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UMC/3-Pin charger tripping fuse board...

Jason71

Active Member
May 8, 2019
2,970
2,898
Shropshire
I’m probably being an idiot but why is it that you’ve got a bunch of circuits that don’t appear to be protected by an RCD at all?

Typically you see sockets and lights distributed between two RCDs, the logic being that if you lose lights or sockets downstairs that you’ll still have them upstairs, and vice versa, etc.

Are those Hager breakers MCBs or RCBOs? Circuit #3 is a RCBO for sure.
I imagine its quite an old installation from back when only sockets tended to be on RCD's In my very limited experience as soon as you let an electrician near a fuse board first thing they always want to do is replace the whole thing before they will do any other work :)
 
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Peter 224

Member
May 9, 2021
262
189
Salisbury
I’m probably being an idiot but why is it that you’ve got a bunch of circuits that don’t appear to be protected by an RCD at all?

Typically you see sockets and lights distributed between two RCDs, the logic being that if you lose lights or sockets downstairs that you’ll still have them upstairs, and vice versa, etc.

Are those Hager breakers MCBs or RCBOs? Circuit #3 is a RCBO for sure.
It used to be that power ring circuits went through RCD protected circuits, lighting and power spur sockets through MCBs. But things may have changed. I tend to support the view that the main RCD probably needs replacing, I notice that there appears to be an unused space in the board, this could be utilised for a 40A spur just for the EV.
 
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Jason71

Active Member
May 8, 2019
2,970
2,898
Shropshire
Hi. did you get to the bottom of this?
I just had the same thing happen to me. testing out my UMC before a holiday.
I have 2 circuits in my house and the UMC is consistently tripping the RCD in one of them but not the other.
I can only assume that the UMC is generating some amount of residual current and
a) I have a faulty RCD that is tripping too low
or
b) I have some level of residual current already in the circuit and the UMC is pushing it over the edge.

its the same circuit as the cooker and kettle so its not related to the current.

bad that the UMC would generate any residual current ( not using an extension lead or anything) but also looks like I need to get my electrics checked out I have had that RCD trip a couple of times in the last 18months for no obvious reason so something is not quite right.
 

VanillaAir_UK

Well-Known Member
Jun 17, 2019
8,235
5,759
Surrey, UK
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 and that my RCD was probably a little over sensitive - tripping around 28mA - the electrician didn't think it was worth fitting a new RCD as there was no guarantee that the replacement would be significantly better whilst moving the circuit would solve it for good.
 
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Jason71

Active Member
May 8, 2019
2,970
2,898
Shropshire
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.
Thanks for that. This is pretty much what I figured. And yes it was at the point of charging start so pretty much exactly what you are suggesting
At some point I will try the unplugging things and see if it then accepts the UMC. I don't have an RC meter unfortunately probably be cheaper than a sparky though If I got one!
 
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JS77

Member
Nov 24, 2019
141
88
England
Just wanted to post a quick update on this in case it helps anyone else...

Tesla actually sent me a replacement UMC (They called me about a separate issue, so I mentioned the trouble I was having and they arranged for a replacement UMC) - however, the same issue occurred: plugging it into my M3 tripped the RCD so it definitely was an issue with my electrics rather than the UMC.

An electrician finally made it over to me and did a lot of checks before having to resort to what others had already advised above ie. Turning everyone off and going through one by one until the RCD tripped. After a fair bit of trial and error, we discovered our recently installed electric hob has an earth leakage of about 3.6 (not massive, but clearly enough to cause the cumulative leakage to trip the RCD). Once that was disconnected, everything worked as expected. So, I can now charge at home, albeit at ~2kw p/hour (and for anyone wondering, we have contacted Neff for a replacement hob :p )
 
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Cnixon

Member
Mar 5, 2020
462
139
Basingstoke
Very informative thread. It looks from your board that you have an rcbo 3rd breaker from right (with test button). If you could swap a light circuit from right side with the relevant 32a socket ring (that you charge with) onto the right side but on a new rcbo (£30) you will not have the earth leakage issues. I doubt the Neff will change much even if replaced.
 

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