During cutting or at idle?
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If during cutting, check grounding, if it is shallow under ground, the ground getting dry will cause that.
If at idle, there are a lot of things that can cause that, like other machines in the shop, changing the power socket, stepping on the wiring, wiring isolation failing somewhere due to fatigue and/or strain, and if in Europe, even your neighbor installing a new air condition unit, etc etc.
A simple solution to remedy the latter, use a 0.01 to 0.022 uF (also known as 100-220nF) capacitor on the THCAD input.
I use a 0.1uF electrolytic one rated for 50V but that might lower the reading speed, although for me it works perfectly fast till 6m/m, i have not tested it faster.
