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Discussion Starter #1
I have a Chargemaster wall unit and BMW i3 both about 18 months old. I haven't been using the car much but every couple of weeks I set the charger to use the off-peak rate. Twice the supply has tripped during the night after about 30% charging (say, 1 - 2 hours). The trip is the 32 A breaker which was fitted by Chargemaster alongside the earth leakage breaker in my main fuse box (so no other supplies connected). I am trying to understand if the car is taking too much current or whether the 32 A circuit breaker is faulty (which seems a bit unlikely?). Any suggestions or advice would be most welcome.

(Note: after the first incident I posted in the BMW i3 forum but wrongly suggest that it was the earth leakage breaker - I didn't look closely enough).
 

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Discussion Starter #2
UPDATE:

I hadn't realised that Chargemaster has a 3 year warranty. Very helpful discussion the outcome of which is that it is extremely unlikely that the breaker is faulty, ditto the car. They can do some checks remotely then will probably swap out the unit. My post a bit premature!
 

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Probably the MCB. They are largely thermal devices.
After 18 months it may be running a bit warmer due to contact aging or a slightly less tight connection - checking the latter is easy enough to do.
Add the warm weather and its effectively getting a bit derated and thus not handling 32A continuously. Is the consumer unit in a warm location?
 

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UPDATE:

I hadn't realised that Chargemaster has a 3 year warranty. Very helpful discussion the outcome of which is that it is extremely unlikely that the breaker is faulty, ditto the car. They can do some checks remotely then will probably swap out the unit. My post a bit premature!
Cross posted with my first above.
I'd be interested to know what fault the charge point has to cause a slight (probably 5A or so) overload on the circuit, which is what you'd need to trip after an hour or two. I doubt the car is drawing more than 32A, so any extra would need to be going somewhere.
 

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Discussion Starter #5
Agree that it’s difficult to see where the extra current is going, if not the car. I will report back.
 

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Probably the MCB. They are largely thermal devices.
After 18 months it may be running a bit warmer due to contact aging or a slightly less tight connection - checking the latter is easy enough to do.
Add the warm weather and its effectively getting a bit derated and thus not handling 32A continuously. Is the consumer unit in a warm location?
TIL - MCBs have recommended torque settings for the connections.
 

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Discussion Starter #7
Well, the guy at ChargeMaster said he used to be an electrician and had never known an mcb failure. It is not in a hot place and charges after midnight. As the wall unit is basically just a contactor with a communication protocol I cannot see how this could cause a problem - there is no communication with the mcb. If they come out I will get them to change the mcb/RCD unit then if it trips again I know it will be the car which is still under warrant.
 

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Well, the guy at ChargeMaster said he used to be an electrician and had never known an mcb failure. It is not in a hot place and charges after midnight. As the wall unit is basically just a contactor with a communication protocol I cannot see how this could cause a problem - there is no communication with the mcb. If they come out I will get them to change the mcb/RCD unit then if it trips again I know it will be the car which is still under warrant.
MCBs do fail! It's highly unlikely that you have a 5A phantom draw, and if it was a leak to earth the RCD would trip. MCBs are electromechanical devices and it's not uncommon for this type of failure. The use of
EVSE with long duration high current duty cycle is not what cheap domestic devices were designed for!
 

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MCBs do fail! It's highly unlikely that you have a 5A phantom draw, and if it was a leak to earth the RCD would trip. MCBs are electromechanical devices and it's not uncommon for this type of failure. The use of
EVSE with long duration high current duty cycle is not what cheap domestic devices were designed for!
Agree, eliminate this most likely cause before looking elsewhere.
 

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Indeed! 32A MCB may well get hot enough to trip at 32A continuous draw after 60 minutes
 

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Can you adjust what the BMW is pulling? When I had a Rolec that used to trip I set mine to 30amps. It look a little longer to charge but kept the temperature down.
 

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Don't have my regs to hand but from memory a 32 amp B type can run forever at 45 amp and not trip.
Theses are electromechanical devices, produced in their millions for a few quid. They can and fo fail.
The electrician who said they never fail must have either had a very short career or been extremely lucky.
 
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Don't have my regs to hand but from memory a 32 amp B type can run forever at 45 amp and not trip.
That's 1.5x rating (150%). Looking at a couple of type B curves it should trip between about 30 seconds and 30 minutes, though at the top end of the tolerance that 30 mins looks like it might be forever. :)
 

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Discussion Starter #19
The thermal device should be there to protect the cable. Any other elements (car, wall unit) should have integral overload protection and trip the wall unit. Agree that a 32 amp mcb should be able to operate continually at 30 amp but I guess they can fail. Assuming they come round I will ask them how they think that a faulty wall unit could trip the mcb feed, given that it only has a contactor and communication protocols with the car. My money’s on a problem with the car charger but if I get the wall unit and mcb replaced and there is still a problem I then have the ammunition to go to the dealer.
 

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My eyesight ain't what it was but that 32 amp curve doesn't look like it goes under 45 amp to me.
Yeah. If that is the Regs curve I'd think it's the 'worst' permitted. In reality there are tolerances. My main source is this pdf from ABB:
Comparison of tripping characteristics for miniature circuit-breakers

The first graph shows the tolerance band in orange and the right hand boundary appears to correspond to the IEE Regs.
I'd expect most off-the-shelf MCBs to be somewhere in the middle when new, so would eventually trip with sustained 150% load.

But I think we agree that a 32A load tripping a 32A breaker is definitely screwy. :unsure:
 
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