Sometimes the included relay is not enough to turn on and off a large load. At this time, you need to additionally use a large extra relay.
To remove spark when extra relay turn on/off, we recommend use Surge suppressor.
Look this power measurement result without Surge suppressor
Surge can see on power line.
If add Surge suppressor like this:
Surge will gone.
if you don't use spark(surge) killer looks like working fine. but huge demage goes to main circuit, finally make a problem.
I saw many user's control panel don't use spark killer, they said “my machine sometime stop or reboot”.
Surge suppressor price is very cheap, and very useful at your circuit.
refer this other company user's manual.