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Power Supply Earthing Problems


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I am currently finalising electrical works on wastewater treatment project.


Firstly, when we connected the the REC earth to our earth bar, we noticed a bit of sparking (i.e. when bolting the earth bar link). At this point the generators were turned off, the REC electricity supply was isolated and the generator earth was disconnected. The earth bar did of course have all protective and equipotential bonds connected to it. There will be some kind of connection to earth via the main bonds out to steelwork etc. I guess there must be enough of a difference between this and the REC earth to give the circulation current.


Next - when we connected the load a further 4A appeared to earth. We found this to be 2A when the MCC1 kiosk board is connected and 2A when the supply to an existing building is connected (as found previously when running on generator).


The plant is now in service with the REC supply connected as above.


Some other information - the site has a TN-S earth. We've learned that power supply were going to (and presumably have) jointed their 4c 185 cable to a 3 core cable at the sub-station. I queried this, making the point that if they changed the earthing arrangement then all our bonds would be incorrectly sized (theoritacally at least). They responded by saying that the earth would indeed be as originaly specified. We certainly have 4 cores and a seperate earth (armour) coming into the cut-out. I don't see what else I can do here.


What I intend to do is carry out some more tests on the 2 supplies mentioned above. I'm pretty certain from previous testing however that we do not have any actual earth faults, particularly on the MCC1 kiosk supplies. These 2 loads are of course unbalanced, they also feed fluorecent lights etc.


As with all earths, many amps could be flowing without our knowledge. We checked here bacause of the problems encountered with the generators.


Other things to note are that the site are directly under a 275kV pylon which is presumably earthed.


Any thoughts on the above, the actions taken so far and the actions proposed would be much appreciated.


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