The maker of the program "Estlcam" mentioned that in a german forum:
https://cncwerk.de/forum/viewtopic.php?t=611

Here is the same info:
http://cnc.a-ueberbach.de/spindle/al...schalter_fircd

Translated with Google:
Residual current circuit breaker (FI / RCD)

In the case of a fault, DC residual currents with low residual ripple or high-frequency pulsating fault currents can occur in frequency inverters, which do not recognize the residual current circuit breakers normally installed in a domestic installation (FI type A). Likewise, the possible DC components in the fault current can lead to a saturation of the coil installed in the FI and render it significantly less sensitive to alternating currents.
According to VDE 0100-530, all-current-sensitive RCCBs (also known as RCD type B) must be installed here, but this entails costs of a few hundred euros.