MCB tripped signal

Should a signal to a plc giving the state of a circuit breaker in a panel be wired via a normally open or normally closed auxiliary contact on the breaker?

I have seen this done both ways.... is there a right way?

Is there a spec which relates to this?

I always wire them so when it is healthy the input is true. this can be important in some situations, for example Stops, some sensors although not safety related as far as trips or overloads as these will kill the power anyway but if these are false on healthy and for some reason power to the contact is lost you would not know if a trip actually happened as far as the plc is concerned. An example is lets say a sensor on a moving conveyor, so the conveyor moves then when it reaches sensor 1 it slows down when it reaches sensor 2 it stops (I know there should be some sort of mechanical stop) if sensor 1 is normally open but the power feed to the sensor is lost i.e. broken wire then when the thing on the conveyor reaches it does not slow down so slams into the stop position. If it is "normally closed" then the conveyor will run slow from the initial start. easy to diagnose, reduces damage etc.
The same goes for levels in tanks although not full proof having the low level sensor as normally open & high level sensor normally closed means that loss of power on the high sensor will stop the feed, again it could go short but this is less likely than going open or loosing power.
As far as breakers are concerned it makes sense for them to be N/O so when energised you get the signal there is often confusion between N/O & N/C depending on the context.
I have seen situations to reduce I/O count where two contacts are in series (motor overload and local isolator) where N/O contacts are used in tripped state so if it trips or local isolator is turned off it alarms, on the other hand I have seen where they are both in parallel so in healthy state they are N/C and open on healthy state.

Normally Open held closed when the MCB is closed that way you have a more fail safe indicator

I try not to use the word "tripped". Healthy or OK is more appropiate , it describes what the input actually represent.

How about NotTripped ?

I would wire it fail safe (normally open, closes when the breaker is closed) and label the input with the text to name the breaker followed by "ON Proof".

Sidenote: I used to label PLC inputs from motor starter auxillaries with wording like "MS1 Aux." or "Motor A Auxiliary" Then I was looking at code written by someone who was speaking English as a 2nd language and she used the word "Run Proof". Hmmm. That is shorter, easy to spell right and more descriptive of what it actually means, so now I use the word "proof" for monitoring contacts quite frequently.

In can tell from your response to "Tripped" you are not for the US
How about "Open" - "Closed"
Over here we consider a breaker either Closed or Tripped
We don't normally use them as a switch.
If it's not Closed it's Tripped
Also a motor AUX is not proof the motor is running
I usually use MC AUX for motors
For breakers is would use MCB AUX

GaryS said:

In can tell from your response to "Tripped" you are not for the US
How about "Open" - "Closed"
Over here we consider a breaker either Closed or Tripped
We don't normally use them as a switch.
If it's not Closed it's Tripped

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I hear the term tripped quite a bit here in the land of bugs, weeds and toll roads. I have even seem some fancy breakers with indicators for all three positions:

Open, Closed, or Tripped.

Proof is not proof of run, but proof the starter pulled in...same as Aux. Probably just me, but I liked it so I borrowed the term.

Doesn't the electrical usage of the word "trip" come from WW2? Like trip mines?

Actually most breakers do have 3 positions to indicate their state
Handle in closed position CLOSED
Handle in mid position TRIPPED
Handle in open position OPEN
some of the them have a red flag that shows when tripped as well

Speaking of the term Tripped dose anybody know where the term DEBUG comes from

GaryS said:

Actually most breakers do have 3 positions to indicate their state
Handle in closed position CLOSED
Handle in mid position TRIPPED
Handle in open position OPEN
some of the them have a red flag that shows when tripped as well

Speaking of the term Tripped dose anybody know where the term DEBUG comes from

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I have heard that in the early days of computers they used large arrays of solenoids and other electro mechanical device. The computers would fill up entire rooms/floors and they would literally get ****roaches and other bugs crawling into solenoids so when something didn't work they would go around and try to remove bugs and other debris that was getting into the devices.

mp91 said:

I have heard that in the early days of computers they used large arrays of solenoids and other electro mechanical device. The computers would fill up entire rooms/floors and they would literally get ****roaches and other bugs crawling into solenoids so when something didn't work they would go around and try to remove bugs and other debris that was getting into the devices.

Click to expand...

It was a "Moth"

https://www.google.com/search?q=DEB...om&aqs=chrome..69i57&sourceid=chrome&ie=UTF-8

The very simple answer is that the PLC should receive a positive indication that something is "healthy", any other condition, whether it be the CB is tripped, or the wires have dropped out, or been cut or broken, is a "fail-safe" indication.

In other words, label and use your inputs for "CB Tripped" as "CB_Healthy", ON = OK.

Always try to use hard-wired field devices in the "fail-safe" mode. Typical example is High-Level probes in vessels should always give an input to the PLC when it is not "wet" or "covered", in that way you protect the system such that if the input is NOT present, you assume the tank is full.

To answer my question origins of DEBUG
To put things in context when I first started out I studied electronics one of my instructors was retired from the government as electronic engineer. He actually worked on the systems.
I learned a lot from him, he was an expert on microwave and radar. (Dew Line)
Those were the days before transistors everything was vacuum tube( Valves for our friends across the big pond)
As was said the first computers were developed in the 1940’s while WW2 was in full swing.
The were all mechanical relay logic huge power consumption 8 bit word. The relays were all open type and at night when the bugs were attracted by the lights would fall into the relays and cause all kinds of problems if they were not removed. To that end they hired local school students to come in before school and before the computer was powered up for the day. Their job was to remove the dead bugs in the relays. There job title was Debugger the name stuck around from there. I expect it will be around long after we are gone.

The most common electrical fault is loss of signal, a wire breaks or a connection comes loose, you get a false 'false' signal. So to see that fault you would need to use a contact that is closed when the circuit is good, and opens when the circuit is bad.

The second most common electrical fault is when a wire comes loose or a cable gets damaged and touches two wires together that weren't intended to touch together, you can get a false 'true' signal. To see that you would need to use a contact that is open when the circuit is good and closes when the circuit is bad.

So the answer that catches both errors is to use both the open and the closed circuits and monitor both. If you get the same signal from both sensors for more that a short period you know that there is a fault. Very few people go this far unless the financial cost or human cost of missing the fault makes it sensible. So if you loose one manufactured part that costs £1.00 because of the missed problem you aren't going to worry. If you damage a £1000,000 machine and loose days of production then it might be best to monitor both signals.