Compact Guardlogix vs Safety Relays

At what point does one jump ship from the standard Guardmaster safety relays within a panel to a Compact Guardlogix or other safety PLC? Im sure there is no ONE answer here, Im curious how all others approach this scenario.

It depends on the panel application, could be cost constraints the list could be endless. Factors, is it really necessary to use a complex programmed system.
What about cost i.e. if a panel only needed a simple cat 4 and all e-stops/guards are local why would you spend a lot of money on something that does the same job. Think about the maintenance guys their expertise probably does not extend to software/controllers. It reminds me of a system which was basically an add on indexing conveyor for a packaging machine had an ASI safety system on it for 3 e-stops the conveyor length was no more than 4 meters When it went wrong what a pain for our maintenance engineers. (Note: the manufacturer stopped using it because it caused more problems in many environments & even their own service engineers hated them).
The term "I have to have the latest model" comes to hand, how many people spend a grand on an Iphone when a 120 pound Samsung can do the same thing for the functions the average person uses. Safety PLC's have their places in industry but sometimes designers get carried away.

JZerb said:

At what point does one jump ship from the standard Guardmaster safety relays within a panel to a Compact Guardlogix or other safety PLC? Im sure there is no ONE answer here, Im curious how all others approach this scenario.

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As soon as the safety scenario requires some logic OR usually after 4 safety relays.

Siemens experience, not AB, but I imagine that it carries over. For us, there tend to be two breakpoints, one based on complexity and one based on features. They tend to go hand in hand. Safety relays are great if the only thing you need to do is cut power to the whole system.



1) If you have a complex safety system, safety PLCs come in very handy. Complex meaning multiple safety functions, interlocking zones, or devices that need to retain power in their safe state.

Say you have a welding cell making automotive parts, multiple robots surrounded by a fence, with a couple conveyors going through. Obviously, if you Estop, everything shuts down. HOWEVER, there are other desired safety modes as well. Each robot needs to run in teach mode, but they often need to coordinate to be sure that only one can Teach at a time. If the door is open, then the robots need to be either safely limited to a low speed, or stopped, but the conveyors may still need to run. Alternately, if a light curtain by the conveyor is broken, then the conveyor may need to stop, but the robots can still keep going.


Hypothetically, all of that could potentially be done via safety relays, but it gets complicated and costly quite quickly.


2) The feature that causes you to go to safety PLCs is networked safety devices. In just about every system I work on, the drives, IO, robots, and other PLCs are all networked together already. There is minimal extra work for many devices to do Profisafe safety communication over Profinet, if the IO data is already going over PN. No physical IO points required, no extra wiring, just over the cable.


In Siemens land, ordering the safety CPU instead of the regular one is only a 10-20% cost increase per CPU. It's pretty easy to balance that HW cost with reduced wiring labor. Because of this, it is pretty rare for me to see a not-failsafe Siemens CPU. I've heard that because of the safety partner, the costs are a bit higher on the AB side. The complexity tradeoff might be a little higher there.

mk42 said:

I've heard that because of the safety partner, the costs are a bit higher on the AB side. The complexity tradeoff might be a little higher there.

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mk, except for real old stuff, safety partners are only required with the 1756 family.

for us, it was this situation.

we had an assembly line with multiple stations, one of those stations was a shrink wrapping oven.
during the design portion, we asked what happens to the product in the oven when the e-stop was pushed and the line stopped. the answer was the product melted and caused toxic fumes and a fire.
so we went with a safety plc. when any station stopped due to an error, fault, or defect being detected, everything past that station kept running; that station and everything upstream stopped.
we had to reset the faulted station, then reset the line to start again.
it all depends on the complexity of your system and other connection to machines.

james

Normal protocol on all ovens I have been involved in is a battery backed DC drive coupled to belt so in the event of failure (mainly power loss or E-stop) the belt can be driven manually to remove product & reduce the risk of fire, so the e-stop stops all functions then manual intervention to run belt. after all if i saw a major problem on a piece of machinery and I hit the e-stop I would expect a belt to stop.

jstolaruk said:

mk, except for real old stuff, safety partners are only required with the 1756 family.

