Siemens S7 Intermitant power loss

Hi All,

We have a PLC which intermetantly losses power. The screen powers down then reboots. THe plant starts and 15 seconds later powers down again. This can happen a number of times, then the plant will run and can run for a couple of days.

Any ideas on what or how to find the problem.

Thanks John127

Bad connections?
Did you ever measure the PLC input voltage? Amps?
Change the power module to a bigger one?

K

The PLC has been in for years with no alterations so I think the power module usless defective should not need increasing?. It is supplied by 240volts transforming / rectifying to 24vdc. There is no signs of 240 voltage drop. The inputs and outputs are 24vdc.

Is it more likely to be the 240v or the 24vdc causing the problem?

You have seen it powering down, but have not looked inside panel and checked the psu at that moment?

We have seen the sreen power down and reboot. We have checked the 240volt supply which is ok. We checked the 24vdc and recoreded a 1.8voltage drop.

Would this voltage drop cause the PLC to power down? If so how do we determine the cause?

It could. it should not. Have you looked for shorts? Try changing psu.

we installed 500ma fuse terminals in each of the 24vdc cicuits. One kept blowing the fuse so we checked associated cables and switches. The switches and part of the cable are located in a cold environment. We found the cable brittle so we changed it and the switch. This still blew the fuse. This weekend we have installed a seperated power supply to this switch, switching the input through a relay to try and remove this directly from the PLC (I should mention that all 24vdc i/o are direct to the PLC not through relays) Today I have been told the PLC still loses power.

Very Frustrating.

How old is current psu? is it tressed how near its max current output?

I sounds as if you have a intermittent load that is not quite shorting your PSU and dropping your 24vdc voltage as soon as this happens the device that is dropping your PSU is then switched off and the PLC and Screen can recover.
You need to supply the PLC and Screen with a separate PSU just until you find the input /Output that is causing the problem. possible a trapped cable, wrong solenoid 240v instead opf 24vdc etc.
If you have min max on your multimeter you can set it up for min and measure the 24vdc when it dips the minimum voltage will be displayed.
It sounds like an electrical problem ie shorting or overloading.

Reading again you found an electrical problem but as you say all 24vdc is direct to the PLC so any problems will pull the common PSU down.
After isolating the supply you will find the fault as either the output or the input will not work.