Try to keep actions in proximity whenever possible.
In your mock, if you go with a single button (or even a second button below), users have to perform several steps:
Select a row
Move the mouse down to the now changing button ('update'). The more workstations added, the further this button change appears from the selected row. They also have to notice this ...
If you really can only use 4 words and you have to convey the whole meaning, all I can think of is something old school like:
VALVE WORKING WHEN LIT
I think 'working' could be replaced with something that signifies 'working correctly' but you'd loose the link to the actual valve. I thought 'serviceable' but I think that word is quite ambiguous (people may ...
I'd chose to highlight the danger or error condition instead of when things are working correctly.
Label for normal condition: OK
Label for error condition : !!FAILURE!!
In monospace fonts and with different text alignment:
Both labels here are short, one word in each.
Capital letters are easier ...
I'd have a gauge ( even just one with a series of leds which light up).
You want to show an actual measure of something.
Not an indication that something has been told to work, but a measure that it is actually working.
How you design this does also depend on the size of the bang if this valve fails. ( cf the Three Mile Island nuclear disaster).
IN RESPONSE ...
I agree with @locationunknown that you may find your question has already been answered, however, I think there's another problem with this design.
According to Apple's Human Interface Guidelines on switches, this element is supposed to be used in a different way.
A switch is a visual toggle between two mutually exclusive states — on and off.
Just use a single, stateless button
Normally, it's important to differentiate the difference between the current state, and what it will be after clicking. However, with a camera, that's very obvious—can the user see themselves? If so, it's the front camera. If not, it's the back. This will also provide familiarity to the user, as this pattern is fairly ...
In your specific example it is best to show the current status, because the style of the control already looks like a status indicator. It isn't clear that the text represent the action of pressing the control.
If you are concerned about general understanding of what the text represents then you would probably be better to move away from a toggle control, ...