A breakdown of saucer separation mentions in TOS
There were only two, and in both cases it almost sounds like Kirk is referring to ditching just the nacelles, not the entire secondary hull.
I still find it interesting that nobody can quite agree on where the hull separation (if any) exists. I prefer the TNG-style "cobra head" left with the secondary hull to the version where part of the neck stays with the saucer.
"The Apple"-Vaal has rendered the antimatter inert. Kirk seems to think that the Enterprise should ditch the nacelles and escape with impulse engines. Cue obligatory "space is big, really big" quote. I suppose the idea is that Vaal can only affect things in orbit and the Enterprise just needs to escape the operational range to get a message out asking for a rescue (I'd imagine once you leave orbit you can just let the planet move away from you). But in that case you wouldn't want to ditch the warp drive if it can be refueled.
"The Savage Curtain"-Yarnek is affecting the matter/antimatter seal. Kirk orders the nacelles to be jettisoned. This situation is more immediate, warp core breaches are nothing to sneeze at. This seems more like a traditional saucer separation is warranted.
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