||NAVSO P-3634: Sneak Circuit Analysis: A Means of Verifying Design Integrity
Appendix A. Examples of Sneak Circuit Analysis
A.2.4 Analysis of Network
Using the simplified representations of the system
during various time intervals, the analyst next determines whether sneak paths
are likely to exist. A major aid in this part of the process is a set of
"clues" identifying conditions that have been associated with sneak circuits
in previous sneak circuit analyses. (A set of such clues is included in this
document as Appendix
. The appendix also includes clues for identifying design concerns.)
One type of clue is the topographic pattern exhibited by the network tree.
Typical patterns are shown in Figure A-5 . Depending on the pattern perceived
by the analyst, he examines the circuits in the light of the clues applicable
to the particular topograph.
FIGURE A-5. TREE
FIVE POSSIBLE PATTERNS
The tree diagrams developed for the example show the H-pattern topograph.
This indicates multiple power and ground paths and a "cross-bar" containing a
switch, diode, or load between paths. Because this condition exists, the
analyst is motivated to examine carefully the operation and the sequence of
events in the circuit. The findings of the analysis for the five time
intervals might take the following form:
Time Interval 1
- No 28 V power available.
- All relay contacts and switches are in their normally de-energized
Time Intervals 2 and 3
- 28 V power is provided externally (Time Interval 2)
or internally by aircraft generators (Time Interval 3).
- The Y1/Y2 winding of relay K2 is energized, holding its contacts in
their existing positions, i.e., contacts 11-12 open, contacts 13-12
Time Interval 4
- Aircraft in flight, internal power.
- Main radar switch is closed.
- Relay K1 is energized.
- Its normally open contacts (16-14) close, and its normally closed contacts (16-15) open.
- Power is removed from relay coil Y1/Y2.
- Voltage is applied to CR7, causing transistor Ql to conduct.
- The X1/X2 winding of relay K2 is energized. Its normally open contacts (11-12) close, and its normally closed contacts (12-13) open.
- Power is restored to the Y1/Y2 coil; however, since power was never removed from the X1/X2 coil, the relay does not change state.
Time Interval 5
- Aircraft lands, internal power maintained.
- Main radar switch is opened.
- Relay Kl is de-energized.
- Its contacts return to their normal de-energized positions. Power is applied to the Y1/Y2 coil.
- Before SK2 contacts 11-12 can open, power is applied to the X1/X2 coil.
- Since the action of transistor Ql is faster than the relay pickup time, the X1/X2 and Y1/Y2 coils are energized at virtually the same time.
- The X1/X2 coil dominates, and the K2 contacts maintain the position of Time Frame 4.
- Therefore, the radar is not de-energized.