Monitored System Vs Non-Monitored System

[bot1357]

Hi there.

I am after some advice regarding a project I am working on at the moment.

My company is supplying a PLC based system on a ship which is going to be controlling hundreds of valves and reading back their status. These valves will be controlling numerous fire safety systems across the vessel. As well as these valves being operated manually we also have an electrical local release panel. These local release panels are wired to the PLC. On the local release panels we have a number of pushbuttons and key switches. So if a person approaches a compartment on the vessel sees there is a fire inside this compartment they can then enable the local release panel (which is mounted outside for the protected compartment) by turning the key switch and then press the push button which operates the system, the logic in the PLC will operate the associated vales and extinguish the fire?

My question is;

Do these local release panels need to be monitored or un-monitored?

Now a non-monitored input is just a straight digital input (bog standard switch) it’s either on or off? A monitored digital input needs to put through an analogue input module which can detect open circuit and short circuit. So if either the key switch or the pushbutton was in fault we would know because the reading we get back will either be high or low. We therefore know that when the local release panel is required it will be in good working order. If it were wired to a straight digital input we would only know if it was faulty when it was used, it would either operate the system or not.

A colleague of mine says they don’t need to be monitored because if the local release panel fails you can always walk over to the valve and operate it manually. I am not convinced, I was taught on day one of my engineering life that any system that is to do with personal safety should be monitored and this is going against everything I have been taught?

I hope I have explained myself well enough and any advice would be greatly appreciated

 

[waross]

One of the reasons that systems and circuits are monitored is to warn of faults so that repairs may be made.
If the standard that you are working to requires inputs to the fire alarm panel to be supervised, your friends logic may not be accepted by the AHJ, or at a post incident investigation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[VEBill]

I presume that Fire Safety Systems on Ships is an area that would simply have to be tightly regulated by Lloyds and the applicable state(s). So the answer should reference those rules.

Not my area. And forgive me if I'm stating the obvious.

 

[dbaird]

the logic in the PLC will operate the associated vales and extinguish the fire

Is this a "safety PLC"? If not, I question using it to perform safety functions. It is well documented how standard PLC electronic input / output signals fail in unknown states. Additionally, the program can become unstable and / or corrupt.

If it is a "safety PLC", it needs monitoring of valve status reporting back to it.

David Baird

Sr Controls Designer
EET degree.
Journeyman Electrician.

 

[DRWeig]

Do what waross and ve1bll strongly implied.

Find out the ship's governing authority and insurance carrier. Each will have fire suppression system specifications that must be followed. Supervision (monitoring) and types of equipment will be covered in those specifications.

Good on ya, and smooth sailing!

Goober Dave

 

[VEBill]

"insurance carrier"

Lloyds is certainly historically and fundamentally related to marine insurance, but they've evolved into a marine standards body sort-of, some-what independent of their insurance roots.

 

[DRWeig]

Cool info, VE1BLL, I had wondered where a lot of those marine rules originated. I've seen citations for Lloyds but never made the connection.

Kinda like Factory Mutual in the US?

Good on ya,

Goober Dave