About Mtbf - Racal Instruments 1260-X121 User Manual

About MTBF

Specifications 1-4
By substituting the actual values:
Total power dissipation =
* (0.1 Ω) * 5] + ( 1.5 W) = 51.5 W at 55°C
2
[(10 A)
This is acceptable power dissipation for an individual plug-in
module. In practice, rarely are more than 25% of the module's
relays energized simultaneously, and rarely is full rated current run
through every path. In addition, the actual contact resistance is
typically one-half to one-fourth the specified maximum, and
temperatures are normally not at the rated maximum. The power
dissipated by each plug-in should be no more than 15 W if all six
slots are used simultaneously. This yields the following guideline:
3 A
5 A
10 A
The numbers in the above table represent worst-case, elevated-
temperature, end-of-life conditions.
Additionally, if fewer plug-in modules are used, more power may
be dissipated by the remaining cards. By using a chassis with high
cooling capacity, such as the Racal Instruments 1261B, almost any
configuration may be realized.
The 1260-X121 MTBF is 783,549 hours at 25°C and 718,344
hours at 30°C, calculated in accordance with MIL-HDBK-217E,
with the electromechanical relays set at 50% rated load at 85°C.
Factors affecting relay life expectancy are:
1. Switched voltage
2. Switched current
3. Switched power
4. Maximum switching capacity
5. Maximum rated carrying current
6. Load type (resistive, inductive, capacitive)
7. Switching repetition rate
8. Ambient temperature
The most important factor is the maximum switching capacity,
Max. 15 relays closed
Max. 6 relays closed
Max. 2 relays closed
Racal Instruments © 2005
1260-X121 User Manual