Cycling Power to PLC's

My background is a continuous automation line where the control power was always maintained unless for extreme circumstances. I have joined a new company that all the PLC's are tied to independent equipment and the power is being constantly cycled. I have yet to find any tech notes or white pages on the downside of this course of action. I this just an urban legend?

Thank you,

Jason

Independent PLCs

We run a similar method here with PLCs tied to individual machines, on many machines the PLC (and HMI) gets shut off regularly, however there is some that keep control power.

Even though I have only been here a short time I have not seen any difference in program or processor failure/malfunction between the ones that maintain power and the ones that shut off.

Independent PLC

One thing I forgot to mention, if you regularly shut down the PLC, battery maintenance becomes very important, that is the only real difference I have seen. But it is an easy fix, new battery, reload program and the machine is back on its merry way.

There is always a small chance of inductive kick damage when cycling the power to any piece of equipment. Then again, there is always a chance of incurring a power line spike while equipment is continuously energized.

As mentioned in a previous post, the memory backup battery suffers somewhat when power is regularly removed from a PLC. For most PLCs, EEPROMs can help with that situation.

What urban legend? Cycling power to a PLC is no different than cycling power to any computer.
That being said, if you have never seen your PLC turned off, you may be blissfully unaware of variables that have values that aren't properly initialized. If a PLC variable has a value that isn't set by the logic, but rather was set by someone using the programming software, upon power restoration it may have a different value. Whether that happens depends on the application and the configuration of the variable.
On machines that have been in operation for any length of time and where the PLC is regularly turned off, any issues along those lines have most likely been resolved.

What urban legend? Cycling power to a PLC is no different than cycling power to any computer.

Click to expand...

I agree. The Law of Entrophy still applies. The longer a computer, electronic device, or any machine is ON, the less time is left of its remaining lifetime. Many computers are used 4 to 6 hours a day (or less), but are left running 24 hours, meaning that the owner will get to use approximiately 25% of the life of of the machine. The % Usage is actually much less, because many machines are left on over weekends, holidays, and vacations. Subtracting those times out, many computers have an effective useful life of about 15% of the possible life.

I see the same type of thing in many plants, where machines, heaters, lighting, and other equipment runs 24 hours a day, whether it is needed or not. Just about any plant could add 10% to its bottom line by cutting off unneeded equpment.

I think the most important consideration now is just the programming implications, i.e. a delay in accepting values from anything until everything is fully powered and checked. It's not so much of a problem as it used to be, but it can still rear it's head under the right circumstances.

The "urban legend" may stem from the early days of PLCs when the power supplies were linear, not switch mode as they all are now. The linear power supplies tended to be more finicky about input power, hence the requirement for CVTs ahead of them, and the output was ramping as you powered it up, which could cause errors if not accounted for. One nasty one was if someone turned on the main power, then immediately hit the Start button as the linear power supply was still ramping up, because things might start out of sequence. With SMPS, there is no output until it is a good stable output, and it is very quick to get there as well.

However with regard to EEPROMs, they all have a finite number of write cycles, so if you are storing to an EEPROM with each power down, you are technically consuming that capability. That was typically in the 10's of thousands of write cycles, which may have been another contributor to the urban legend, but modern EEPROMs or Flash Memory is now in the millions of write cycles, so the likelihood of consuming it in the life of the machine by powering it off once per day or even once per shift, is so low as to be insignificant any more.