Ups

Hi,

I'm wondering, does plc need an external ups? why or why not?

It is entirely up to the process requirements.


If you have a process where a major power failure will take out the whole process plant, and the product can simply be dumped to the spillage sumps and cleaned up after power returns, you wont necessarily need a UPS for the PLC.

If however you run a process where for example batching sequences or multiple redundancies are part of the process, then you would need a UPS.

The general rule is, yes, do install UPS to keep both the PLC controllers and the SCADA computers online. At the very least it will give you enough time to give the control system a controlled shut-down in case of major power failure.

Noting as bad as trying to recover a server PC that’s crashed because of a power failure.

I see. Thanks a lot.

I always use 24VDC for generator systems and large batteries. The battery supply is grounded normally and then passes through a 24/24VDC switch mode power supply. The output of the switch mode is allowed to float. For other jobs I still use 24VDC off a switch mode power supply. The 24VDC is allowed to float. Grounding the negative of the switch mode output introduces ground noise and destroys the isolating characteristics of the switch mode power supply.

BobB said:

I always use 24VDC for generator systems and large batteries. The battery supply is grounded normally and then passes through a 24/24VDC switch mode power supply. The output of the switch mode is allowed to float. For other jobs I still use 24VDC off a switch mode power supply. The 24VDC is allowed to float. Grounding the negative of the switch mode output introduces ground noise and destroys the isolating characteristics of the switch mode power supply.

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I'm sorry, I kinda confuse. Do you mind if I ask for a diagram?

My opinion is that a UPS is a waste of money for most applications. If the power is out, what is the PLC going to control? Further, if you have a UPS then you need special logic to "idle" the control during the power failure and implement a start-up sequence when the power comes back on. This is particularly critical if there is a short duration power blip - say a second or so. Further, a UPS only delays the inevitable. It is uncommon to have a UPS able to keep the PLC on for more than an hour. After that you still have the same issues. Can you guarantee that power problems will always be resolved in less than 30 minutes? Finally, a lot of folks install a UPS for transient protection, EMI noise filtering, and surge protection. There are lots cheaper ways to accomplish that objective!

This opinion is contingent on a couple of assumptions:
1 - the PLC has battery backed memory and the programmer included all critical setpoints within it
2 - The controlled process doesn't need PLC intervention to implement a safe shutdown on power failure (and it is a rare process that would need that!)

"Uninteruptible Power Supply" is one of those phrases that tends to give one a warm, fuzzy feeling. It sounds really good, right? However, if you think through the application thouroughly, most of the time it is a false sense of security being provided.

Note: an exception to the above is an unmanned remote location connected by telemetry to a main control station. Then you may need the UPS to keep the remote PLC alive long enough to send a "power failure" message to the main control, allowing an alarm to be logged and alerting operators there.

I have installed a few UPS units just to help ride through the voltage droops. We have fairly bad thunderstorms in the summer that affects our power in the plant. The PLC 5/40's always fault out when the power cycles two or three times in a second. The UPS keeps the processor alive but drops out the control circuits and stops the process. I like them because I'm not called in the middle of the night to reload a program. Other than that, they are pretty useless.

We experienced a situation where a UPS was needed for our PLC. We already had surge protection installed to protect it, but the power transients we were getting was causing the PLC-5 processor to clear its memory ( and of course all of the product data). On advice of a trusted systems integrator we installed a UPS for the PLC and the problem went away (this was after changing everything including the kitchen sink).

We have since made it standard for all of our control processors, whether a PLC or a PC, to have a UPS supplying it. It probably was a unique situation with that one PLC-5 (which has since been upgraded to Contrologix for an overall upgrade), but it is better to be safe than sorry.

SOLA makes a relatively inexpensive UPS which is din rail mountable. It doesn't have a lot of battery time, but for short blips it works well.

P.S. I did try to get the GM to sign off on a 100 Million VA UPS so the entire place could still run, but he balked at that one.

Tom,

For most applications a UPS may not be needed. But there are always a few where it really is useful. The last one I did where I thought it was probably needed was for a Thermal Oxidizer for the Hawthorne Army Ammunition Plant. They were melting out the residual nitrocellulose and propellants from old 105 mm tank rounds and also rocket motors (after most had been removed manually). A shutdown in the middle of a batch could have resulted in overheating and a fire, or worse. The UPS has to supply power to all controls for 1 hour, and the backup generator had to be online in 5 minutes or less. The UPS was rated 10 KW and measured about 6' x 6' x 6'

After 2 years they did have a large fire because some rocket motors got put in the batch that had a full load of propellant. The operators said it looked like a 4th of July fireworks show. The plant was 60% destroyed and had to be rebuilt.

Tom Jenkins said:

My opinion is that a UPS is a waste of money for most applications. If the power is out, what is the PLC going to control?

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If you need a UPS\battery backup you will generally know you need it, agreed its not a standard requirement but the above is a bit of a sweeping statement to say the least.

If power is out you may need to initiate a generator start, how are you going to do that if you have no power to your PLC ?

If power is out you may need to check the status of critical equipment on site, how are you going to do that with no power to your PLC ?

We installed them for the control systems on our kilns. We're in a bad spot for power and endup with a lot of quick brown outs in the summer as the utility company switches the feeds. The UPS allows us to weather the brown outs without having to reset all the automation associated with the kiln. It also gets us through the downtime during a major power outage until the backup generators can start and switch over.

tragically1969 said:

the above is a bit of a sweeping statement to say the least.

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Which is why I made a point of saying "for most applications"!

I agree with Tom. In past jobs, I've removed all UPS. They were a maintenance nightmare.

As for initiating a generator start, I would think that the better design is to initiate on a loss of power, not a control signal from the PLC to the generator that relies on having power at the output of the PLC.

As for checking status of equipment, unless the entire plant is on a UPS, there won't be much to check.