There are RCD units for DC exist. How do they sense the small DC current difference?
There are GFCI (RCD) units for DC. How do they sense the small DC current difference?
I understand how GFCI for AC works. It uses a differential current sensing transformer with two opposing primary windings and a sense winding. The turns ratio would be on the order of 1:1:1000. But a transformer is an AC thing. What’s done for DC?
There’s this board-level module which can detect 6 mA DC currents.
Then this high level white paper mentions GFCI for DC on p.5 . But it sounds more like a neutral-ground fault detection. In addition, the secondary winding on the left of the diagram has a third terminal for earth ground, which confuses me.
It may seem strange to use a current transformer to detect a DC ground fault current. However this is achieved by driving the CT [current transformer] core into and out of saturation at a certain frequency, and using the DC fault current to generate a detectable change in the saturation characteristics. [page 5]
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There are GFCI (RCD) units for DC. How do they sense the small DC current difference?
and
But a transformer is an AC thing. What’s done for DC?
The "board level module" you link requires a power supply (if you read the details) hence, it's not like a simple passive CT that is only capable of measuring AC current quantities. Of course, I can't exactly tell you what it uses but, it seems totally within sensible bounds that it contains a hall-effect sensor to detect the differential magnetic signal from forward and return conductors.
Hall-effect sensors work at DC as well as high frequency AC currents so, this approach seems an eminently suitable technology (and inexpensive these days).
The device in the "high level white paper" could use dual relay coils that are wound anti-phase. The net flux (whether AC or DC) could attract an armature and close/open relay contacts. I'm not saying this is definitely what it does but, given the nature of what that product appears to be it could use a form of latching relay or, like the previous question, use a hall-effect sensor.
But, using a core that saturates seems a reasonable approach also.
In addition, the secondary winding on the left of the diagram has a third terminal for earth ground, which confuses me.
I have no idea what you refer to.
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