Dear Mr. waveboy (Electrical)
" ...We wish to make an RCD to detect 30mA of imbalance current between line and neutral, and also detect 6mA (and more) of DC leakage current...."
Che. 1. When you mention 30mA of imbalance current between line and neutral, we take it as an AC 50/60Hz system. .... Then, you want also to detect 6mA (and more) of DC leakage current...
2. I am confused. Do you mean to detect/trip at 30mAac and also >6mAdc by the same RCD?
3. There are RCDs designed for different loads/with different tripping characteristics on the market; but don't seems to fulfill your requirements.
4. Please advise what is your equipment that is having AC and DC in current in it, that need AC and DC protection by a single? RCD .
Che Kuan Yau (Singapore)

Quote (che12345)

. I am confused. Do you mean to detect/trip at 30mAac and also >6mAdc by the same RCD?

Thanks, Yes
Its for a 32Arms mains distribution box.
As you know, our application calls for a fluxgate current sensor....but please give guidance so we can do it without feedback, and without bothering with a "square law" core?

...so...

We are going into business making RCDs. We cannot undertsand why this massive gap in the market has been missed? All existing offtheshelf RCDs are >£30 and massively over-engineered!!!.

Our method is a simple version of a "Fluxgate current sensor".

That is, we use the fluxgate principle, but dont bother with the feedback, and dont bother using a "square law" core Torroid (we just use a standard , cheap as chips, AC CT torroid)

Its for 32Arms mains distribution boxes, that need an RCD.
They must detect >30mA of imbalance current (between line and neutral) , and 6mA or more of DC "leakage" current.

....seriously ....all you have to do is you get your mains line and neutral running through the torroid aperture.......then you simply shove a square wave voltage into the sense coil (the sense coil "secondary" that's wrapped round the torroid).......then the voltage that gets looked at, is simply the voltage across a low value resistor in series with the sense coil.....(its put in series with the coil "gnd" connection and ground)........and you then see either a symetrical waveform if no DC and no imbalance...or otherwise its non symetircal..........in fact, all you have to do is buffer it, filter it, add a small offset voltage so its positive all the time......then read it into an ADC....and its job done. Why is not everybody doing it like this?.....no feedback is needed.
No "square law" core is needed.

So why is nobody doing this?
Why is everyone paying >£30 for an offtheshelf RCD? (i am speaking about RCDs that dont contain the contactor, but just give the signal)

..the way described is inaccurate a bit, but you easily calibrate that out in production....just measure the coil core characteristics of the parts before building them up....

(this is for a sensor which will detect 30mA imbalance current in 32Arms wires...and also detect 6mA DC and more.)

Dear Mr. waveboy
"... Yes. Its for a 32Arms mains distribution box. ..."
Che. 1. I am still confused. In which IEC, NEC, ANSI or state/regional standard called for 30mAac AND >6mAdc; for life protection. Recommended to be installed in a AC distribution box.
2. There is " Rogowski coil" principle that is capable of measuring AC
AND DC currents; but with very low voltage output.
Che Kuan Yau (Singapore)