Hi,

Recently i've stumbled upon a low sensing pressure switch that confuse me about NO and NC contact. the wires is connected to common and NO terminal. When pressure is above 16bar (normal required condition), contact is OPEN. Contact closes when pressure below 16.

From what i understand in RELAY, NORMALLY is when coil is not energized. So normally open is CONTACT OPEN when coil not energized.

So, what is the general definition of "NORMALLY" in process switches? is it not the same with RELAY? or did manufacturer defines "NORMALLY" differently?

thanks

 

A relay is energised by an explicit action - applying power to the solenoid circuit. So it is easy to separate the energised and de-energised states. "Normal" refers to the bench state of the relay when no current is applied.

There is also a conflict between the electrical use of "Normal" to mean a de-energised circuit; and the process sense where "Normal" applies to the process operating at its design conditions.

But a pressure switch is activated by the pressure in the sensor element. A "low pressure' switch may activate say when pressure goes from 5 to 4 bar. If left on the bench, with atmospheric air pressure applied to the element, it will then see a low pressure. The second question you need to ask is what is the required failure mode of the switch - remembering that in general an open circuit will result if there is any electrical fault.

I have found it more useful to define switch status in terms of "Process Normal" - what should be the state of the switch when operating at design conditions; "Active" - what happens if the pressure is in the range to operate the switch; and "Failed" - what happens if there is a system failure?

Generally a low pressure switch will be Process Normal - made or closed; Active - open; Failed - open.

Bruce.

 

Thanks Bruce.

Actually, there are nothing wrong with the application of our switch. it was configured to do what it should do. Active -closed & Failed -open. i.e. stop the machine when low pressure or system/signal failure.

What i seek is the general definition of NC and NO in pressure/process switches used in the industry. most of process switches i saw have these NO and NC terminal. but it confuses me.

Normally = condition with no external disturbance (shelf state)

OR

Normally = Process normal as explain by Bruce (all this while, i'm with this definition)

OR

BOTH depending on user/manufacturer?

thanks

 

NO Normally Open - contact is open - "light off"

NC Normally Closed - contact is closed - "light on"

The state of the switch in a normal state, without external influence eg. in the box, on the bench etc

 

From Manufacturer's point of view, the NORMAL status is the condition of the microswitch in shelf condition (i.e. at atmospheric pressure). So if a pressure switch is used for sensing low pressure, say below 16 bar, and it is desired that the contact OPENS when pressure falls, it is required to connect the "NO" contact to the circuit. This contact will be closed when the pressure > 16 bar and will open below 16 bar(which is also the shelf condition of that contact). For process switches it is better to refer switch contacts as "MAKE TO SIGNALIZE" or "BREAK TO SIGNALIZE" instead of NO or NC. Which one to wire up will depend on the process conditions and the circuit requirement.

 

NC or NO contacts are generally defined when no power to relay or normal process.

Connections depends on what kind of system you are following
1. Break to Alarm means Open to Alarm or
2. Make to Alarm means close to Alarm.

Let us say Break to Alarm (BTA)

Low Press/Low Process signals will be connected between NO & C terminals. Now what happens when process goes abnormal (here low) it will open the contact and will Alarm.

Now let us say you are using instead of Make to Alarm (MTA)/Close to Alarm.
In this connection you will need to connect your signals/wires at NC & C.

So it is very important that you follow how your system has been set up but again someone can argue stating that one can invert this by negation block in the system and ...yeah it is possible but fundamentally one need to identify what system you have been using..
In brief again for BTL Low Process switch .. use NO & C and for High process NC & C.

If MTA for Low Process use NC & C and for High Process NO & C.

Good Luck.

 

Dear Sir,

Normally means then initial condition of any switch or relay without pressing or without giving any supply is called normal condition.

Thanks