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Good info. Google tells me that 1756 translates to Controllogix? So you mean that (Control/not compact) GuardLogix class PLCs need a safety partner for SIL3, whereas Compact GuardLogix don't need a safety partner for SIL3?



Slightly off topic question I've been meaning to ask forever. What IS a Rockwell "Bulletin"? Is it a catalog? It seems like the part numbers often, but not always, start with the bulletin number. Is it a product family?

mk42 said:

Good info. Google tells me that 1756 translates to Controllogix? So you mean that (Control/not compact) GuardLogix class PLCs need a safety partner for SIL3, whereas Compact GuardLogix don't need a safety partner for SIL3?



Slightly off topic question I've been meaning to ask forever. What IS a Rockwell "Bulletin"? Is it a catalog? It seems like the part numbers often, but not always, start with the bulletin number. Is it a product family?

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The compactlogix offers several different SIL and PL choices, the older 1768-L4xS were SIL3/PLe and the newer ones have a lot of SIL2/PLd choices. You have to weed through them. All are/were without safety partners.

Yeah, "bulletin" is old school reference to product family. I'm not sure they use the term bulletin anymore.

So there's some good info in here already. Let me lay this out a bit further for my specific scenario.

We build machines that blends powders, so it rotates a vessel around at a low-ish speed 10-25RPM and also lifts said vessel up and down vertically. The machine is protected by a Keyence laser area scanner. Great product, but we run into issues when customers want to 'jog' or 'bypass' the laser to get the machine out of an unlikely, but still possible, 'lost' scenario. Now if the entire safety system is Cat3, you cant just go ahead and throw some sort of non safety rated relay in the panel that can be fired from a non-safety rated PLC for the purpose of bypassing the safety module that's typically satisfied by the two OSSD's from the laser. The laser has a muting function, but this can only be triggered if there is nothing in the protection zone and typically thats when the customer wants to bypass the laser. Hence my wanting to look into using a safety PLC for these applications.

jstolaruk said:

The compactlogix offers several different SIL and PL choices, the older 1768-L4xS were SIL3/PLe and the newer ones have a lot of SIL2/PLd choices. You have to weed through them. All are/were without safety partners.

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Thnx for the clarification!

jstolaruk said:

Yeah, "bulletin" is old school reference to product family. I'm not sure they use the term bulletin anymore.

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Their webpage uses it, at the very least.



https://ab.rockwellautomation.com/IO/Chassis-Based/1756-ControlLogix-IO

JZerb said:

So there's some good info in here already. Let me lay this out a bit further for my specific scenario.

We build machines that blends powders, so it rotates a vessel around at a low-ish speed 10-25RPM and also lifts said vessel up and down vertically. The machine is protected by a Keyence laser area scanner. Great product, but we run into issues when customers want to 'jog' or 'bypass' the laser to get the machine out of an unlikely, but still possible, 'lost' scenario. Now if the entire safety system is Cat3, you cant just go ahead and throw some sort of non safety rated relay in the panel that can be fired from a non-safety rated PLC for the purpose of bypassing the safety module that's typically satisfied by the two OSSD's from the laser. The laser has a muting function, but this can only be triggered if there is nothing in the protection zone and typically thats when the customer wants to bypass the laser. Hence my wanting to look into using a safety PLC for these applications.

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Typical, we provide a handheld enabling device that allows the safety scanner to be bypassed so that he/she can do maintenance with motion power. The "deadman" basically replaces the safety scanner.

jstolaruk said:

Typical, we provide a handheld enabling device that allows the safety scanner to be bypassed so that he/she can do maintenance with motion power. The "deadman" basically replaces the safety scanner.

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So my main issue is that the protection zone needs to be clear before 'muting' the laser and usually the only time the customer requests a bypass is once there is an obstruction within that protection zone.

I've been out of the large systems for a few years, but from what I recall, I could get a safety PLC (like a Jokab) for just under $700, and it would provide me with somewhere around 3 safety 'zones'.

The general rule for me was about 3 safety relays. At that point, most times I could break even at a minimum and anything above I was saving money and space.

FYI, my typical safety applications were light curtains, emergency-stops, and pull-cords on assembly machines.(safety relay + 2 aux relays in all cases for each device - usually ~ $300 for the set.)