Why did my oscilloscope hookup trip my RCD?
The oscilloscope's probe ground is connected to the earth (0V). It's likely that the "ground" of the power meter is not really ground. It's likely that the board's "ground" is actually at the neutral or line voltage, so it would create a circuit between neutral and ground or line and ground, which the RCD detected. But, without a board schematic, it's difficult to debug.
To debug the circuit, a differential voltage probe would be best. Otherwise, the probe ground could be connected to the earth prong in the power meter.
GND in the context of an electronic circuit usually reffers to the reference rail against which things in that circuit are measured. It may be floating and isolated, it may be tied to mains earth, or if you are unlucky it may be tied somewhere else. GND on external connectors will nearly always either be floating or tied to mains earth but once you get out your screwdriver and start connecting to ports that were only meant to be used for factory program/test/debug or connecting internal modules then all bets are off.
If you are building a power meter with no external data connections then it's generally easiest to tie your "circuit ground" to mains live. That way you just need a simple capacitor based transformerless power supply for powering the circuit (I expect thats what thosebig caps are for). A series resistor for measuring current and a resistive divider going off to the neutral for measuring voltage.
Even if I did have a data connection I'd be tempted to do it this way and then optoisolate the data connection.
Scopes usually tie the ground of their inputs to mains earth. You can get scopes with floating inputs but they are uncommon and expensive. You can also get isolated probes but again expensive.
Circuit ground tied to mains live, scope ground to mains earth, the result is a short circuit from mains earth to mains live. BANG.
I expect the way this device was debugged during developement was to feed it from a floating output isolating transformer. Once that was done the circuit ground could be connected to the scope without causing a short circuit.