Help needed! Z changes after touching the metal, to a value above the limits.

Hello, I have built a new plasma cutting table. I am working with Linuxccnc 2.9, Qtplasmac, and an LPT board. A problem has arisen: the torch touches the metal on the floating platform, the sensor is triggered, the Z value on the DRO suddenly changes by 100+, the machine goes into an emergency due to exceeding the Z‑axis limit, then the Z‑axis moves away and the readings return to normal. Please help me solve this problem.