Which Contact is Best NO or NC

Generally the installer should decide if it is better to have the contacts open or closed in case of power failure to the contact.

So, it depends on the application.

I agree with fixitorelse.

The general rule in an interlock system for dumb sensors is that the "safe" or operate condition is a contact closure. A significant exception (I'm sure not the only one) to this rule is fire alarm systems for precisely the wire shorting concerns you've stated. However, this exception is valid because a fire alarm system does not know where a fire will happen. Because of this any of the wires might short at any place in the cable run. In contrast you have a specific cable run where high temperatures are breaking down your cable insulation. You must now use the fact that you know where temperature is a problem to either prevent wires from shorting or finding a way to discriminate between a contact open, contact closed, broken wire, and shorted wires.

Preventing your wires from shorting out is certainly the preferred fix because you will not have to repair the wires as often. If this really is impossible, you could go with an intelligent safety sensor of some type that your safety PLC talks and listens to. A loss of communication will be an unsafe condition. A less costly alternative that requires more on site knowledge than I have, is to place a load in series with NC contacts and measure for this impedance at the PLC.

Safety switches should always be 'Fail Safe'. That is to say during normal operation the circuit is complete, a failure is sensed the circuit is opened or interrupted.

Fixitorelse is correct: fix it (the cable problem), or else...

[edit] Redfred, you said "A significant exception (I'm sure not the only one) to this rule is fire alarm systems for precisely the wire shorting concerns you've stated." Can you expand on this? In the electronic fire rated door stuff, this is not the case.

Some alarm systems (certainly the intruder alarm at the factory where I contract) have series resistance in each switch input, with another resistor in parallel with the contacts. This is for tamper/open/shorted wire protection.

Safedge™ systems use a 6K resistor across the contacts - the controller must see between about 5K and 8K to indicate a safe state. A 'special' Safety relay is needed (when Nesa maked - i.e. before AB absorbed them - they had a thing called a "safedge evaluator", which looked nothing like a Safety relay).

I don't know offhand of any other machinery Safety wiring using this approach (tho' there may well be).

I agree, the dual loop is a common application in an intruder detection scheme.

I have not seen this used on fire detection and annunciation... is it commonly used and I am simply behind the times? I do live in North Dakota....

No idea, I'm afraid - not my area (technically or geographically speaking).

Can you post a loop wiring sketch/diagram, including the power supply, process input and grounding?

There might be a wiring solution, but it requires knowing what your current wiring is.

If your wiring & environment is such that neither NC nor NO alone can be considered safe, you could consider using 1 NO and 1 NC input from each switch. This would need dedicated Safety relays (e.g. Rockwell MSR127) unless you're using a Safety PLC. You'd obviously have to rewire each switch using 4-core cable.

Yes, like voting. Or a form C switch and three core cable. For a PLC, I like it.

Are we considering/solving the wrong problem? I can't really tell from arunbabu's OP: is it desired that the circuit be supervised to treat a circuit trouble different than a detected trouble?

Good question!

arunbabu,

The concept of using NC contact for safety device is to provide positive fail safe protection system making it independant of the relay system . "NO" contact will be operative only when the relay is in operation and protection will be available only when the system is ON. and making the safety system dependent. If we use "NC" contact the safety system will be totaly independent of the relay operating system.For checking of the faulty cable the cable have to disconnected from NC contact.

Hi All,

This problem is present in our conveyor system hot product are transfer from one place to another place, in all control and power cables fire protect paint also applied and some route of cable also re-routed but still fire is happen in cables due to hot fines are accumulation in cable tray for this cable getting fire. Due to NC contacts in all safety devices if cable got fire and short I can't know instantly (due to NC) this leads a equipment's breakdown and loss of production also.

In safety devices NC is used because if cable got fully damage/any loss connection in field problems are identified easily and solve quick for this only all safety devices taken in NC. In NO if cable got fire also it will give alarm in quick but problem is if the cable got fully damage/any loss connection field also its shows healthy only due to NO

I suggest management to take these area safety devices from NC to NO, form this they said NO is not good, but comparing fire and all production losses NO is better for this area safety devices. Suggest me which one is good NO or NC and give the No and NC contacts advantages and disadvantages.

This has turned into a classic CR4 problem. Our OP has a problem but cannot recognize all of the facets of the problem. Our OP can only see one other option so they think that this must be the solution to their problem. Their management accurately tells the OP that their idea won't work in their native tongue, so they come to us and in broken English they tell us their problem. We recognize more facets of the problem than the OP and try to explain both the added facets and our recommendation to solve the problem. The OP still wants to know if their idea will work.

You can bring a horse to water but you cannot make them drink.

GA jhhassociates, Doorman & JohnDG

Thanks to all...

You're not solving the problem by going from NC to NO. Changing to NO defeats the purpose of the "fail safe if broken" configuration.

Your problem is the use of unsuitable material and a cable management system that not only provides no protection but actually works as a sh*t magnet for the problem.

Re-routing and ducting has been suggested by others.

I would add that in the operating environment described by the OP that a seriously heat resistant cable or cable assembly is warranted.

My recommendation would be to use PFTE (Teflon) insulated conductors inside a metallic conduit. Alternatively, mineral insulated cabling (Pyrotenax)could serve this purpose just as it does in fire resistant critical signaling and power distribution systems.

Is there not a few other alternatives?

The old solution to this was to run DC protection relays powered by AC and to put a diode at the end of the line (eg at the emergency stop). The diode allowed the DC relay to operate on half wave AC. A cable short would result in a full wave AC supply, hence the relay would drop out.

Modern safety relays send different "pulsed" supplies down two separate circuits, a short of all would cause the relay to drop out as differing pulses would exist in each signal.

Also consider (a) the two cores of each single signal going to the device not being run directly side by side, either individually or in a multi-core (b) using individually screened cores, with the screens being earthed and a centre tapped +/- 12V supply providing the positive and negative legs. Such systems would result in at least one leg generally fail to earth and hence not providing 24V across the relay.

I'm not an expert in emergency stop relays but if you discuss with typical manufacturers (Pilz, Rockwell, many others) this type of failure is well known and they have some ingenious solutions.