Here you find consumer RCD units trip at 30mA for single or three phase units, but you can get industrial units that are 100mA, 1A or 2A, mostly used for protection of 3 phase heating banks or industrial loads where you can get a high leakage current but want protection against a heater shorting to ground at the low end. They often have a delay built in to reduce nuisance tripping at the set current for a few seconds, but trip fast at high imbalances.
I had some old 1970's vintage units i took apart years ago. Inside was the core with the 2 mains windings on it wound so as to balance the magnetic field to zero in the nice permalloy tape core, and a sense coil of a few dozen turns of fine wire and a test wire of a dozen or so turns of fine wire with a resistor to limit the current through the test switch when pressed. The sensed current went to a small PCB with a bridge rectifier on it, made from 2 1N4001's and a pair of 12V 400mW zener diodes. One tantalum dipped capacitor for smoothing, then a trigger circuit made from a SCS to dump charge into the trip coil, a small coil with an Alnico magnet holding the latch mechanism in the closed position. When the imbalance current was enough the SCS would fire, dumping the stored charge on the capacitor into the coil, and opposing the Alnico magnet so it would be pushed off the coil by a light spring and then via a plunger pin tripping the main mechanism to switch the breaker to off. The newer ones do not have the PCB, the sense coil directly drives the trip coil, driving it into magnetic saturation and releasing the magnet irrespective of the pulse polarity.
On a side note these trip mechanisms were used on the first generation of prepaid electricity meters made by a local company, as they were already approved. These had a flaw in that they covered the existing test switch with a label, and the enterprising users found they could defeat the meter by pushing a pin through the label and then disabling the trip mechanism with it, thus getting free power. The next versions all were split, with the indoor unit only being a keypad that communicated via a 2 wire interface to the power switch outside up the pole, the 2 wires being built into the split concentric supply cable used for them as 2 0.5mm copper cores placed next to the earth strands. A central 7 core 10mm line, 7 individually insulated neutrals making up 10mm and 3 cores making up the earth of 6mm with a blue and white 0.5mm control cores.
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