I have 3 active circuits that I need to measure current of, If I use a rotary switch to inerupt circuit one (!), No problem, but when I go to circuiot two (2), how do I return power to circuit one (1)?

 

U have to use separate shunts to all of the circuits and use the DP rotary to switch the ammeter

 

You could use a DPDT relay for each line as shown below. You just add one relay for each line you want to monitor. You energize the relay of the circuit you want to monitor and leave the other relays off.

Be aware though, that if you switch the circuit under load, the load will be momentarily interrupted during the time it takes the relay to switch.

If you need to switch under load and don't want the load interrupted, one way to minimize the transient is to connect a diode across the relay load connection (if it's an AC load you need two diodes back-to-back). That will maintain the current flow at the cost of a momentary voltage blip of about 0.7V across the load connection when the relay switches. This works only if the drop across the ammeter at full load is less than a couple hundred millivolts.

Also make sure you don't have more than one relay energized at the same time, otherwise you will short the two circuits together. A slight delay between turning off one relay and turning on the next is advised since the relay drop-out time is typically longer than the pull-in time (particularly if a diode is used across the relay coil for transient suppression).

Edit: If you use a rotary switch to control the relay coils with non-bridging contacts, then the switching delay between switch contacts to release one relay and energize the next may be sufficient to avoid any problems with two relays being on at the same time.