Level Switch Contact

It sounds like it's designed to be fail safe. E.G if the wire gets cut/pulled off etc this will show as a high level.
Bottom line is you can do whatever you like if you are designing the whole system, but if you are interfacing to an already existing system you need to wire it the way it's designed.
Del

And, so it seems

it would seem so.. they gotta remember.. Red to Red...Green to Green and blue to bits!

LMAO!!

PMSL.

The question goes to the hearth of the specification of the serial current loop.

4 mA / 20 mA for the two digital states. Any current at all means the loop is alive. But, deviating much under 4mA means resistive problems, deviating much over 20mA means short type of problem. I leave signal level deviations inbetween for you to work out. They are there.

As you see there is more, than simple NC or NO. Two same switches? Two opposing switches? Min/max currents monitored? The monitors monitored?

Failsafe circuits are a fascinating area, but you need the temperament for it. And tons of patience.

One reaon could be that most level switches are used to fill a tank when the level is low. An NC contact will come on in this state and go off when the level goes high.

bioramani, your answer is best.

For filling a tank, the NC would drive the fill solenoid coil when the level is low.

For pumping out a tank (i.e. sump pump, sewage lift), the NO contact would turn on a pump or drain solenoid when the level is high.

A single device with either NO or NC will not necessarily provide "fail safe" as the device could fail so that the contacts are always closed (stuck float, welded contacts, etc.) or always open (broken lead, stuck float, dirty contacts, etc.). Determining which failure is safer requires knowing more about the system. A truly safe system requires redundancy and a means to detect and alarm the disagreement so that the failure can be repaired before a second failure occurs.

In any switch being used, if the mechanism fails and you are using NO you will not detect the failure, if you use NC and the switch opens it means either it reached its trip position or if failed, in either case the trip will actuate correctly

Hi Imartin,

Fail safe is very important, I did read some where that the reason for the Chernobyl's Disaster was due to non fail safe wiring, but can't remember where.

For instance if we relied on a switch closing to stop a machine and the wire had been broken for what ever reason the machine would not stop.

If we used a NC contact and for any reason including that the circuit was opened to stop the machine it should stop, this is the reason that stop buttons are usually NC in there non active state.

It all depends on your application as to how use your level sensor, for instance if you want to fill a tank and stop it at a fill level then use the NC contact, but if you had an application that when the level has become high you need to pump the material out of the tank you could use the NO contact. This would also depend on the application that it was used in.

If the application that it is used in could cause harm to people, you then have to evaluate the harm that can be done and it becomes a Category Standard up to Cat 4 you then have to install a safety relay of the approved ratting. This would mean more control wiring and correct installation, if this is required please take advise of the a person familar with this situation.

Best Regards

Joe

When electrical switches fail, they tend to fail open more often than fail shorted. Therefore, it makes sense to design a safety shutdown circuit in such a way that the most probable failure mode (open) initiates a shutdown to avoid a "dangerous undetected fault" condition.

In the case of your high-high level switch, it is wired NC so that it will open when the level gets too high. If the switch happens to fail (open), then the control system will see that fault as a high-high level condition and also shut the process down.

The basic rule of fail-safe system design is to design the system in such a way that the most probable equipment failure mode(s) initiate a safe shutdown.