High (?) Ze on TT, EV charge point

[littlespark]

Not had much experience with earth rods in the past 30 odd years, so I’m asking here before going home and reading a book.

Was asked to look at an older style ev charger (bp charge master) and after a recent visit to another job with the same problem, I thought I’d check the Zs first at the charger. Came in at 190 ohms. Without knowing the earth system at the time, I then checked same at the board, and a random socket in the kitchen…. Both around 190 mark.

Asked where the meter was, and got a surprise when they directed me halfway down the garden to a meter box on the wall, at the bottom of an overhead line pole.

Also checked Ze there off the service isolator and rod, and still got 180 ish ohms.


Like I say, zero experience I’ve the last 30 years. This is the first domestic TT I’ve ever come across in Scotland.


Apologies for pic, I might edit it later to turn it around.

I believe the max Ze for TT could be as high as 200 ohms… (correct me if google is being wrong) so those 190 readings are ok…. But maybe not for the ev charger?


Is this suppliers responsibility to give a better earth, or will I need to drop another rod?

 

[PS Electrics]

Not had much experience with earth rods in the past 30 odd years, so I’m asking here before going home and reading a book.

Was asked to look at an older style ev charger (bp charge master) and after a recent visit to another job with the same problem, I thought I’d check the Zs first at the charger. Came in at 190 ohms. Without knowing the earth system at the time, I then checked same at the board, and a random socket in the kitchen…. Both around 190 mark.

Asked where the meter was, and got a surprise when they directed me halfway down the garden to a meter box on the wall, at the bottom of an overhead line pole.

Also checked Ze there off the service isolator and rod, and still got 180 ish ohms.


Like I say, zero experience I’ve the last 30 years. This is the first domestic TT I’ve ever come across in Scotland.

View attachment 123258
Apologies for pic, I might edit it later to turn it around.

I believe the max Ze for TT could be as high as 200 ohms… (correct me if google is being wrong) so those 190 readings are ok…. But maybe not for the ev charger?


Is this suppliers responsibility to give a better earth, or will I need to drop another rod?

I'll add my 2p worth

Yes 200 and under is acceptable. The problem with Earth Rods are that they are dependent on soil conditions.

So first question is how dry has it been there?

If you are close to 200 and it has been quite wet then the worry would be that during dry spells you could get much higher readings. If it has been quite dry for a while then it is not likely to get much worse.

I believe the thinking is that at 200 Ohms or lower then the max Zs of 1667 for an RCD 30mA will not be exceeded even in the poorest of conditions.

 

[westward10]

From my understanding the 200 ohm (Ra) is the resistance between the electrode and general mass of earth and can only be measured accurately with an earth electrode tester. Using an earth fault loop impedance tester it records the full earth fault loop path including the supply line, transformer, substation electrodes and Ra. A true test of Ra should generally be lower than Ze readings as it takes away the other parts of the fault path so I believe GN3 makes an assumption that if you use the Ze method and in your case with a reading of 180 ohms Ra will be lower. Regardless it falls below 200 ohm.

 

[littlespark]

Customer got back to me to say an SP engineer would be out today sometime. So will go from what they say.

Never expected a rod, so ELI was only thing I could test with.
Weather has been incredibly dry… so that might bring the readings up.

I’m assuming the charger itself is sensing the high resistance and not allowing charge….. didn’t think an older chargemaster had that function though.

Apparently it’s been not working for weeks, but they only thought to phone me last Friday.
Maybe a power reset might have fixed it for a while.


Lots of detouring today as a lorry has overturned on A1 between me and all my wholesalers.
Northbound traffic is being routed through Berwick. Nightmare.

 

[westward10]

What provides ADS to the armour of that cable?

 

[littlespark]

What provides ADS to the armour of that cable?

Glanded into the metal consumer unit at the house end, only OCPD is the main fuse, which, if it is 100A like it says on the carrier, then that’s an issue.
Could do with a switch fuse at this end to protect the cable, (16mm) which I will be suggesting to them. At least 10m of cable between meter and consumer unit, buried under the driveway.

I got a text back from customer saying that SP have disconnected the rod and made it TNCS.

I might nip back and see what new Ze is…. But customer is just happy her car is charging.

 

[westward10]

As TT the SWA would have needed RCD protection but that is irrelevant now. Being an old charger it may not be suitable for a TN-C-S earth.

 

[PS Electrics]

From my understanding the 200 ohm (Ra) is the resistance between the electrode and general mass of earth and can only be measured accurately with an earth electrode tester. Using an earth fault loop impedance tester it records the full earth fault loop path including the supply line, transformer, substation electrodes and Ra. A true test of Ra should generally be lower than Ze readings as it takes away the other parts of the fault path so I believe GN3 makes an assumption that if you use the Ze method and in your case with a reading of 180 ohms Ra will be lower. Regardless it falls below 200 ohm.

GN3 says there are 3 ways to do it and you are correct it says using a ELFI tester might not be accurate.

Having said that it doesn't say you cannot use an EFLI tester.

The only mention I could find of the 200 Ohms in GN3 was that readings above 200 may not be stable. It does also say the test should be done in the least favourable conditions if possible which makes sense.

As TT the SWA would have needed RCD protection but that is irrelevant now. Being an old charger it may not be suitable for a TN-C-S earth.

It appears that unit (BP Chargemaster) has no in built protection at all! So requires an RCD and now also PEN Fault protection.

Found an install doc for that charger interestingly it says

For a TT insulation (sic), the Ra earth rod impedance shall be less than 100 ohms.

The unit appears to have the 6ma DC RCD built in though.

 

[littlespark]

yes, it does have rcd protection but nothing else.
These were the first chargers to come on the market, roughly installed for free by whoever, just to get a car sold.

 

[westward10]

Should have been left as TT or it is going to need PEN fault protection.

 

[littlespark]

I have told the customer that new chargers are a lot more safer, and will send a quote for replacement.

But, ultimately, it’s their decision.