How can you get >6mA DC in the mains from an EV?

[cupoftea]

Hi,
EV chargepoints need >6mA DC detection in the mains, so the Type B RCD's dont get blinded..
But how do you end up with 6mA DC in the mains from charging an EV?

 

[dick_freebird]

Is this really a DC spec or is it more like ground fault interruptors
where you're looking for an AC current imbalance between line
and neutral?

Anyway, since the first or second thing you hit coming into the
EV is rectification, an imbalance in "taken" cuttent in some leg
of the rectifier bridge might result in (say) half-cycle rectified
current producing a time-averaged pulse imbalance that would
equate to 6mA DC?

 

[cupoftea]

Thanks, yes , i see what you mean, but in that case, wouldnt all things needing a rectifier at the front end need a "6mA DC detector"?...not just EVs.

 

[dick_freebird]

Well, many "pluggable" electric items seem to come with
built-in GFIs (like my pressure washer and window A/C
units - both of which have some "wet risk" of developing
a ground fault).

I expect this is a stab at human safety (or at least, at
"checking the box" re liability. And so it is as much (or more)
about "squishy" stuff like legalities and human propensity
to stupid acts, than any engineering "why?".

 

[cupoftea]

Thanks, i see your point.
The rectifier case is an interesting one...because it does indeed produce an overall DC level, when each "leg" conducts differently..............but that woudlnt be detected by eg a fluxgate current sensor, because at all instants in time, the go and return current will actually be equal.
This makes it even more interesting because the only cheap way to detect DC in the mains is a fluxgate current sensor.....and in fact, thats what EV fraternity uses..........so that most likely means that the "Non-symetric" rectifier is not what is producing the DC level in EVs.......which brings the question ...what is?.......but i wonder if you have already answered it....this is just a "hand waving attempt" at providing safety.

 

[FvM]

As far as I know, we are looking for > 6mA DC ground fault (= common mode) current, not "DC in the mains". DC mains current is also limited to a low level by power quality standards, but not so low.

The specific problem of EV OBC is possible ground fault of the DC HV circuit and lack of safe PE for the vehicle chassis, not "non-symmetric rectifier".

 

[cupoftea]

The specific problem of EV OBC is possible ground fault of the DC HV circuit and lack of safe PE for the vehicle chassis,

Thanks, yes, i see what you mean, though i must admit i cant foresee how the HV batteries could end up having the effect of pushing DC into the mains?......is it some kind of scenario where a car gets run over by a truck as it sits on the drive, outside the house...and then the "6mA DC in the mains" happens?

 

[Easy peasy]

active front end on an EV charger can draw currents that are not symmetric about zero current - the standard spec is < 5mA ave over 24 hrs - in the neutral - to avoid earth stake corrosion.

For 3 wire systems there is no Neutral - but drawing DC from 2 phases can really upset the local supply transformer ( e.g. single diode rect to a load ) - so also a no no ...

 

[cupoftea]

active front end on an EV charger can draw currents that are not symmetric about zero current - the standard spec is < 5mA ave over 24 hrs - in the neutral - to avoid earth stake corrosion.

Thanks, by "active front end" i believe you mean the diode bridge(?)
Yes i see the point.....one 10ms sinusoid could be slightly different to the nxt...and overall theres then DC flowing....thats interesting, because that wouldnt saturate a Type A RCD....because go and return currents are at any instant, still the same.

So i am still wondering what scenario could result in the >6mA DC that would saturate ("blind") the Type A RCD?

 

[FvM]

"Active front end" is referring to active rectifier stages (PFC), usually bidirectional.

As already mentioned, DC mains current component has nothing to do with DC residual current.

I have no problems to imagine an OBC ground fault scenario that causes a DC current. You might ask if the risk is high enough to justify the DC sensitive RCD requirement for EV chargers.

 

[cupoftea]

Thanks, its hard to consider a fault that could transfer that extra low voltage DC on to the mains without the mains being connected to the DC output , these things have areas of separation on the circuit board , sometimes with air gaps to prevent this....
.. you would need a very peculiar fault to allow a typical SELV supply to be driving N-E and not go bang! very shortly after.

Also, its beed said that 5mA of DC leakage current in the mains is acceptable.....but EVs have to trip on 6mA.......so the DC leakage detector has to be able to differentiate between 5mA and 6mA of DC leakage current..(so it doesnt nuisance trip on 5mA) ..this would require a really expensive device?

 

[FvM]

Why SELV? The DC sensitive RCD is there to handle possible ground faults of the primary and secondary HV circuit. You are right that a secondary circuit fault would be only seen by the RCD if the OBC isolation is also broken (double fault).

--- Updated Sep 24, 2022 ---

Also, its beed said that 5mA of DC leakage current in the mains is acceptable.....but EVs have to trip on 6mA.......so the DC leakage detector has to be able to differentiate between 5mA and 6mA of DC leakage current..(so it doesnt nuisance trip on 5mA) ..this would require a really expensive device?

No. IEC 62423 specifies a minimal non-operating current of 0.5 In (3 mA).

 

[cupoftea]

No. IEC 62423 specifies a minimal non-operating current of 0.5 In (3 mA).

Yes i agree, you can have down to 3mA and its got no need to trip.
The thing is, when the DC current is 5mA.....you dont want it to trip, as that will be a nuisance trip (and shorten the relay life) ......but how do you make it so sensitive that it trips at 6mA , but not at 5mA?

 

[FvM]

You don't get 5 mA residual DC current without a serious isolation fault, e.g. water intruding the OBC case.

 

[cupoftea]

Thanks yes, and the DC in the mains that you get from eg a "non symetric " rectifier would not saturate ("blind") a Type A RCD, because the go and return currents are at all times the same.....even though there is a DC level in the mains....as the attached LTspice sim shows.

The mind boggles as to how you could get DC in the mains that would saturate a Type A RCD.

 

[cupoftea]

The attached shows how an EV charge scenario could get DC in the mains...but as can be seen , the chances of this are remote in the extreme....the battery has somehow managed to get a leakage path to both primary side of SMPS and also to earth, .....at the same time!!!.........What the _____

I guess when this happens, you also get a total lunar eclipse and a visit from extra terrestrials!

Though i suppose, perhaps an EV could be involved in a multi car pile up......get totally mangled.....then someone might try and charge it up.

Presumably the attached is how the metrel tester injects DC into the mains, for the testing of the "6mA DC " detector?

Metrel EV tester

MI 3155 EurotestXD

MI 3155 EurotestXD is the newest flagship of Metrel’s most advanced line of multi-functional measuring instruments and is designed specifically for testing in industry. What differentiates this instrument from the rest is its ergonomic design and an intuitive user interface, encompassing a...

www.metrel.co.uk

 

[FvM]

A ground fault of the primary DC bus would be the simple case.