interlock vs trip: synonymous?

[sbibiz]

hi all,

first I want to apologize if this was covered somewhere else but... I didn't find it.
My question is really simple but I couldn't find an answer on the net.

in control engineering, is the work "trip" different from the word "interlock"? in other words, are they synonymous?

Do they both refer to preventing an undesired state in a state machine so they both act under "anomalous" conditions (for example the LL level inside a tank will trip the relative pump to prevent it from running dry).

Sorry but I'm new to this kind of subject and I want to make sure I understand it correclty.

Regards,

Alberto

 

[zeusfaber]

Where I come from, a Trip changes the state of an output (usually by shutting a machine down, or setting off an alarm) in response to an undesired combination of conditions.

An interlock prevents a change of state (often by not letting you start a machine) in response to a combination of conditions.

A.

 

[sbibiz]

thank you zeusfaber.
So basically your reply confirms my understanding: they act because of anomalous/undesired conditions and not in response to normal operation (ordinary conditions) for a machine/ equipment.
Their involvement is because something "strange" happens. Something that should not occur but has been considered as "possible" during design.

for example: we want a machine to work within 2 thresholds (ordinary conditions) but we know that, for some reasons, it could go beyond one of the two and this could cause problems to the machine or even to the operators. Here it comes the trip.

But (another question) how do you call the 2 thresholds under which you have the ordinary operation?

thanks
A.

 

[jraef]

That's about as good of a definition as you will likely find.

Shorter version:
Trip = action taken after an anomaly.
Interlock = action based on some other action, anomaly not required.

"Will work for (the memory of) salami"

 

[sbibiz]

thanks jraef.

so now I have a bit of confusion in my mind (less than before )

you exactly hit the point I have doubts. the trip is used then for anomalies while then interlock for normal operations. Correct?

i can't understand why I can't find anything satisfactory on the net. Do you have some links?

 

[jraef]

How about this:
All trips are technically "interlocks", but not all interlocks are trips.

This might actually be defined, do a search on "ANSI/IEEE C37.2" and look for the definitions section where it is discussing what are called "device numbering" conventions.

"Will work for (the memory of) salami"

 

[electricpete]

Trips are not included within interlocks to my thinking. You can see this by looking at the definition offered by zeusfaber. Interlock prevents change of state, whereas trip initiates a change of state.

Here is my response to op: Think about what trip and interlock do and it will help you understand why they are used.

A trip automatically stops equipment. Why would you do that? Yes - operation is out of bounds (anomaly).

Examples of trip conditions: High current. Sometimes high temperature, high vibration.

An interlock prevents something from happening. Why would you do that? Not necessarily an anomaly. It is a control to prevent manual action from putting the equipment in an undesirable configuration.

Examples of interlock:

Sometimes lube oil system must be running prior to starting a pump. Because the lube oil system supports the pump or motor and it would be silly to start the main pump without lube oil system running.

Sometimes pump discharge valve must be closed (excepting recirc path) for prior to starting a pump. It can reduce water hammer and sometimes is necessary for successful starting of the pump.

Sometimes pump cannot be started unless seal water is sensed because pump will be damaged without seal water.

Sometimes there are two machines that should not be run at the same time. Interlock can enforce such requirement.


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(2B)+(2B)' ?

 

[ScottyUK]

Interlocks are designed to prevent undesirable events occuring.

Trips are designed to take executive action once an undesirable event has occured.

 

[jraef]

So if I turn on conveyor A, and as soon as it runs, I need for conveyor B to turn on, would that not be an "interlock" between those conveyors? Presuming that it is OK for conveyor B to be running even though A is not, there is no "undesirable event" going on there, I just need to call for conveyor B if A is started and B is not already running.

"Will work for (the memory of) salami"

 

[jraef]

LOL, as soon as I posted it I saw the flaw in my hypothesis...

A but not B is the undesired event.

"Will work for (the memory of) salami"

 

[electricpete]

http://en.wikipedia.org/wiki/Interlock_(engineering)

…Similarly household washing machines will interrupt the spin cycle when the lid is open

That example throws a wrench into the works. What they're calling an interlock does initiate a change in state from running to not-running (although it would restart when you reclose the lid, which perhaps helps differentiate it from a trip). That makes the piece of the definition that focuses on change of state or preventing change of state a little muddier.

=====================================
(2B)+(2B)' ?

 

[electricpete]

Somehow my link got mangled. Here was the link I meant to give:

http://en.wikipedia.org/wiki/Interlock

=====================================
(2B)+(2B)' ?

 

[waross]

Machine sequencing interconnections are interlocks. An interlock may prevent an action until another action has occurred or may cause another action to happen automatically.
Trips are for protection circuits.

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

 

[jraef]

Trips are for protection circuits.

So if I set up an over torque trip in my VFD, what "circuit" am I protecting?

Rhetorical question Bill, just pointing out that you might be falling into the same "painting with a wide brush" trap that I fell into.

"Will work for (the memory of) salami"

 

[waross]

Take the word protection off my post Jeff. You are providing over-torque protection. There is some overlap in the terms. But I don't feel strongly about it either way. I'll try to adapt to the other guy's terminology. (On a good day.) grin

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