Across the L1 and L2 is 208V
Across the Ground to each Line is 120V
Feed from 3 Phase 208V Panel
Is turning 208VAC on and off with only controlling one line with a solid state relay, actuated by 24Vdc, a common practice or should both lines be turned on and off at the same time at all times to control 208VAC? If I am understanding correctly there would still be an active 120V AC looking to ground out. Although it would work it is NOT the correct way to turn on or off a device running on 208 three phase.

Thanks for the help in advance.

 

I am understanding correctly there would still be an active 120V AC looking to ground out.

Yes.
For safety, you need to switch both lines.

 

Unfortunately, it is a fairly common practice to only switch one side of the 208 volt line feeding a single phase motor.

I CONSIDER THAT A VERY POOR PRACTICE at all times and in all applications!!
THE ONLY reason it is done is for cost reduction. There is no other benefit for normal applications, such as motor control.
The obvious big DISADVANTAGE is that, for motor control applications, it leaves the entire motor windings subject to damage from transient line voltage spikes, such as nearby lightning strikes.

I have seen that sort of danage first hand, in a A.C. compressors with 3-phase motors, with only two of three phases opened. ALL of the 3-phase motors damaged by arc-over from the windings to the frames.
So while breaking only a single pole for a 240 volt single phase application can function satisfactorily, it is a POOR CHOICE!! The ONLY motivation is initial cost reduction.

 

Across the L1 and L2 is 208V
Across the Ground to each Line is 120V
Feed from 3 Phase 208V Panel
Is turning 208VAC on and off with only controlling one line with a solid state relay, actuated by 24Vdc, a common practice or should both lines be turned on and off at the same time at all times to control 208VAC? If I am understanding correctly there would still be an active 120V AC looking to ground out. Although it would work it is NOT the correct way to turn on or off a device running on 208 three phase.

Something does not sound right for a N.A, 120v supply, how do you get that from 208 v ?
Typically N.A, does not use 3ph to produce the typical 240v centre tapped 120-0v-120 supply