MTBF of PLC Systems 1

[Rodmcm]

Using supplier published MTBF and the e^(-lambda*T) methods for a PLC rack with CPU/Backplane and I/O the unavailability comes to a staggering 1900 hours a year.. Has someone got any sensible fault tree analysis information for PLCs

 

[IRstuff]

How did you arrive at that number? That number sounds implausible, since it would imply a net MTBF of about 260 hr, assuming a 3-day MTTR.

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[ScottyUK]

IRstuff,

thread238-406328

 

[IRstuff]

OK, but how many devices are we talking about? If ultra-reliability is required, there are redundancy approaches that can be implemented. Moreover, unless the OP is talking about thousands of components, the 1900 hrs of downtime is extremely large. For critical systems, the expectation is mean to to repair (MTTR) on the order of an hour or less.

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[Stanley36]

Some failure rates (in hours) of PLCs below, with the source of the data. This includes spurious trip failures and failures that will prevent the system operating but will not cause the PLC to trip (unrevealed failures). Apologies for the format it is from a spreadsheet I have.

CPU 1.0E-05 Exida Safety Equipment Reliability Handbook, 2003, ISBN 0-9727234-0-4
PSU 5.0E-06 Exida Safety Equipment Reliability Handbook, 2003, ISBN 0-9727234-0-4
AI 2.0E-06 Exida Safety Equipment Reliability Handbook, 2003, ISBN 0-9727234-0-4
DI 1.0E-06 Exida Safety Equipment Reliability Handbook, 2003, ISBN 0-9727234-0-4
AO 2.0E-06 Exida Safety Equipment Reliability Handbook, 2003, ISBN 0-9727234-0-4
DO low 1.0E-06 Exida Safety Equipment Reliability Handbook, 2003, ISBN 0-9727234-0-4
DO High 1.0E-06 Exida Safety Equipment Reliability Handbook, 2003, ISBN 0-9727234-0-4
Industrial PLC - Analogue Input (single) 3.6E-06 SINTEF, ISBN 9788214-048490
Industrial PLC - CPU (1oo1) 1.8E-05 SINTEF, ISBN 9788214-048490
Industrial PLC - Digital output (single) 3.6E-06 SINTEF, ISBN 9788214-048490
Programmable safety system - Analogue Input (single) 3.2E-06 SINTEF, ISBN 9788214-048490
Programmable safety system - CPU (1oo1) 9.6E-06 SINTEF, ISBN 9788214-048490
Programmable safety system - Digital output (single) 3.2E-06 SINTEF, ISBN 9788214-048490
Hardwired safety system - Trip amplifier / analogue Input (single) 4.4E-07 SINTEF, ISBN 9788214-048490
Hardwired safety system - Logic (1oo1) 3.3E-07 SINTEF, ISBN 9788214-048490
Hardwired safety system - Digital output (single) 3.3E-07 SINTEF, ISBN 9788214-048490
PLC (DO) - Simplex system 5.0E-06 FARADIP-THREE V6.4