urban areas at the moment – but who knows what the future will bring
I live inside the M25 - outages are rare and short. Not sure if we average even 1 a year, and I honestly cannot remember one lasting more than a few hours.
But as you say - who knows.
My preparation for the "who knows" is to install the infrastructure to allow me to use a generator. Some other work planned for the near future will give me an ideal opportunity to install the earth tape, put a 60309 inlet on the outside of the house, a C/O switch inside, etc.
I may well never use any of it - in fact I hope I never need to, but if things do become flaky I will be in a position to plug a portable genny in.
My preference would be for a diesel, one that I could run on a range of fuels.
Cos they is also quieter than petrol and less fumey....
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