Likely failures in proven systems
The following problems are arranged in order
from most likely to least likely, top to bottom. This order
has been determined largely from personal experience
troubleshooting electrical and electronic problems in
automotive, industry, and home applications. This order also
assumes a circuit or system that has been proven to function
as designed and has failed after substantial operation time.
Problems experienced in newly assembled circuits and systems
do not necessarily exhibit the same probabilities of
occurrence.
Operator error
A frequent cause of system failure is error
on the part of those human beings operating it. This cause
of trouble is placed at the top of the list, but of course
the actual likelihood depends largely on the particular
individuals responsible for operation. When operator error
is the cause of a failure, it is unlikely that it
will be admitted prior to investigation. I do not mean to
suggest that operators are incompetent and irresponsible --
quite the contrary: these people are often your best
teachers for learning system function and obtaining a
history of failure -- but the reality of human error cannot
be overlooked. A positive attitude coupled with good
interpersonal skills on the part of the troubleshooter goes
a long way in troubleshooting when human error is the root
cause of failure.
Bad wire connections
As incredible as this may sound to the new
student of electronics, a high percentage of electrical and
electronic system problems are caused by a very simple
source of trouble: poor (i.e. open or shorted) wire
connections. This is especially true when the environment is
hostile, including such factors as high vibration and/or a
corrosive atmosphere. Connection points found in any variety
of plug-and-socket connector, terminal strip, or splice are
at the greatest risk for failure. The category of
"connections" also includes mechanical switch contacts,
which can be thought of as a high-cycle connector. Improper
wire termination lugs (such as a compression-style connector
crimped on the end of a solid wire -- a definite
faux pas) can cause high-resistance connections after a
period of trouble-free service.
It should be noted that connections in
low-voltage systems tend to be far more troublesome than
connections in high-voltage systems. The main reason for
this is the effect of arcing across a discontinuity (circuit
break) in higher-voltage systems tends to blast away
insulating layers of dirt and corrosion, and may even weld
the two ends together if sustained long enough. Low-voltage
systems tend not to generate such vigorous arcing across the
gap of a circuit break, and also tend to be more sensitive
to additional resistance in the circuit. Mechanical switch
contacts used in low-voltage systems benefit from having the
recommended minimum wetting current conducted through
them to promote a healthy amount of arcing upon opening,
even if this level of current is not necessary for the
operation of other circuit components.
Although open failures tend to more
common than shorted failures, "shorts" still
constitute a substantial percentage of wiring failure modes.
Many shorts are caused by degradation of wire insulation.
This, again, is especially true when the environment is
hostile, including such factors as high vibration, high
heat, high humidity, or high voltage. It is rare to find a
mechanical switch contact that is failed shorted, except in
the case of high-current contacts where contact "welding"
may occur in overcurrent conditions. Shorts may also be
caused by conductive buildup across terminal strip sections
or the backs of printed circuit boards.
A common case of shorted wiring is the
ground fault, where a conductor accidently makes contact
with either earth or chassis ground. This may change the
voltage(s) present between other conductors in the circuit
and ground, thereby causing bizarre system malfunctions
and/or personnel hazard.
Power supply problems
These generally consist of tripped
overcurrent protection devices or damage due to overheating.
Although power supply circuitry is usually less complex than
the circuitry being powered, and therefore should figure to
be less prone to failure on that basis alone, it generally
handles more power than any other portion of the system and
therefore must deal with greater voltages and/or currents.
Also, because of its relative design simplicity, a system's
power supply may not receive the engineering attention it
deserves, most of the engineering focus devoted to more
glamorous parts of the system.
Active components
Active components (amplification devices)
tend to fail with greater regularity than passive
(non-amplifying) devices, due to their greater complexity
and tendency to amplify overvoltage/overcurrent conditions.
Semiconductor devices are notoriously prone to failure due
to electrical transient (voltage/current surge) overloading
and thermal (heat) overloading. Electron tube devices are
far more resistant to both of these failure modes, but are
generally more prone to mechanical failures due to their
fragile construction.
Passive components
Non-amplifying components are the most
rugged of all, their relative simplicity granting them a
statistical advantage over active devices. The following
list gives an approximate relation of failure probabilities
(again, top being the most likely and bottom being the least
likely):
-
Capacitors (shorted), especially
electrolytic capacitors. The paste electrolyte tends
to lose moisture with age, leading to failure. Thin
dielectric layers may be punctured by overvoltage
transients.
-
Diodes open (rectifying diodes) or shorted
(Zener diodes).
-
Inductor and transformer windings open or
shorted to conductive core. Failures related to
overheating (insulation breakdown) are easily detected by
smell.
-
Resistors open, almost never shorted.
Usually this is due to overcurrent heating, although it is
less frequently caused by overvoltage transient (arc-over)
or physical damage (vibration or impact). Resistors may
also change resistance value if overheated!
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