Safety PLC Question

I am reading about and trying to wrap my head around Safety PLC and Safety Modules like the 1734-OB8S, 1734-IB8S, and the 1734-OB4E.

I have (read/am reading) the manual for the these modules and had some questions for those of you that have experience with them.

So from my understanding and I may be way off base here, is that

1.) The safety plc only controls safety inputs/outputs while a normal plc only controls standard inputs/outputs. Is this correct?

2.) The safety module inputs are built to receive signals and to detect if there are faults on them or not for instance an E-Stop button or some other machine fault notification. Is this correct?

3.) The safety output modules are used to power the machines that the inputs are monitoring. Is this correct?

4.) I realize that you use a pulse test but if this is the case what is the muted pulse that appears on the 1734-IB8S?

Thanks for taking the time to read through and give me any help/advice or insight.

Thanks,
JustAGreenHorn

liberati said:

1.) The safety plc only controls safety inputs/outputs while a normal plc only controls standard inputs/outputs. Is this correct?

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The GuardLogix Controller runs two tasks, a safety task and a standard task. So, the same physical pair of controllers can handle both safety and standard I/O. The main controller runs both tasks while the safety partner runs only the safety task. So, the safety task is run on each for redundancy.

2.) The safety module inputs are built to receive signals and to detect if there are faults on them or not for instance an E-Stop button or some other machine fault notification. Is this correct?

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That is correct. The I/O modules have pulse testing, which can detect shorts between channels, shorts to ground, shorts to +V. The system can also detect faults in the transmission of data from the remote I/O to the controller, such as network loss and/or delay.

3.) The safety output modules are used to power the machines that the inputs are monitoring. Is this correct?

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Generally, the safety outputs control safety-rated relays to remove power from machine actuators.

4.) I realize that you use a pulse test but if this is the case what is the muted pulse that appears on the 1734-IB8S?

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The pulse test outputs of the 1734-IB8S can (if not used for pulse testing) be used as standard output points. One common use is for the muting lamp of a light curtain (which generally do their own pulse testing) this test output can detect a burned out light bulb.

Okay so I'm not that far off then : P
Thanks for the fast reply.
So does that mean you can't control output power at all regardless of the voltage?

So how is the physical hook-up of everything?
You have the Safety PLC and it is on the same rack as your safety modules.
Say you have one E-Stop and one Motor.
So the E-Stop would make the connection from the input and the test bit be normally open and when the push button is pressed the plc would know that the button was pressed because it will now see the test pulse and can kill the motor using the safety output module correct?

Why do you need a safety output module if that is the case? Why not just use a safety input and use a standard output or am I missing the point?

liberati said:

So does that mean you can't control output power at all regardless of the voltage?

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Maybe I'm misunderstanding, but the output of the 1734-IB8S is only 1A @ 24VDC. What machine is running on 1A? You're gonna need a power contactor (safety-rated).

So how is the physical hook-up of everything?
You have the Safety PLC and it is on the same rack as your safety modules.

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The 1734 POINT I/O is remote I/O module, it will never be in the same rack as the PLC. I use POINT I/O over Ethernet.

Say you have one E-Stop and one Motor.
So the E-Stop would make the connection from the input and the test bit be normally open and when the push button is pressed the plc would know that the button was pressed because it will now see the test pulse and can kill the motor using the safety output module correct?

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No, with safety modules you always wire the normally-closed contacts. That way the pulse testing is constantly checking for ground faults and shorts (the pulses are very short so are not seen). If the wire breaks that appears the same as someone pressing the button.

Why do you need a safety output module if that is the case? Why not just use a safety input and use a standard output or am I missing the point?

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The biggest difference is that safety output modules are designed, tested, and rated to be used in safety applications.

The pulse testing still applies so you could detect that your main power relay has shorted to +V for instance or that the module can indeed turn off the power contactor.

Also its more than just using safety-rated components, they have to used in the proper configuration. The application of the safety outputs are that the power contactors are usually monitored as well, so the PLC logic knows if a contactor is misbehaving.

