BACKGROUND
We have been having a problem with unexpected stoppages of PLC recently. Another Contractor has possible tracked down the source of our problem to what he refers to "Dark Period". We are using a Siemens S400 Safety PLC and apparently as a safety check the plc will send pulses out to confirm continuity on all Outputs. What the contractor believes is happening is that sometimes a particular relay (always the same one strangely enough) manages to recognise this "dark period" pulse and drops out. His suggestion to fix this issue is to parallel to safety ouputs up so that at least one of the outputs will be able to hold up the relay during the others "dark period". The documentation he has provided appears to be copied from a Siemens Manual.
I think that the contractor is correct in diagnosing the problem but what concerns me is the proposed solution. The plc output in question is driving a solid state realy that is switching 240V AC to control another relay. I feel that the problem amy be caused by the solid state realy being a "bit to smart" for it's own good and that we may be able to fix the problem by using a mechanical relay (one that cannot react fast enough to drop out on the dark period). Problem is Safety PLC's are not my "field" so I thought it best to ask the people that know. Has anyone out there had similar experiences ? It seems a bit much to me that we may need to use two outputs for every relay we want to control.
We have been having a problem with unexpected stoppages of PLC recently. Another Contractor has possible tracked down the source of our problem to what he refers to "Dark Period". We are using a Siemens S400 Safety PLC and apparently as a safety check the plc will send pulses out to confirm continuity on all Outputs. What the contractor believes is happening is that sometimes a particular relay (always the same one strangely enough) manages to recognise this "dark period" pulse and drops out. His suggestion to fix this issue is to parallel to safety ouputs up so that at least one of the outputs will be able to hold up the relay during the others "dark period". The documentation he has provided appears to be copied from a Siemens Manual.
I think that the contractor is correct in diagnosing the problem but what concerns me is the proposed solution. The plc output in question is driving a solid state realy that is switching 240V AC to control another relay. I feel that the problem amy be caused by the solid state realy being a "bit to smart" for it's own good and that we may be able to fix the problem by using a mechanical relay (one that cannot react fast enough to drop out on the dark period). Problem is Safety PLC's are not my "field" so I thought it best to ask the people that know. Has anyone out there had similar experiences ? It seems a bit much to me that we may need to use two outputs for every relay we want to control.