PLC Major Error when Switching to Emergency Power

arocha06

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Join Date
Oct 2012
Location
Boston, MA
Posts
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Upon switchover to emergency power the PLC (Micrologix 1400) enters fault mode, major error code 0020h. The PLC is connected to a UPS and never lost power.
I switched the plc to mode program and then to run mode in order to reset the fault and it is operational again.

Any thoughts as to what may have casued this failure and how to resolve it? The only input that the PLC should have recieved due to the power switchover is loadsheds for the three pumps. (pumping station)

Thanks!
 
Is the PLC working through the UPS and does the UPS have enough power to run all the associated PLC equipment.
If you power the PLC with the UPS only - how long will it last before it fails?
the PLC might be seeing either a spike or drop in supply voltage
 
Thank you iant,

The transfer to emergency power takes less than a minute and the UPS has enough charge to last much longer than that. Good thought on the spike/drop in voltage, I will check on that.
 
Upon switchover to emergency power the PLC (Micrologix 1400) enters fault mode, major error code 0020h. The PLC is connected to a UPS and never lost power.
Another name for this set-up is a "UPS battery tester". How long has it been since your PLC UPS battery was discharged-and-recharged, or replaced? Batteries develop memories (resistance to recharging), become weak so that the voltage level cannot be maintained for very long.
 
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The transfer to emergency power takes less than a minute and the UPS has enough charge to last much longer than that.
But does the UPS have enough capacity in Amp-Hours to handle the current for all connected loads? If not, the voltage will sag and your PLC will drop out (just as it did). I hope you have at least a 90-day warranty! Just because it shows fully charged does not mean it can run your PLC (and control devices) for even 1 minute.

If you power the PLC with the UPS only - how long will it last before it fails?
Run Iant's test next, with power to the UPS turned off. Then turn power back on, and measure the total current load with an ammeter. It could be more than your UPS is rated.
 
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I have two somewhat similar installations but have not had any problems with them (so far).
I did receive a warning from the UPS supplier's tech support that some of their models do not work properly on generator power.
You might want to check with your UPS supplier's tech support to see if they have any warnings on this issue.
 
Upon switchover to emergency power the PLC (Micrologix 1400) enters fault mode, major error code 0020h.

See picture and PDF

20Hfault.png
 

Attachments

  • minorfaultS5bits.pdf
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Is it an online UPS or a bypass UPS? If bypass the PLC will cop harmonics from the generator. Have seen that before and is one reason why I use 24VDC PLCs and switch node power supplies in the middle.
 
Read Mickey's response very closely: Error code 20h is not a typical "memory lost" errot that you might associate with a "bump" or "spike" in supply power.

It's a math error, usually associated with overflows.

Could a signal coming into the controller be dropping to a lower voltage that would cause such an overflow in a math instruction ?

Trapping S:5 errors is notoriously difficult in the SLC/MicroLogix platform, but there are resources in the user manual and on the Forum to help.

Does this fault happen every time the switchover is done ?

A diagnostic that would prove it's not a noise or spike problem is to place the controller in PROG mode and perform a UPS switchover.
 
Thank you all for the help, Mickey's post and supporting docs were very heplful. I corredcted the 20h overflow error by adding an S5 OTU at the end of the file. Emergencys switchovers now occur without a hitch! (y)
 
Thank you all for the help, Mickey's post and supporting docs were very heplful. I corredcted the 20h overflow error by adding an S5 OTU at the end of the file. Emergencys switchovers now occur without a hitch! (y)

I would advise that you find out the root cause of the minor error, rather than unconditionally unlatch (ignore) it.

What you did may be harmless, but it would be better to know exactly what caused it and fix that.
 

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