On the question of safety, there is no difference. Both proportional and ON/OFF can fail.

Rule of thumb for controller choice:
If you are using an ON/OFF controller on a setup, and the controller relay is clicking on and off more than once every 30 seconds, then a proportional controller is a better choice for your setup in terms of relay wear. Note that proportionals cannot be used for fans or heaters with moving parts, like space heaters.

When proportionals fail, they often fail ON. The most common cause - voltage spike or inherently defective output power device (triac, SCR). A surge protector is not a bad idea.

For safety, an ON/OFF in series with a proportional's output is the real way to have a safe system, as described by the other poster in this thread.
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Mark

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