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zero cross switch is used to protect both device and sinusoidal of mains current when you are switching significant load, e.g. heaters, motors, lamps, switched power supplyes...
Imagime connecting heater into the middle of sinusoid of 230AC.Resistance of heater raise with temperature so at first moment when temp is still low, you have very high current shock which deform sinusoid of AC transferrin many harmonics with EMC into mains and this shock is also lowering the lifetime of contacts of relay for example, cables and connectors and heating spiral too.
ZC will in this case wait for sinusoid to cross 0V and when voltage is lowest possible, attach the load so the current shock will not appear. This is often used in SSR with combination of triacs or double thyristors.
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