What could be a reason for RCCB triggering?
An RCD (in any form) is always looking at the current going out vs the current coming back, which SHOULD be zero difference, so if they are different by the amount of your RCD (usually 10 to 30mA in your part of the world), the device trips. It might be that your type of meter is one that measures by looking at the potential difference by putting on a small load, so the act of trying to read voltages was a condition that the RCD interpreted as a short.