Thanks for the responses. Okay so you would use the output to energize a relay of some sort for the actual power supply then.

So if I were to have a E-Stop with a channel 1, channel 2 and a stop reset signal is below the correct way to create the module configuration? ( sorry for all the questions I just want to make sure I am thinking and going about this the correct way.)
Point Operation Type Point Mode
Channel 1 Equivalent Safety Pulse
Channel 2 Equivalent Safety Pulse
Stop/Reset Single Safety
and this is done through the IB8S module properties.

Also let me know and I'll ship a beer out to you. Thanks for all the info.

Yeah, that configuration looks good. The two channels must use separate test sources (0 and 1).

Take a look at the Allen-Bradley application note on the subject. It covers a basic one motor e-stop scenario.

No problem about asking questions. This is safety-rated stuff so, you want to make absolutely sure what your doing is right.

I was supposed to be in Atlanta this month and could have taken you up on the offer. One government shutdown later and instead I get to go to Detroit (well, actually Windsor).

Yeah I had seen that before ( skimmed it ) and wasn't sure if what I was trying to accomplish is the same as what they were.

So from the example that they have they are using on module for just one E-Stop and they are using safety as opposed to a safety pulse for the fault pushbutton resets.

Is there a reason that safety is being used as opposed to a safety pulse?

Wouldn't you want to know if there was an issue with the E-Stop button instead of just assuming it will work by just using the safety?

Doing government contract work?
Yeah I wonder how long it will go on for.

Also you said you must use different test points if they are dual channel.

If I were to have 2 dual channel E-Stops would this configuration be okay

Channel 1 E1 Equivalent safety test Test Point 0
Channe 2 E1 Equivalent safety test Test Point 1
Reset E1 Single safety N/A
Reset E2 Single safety N/A
Channe l E2 Equivalent safety test Test Point 0
Channel 2 E2 Equivalent safety test Test Point 1


Also is there a reason to use complements over equivalent besides a case by case basis?

liberati said:

Yeah I had seen that before ( skimmed it ) and wasn't sure if what I was trying to accomplish is the same as what they were.

So from the example that they have they are using on module for just one E-Stop and they are using safety as opposed to a safety pulse for the fault pushbutton resets.

Is there a reason that safety is being used as opposed to a safety pulse?

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The funny thing is that the POINT I/O installation manual says that 'standard' should be used for a "standard device, such as a reset switch."

Wouldn't you want to know if there was an issue with the E-Stop button instead of just assuming it will work by just using the safety?

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That is why the e-stop buttons themselves are pulse tested. The reset button on the other hand, does not need to be tested (if the PLC is programmed correctly). If the reset button is shorted to ground, you can't reset, but you're safe. If the reset button is shorted to +24V (or held in), you don't have a falling edge to trigger on, so again no reset, you're safe.

For a single channel, there is no difference between 'safety' and 'standard' as far as fault detection goes.

Doing government contract work?
Yeah I wonder how long it will go on for.

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Just long enough to completely bugger the schedule.

liberati said:

If I were to have 2 dual channel E-Stops would this configuration be okay

Channel 1 E1 Equivalent safety test Test Point 0
Channe 2 E1 Equivalent safety test Test Point 1
Reset E1 Single safety N/A
Reset E2 Single safety N/A
Channe l E2 Equivalent safety test Test Point 0
Channel 2 E2 Equivalent safety test Test Point 1

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That would work, although if this is on one module you should use Test Points 2 & 3 for the second button. As there is the however minimal chance that Channel 1 E1 and Channel 1 E2 could short and if they did since they use the same test source the fault would not be detected. It is recommended that all pairs use different test sources.

Also is there a reason to use complements over equivalent besides a case by case basis?

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It is almost always better to use equivalent normally-closed contacts in safety circuits. A normally-closed circuit can be continually pulse tested, whereas a normally open circuit can not.

Of course, as with all safety circuits, you have to look at what will be safest on a case-by-case basis.