Generator earthing

David


My changeover panel will have have rcd protection built into it but the generator does not have rcd protection in it. As such, there will be about 10 metres of cable from generator to changeover switch without rcd protection

All the Genuine TNS farm systems I have seen had a transformer next to the load, usually large for grain drying (150-300 kVA), and an 11kV supply. You cannot assume anything else is TN-S particularly nowadays when random changes seem to be made! And whilst I agree with Graham for any cabled installation, it is not a problem with the local TX and an incorporated Earth mat, which meets the within your control specification. I think you may be right John about the generator Earth bars going into the rock, It is very difficult to get a 1 micro-Ohm Earth resistance although the water source might be good too, as a HV fault in the power station could be very dangerous! If you only have a small generator and no livestock an Earth rod and RCD is probably adequate, but I have had a lot of experience with bigger ones and I much prefer an Electronic MCCB as protection. Certainly bonding every bit of ground connected metalwork would be a good idea, to get a low Ze value.

551.4.3.2 says a system for distribution of Electricity which does not apply to a private transformer.

IF this is not a private transformer job, and the supply is from somewhere else then, of course, you will need some form of Earthing for the generator supply but this probably will be low enough resistance from the bonding Mike and  I mentioned above, add another rod if you like it does no harm, but the supply is not safely TN-S.

There is another point and that is the position of the generator N-E bond. The generator will be insulated from the Earth if on the linkage (normal), and failure of the Earth cable core quite possible, and will not be noticed. The safest place for the bond is therefore somewhere fixed in the switchgear to ensure ADS will still operate correctly. Again this is not a problem if the generator is fixed down to a base and a long  PTO shaft connected to the tractor.

I am not sure how big this generator is? However I would want to see both over current and earth fault protection on the generator panel and an independent earth electrode. Ideally I would want an adjustable earth protection device at the generator to have some selectivity with downstream devices for larger installations.

David,


Perhaps there is a clue here, in the third post in the thread:

 

The farm is 200a single phase TN-S and as such does not have an earth spike.

Which could be a "private transformer" but is perhaps unlikely to be ... in this particular case, it doesn't matter whether the supply is "true TN-S", or "was TN-S but now repaired in places with CNE cable".

Hi all and thanks for your replies


The supply transformer only supplies this one property and was only installed last year During the upgrade to 200a and is definitely tn-s. some have mentioned a ‘private transformer’. I’m a bit confused by this, does it mean it is a dno transformer and only supplies one property Or it is privately owned and ran? Yes the generator is basically an alternator with a Pro shaft out of the end. It is bolted to the floor of a plant room and the pto shaft goes out through the wall. So there is no control panel on the gen itself. My changeover switch is incorporated into a larger switchboard which includes ct chamber and meter cubicle. Also incorporated in this is the over current protection for the generator and I intend to fit a selectable rcd aswell. I would have liked to have had these directly on the generator but that is not possible so there is about 10 meters of cable between the genset and the gen breaker/rcd in the switchboard. Any views on the original questIon of whether the earth cable from the spike can go directly onto the generator earth terminal or if it must go to the MET?

What is the rating of the generator? Is one pole of the winding connected to the generator frame? Or is it a separated winding? What is the rating of the over current device if any on the generator output?

Hi John


It is 80 kva connected for single phase. They actually ordered 40kva but were sent an 80. The neutral and earth are connected together in the set which are then connected to the frame. There is a 160a Mcb downstream of the gen in the switchboard and I intend to add a 300ma time delay rcd here aswell

some have mentioned a ‘private transformer’. I’m a bit confused by this, does it mean it is a dno transformer and only supplies one property Or it is privately owned and ran?

privately owned and run - i.e. electricity is purchased at HV and the entire LV side, including earthing, is part of the consumer's installation. That way there's dependency on the DNO's equipment for the earthing.


  - Andy.

Put a spike close to the NE bond - by all means have another one somewhere else as well if you like, so if the CPC snaps it becomes TT, not floating,  but you really should have at least one at the point where N and E separate. From your description, that is near the genset.


Treat all the cable between genset and the first  RCD (the  300mA time delay monster ) as you would a meter tail in that environment, which probably means  2 layers of insulation/sheath and careful routing.

  If you need cables to cross the ground a tough plastic duct will do if mechanical damage is possible (its a farm so someone will probably suggest old water pipe - it really should be black but if its obvious what is inside it no-one will care...)

Also make sure that the change-over switch breaks the neutral, or any RCDs on the genset will trip due the mains NE link looking like an NE fault, and similarly any RCD on the mains supply side will see the genset NE link as if it was a fault and trip. You can use switched NE links instead, but it needs interlocks and alarms and is almost never the best way  to do it, switched  neutrals ganged to the phases is much better.

Mike.

Mike


i am planning on using SWA cable between the genset and MCB for the genset in the switchboard (Around 10 metres) and this will be carefully routed in a plant room so as potential Damage will be minimised. 

The neutral and earth of my SWA join at the neutral/earth terminal in the generator but I suppose you could say that this is Where they also separate as they leave as two separate cores Going to the switchboard. As such, if I take your description of where the neutral and earth separate correctly, then that would be in the generator terminal box. Would you consider SWA suitable cable to use between generator and generator MCB as this would be upstream of the overcurrent protection?