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Charging amps memory issue

Caterman

Member
Nov 13, 2020
73
33
Birmingham
I have a problem on my 2021 MS LR that when I charge at home on a 3 pin charger, if I set the amps to 10A it defaults to 24A when I plug in which trips the RCD.
I haven't had it long and not had to charge that often but it didn't do that on the 1st 2 charges only on subsequent ones.
I've had the home electrics checked over and they are ok.
I have read other threads where this is a problem but none seem to get to a solution? Not sure if I'm being an idiot or I need to speak to Tesla about it.
 

26ct2143

Member
Nov 22, 2020
202
81
Burton-on-Trent, UK
Not sure what's happening here,
But the charger should tell the car that it's only able to supply 10amp when you connect.
Do you plug in the charger at the wall first and switch on , then connect to the car after? (in that order?)
If yes,
Then either the charger is not telling the car it's a 10amp max situation, or
the car is ignoring the charger's request.

Do you have another charger to test?
 

Adopado

Active Member
Aug 19, 2019
3,807
2,914
Scotland
I have a problem on my 2021 MS LR that when I charge at home on a 3 pin charger, if I set the amps to 10A it defaults to 24A when I plug in which trips the RCD.
I haven't had it long and not had to charge that often but it didn't do that on the 1st 2 charges only on subsequent ones.
I've had the home electrics checked over and they are ok.
I have read other threads where this is a problem but none seem to get to a solution? Not sure if I'm being an idiot or I need to speak to Tesla about it.

I don't think it should trip the RCD though it might trip an RCBO (an RCD and circuit breaker combined) so I'm guessing that's what you mean. (AFAIK an RCD shouldn't trip with overcurrent alone... maybe this is indicative of another problem.) I am also assuming that the "default to 24A" is what you are seeing on the car's screen. Are you using the supplied Tesla UMC? At first sight it looks to me as if the UMC is not communicating with the car correctly. Could you borrow another one to test?

Hopefully @Glan gluaisne or one of our other electrical wizards will spot your post!
 

Durzel

Active Member
Jul 17, 2019
3,264
2,260
Bath, UK
As above, RCDs don’t trip due to load (overcurrent), they trip due to line faults. Are you sure it’s the RCD tripping as opposed to a MCB/RCBO?
 

Glan gluaisne

Supporting Member
Sep 11, 2019
2,782
2,708
UK
I have a problem on my 2021 MS LR that when I charge at home on a 3 pin charger, if I set the amps to 10A it defaults to 24A when I plug in which trips the RCD.
I haven't had it long and not had to charge that often but it didn't do that on the 1st 2 charges only on subsequent ones.
I've had the home electrics checked over and they are ok.
I have read other threads where this is a problem but none seem to get to a solution? Not sure if I'm being an idiot or I need to speak to Tesla about it.

This is possibly a fault in the UMC, a fault in the charger or perhaps may be related to the house electrical installation, even though you've had it checked.

What's supposed to happen is that the UMC signals to the charger that there is only 10 A available. It does this before the UMC contactor closes to supply power to the charger, by setting the duty cycle of the Control Pilot signal (a low voltage 1 kHz control signal). The charger is only supposed to load the Control Pilot down, so commanding the UMC to close the contactor and supply power to the charger, after it has adjusted its own maximum demand down to 10 A or less.

If the charger is demanding 24 A, with the UMC signalling that only 10 A is available, then when the UMC contactor closes it will almost certainly blow the fuse in the UMC plug a short time later, as the current ramps up above plug fuse rating. It should not create an earth leakage fault and trip the RCD, but I have seen RCDs trip as a consequence of odd load spikes, so it is possible this could be the cause.

To determine what's going on will need some investigative work, starting with eliminating the charger (which is built in to the car) as being at fault. Connecting the UMC via a meter to measure the current being drawn might be a good first step. If this rises above 10 A then that tends to indicate a problem either with the UMC or the charger, rather than the house wiring. Determining whether its the UMC or charger then needs a test box connected to the UMC, so that the duty cycle of the Control Pilot can be measured. If this is signalling that a higher current than 10 A is available then it's a UMC fault. If it's signalling that 10 A is the max available then it's a charger fault.

The fact that the RCD is tripping, though, suggests that the 24 A thing may be a red herring. The car can display the incorrect charge current initially, but should settle down to show 10 A once the charging process has started. Testing for earth leakage is pretty easy, just needs an earth leakage clamp meter and a test extension lead with the conductors broken out to enable the clamp meter to measure the leakage current. I checked the UMC leakage current a year or so ago, and posted about it here somewhere, and IIRC it was very low, just a mA or so, well below the ~30mA needed to trip an RCD.

Any competent electrician should be able to do an earth leakage test, with and without the car plugged in, to try and see if that's the problem. What does happen a fair bit now, as more and more electronic gadgets are in use, is that the cumulative earth leakage current on a circuit can rise to a point just below that which trips the RCD, so that just a tiny bit more from something like a charge point, can trip it. This is a particular problem on installations that only have a single RCD, or that have most of the in-use power outlets connected to one RCD in a split CU. It's also possible for RCDs to partially fail such that they trip at a lower current than they are supposed to making them more susceptible to nuisance tripping.

My own fix for this, when I built our house, was to fit an all-RCBO consumer unit. This pretty much stops the cumulative leakage problem, but still protects each circuit adequately.
 

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