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

 

To the TS:
All of the Worldwide electrical codes I've browsed require, without exception, that always all of the live phases MUST BE SIMULTANEOUSLY disconnected. Full stop.
Some situations may even require that the neutral to be disconnected.
The only thing that can remain connected is the green grounding wire.

 

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

The answer is clear and obvious to many of us!! The 240 volts system and nomenclature relate to SINGLE PHASE/SPLIT PHASE systems, which have been common for many years!!
The 208 volts/120 volts nomenclature and arrangement are found with THREE PHASE systems ONLY!! With three phase power the math is quite different!! Suddenly VECTOR MATH is required.
Instead of the phases being 180 degrees apart they are now only 120 degrees apart! So the old "center tapped mains" does not work that way any more, IN SYSTEMS WITH THREE-PHASE DISTRIBUTION.
IT IS MORE EFFICIENT because the same systems can provide a lot more power with only one more wire.
What gets confusing sometimes is that 440 is now closer to 480 between phases and 277 between each phase and neutral. AND, acording to those who have experienced it, "277 bites a lot worse than 220".

 

The answer is clear and obvious to many of us!! The 240 volts system and nomenclature relate to SINGLE PHASE/SPLIT PHASE systems, which have been common for many years!!
The 208 volts/120 volts nomenclature and arrangement are found with THREE PHASE systems ONLY!! With three phase power the math is quite different!! Suddenly VECTOR MATH is required.
Instead of the phases being 180 degrees apart they are now only 120 degrees apart! So the old "center tapped mains" does not work that way any more, IN SYSTEMS WITH THREE-PHASE DISTRIBUTION.
IT IS MORE EFFICIENT because the same systems can provide a lot more power with only one more wire.
What gets confusing sometimes is that 440 is now closer to 480 between phases and 277 between each phase and neutral. AND, acording to those who have experienced it, "277 bites a lot worse than 220".

I am well aware of the difference!!!?

I had issue, or rather question I had where was the OP obtaing those figures if derived from a N.A. Domestic supply?

Re OP:
USA 240V three wire L1, L2 and a Ground

 

I am well aware of the difference!!!?

I had issue, or rather question I had where was the OP obtaing those figures if derived from a N.A. Domestic supply?

Re OP:
USA 240V three wire L1, L2 and a Ground

Those figures were correct back in 1954, and as recently as 1965, IN SOME RESIDENTIAL AREAS that had not yet been updated.

 

Those figures were correct back in 1954, and as recently as 1965, IN SOME RESIDENTIAL AREAS that had not yet been updated.

The discrepancy is between the title of the thread, which states 240 V three wire (i.e., split phase), and the actual content of the first post, which only mentions 208 V three phase.

 

same systems can provide a lot more power with only one more wire.

It provides 1.5 times more power for the same mass of copper wire.

 

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.

It actually is, or was, common practice, within approved envelopes. Heating circuits come to mind, which I've come across many with single pole switching. Outside of those envelopes, codes require all poles be switched, as mentioned.

 

I think that the question in post #11 has been asked already. The short answer is YES, it has been done, the longer answer is that it CAN WORK BUT IT CREATES A SAFETY HAZARD that is hidden.
Consider a 208 volt heater circuit: openiing either side of the circuit stops the heating current completely. BUT ALL of the heater circuit is still live at the full supply voltage. The heater is ot heating but it is still electrically live, just like when it IS heating. To be safe, the control must break all of the hot feeds.

 

I specified and used 208V 3ph panels because we co-generated 3.5MW 19200V 3 phase that was synced with Georgia Powers grid. Even with our capacitor banks GA Pwr. was not happy with us due to unbalanced phases. Almost all of our motors were 3 phase 480V with a few 9600V both delta and wye. But all of the older shops power, lighting, and area heating/cooling were single phase and keeping the phases balanced was a nightmare.

 

The question no one has asked is what is the application/purpose?
If the purpose is to control a load (heater, motor, light) then switching just one of the active legs is OK.
If the purpose is to provide some sort of isolation to the load, then DONT use a SSR for that prupose. They leak current and any part of the circuit has to be considered live. If its for isolation, then the only option is a two pole isolator on the active lines.