Surge suppressor, 440 Volt. The Calmotion version correct a design error in the Fadal device. The Fadal version paralleled the varistors and rated them as if they were resistors. This is discouraged by the manufacturer. 
 
1. They don’t share current evenly
MOVs have wide tolerances in their clamping voltage. Even devices from the same batch can differ noticeably. The one with the lowest clamping voltage turns on first and takes most of the surge current.
2. Thermal runaway risk
As that “first” MOV conducts more current, it heats up. MOV characteristics are temperature-dependent—heating lowers its effective clamping voltage further, so it hogs even more current. That feedback loop can push it into failure while the others do very little.
3. Aging makes imbalance worse
MOVs degrade with each surge event. The one that has taken more hits will age faster, lowering its clamping voltage over time. That makes it even more likely to dominate future surges, accelerating failure.
4. No inherent current-balancing mechanism
Unlike resistors or some active devices, MOVs don’t have a built-in way to equalize current. Without added series impedance (like resistors or inductance), parallel operation is unstable.
5. Failure modes aren’t cooperative
When one MOV fails (often short or leaky), it can dump continuous current or overstress the remaining devices, leading to cascading failure.