tl;dr - low values of cumulative earth leakage is perfectly normal, safe and does not indicate a fault. The UMC can have a transient leakage of around 6mA at the point that the car starts charging. If you have a 30mA RCD and cumulative earth leakage for all associated circuits is already in the low 20mA, then there is a risk of the RCD being tripped.
Earth leakage is very normal for devices. I believe that a device can leak 3mA before it becomes out of spec. From my experience when I was measuring earth leakage was that the Tesla UMC generates less than 3mA of leakage except the very moment that the car starts charging. I don't know what value as it was too fast to measure but as an example of my installation using figures that I remembered, I had measured cumulative leakage of low 20mA's on the RCD (its cumulative on all circuits that are protected by the RCD) which was all normal, but marginal on its 30mA trip. I believe that regs stipulate 2/3rds of the RCD stated leakage should be the target for the RCD but its all pretty moot as most of the things that contribute to the leakage are things like power supplies so non fixed appliances.
Anyhow, the moment my car plugged in and the relay in the UMC activated, the RCD tripped so that exceeded to RCD leakage limit which should be 30mA but may be less if over sensitive. So at moment that car started charging, UMC leakage exceeded 3mA (I believe a brief transient is within spec even if it exceeds 3mA) and tripped the RCD. However, by unplugging a few items with power supplies, I could bring the cumulative leakage below the threshold needed to start the car charging - at which point I could measure the leakage attributed to the RCD. As the UMC only had a high transient leakage, once charge had started, I could then plug all my devices back in and everything would remain below the nominal 30mA limit - I think it was around 26mA cumulative. Unfortunately this was not a solution as the moment the car needed to have charged again it would have tripped the RCD. I was able to keep the car charging on UMC with all circuits active for over an hour, at which point I stopped charging.
As there was no fault, just cumulative leakage, there was nothing to fix as such. The solution was to reduce the leakage on all circuits for the RCD by moving a circuit on to a different RCD that had negligible leakage. My electrician confirmed the cumulative leakage and ran a few safety checks to see if anything was obviously contributing at a circuit level. Problem sorted but I do have just the one socket that I can use to charge the car if using the UMC.
Thankfully I had a meter that could measure earth leakage - most cannot. I spend several hours one afternoon trying to identify a specific culprit with lots of head scratching - I am just glad I was not paying for a sparky to come to the same conclusion. There was no one device. The UMC just so happened to be the one that broke the camels back so to speak, and only at the moment that the car started to charge. Once charging, all was good. My observation was that the UMC probably had a transient leakage around 6mA after which it was around 2mA.
