Appendix C. Clues for Use in Sneak Circuit Analysis
C.2 Tree Topograph
Five types of tree topograph patterns that were shown in Appendix A are used
as aids in analyzing the network trees. Application of one or more of these
topographs to each tree can help to alert the analyst to the possible existence
of certain kinds of sneak conditions.
Of the five patterns, the "H" pattern, in which current can flow in opposite
directions through the crossbar, is most frequently associated with significant
In the following pages, clues are listed for analysis of all the tree
topographic patterns except the single power/single ground configuration,
which is the least likely to present sneak circuit problems.
Single Power/Multiple Ground
Clues to Sneak
Might a switch be closed when a load is not
desired (and reverse)?
Can a load be enabled through more than one switch
path? (Can a load fail to be disabled by a single switch
Can transitory current paths exist during change
of state of switches?
Can incorrect sequence of switch operations cause
undesired result? (Are interlocks necessary?)
Can relay races occur? (Are interlocks
Can timing gaps or overlaps exist in relay
circuits (e.g., break-before-make)?
Can relays or series loads be unintentionally
energized/de-energized due to critical pick-up or drop-out
Do power-to-ground paths require electrical
Can diodes or other current direction limiters
conduct when current flow is not desired (and reverse)?
Can current flow through suppression diodes cause
Can press-to-test circuits energize circuits other
than those intended to be tested?
Are grounds at different potentials?
Do labels reflect true functions?
Do any circuits loop back through same
Are there any unnecessary
Multiple Power/Single Ground
Clues to Sneak
All clues of Case I except item 12 plus:
Does loss of ground allow one power source to feed
the other, resulting in high current flow if impedance is low?
Can multiple power sources enable a given load
(and reverse: can disabling one power source not disable load)?
Does loss of one power source so mis-bias the
circuit as to allow unwanted paths?
Does momentary loss of ground generate transients?
Multiple Power/Multiple Ground
All combinations of Cases I and II.
Multiple Power/Multiple Ground With Crossbar
Connection Having Impedance and/or Switch and/or Diode in Crossbar
All combinations of Cases I, II, and III plus:
Does reverse current flow exist in the
Is the configuration such that a ground-seeking
branch for any node contains another node which in turn can lead to power by
a different path?