Siemens 3TH42

Some people spoil all the fun!

Do NOT use it as a 'Go' button!

Why not? If it's used as a control relay, not switching power loads, you can use the black button to:

1. Test or bump the relay.
2. Activate the relay, if it's wired with a momentary contact that parallels one of the relay contacts to create a holding circuit. ("start button")
   

Best to use an insulated screw driver. Push firmly, release quickly.

Interesting.

I have been using those little black buttons as manual overrides all my life and I have yet to ever fry a set of contacts by doing so.

Why would the forces placed on the contacts while doing it manually be any different than those produced by the coil?

Every contactor and relay I have ever taken apart has had small springs or similar devices that hold the contacts together when closed which to me says their force is what the contacts themselves are being subjected to. Not the holding forces of the actual coil.

I Guess you never closed a contactor manually onto a short....

Those springs are there to dampen contact bounce until the armature closes completely. Once compressed by the electromagnet, the connection is virtually solid.

If you have ever seen an electric Fire Pump Controller, there is a manual lever that can be used to close the contactor in case there is a control circuit failure. To make that work, there is a mechanical advantage designed into it, plus a locking knee to maintain that pressure so that the operator can leave. The mechanical advantage designed into that lever must be no less than 3x the amount of pressure that an operator can exert directly (I worked for a time for a Fire Pump Controller mfr.).

All that said, I will concede that if one is talking about a small contactor, i.e. a re-rated control relay, then I'll accept that pushing the plunger likely has no detrimental effect. But the OP did not stipulate the size of his "contactor", so we don't really know.

How big of contactors are you talking about?

The one the OP reference is a good for 10 - 15 or so amps which to me is on the small end of things.

I have a number of good sized #PST units rated into the 200 - 300+ amp range that I have salvaged over the years and they still use the typical spring loaded contact systems.

The OP used the term "contactor", so it's safe to assume he is using it for power.

Yeah? Siemens calls them "contactor relays" because, although they look like contactors, they're really relays.

The OP's model 3TH42 is an 8-pole device. It's available with various combinations of non-replaceable NO and NC contacts, indications that we're dealing with control and not power.

In any case, I qualified my comment accordingly.

It activates the eject seat! Push to test it!