I'm not too sure how you would use a "low ohm meter" to test the adequacy of an installation's earthing, no matter what test current it used.
Kind Regards, John
You know as well as I do enquiry is a valid method, so if the DNO says your earth is 0.35Ω and you measure 0.90Ω then you are permitted to assume the earth loop impedance is 1.25Ω. With a TN supply this is only method that can be used until the DNO supply is activated.
I'm going to have a EV charge point fitted next month I think they can be a bit finicky about earth points,
This is very different. Depending where the EV charge point is fitted, in a garage it could well use the TN-C-S supply without any real problem, but if being charged outside the equipotential zone within the home, i.e. on the drive, then the earth system must take into account different voltages between two equipotential zones.
So under fault conditions (loss of PEN) the house and garden could have 230 volt difference, unlikely that much, but could well exceed the 70 volt permitted with EV charging points or 50 volt with other items, seems users of EV points can stand 20 volt more than all other users, no idea why.
So where a TN-C-S supply is used there are in general three options, one use a TT supply for the EV charge point, two use a monitoring earth rod to auto switch off the EV charge point in the event of a fault, and three monitor the voltage and if it is not within 207 to 253 volt auto switch off the EV charge point. When I say switch off I also mean disconnect the earth one the lives have been disconnected.
As well as the earthing problem with some thing which can allow DC to be imposed on the system you also need to consider will the RCD's still work. There are two main methods, one is fit a type B RCD,
which are very expensive, the other is use a 6 mA disconnection unit, if the DC component exceeds 6 mA it will auto disconnect, this allows a type A RCD
to be used. You can't use type AC
RCD's on the feed to EV charge points or solar panels.
My old house the RCD does not have the type written on the RCD's, neither as a type, or the sign, this house the packets for the RCBO's were clearly marked type B, however it transpires this was referring to the magnetic overload not the RCD within the RCBO and they were type AC.
With a TN supply the RCD is secondary protection, so I have still got 14 type A RCBO's fitted, but with a TT supply then the RCD is the primary protection device, so I would need twin pole switching type A RCBO's.
This is why EV charging points are fitted by electricians who have had special training, he needs to do an assessment as to what needs to be fitted. Last thing we want is to give the post lady a shock, or any other bona fide visitor, milkman, meter reader, social serves etc.
Or more to the point, if it does happen, you want to be able to say not my fault. Not sure if the law has changed? I know when I was at collage in the 70's you could watch a intruder injure himself for example fall down a vehicle pit, but you could not set a trap for him, like dig a pit. Except where local by-laws said different, glass on top of a wall visible from outside which would clearly need illuminating was permitted, but if not visible until you climb the wall it was not.
So where I live my rear drive is not used by visitors, so likely I could do what I want, but front drive is used by visitors so needs to be safe for them to use. Although since I live in Wales any work in the garden needs notifying to LABC so in real terms could not fit my own EV charging point.
