External fault displayed on PLC

[TFlow]

Hello Guys,
I'm new on the forum. There was power outage on my production line some days back but after it was restored, the line did not start up.

While troubleshooting,we noticed an EXTERNAL FAULT displayed on the PLC. We eventually discovered the UPS for the PLC was not on before the power outage so we put it on and reset the PLC then the line could start up.

My question is; what are the likely causes of having an External Fault on a PLC?
And how do I fix it when it comes up.

Thank you

 

[iant]

please give us more info
I am guessing the PLC is a Siemens - Is it?

 

[TFlow]

The PLC is Siemens S7-400.

 

[iant]

the eternal fault is caused by the communications dropping out before the PLC new there was a power failure.
That is what the UPS is for.
Basically the Profibus or Profinet detected a broken cable or device.
even device power failure
no problem but an external fault would be displayed

 

[TFlow]

Thanks a great deal Iant. The guy I met on the job actually insists it was a 24V supply from where that was missing is the cause of the external fault. And he said, if we had RESET the main breaker for the line as many times as possible the fault will disappear.

Thanks for the input. Can you give me any material to read up on this?

 

[iant]

the manuals are available through Siemens
http://support.automation.siemens.com/WW/llisapi.dll?func=cslib.csinfo2&aktprim=99&lang=en