Solution:

It is caused by anti voltage. It coccurs when the relay switch a inductive load. And its amplitude(V) is relative to the induction coil(L), current variation(di) and time(dt). Formula V = -Ldi/dt. For example, if the load is a relay-OMRON MY24NJ, its current(i)=36.9mA, inductance(L) =5.72H.operation time(t)=4ms Then, the formula is V=-Ldi/dt and get the anti voltage V= -5.72 * 36.9m/4m = -52.75V. Due to the switch time will be faster than operation time, the actual value measuring by instrument is up to -320V. It will be over reply's specification. To solve this issue, user can add a diode in parallel to discharge this potential.people call it as freewheel diode.