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Connect the oscilloscope probe to the relay wiper and ground the other plate of the contact. Without an RC network across the contacts, check the amplitude of the transient voltage on contact break and the amplitude of the current on contact make. If the voltage is less than 300V and the current less than the maximum switching current rating of the relay, and if you don’t see any arcing, you may not need the contact protection at all. If you spot arcing, connect a .1 mF + 100 ohm, 250 VAC, Quencharc across the contacts, and observe the levels of suppression, voltage on break and current on make. The suppressed voltage should be below 250V, which provides 70 volts of safety margin from the breakdown potential of air. If the voltage is still above 250V, try a .25 mF + 220 ohms or a .5 mF + 330 ohms range. If you need a higher capacitance than 1.0 mF, you may be better off with a Zener or a varistor in terms of cost and space. For most relays and triacs .1 mF + 100 ohms provides a satisfactory suppression.
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