Normally Closed Application

I have an old wired home alarm system that has one of the window sensors wiring damaged and its inaccessible.

The old sensor would be closed when window is closed and open when window is opened.

I want to replace this with a Shelly Window Sensor and Shelly Relay - the sensor will trigger the Relay condition where the relay's switched contacts are connected to the alarm control board in place of the original wired sensor.

The problem is that the relay must be closed when the window is closed and open when the Sensor Triggers it.

I recognize I can set it up like that, but what it means is that the relay will be virtually constantly enabled and only turned off when the window opens.

A Normally-closed relay would be just perfect (I have submitted product suggestion for that - I'm sure there are lots of other applications for this)

In the meantime, what is concern over lifetime reliability, if the Shelly 1 is set up to be in that predominantly enabled state?

Is there anything else I'm missing to achieve what I want to do? I suppose a second (non smart) relay driven by the Shelly could be employed to provide the correct logic but it's a bit messy. i.e. the Shelly 1 output is only when the sensor is triggered open and the closed switch

ASi-Master - I recognize that a simple wired sensor replacement would be ideal - the problem is accessing the original wire, which means breaking into the drywall and associated refinishing (I did mention in my opening post that it was inaccessible) and that is why I am looking at a wireless solution.

It's not the wireless sensor that is the issue in this case - it's the relay. (which is in effect the receiver/actuator/slave to the Alarm Panel) - the sensor is somewhat moot - whatever is used it simply has to direct a 'local' device at the Alarm Panel to replicate what the Window status is.

For further info, the old panel itself is actually being replaced by a Konnected board https://konnected.io/ - it's a wired-system replacement so doesn't take wireless inputs directly hence the requirement for a slave switch driven by the wireless sensor.

There will be battery back-ups on my Router and the Konnected Panel (and it will supply whatever relay I use). But whether Konnected or the Original system, the solution really is the same either way. With the Konnected I'm fine with the Failsafe of this scheme - the sensor is battery operated; the panel and relay and router are all with battery-back-up. No different than the old wired panel which is also AC with a battery back-up.

Thank you gevauem - that is at least somewhat re-assuring. There really shouldn't be a problem with the coil failing and since the voltage/current is minimal don't really have to be concerned with the contacts either. Realistically, a relay that is constantly on is probably more reliable than one that is switched frequently! Maybe in the next 5 years Shelly will have a NC option - or better, a Solid State switch

I really like the Shelly products and it's attractive/convenient keeping sensors & slave in the 'family' especially as already have the app employing other devices,

If I'm set on the Shelly for the 'control' side the other option would be to add ANOTHER (non-smart) relay being driven by the Shelly

That Truth table for that would be

Sensor Closed - Shelly OFF - Second Relay OFF (using N/C contact)

Sensor Open - Shelly ON - Second Relay ON (switches to unconnected NO contact)

But it may just be simpler to stick to using the Shelly in constant 'on' mode, otherwise use a Sonoff device instead which does have an NC contact rather than adding that secondary relay.

Just writing this out, helps me process my thoughts and decision