Leackage breaker trips when probe is connected to main line

Description

We want to measure the voltage of the main line with our oscilloscope. When we connect the ground lead of the probe to the neutral lead the earth leackage circuit breaker is tripping. What is the reason?

Resolution

The ground of every channel of the oscilloscope is connected to the protective earth conductor. Then the neutral lead and the protective earth conductor are connected.
Circuit breakers of the newest standard detect that and activate the trip.

To measure the mains voltage please use an oscilloscope with isolated channels i.e. the Scope Rider RTH.
The RTH fulfills the CAT ratings up to IV.