MTBF of Protection Relays and other things
MTBF of Protection Relays and other things
(OP)
Micom have a published paper that says that the MTBF of a P341 relay as 30 years. This gives a probability of unavailability of 287 hours/year (12 days) which seems ridiculous. Similarly a 16 module PLC rack, using the same idea of summated lambdas (1/MTBF) and probability of reliability as (e^(-lambda*T)) gives an unavailabilty of 1946 hours/year!
Obviously these figures do not seem to match reality. Can anyone explain please
Obviously these figures do not seem to match reality. Can anyone explain please






RE: MTBF of Protection Relays and other things
But, if your MTBF is 30 years, it shouldn't mean that the device is out of service for such a long time as you say. There is no mentioning of MTTR, which is also an important number when it comes to availability.
Schneider says that these data are based on number of sold units and number of returned units. If Schneider then uses the numbers in a too simplistic way or if you use them in a way you shouldn't, I cannot really say. But it is obvious that something isn't right. The unavailability numbers you mention are worse than they were for old vacuum tube computers like the ENIAC and other block heater monsters where there were a few failures each week and the time to repair was very short - repair equaled usually looking for a dark filament and changing a vacuum tube. Mostly done in a minute.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: MTBF of Protection Relays and other things
RE: MTBF of Protection Relays and other things
The young relay tech's want NEW equipment.
The accountants expect 30 years for equipment life.
The budget guys always want one more year.
The scheduling guys want to wait until the next maintenance cycle.
ETC.
RE: MTBF of Protection Relays and other things
Still, lets say the MTTR (mean time to repair) is about 11 days, just because it makes for a nice calculation. So we have,
MTBF = 30 years = 1560 weeks
MTTR = 1.56 week
Availability = 1560/(1560+1.56) = 99.9%
So, assume the gear is expected to last 20 years or 1040 weeks. Then, you can expect 1039 weeks of availability during that 20 years or 1 week of downtime due to the relay over that 20 years. So, I suppose this could mean 0.05 weeks or 8.4 hours of downtime or unavailability per year.
Now calculate again, but change the MTTR to 2 or 3 days since you stock or can order parts and change out the bad piece within days of a failure instead of needing 11 days.
RE: MTBF of Protection Relays and other things
With 16 units then, the failure rate is 60.8 fails per million hr. That comes out to 0.53 fails per year. Assuming a 1 week repair time, the average unavailability is 89.6 hr/yr
TTFN
I can do absolutely anything. I'm an expert!
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: MTBF of Protection Relays and other things
http://files.engineering.com/getfile.aspx?folder=1...
Based on my experience, unavailability of IED is a matter of minutes, not days!
the last figures from Schneider Electric for P341 are MTTF 65 y/MTBF 569400 h.
regards.
RE: MTBF of Protection Relays and other things
TTFN
I can do absolutely anything. I'm an expert!
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: MTBF of Protection Relays and other things
So somehow I suspect those numbers for testing time are guessed, standardized, or some estimate based on the manufacturers reasoning of how long testing should take. This could make a difference in the numbers without changing how reliable the relay really is.
Another factor is what is the expected life of a device. Is it 20 years, 30 years, or is there a formula we don't know?
With all that said, it really comes down to what experence a company has had with a manufacturer.
RE: MTBF of Protection Relays and other things
TTFN
I can do absolutely anything. I'm an expert!
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers