3 way switch -common connection ?

Hi - I am replacing a 3 way switch with an electronic switch with a motion detector. The wiring instructions show that you leave the other mechanical switch in place and make a few modifications to its wiring. All of the wring makes sense to me except for one small part. The thing that seems odd is that the remaining mechanical switch has a wire going from the common to what seems to be a completely unused non-common terminal. Wiring the common to a non-common terminal just seems redundant. Can you think of a reason? I know electrical codes can be weird sometimes, so maybe there is a code where you can't leave terminals unused?

Here is the diagram:

 

Your only using the mechanical switch to send a signal to the electronic switch. The signal is the momentary interruption of power when the mechanical switch is switched. That signal tells the electronic switch to change states, either from closed to open or open to close. Or that's my guess and I'm sticking to it til Pete corrects me.

 

Sounds like a valid reason Ray.

Usually only one traveler is used from the existing three way switch but things are always changing. Without the model number of the switch.... can't be of much more help.

 

I would imagine that job would be done by the wire marked "First Traveler" which can be switched. The wire marked "Second Traveler" is connected to constant 120V and cannot be switched (it appears as though that line is just supplying constant power to the electronic switch). So I'm not understanding why that constant 120V would be wired to one of the outputs of the first switch (I can't see how flicking the switch would affect that output.. it would always be a constant 120V).

The switch is a Leviton IPS06.

 

I was looking further into that control. That one does use strange wiring when compared to the single pole connections.

The sensor is always powered thru the red wire that comes from the standard 3w switch.
In one direction there is no power on the 3w/brass connection and in the other position there is power on the 3w/brass connection.

 

Yes all of that looks normal to me, no problem there. I am just curious about that one extra connection going to that unused output of the mechanical 3 way switch.

It's not a big deal, I can either connect it or leave it disconnected (it is the same either way). Just curious.

 

Yes..... you probably don't need to bridge common to one traveler.

All the occupancy sensor is looking for from that switch is power on or power off to the 3w/brass terminal. Technically you could change the three way switch to a standard switch and it would still work.

 

Thanks, makes sense.

It saves me from a big ugly 4 wire marette connector and an extra pigtail (hard-nuff to fit things into the box as it is).

I just mocked-it-up on the bench and (as expected) it works the same either way.

The only thing I can figure is that they just don't want homeowners getting freaked-out as to why there is an empty terminal, so they just connect it "something".