@Timothy_Rothman I do not see how power diodes would do the same thing.
Check out the power control tab on this schematic and see if that helps with some ideas.
https://www.digikey.com/schemeit/project/k40-s-21-8SM4SO8200E0/
@donkjr here’s the wiring diagram for the Laser On switch using a relay. Everything defaults to “open”. So a failure should stop current flow.
The wiring would be the same for Laser Test. Just with a momentary switch and wired up to LH and G instead.
@Bill_Keeter looks rational but I still do not see an advantage of this approach over the circuit I gave you without relays 
:).
@donkjr total isolation and can use separate power supplies.
@HalfNormal total isolation of the switch from the LPS? How do you see that as important? The inputs to the LPS are optically isolated? :). My circuit can also used separate power supplies all they share is a ground?
How can it be bad? 
A few more parts, yes.
@donkjr for myself, I wanted everything separated out for my own sanity. The relay, while a hassle, made it simple for me to mentally go through the process of testing each mechanism by itself. I made a few mistakes with my wiring last week (unrelated to these switches) and blew a few fuses. So now i’m being more careful.
Please don’t misunderstand. Your wiring diagram was very helpful. As I was thinking about these switches all wrong.
One benefit, for myself, with this new wiring diagram is that I plan to run the door switches, water flow switch and Laser On switch all in series. So now with this relay, I’ll be able to see the red LED “on” light even when the door is open.
@Bill_Keeter Not to confuse you more, but one way to make sure you are absolutely safe with interlocks, is to make the relays self latching and make the power for the relays go through the interlock switches. What this accomplishes is that the relays will drop out when an interlock is activated. Just another idea but not necessary.
http://www.reuk.co.uk/wordpress/electronics/latching-relay-circuit/
@Bill_Keeter no worries, I just ask because I like to learn why one or the other approach is attractive to a maker and check that I didn’t miss or should learn something. There is always multiple ways to solve a problem.
Your point about the led being on when the laser sw is in the enabled position and the interlock open … shouldn’t the enable light go out when the interlock is opened … :).
On mine when an interlock was open and I could not see it, it drove me up the wall. I found a way to put an led in the circuit that was lit anytime the loop was enabled. Its saved me from standing on my head when I thought my LPS or laser was dead.
@HalfNormal lol … does >>> “not bad = good?” :).
I will explain if you send a picture of yourself standing on your head!
@donkjr haha… yeah, true the interlock and the Laser On LED could get confusing in a hurry.
Luckily I have another LED (green) i’m setting up that will tell me when the waterflow & interlock switches are closed.
@Bill_Keeter of course ALL of this could be avoided with DPDT switch/pb and parallel interlock switches. And you get true redundancy for free…
@HalfNormal …ok now what?
@donkjr you have left me speechless!