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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

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Posted by: Alan Wild ( )
Organization:Ferranti Technologies Ltd
Date posted: Tue Mar 6 11:18:34 US/Eastern 2001
Subject: Mechanical or Solid State Relays?
Can anyone supply me with information to support a decision on component reliability for a rail application? Which are more reliable, mechanical or solid state relays? Mil-217 analysis (GM environment @ 50 deg C) shows mechanical solonoid types (based on predicted design stresses etc.) to be 0.14 FPMH against Solid state type of 0.35 FPMH. Does anyone out there have any actual recorded results to support or disagree with these predicted results?


Subject: Relay Reliability
Reply Posted by: Bruce Dudley ( )
Organization: Reliability Analysis Center
Date Posted: Tue Mar 6 14:09:08 US/Eastern 2001
Mil-Hdbk-217 is based on reliability data that could be over 10 or 20 year old. The reliability model for the solid state relays was developed in 1992 and published in the technical report RL-TR-92-197, "Reliability Assessment of Critical Electronic Components". In the model development, base failure rates, after all contingency factors were removed, show a rate of 0.034 failures per million hours (FPMHR) for general purpose relays and 0.029 FPMHR for solid state relays. The report indicates that a base failure rate of 0.02 FPMHR was proposed for both solid state and mechanical relays. Why the model parameters were changed for the mechanical relays I do not know. As a result, Mil-Hdbk-217 is more pessimistic on the solid state relays and it looks like there is no good justification of the condition. The RAC Reliability Databases, EPRD (Electronic Parts Reliability Data) and NPRD (Non-Electronic Parts Reliability Data), indicate that the solid state relay could be better than the rate from Mil-Hdbk-217. A ground fixed data source for solid state relays shows a failure rate of 0.03 failures per million hours based on trillions of hours and hundreds of failures. If the value was converted to ground mobile using the translation factors in the Reliability Toolkit, the failure rate would be 0.075 failures per million hours which is better than the mechanical rate from Mil-Hdbk-217.

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