Inrush current affect on a PLC

MET8 · Aug 18, 2009

Hey I was just wondering if anyone knew what kind of affect a high inrush current might have on a plc (mainly the CPU).

I was also wondering if you shut off a panel with a plc in it for a while then turn it on, would a high inrush possibly produce some kind of magnetic field that could mess up a PLC's program? If this is a possibility, could a faraday cage help?

rdrast · Aug 18, 2009

Spikes on the AC line are about 99% more likely to affect a program then inrush current or magnetic fields.

Use a high-quality AC Line filter ahead of the AC source to the PLC (Power supply AND I/O). This is not a typical strip surge suppressor, but something like an industrial strength Corcom filter or similar.

You can also try a UPS, but if so, it really should be a fully on-line version; not one that switches in on line disturbances.

MET8 · Aug 18, 2009

How likely is it do you think that we (several times a week, mainly during the 3rd shift hours) get spikes on the AC line that would be enough to make the plc lose its program? Typically it seems this only happens after the entire cabinet has been shut down for a while.

rdrast · Aug 18, 2009

MET8 said:
How likely is it do you think that we (several times a week, mainly during the 3rd shift hours) get spikes on the AC line that would be enough to make the plc lose its program? Typically it seems this only happens after the entire cabinet has been shut down for a while.

If it happens after the entire cabinet has been shut down for a while, I'd look first to the backup support for the processor, battery or capacitor.

rdrast · Aug 18, 2009

You also didn't mention the brand/type of PLC, Some ControlLogix processors had a firmware conflict a while back that could cause the program to clear randomly.

TheStarr · Aug 18, 2009

As rdrast said, a line filter is very important.

We had a new machine come in that used a 480v to 120v transformaer. When turning the power on, the spike from the transformer stabilizing would zap the program out of the PLC (Compact logix L32)

A quality line filter fixed the problem.

MET8 · Aug 19, 2009

The PLC is an A1SHCPU and the power supply is A1S61P. We've replaced the battery, CPU, black plane, and power supply. Going to see if we have a line filter in there or not. By the way I'm 3rd shift so my posts most likely won't update other than around 6 in the morning

rbtorres · Aug 21, 2009

Most PLC are quite sensitive in all sort of electrical noise or interference. Try to use a better AC line interference filter it may work and avoid your supply line for PLC close to any high power line cables.

BobB · Aug 23, 2009

I generally use a 24VDC powered PLC with a switch mode power supply from AC to DC. Have never had a problem.
Whenever I have used AC powered PLCs I have had the occassional problem - mostly with Hitachi would you believe.
I guess I have adopted the 24VDC rule from working with generators a lot of the time and batteries to power the system.
Even then I use a 24VDC/24VDC switch mode in front of the PLC.

Inrush current affect on a PLC

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