Fred Bloggs wrote:



Switch closure needs to be monitored. On one side of the switch is
mains voltage (120vac); on the other, the load.

Simple enough to monitor the switch's status: when mains voltage
appears on the load side, it's closed.

But how to monitor this switch during a power failure? I'd like to be
able to report to an alarm sensor's input (simple current loop) when
this switch closes, whether mains power is applied to the switch, or
power has failed.


Simple enough to do in practice- but the environment can make things
"sticky"- be sure your sensor input is isolated or else fuse it:

View in a fixed-width font such as Courier.




+-------------------------------+
| |
+----DPDT---+ +-----------------+ |
| | | | | |
|NC| | | | |
| o COM | | level sw | |
| \ o-----|--------o o------+ | |
LINE-+-------|--o | | \ | | |
| |NO | | ^ | | |
| | | | | | | |
| | | | --- | | |
| | +--------|--+ | | | | |
| | | | | | | | |
| |NC| | - - | | |
| | o COM | | | |
| | \ o-----|-----------------+ | |
| +-|--o | | |
| | |NO | | |
| | | | | |
| +-|-----------|-----------------+ | |
| | | | | |
| | ====== | | | |
+-------|---UUUUUU--|--+ | | |
+-----------+ | | | |
+----------------------+ | | |
| | | |
| +----+ | | |
NEUT-+------------+LOAD+-------------------+ | |
| +----+ | | |
| | | |
| | | |
| | | |
| lvl sw monitor relay | | |
| +--------+ | | |
| | ====== | | | |
+----------|-UUUUUU-|-----------------+ | |
| | | |
| / | | |
+--|--o o---|--------------------+-------
| | NO | |
| +--------+ | to sensor
| |
+----------------------------------+-----



You can do a better job of preserving the level sw contacts like so:
View in a fixed-width font such as Courier.