Just a quick question. I have a Cat 4 safety installation.

I buy a Cat 4 safety relay, hook it up with two-channels, manual reset, e-stop, and connect monitoring etc...

But I want to shut down a 3-phase motor....

My external K1 and K2 relays. Do they need to be safety contactors that can handle the amperage of the 3-phase motor (fractional HP) or can I use a standard motor contactor with a N/C contact to monitor back to the safety relay?

The safety relay channel one connected to K1 and channel two connected to K2 will control the coils of external K1 and K2 contactors, in this case which are in series to a 3-phase motor.

No code knowledge here Chako, but common sense says that K1 and K2 need not be safety rated, but must be what I would call "control reliable". In other words it must not be possible for the aux to open and one of the mains to stay closed.

I usually oversize my motor contactors so they don't weld shut after cycling power to a big load like a 3-phase motor. If it's something that I want to make sure will turn off I'll use twice the HP rating contactor. Unless space is a major issue I'd do that in this situation if you want to make sure that it will shut off the motor.

All of the safety circuits that we do have two safety contactors in series with the power feeding the motors and these contactors are controlled by the safety relays.

Hope this helps

How would you determine when the contactor is "big enough" to prevent the contacts from welding shut?

The contacts in the starter rely on a spring...they are not force-guided. What happens if the spring breaks?

You need to guarantee that the contactor contacts are positively guided i.e. the auxiliary feedback contacts reflect the condition of the main power contacts.

In most contactors the contacts are spring loaded which means that some may weld closed and the others can return to their normal at rest position quite happily, if this where the case and your main contacts welded then the aux contacts would return to the rest position and not signal this descrepancy, the safety relay would not be aware of a main contact weld.

Also safety contactors prevent manual override, there is no little button on top of the contactor to push and activate the contacts.

If you are at cat 4 I would think that a cat 4 rated contactor is required.

Use the safety relay to shut down the output power to your motor contactor. You could wire in the contactor to your safety relay so that you can't reset the relay if the contactor is pulled in.

This doesn't work. You've got to provide safety rated control throughout the system. The non-safety contactor attached to a safety relay doesn't cut it.

If you can't do it with a 700S relay, then you need something much bigger.

I can only think of three alternatives. With drives (and presumably soft starts), you can disable power to the IGBT's which disables power to the motor in a solid state "contactor". That is the purpose of the "safe off" or "safe torque" off connection. Several drives including some of the Powerflex drives have this set of contacts which can be driven from a safety relay. This includes the 40P, 70, and 700S. I believe you can get it on some of the Kinetics drives as well. This sounds expensive but at the lower end of the power spectrum (<10-15 HP), drives tend to actually be less expensive than starters in many cases. You may want to check other manufacturers, too, as they may offer alternatives that Rockwell doesn't.

Second, Rockwell sells a device (forgot the exact name) which is essentially a big safety rated contactor to kill power in an area plus it has options to disable hydraulic and pneumatic power as well. It's called "Electroguard". It's not cheap but it performs the function quite well.

Finally, the only other choice is to work with a fail-safe brake or clutch that allows you to forcibly stop a shaft from turning. Normally the clutch/brake just sits there innocently doing nothing so it won't be "wearing out" any time soon. Whenever the system calls for it though, the brake/clutch disconnects mechanical power.

I'm not sure if this helps, but I found this article interesting:

http://www.bin95.com/safety_relays.htm

I think, the most important question is what will the circuit do in case of fault of elements.

I didn't say that it was a fool-proof solution, I was just saying that you can reduce the chance that a contactor will weld itself "on" if you use a bigger contactor than is required.

I am not an expert but my understanding is the same as Paul's, you have to provide safety rated control throughout the circuit. Better safe than sorry, for this reason I have been looking to only use safe off options, that paul has mentioned on drives and softstarts and also killing the input power to the motor, or drive, we have now used then a few time and I am very impressed with them.

One issue I've run into with safety rated control circuits...with the exception of the fairly expensive device I mentioned before, you can't "fail safe" hydraulics and pneumatics. The moment that you think you have it licked, imagine what happens if a hose blows somewhere in the system...you just lost control. As near as I can tell, SIL-whatever means "no hydraulic or pneumatic controls" in almost all cases. There are "safety rated" shutoff valves for both, but these are not very easy to come by and I can't remember if I've actually seen a "control reliable" rated one yet.

I also use the safe-off option on AB drives a lot but on a side note, the safe-off pins are different on the 700S drives compared to the 70 and 40P.
Example

70/40P
1-Monitor-NC
2-Common-NC
3-+24VDC
4-24VCommmon

700S
1-+24VDC
2-24VCommmon
3-Monitor-NC
4-Common-NC

I agree paul, and have see it first hand when a pneumatic valve failing to do its job and fail half way between pressure to device and pressure exhaust. The air got stuck in the line and the brake stayed open, i.e. off, when a e-stop was pressed. Thank G*d the e-stop was pressed for a different area and no one was around it, but it could have gone bad. After that it was suggested to put a safety valve for all the brakes to kill air just for a e-stop. As paul has said there is always what if's floating out there.

When hydraulics and/or pneumatics are involved, guarding becomes escpecially critical. The use of solenoid-locking type guard switches with redundant contacts goes a long way in SIL ratings. Remember SIL ratings are based on human injury, not damage to the machine or product.

Wouldn't a risk assessment have identifed the system the brake was attached to has a high risk category (3 or 4) therefore a dual monitored air dump system be installed??

We build our own automation in house and every machine is atleast category three, we use dual monitored air dump valves from fluid centry. We use a pilz PNOZ Multi to activate these valves and then monitor the spool movement in the Pilz. We use the the multi because within the one machine we may have up to 6 safety zones that work independantly from one another, each zone has a set of dual dump valves. Expensive but the redundancy minimises the risk of a dangerous situation arising from valve spool sticking. If the spool does not do as commanded the feedback will be reflected to the Pilz and it will lock the reset function out.

We use the same dual redundancy for DOL motors, VSD's and Servos, we use the AB 100s range of contactors. As far as servo's and VSD's are concerned we break the load side of the amplifier however we disable the outputs 100mS before disconnecting the motor. Our risk assessments consider the capacitors of the drives a stored energy source.

What I keep wondering about is on a safety relay what design is there to detect and take corrective action of one of 3 motor contacts does weld shut. I have seen the contacts burnt to near non existance and I have seen the relay get plugged full of dust preventing one contact or more from opening. If I wanted assurance the all contacts would open I would oversize the relay and if I wanted to make double sure two in series.

Dan Bentler

I don't believe there is mean't to be any "corrective action" in the case of a contactor.....if it fails it fails, simple as that. The more desirable feature of the contractor is to be able to broadcast its faulted condition. If the situation is so critical that single point of failure is unacceptable then two contactors are required. If you are in an enviroment where the fault that effected the first contactor can easily effect the second then you need to to take further action to prevent failure of both items simulateously which could be a further set of contactors in another cabinet???? your risk rating will drive the solution.



A single contactor even over rated is a single point of failure. In cat 3 and cat 4 systems, in my opinion, this solution is not acceptable.

It seems I stumbled upon a product "gap".

Definitely I agree with the safety contact, but traditionally with 700S-CF relays, they only go up to 10A.
http://www.ab.com/en/epub/catalogs/12768/2...29737/tab4.html

Looking for a 14A rating is tough in a control relay. I'll check the other solutions, but looks like this one is limited the higher amperages you go.

Look into the 100S- contactors