Fail safe contact for motor

[Indy]

Hi
Usually with the term failsafe I take this to mean a no contact being held closed when conditions are ok and if something goes wrong with the sensor it reverts to no and stops the machine. I have recently came across a design showing a nc contact which opens when conditions are not ok and stops the machine. The reasoning I read was that the machine was only to be stopped if a set point was reached by the sensor and was to remain running at all other times even if the sensor was to fail.

Has anyone came across this before because as stated I have usually seen the opposite?

Thanks

 

[BrianPetersen]

It depends what failure mode you are protecting against!

But ... A process that critically has to be kept running is inherently risky because there are other failure modes, e.g. blown fuse or broken wire, which will lead to the machine stopping regardless of that critical sensor.

There are processes out there which have to stay running in the presence of a failure (a simple one being an aircraft) but those require much more critical attention to their design than merely keeping a single switch contact normally closed.

FMEA is unavoidable for those situations.

 

[Indy]

Thanks for this. From what I’ve read I think this is strange case, I agree with the FMEA needing to be carried out.

Thanks

 

[waross]

A fire pump is one example of a motor that may be allowed to run to destruction.

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

 

[itsmoked]

Most chains of 'protection' are NC so a broken wire fails the system.
The most common ones on the planet are that way, thermal over-temp switches that open on high temp. Every oven and dishwasher has one.

Refrigeration systems are filled with NC pressure and temp switches often in series so over or under pressure or over temperature or under air flow or excessive ice or over current all OPEN and trip off the system.

Using a relay backwards so that it has to operate to signal a problem is definitely not optimal and risks failure to signal-a-failure or to protect something.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I agree with you Keith. There are exceptions.
Many power breakers such as found in substations often use battery power to energize a trip coil.
Others are more familiar than I concerning the check and back-up circuits.

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

 

[LionelHutz]

Yes, it's possible. It depends on the what the protection is, and also sometimes on how the control scheme is intended to work.

 

[jraef]

The operative issue here is whether the STOPPING of this machine presents a more critical failure than the inability to shut it down. The way I see it implemented more often though is a mechanically held contactor that requires a Close input and an Open input separately (two coils). One such application I was involved with was an 800HP MV motor on a chemical mixing operation that, if the agitator motor turned off prematurely, would result in partial solidification of the product and the loss of the agitator and tank. The motor was designed to run at what would otherwise be considered overload conditions continuously if necessary, with lots of alarms and warnings embedded in the control systems.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Skogsgurra]

I usually go to Pilz

https://www.pilz.com/en-US

They have been in this field since B.C. Almost. And have lots and lots of experience and examples.

You may have to search their archives, but I'm confident they answer most questions you have.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.