SRC Forum - Message Replies
Forum: Reliability & Maintainability Questions and Answers
Topic: Reliability & Maintainability Questions and Answers
Topic Posted by: Reliability & Maintainability Forum
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998
Posted by: Daniel Mahoney
Date posted: Thu Jul 8 15:13:03 US/Eastern 1999
Subject: Relay reliability
MIL-39016/13 Relays have a life requirement of 100,000 cycles at high level (1 ampere at 28 V dc) and 1,000,000 cycles at low level (10 to 50 milli amperes).
Question 1: Can an operational usage at .85 amperes be extrapolated to arrive at a life of 200,000 hours (i.e., a straight line fit of the two requirements above)?
Question 2: Is it the load at opening/closing of the relay that is driving this requirement or is it the load while the relay is sitting there in the opened/closed position?
A quick response would be grately appreciated.
Subject: Relay Reliability
Reply Posted by: Bruce Dudley
Organization: Reliability analysis Center
Date Posted: Fri Jul 9 10:27:17 US/Eastern 1999
After researching the characteristics of the relay under consideration, I found that if the relay has an AC resistive load the current function is nearly linear, that is I^1.14. If the load is DC then the current function is not as linear, that is I^1.3. So, if you have a AC load, the extrapolation of the 100,000 cycles to 1,000,000 cycles based on load current would be reasonable. Equations for the contact life expectancy for AC and DC loads are cited in a technical report RL-TR-92-197, "Reliability Assessment of Critical Electronic Components" by Mr. Denson (RAC) in table 4.4-12. My estimate for your 0.85 amp usage is 242,000 to 250,000 cycles. The answer to your second question is yes, the switching of the load causes the degradation through arcing and pitting of the contacts.