Electronics > Projects, Designs, and Technical Stuff
How do I rate a CT for leakage detection use? Also a DC leak detector coil?
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741:
Many thanks indeed for this detailed information.

When you say
--- Quote ---I would challenge the assumption that you need to measure AC, what are you measuring AC for? The standard specifies that you cannot trip on AC fault, that is the RCCB's job upstream, not yours.
--- End quote ---
but also that
--- Quote ---Expect to spend >$20 per sensor, if you're lucky it will be able to do both the 20mA AC and 6mA DC.

--- End quote ---
that does confuse me a bit. My understanding now is I consciously ignore AC leakage detection as a design goal.

Re:
--- Quote ---There are plenty of off-the-shelf sensors
--- End quote ---
is that the kind of thing Farnell, Mouser etc stock - and what is the corrent search term?
NiHaoMike:
How about a large pair of inverse parallel diodes in the ground line with a 10 ohm or so resistor across them as a shunt? A small fault current will create a small voltage drop across the resistor while a large fault current will be shunted by the diodes. It would be very unusual to have a ground fault in an EV that doesn't also fault to the chassis. A separate ground connection can be used to inject a test current to verify the ground fault sensing works and that the ground connection is good. An AC GFCI/RCD would still be present as a backup.
uer166:

--- Quote from: NiHaoMike on February 13, 2020, 01:50:06 pm ---How about a large pair of inverse parallel diodes in the ground line

--- End quote ---

A residual current fault doesn't generate any current in the device's ground, this won't pick up anything when someone's getting shocked. Only way is to measure current imbalance in the live/neutral, or the phase1/phase2/phase3/neutral lines to pick it out.

To OP: check relevant standard that you're certifying your EVSE to. In north america, we need 20mA AC detection, in EU, 6mA DC detection. A device that does both can be useful if you're selling it in both regions (edit: what I mean is it can have separate GPIO outputs, one for 6mA DC trip, another for 20mA AC, and you use them selectively). In EU you most likely will only need a 6mA detector. Digikey etc won't have these since it's pretty specialized, you'll have to talk to the manufacturers directly, here's the most popular one I've seen in field:
https://www.bender-uk.com/company/news-item/6ma-dc-sensitive-residual-current-monitoring-for-ac-charge-stations

Based on your questions I wouldn't attempt making one yourself yet, but instead integrate an off-the-shelf sensor.
duak:
Uer166 pointed out some things that I should have been more clear about.  LEM makes a number of current sensors using different principles of operation.  One is the high precision FG type.  Another is a Hall sensor type where an amplifier and compensation winding null out the flux from the sense winding.  It was these I was thinking about in my reply. 

I have experience with the LEM non FG type from the 80s where I used them in a BLDC motor servo drive.  They had sufficient precision and stability to allow the output current to be set to within +/- 10 mA out of 13 A ie. 0.08%, and usually much better than that.  Field service would occasionally recalibrate a servo drive and find that the zero setting was off by 30 to 50 mA but full scale was fine and that adjusting electrical zero restored the spec.  Subsequent tests showed that if the line power to the servo drive was cut while the motor was turning fast, the current  from the motor backfeeding the drive could leave a slight remanent flux in the current sensor core that manifested as an offset.  If the power to the LEM sensors could have been maintained while the motor spun down, their self nulling would have prevented them from being magnetized.  During power loss the drive was designed to brake the servo motor by rectifying the back EMF from the motor back to the DC link while aggressively discharging the DC link with the braking resistor.  In this mode, an asymmetrical pulsed current was applied to the sensors rather than a symmetrical decaying sine wave.

As uer166 reminds me, an FG sensor drives the core into saturation all the time, so the above effect should not occur.  I have a Tek P6042 current probe that can resolve mA.  It is not an FG design and it has a degauss switch.  It can pick up a DC offset just by opening and closing the probe jaws.   I looked at the hp 428B current probe manual and although it is an FG design, it too has a degausser and hp recomends using it after measuring Ampere level signals.  I'm bringing up the possible problem of a remnant field because if the sensor isn't designed correctly, it could hide a DC leakage current that could be a hazard.  Just sayin'.

Uer166 is absolutely right in saying that the flux is never zero except when changing polariy.  I should have said that the average flux over the excitation cycle is zero and is a part of how the LEM sensor works - look at the last line of the 2nd paragraph on p.5 of the LEM document I linked to above.  Other designs may not do this.
NiHaoMike:

--- Quote from: uer166 on February 13, 2020, 10:05:36 pm ---A residual current fault doesn't generate any current in the device's ground, this won't pick up anything when someone's getting shocked. Only way is to measure current imbalance in the live/neutral, or the phase1/phase2/phase3/neutral lines to pick it out.

--- End quote ---
I'm having a hard time thinking of a realistic fault with an EV that would cause a ground fault not involving the chassis. All the high voltage wiring is well protected for a good reason.
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