For reference, the input terminals on the OMTech anonymous 60 W HV laser power supply:
AFAICT, that’s the default layout for all similar power supplies.
The H and L pins are the High- and Low-active enable inputs that, when it’s working right, control the laser output. The KT332 controller (and, most likely, all RuiDa controllers) produce a low-active output, so you just wire the controller’s output to the L input and you’re done.
That was the original failure that got me to this point: the power supply ignored its L input and turned the beam on at whatever power the PWM signal on the IN terminal called for. Having that happen was surprising, having it happen with the cabinet lid open was … disturbing.
The P input is intended for the Water Protect signal from the flow sensor on the laser cooling plumbing. When the water is flowing, the IN terminal will be low and the power supply will pay attention to the L input.
The power supply arrived with a jumper between the P input and the G ground / common terminal:
The jumper holds the P input low = active, meaning the power supply thinks the water is always flowing.
It turns out that the Water Protect signal goes only to the controller. When it’s inactive = no water flowing, the controller will refuse to fire the laser and also sound an alarm. Running the signal directly to the power supply would result in a puzzling failure-to-fire with no diagnostic from the controller.
I removed that jumper and added a (green) wire from the Lid Interlock signal at the controller:
To the power supply’s P input:
In principle, if this power supply fails the same way as the previous one (with its L input always active), then at least it won’t fire with the lid up.
Believing that may display a childish naivety, but at least the thing seems marginally safer than it was before.
The Smell of Molten Projects in the Morning 
The fda advocates passing the power supply to the power supply through a shunt in the lid. I’m paranoid enough to think that’s a damned fine idea … i’d never believe one logic level input to chinese garbage is more reliable than the other.
Aye!
I considered putting an SSR in series with the existing AC wiring to turn on the HV supply only when the lid is closed and everything else thinks the supply should be on. I’m not sure why I trust the lid interlock sensor more than I trust the HV supply’s guts, but I think requiring a double enable to fire the laser should suffice.
Trust, but twist the key switch while fiddling inside …