failure design

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jward
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failure design

Post by jward » Sun Jan 22, 2017 3:08 pm

What are standard failure practices and designs?

For instance, my desire is to keep relevant equipment turned off if a float switch indicates an error condition. I was thinking about how the switch should be wired. It seems one might set the din to normally closed so that if the sensor/switch wasn't present it would be an error, but this would be susceptible to a short creating the closed state too. I guess one would have to look at failure mode for the switches too. What is best practice?

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JonW
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Re: failure design

Post by JonW » Sun Jan 22, 2017 6:13 pm

NO vs NC? There's really no best practice to this. The best choice is always going to depend on a combination of what your switches support and what is the overall logic of the processes/ladder logic. No one answer.

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Re: failure design

Post by jward » Sun Jan 22, 2017 6:46 pm

normally open (NO) or normally closed (NC)

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Re: failure design

Post by JonW » Sun Jan 22, 2017 7:02 pm

jward wrote:normally open (NO) or normally closed (NC)
"NO vs NC?" was meant as a rhetorical question.

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Re: failure design

Post by bzomerlei » Mon Jan 23, 2017 11:47 am

Not sure if this exactly applies to your question, because I think you might be referring to using input from the float switch into the BCS.

I wired my float switch so that when the water in the hlt drops below a certain level, it breaks the circuit for the low voltage control on my SSR. This way I'm bypassing anything happening in the BCS. The element will not fire without the right amount of water, even if I made a mistake in programming the BCS, or if the BCS gets hung for some reason.

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Re: failure design

Post by jward » Mon Jan 23, 2017 5:45 pm

Interrupting the relay input is a good idea for safety. For my use I have 2-3 relays to interrupt. I'd need a triple pole triple throw switch.

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Re: failure design

Post by oakbarn » Mon Jan 23, 2017 9:13 pm

jward wrote:Interrupting the relay input is a good idea for safety. For my use I have 2-3 relays to interrupt. I'd need a triple pole triple throw switch.

Think Relay Board if making one float a "safety" for more than a single SSR. I do use a Float Switch as an Exit Condition for a Water Valve. In that Case, I have the DIN "On" and the Valve State Exits when the DIN is OFF. That way I know that the float Wiring is good before I open the Valve. In my case, I am using the UI to Assert the State.

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Re: failure design

Post by jward » Tue Jan 24, 2017 8:35 am

I thin I am sort of doing that now. Currently, I have the glycol process as the highest process. The entry state controls the temp. The exit condition for that state is the float switch. The next state turns off all of the protected devices.

Speaking of failures.... My AC unit is no longer cooling. The compressor doesn't seem to turn on.

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