varistor calculations needed

I think you're chasing ghosts with regard to the speed of the relay(s) dropping out. But in any event, simplify the issues. As I understand it: You have an MCU driving a small relay coil through a bipolar. (Based on your figures, 5V 120mw that's 60 mA, so use a transistor between the mcu and the relay coil.) The small relay switches a big relay.

Draw us a circuit - something like this:

Big relay V+ supply V+ --+------------+ | +----+ | | | | | | [Rly1] ---o [Motor] | | | ^--o | +--- AC --- /c | ---o +--- Mains |MCU|---[R]---| [Rly2] ^--o | --- \\e | | | | | | +----+ Gnd -+------------+ Big relay Supply Ground

You have mentioned 4 issues (I think - there may be more):

1) Protect the MCU from too much current draw 2) Protect the NPN from spikes 3) Protect the contacts of Rly1 4) Protect the contacts of Rly2

For 1, the NPN is the solution For 2, a diode across the Rly1 coil is the solution For 3, a diode across the Rly2 coil is the solution assuming the supply to Rly2 is DC

The above assumes that drop out delay is not an issue.

For 4, you may need a different relay.

There is an inconsistency in what you have posted. You said > The motor operates every minute one time > and the relay 50 times per day.

There are 1440 minutes in a day, so something isn't right, unless you mean that the motor operates for a duration of 1 minute, 50 times a day. Or ...?

So please give us a fighting chance. Create (or modify) a diagram that shows your circuit, and give us links to the datasheets for each relay.

nding

small

This Ross paragraph is quite accurate. Using relays simply creates more problems (ie massive noise transients from the electric motor), less reliable operation, and noise problems for the microprocessor that require additional transient protection.

For example, a simple MOSFET transistor between the uprocessor and relay already has a relay suppression diode inside the package. And since the motor is so trivial, then a zero crossing triac means more reliable and easier to implement design.

Ross also suggests using an optocoupler. Also a good suggestion that increases galvanic isolation (but increases costs). Using relays in this situation is often considered obsolete design. Relays typically are not used until 120 / 240 volt switching currents are tens of amps.