valve limit switches recommended wiring in protection systems

We are working in a modification project on a chemical plant where we have to implement a logic for a protection system where we have valves
limit switches involved. In the Hazop arised a question related to the protocol used to wire them. For the alarms we use negative logic, that is to say no signal => problem, and for the limit switches in other parts of the plant that are not involved in the protection system we use the closed switch of the SPDT to indicate the position of the valve. There are some discrepancies between different engineers in the best protocol to follow.
Someone of you knows about a methodological recommended practice to follow for the wiring of those limit switches involved in a plant protection system?
Thanks in advance.
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Not sure what happens in your part of the world, but our standard wiring practice is to use normally-closed contacts of valve limit switches and wire the signals back to the control system as 'not open' and 'not closed'.
This has the added advantage that you can wire these signals directly to indicators at the switchboard - two lights indicates valve travelling and no lights indicates limit switch fault.
HTH, Cameron:-)
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Cameron Dorrough wrote:

Most often I am using the limit switches in a complementary switch arrangement (separate Normally Open and Normally Closed contacts) and have the micro check both parts of the switch.Using a regular pulse power supply to the switches improves your ability to check the wiring to the switches also.
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jorditg wrote:

See the material at Walter Driedgers site.
<http://www.driedger.ca/limitsw/LimitSw.html
The link, above, points directly at an article entitled "CONNECTING and INTERPRETING LIMIT SWITCHES". As Walter deals in developing process instrumentation you should find the information very helpful.
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Speaking of Walter, where is he these days?? He used to be a regular poster here...
Cameron:-)
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Cameron Dorrough wrote:

He does seem to have taken a slightly more back seat these days. I think his last posting here was 3 Jan 2005 at 04:40 GMT. I'll have to email him and see what is happening. .
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Unfortunately, his "dirty pictures" link seems to have disappeared ... Love those water-powered traansmitters.
Bruce.

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He is still very active on the ISA Discussion Group
Steve
Steve YATES MTL Instruments, Luton, UK Email snipped-for-privacy@mtl-inst.com Website - www.mtl-group.com New Hazardous Area Blog - www.mtlblog.com
Cameron Dorrough wrote:

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Steve Y wrote:

I received a response from him today and he says that, owing to some problems with his new PC gaining access to newsgroups he decided that he would stick to just the ISA group. Seems he is still very busy in his work-life.
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It's good to know he's still around.. Thanks.
Cameron:-)
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