Table 2. Normalized
Failure Mode Distributions for
Table 2 shows the relative probability of the three principal
failure modes for relays. Relays most
commonly fail in the "stuck open" position where the mechanical switching
element fails to close and the relay fails to carry a current. Relays are less likely to unintentionally
close or remain closed after the switching current is released. For this reason, the reliability of relay
circuits can be improved by using parallel
Unlike most of the other electrical parts, relays
(with the exception of solid state relays) contain a switching element that
physically moves to make electrical contact.
This makes them less likely to follow a constant failure rate or
traditional "bathtub curve” profile.
Instead, they are more prone to follow the failure rate curve for a
mechanical part, with an increasing failure rate with age. Except for special
high voltage and high temperature applications, solid-state relays are
inherently more reliable and predictable for long life applications.
The two most common failure mechanisms of relays are
contamination and mechanical wear of the internal switching elements discussed
a. Contamination is a major
cause of early life failures. Sources of
contamination are numerous, but they are often from the various chemicals used
in the manufacturing operation (e.g., soldering fluxes and cleaning
agents). Types of contamination can be
divided into two categories: metallic and non-metallic. Metallic contamination causes shorted
conditions or blocks the physical movement of mechanical elements. Non-metallic or gaseous contamination creates
open circuits when it periodically deposits itself on contacts.
b. A second major cause of early
life failures in relays is mechanical wear of internal switching elements. In fact, the life of a relay is essentially
determined by the life of its contacts.
Degradation of contacts is caused from high in-rush currents,
high-sustained currents, and from high voltage spikes. The source of high currents and voltages, in
turn, are determined by the type of load.
Inductive loads create the highest voltage and current spikes because
they have lowest starting resistance compared to operating resistance. This is especially true for lamp filaments
and motors, which is why derating is more severe for these types of loads. The life of a contact can be further degraded
if contamination or pitting is present on the contact. Physical wear can also
occur to other elements within the relay.
Some relays contain springs to provide a mechanical resistance against
electrical contact when a switching current is not applied. Springs will loose resiliency with time. Relays can also fail due to poor contact
alignment and open coils.
Selection of the proper relay type for a given
application is the most significant factor affecting relay reliability. Many poor design practices are used when
designing them into circuits. This is
because relays are a relatively uncommon circuit element and often receive
little attention during the design process.
Whereas most designs will use hundreds each of microcircuits, resistors,
etc.; relays typically number in the single digits. Therefore, designers are often less familiar
with the intricacies of selecting the proper relay type and rating for a
particular application. Some of the more
common poor design practices are listed as follows:
a. Paralleling contacting is where two relays
are placed in parallel to handle the current that one of them cannot handle
alone. The problem with this type of
design is that mechanical switching occurs at relatively slow switching
speeds. Therefore, for a brief instant,
only one relay needs to carry the full current load. Further, switching speeds tend to slow with
age, amplifying the affect over time.
The preferred method is to use a single relay of sufficient current
handling capability. If dual relays are
used in parallel in increase reliability, each relay should be capable of
handling the full current load.
b. Transient surge currents
are often difficult to measure and predict, especially when switching inductive
loads. It is not uncommon for surge
current to reach ten times steady state current. Protective devices should be used to limit
surge current. The simplest solution is
to use a relay with a substantially higher rated surge current than
c. A cold filament lamp draws between 3 and 10
times the steady-state current until warmed up.
Relay contacts used for switching lamps should be able to withstand
such current surges without the possibility of welded
d. Relays are sometimes used
in applications where they switch a redundant circuit element or an additional
power supply current into a circuit.
High surges occur in ac applications when the redundant current is not in
synchronization with the original current.
e. A typical misapplication
is the use of small multipole relays in 112/200 volt 3-phase ac
applications. Phase-to-phase shorting at
rated loads is a strong possibility, with potentially catastrophic
f. Caution must be applied when using relays to
reverse motors, particularly where the motor can be reversed while running
(commonly called "plugging"). This
results in a condition where both voltage and current can greatly exceed
nominal. Only power relays rated for
"plugging" and reversing service should be utilized in these
g. A relay race condition occurs when one relay
must operate prior to another from a separate drive circuit, but fails to do
so. The problem usually occurs after the
equipment ages or temperatures rise.
Potential race circuits should be avoided. Where they must be used, extra consideration
must be given to wear considerations, coil suppression circuitry, ambient
temperature, drive power, and operate and release times.
h. A slowly rising triggering
current has an increased likelihood of causing chattering conditions. A problem occurs because back electromotive
forces (EMFs) are produced when the armature closes to the pole face. This voltage is opposite in polarity to the
driving voltage and can cause the relay to release immediately after initial
contact. This process repeats and causes a chatter condition until a sufficient
amount of drive current is available to overcome the back
Derating of continuous current is dependent upon the
load type and is shown in Table 3. Derating is more severe for inductive and
filament loads, due to high current demands upon initial startup and increased
propensity of voltage spikes. If a relay
is used to switch a combination of loads, the most dominant load should be used
for derating purposes.
Some relay specifications will contain
individual current limitations for capacitive, inductive, motor, and filament
loads. For such specifications, limit
current to either the current derived through Table 1 or the maximum current rating for the particular load type given in the
specification, whichever is less.
The voltage to energize or trigger the relay should
be at least 110% of the minimum rated energizing voltage. Coil energizing voltage is not derated in the
traditional sense of the term because operation of a relay at less than nominal
ratings can result in switching failures or increased switching times. The latter condition introduces contact
damage and can reduce relay reliability.
Coil Dropout Voltage
The voltage to dropout or un-trigger a relay should
be less than 90% of the maximum rated coil dropout voltage.
Limit ambient temperature to maximum rated ambient
temperature as shown in Table 3.
Note: Relay ratings may be given
under the assumption that the relay case will be grounded. If such relays are used in applications where
the case is not grounded, additional derating should be considered because the
relay may lack arc barriers and contain smaller internal
Table 3. Derating Requirements for Relays
% of Resistive Load Rated Value in Environment
Category 1 Protected
Category 2 Normal
Category 3 Severe
70 -- Resistive Load
70 -- Capacitive Load
50 -- Inductive Load
30 -- Motor
20 -- Filament (Lamp)
60 -- Resistive Load
60 -- Capacitive Load
40 -- Inductive Load
20 -- Motor
10 -- Filament (Lamp)
50 -- Resistive Load
50 -- Capacitive Load
30 -- Inductive Load
20 -- Motor
10 -- Filament (Lamp)
Coil Energize Voltage
Coil Dropout Voltage
10oC of Max Rated
20oC of Max Rated
30oC of Max Rated
Design and Material
construction methods and materials of each type of relay differ. Considerable
differences exist between the materials and processes The construction methods
and materials of each type of relay differ. Considerable differences exist
between the materials and processes used to manufacture relays. A relay, in its
most basic form, is a combination of a switch and an inductive element. In solid state relays, the inductor is
replaced by a semiconductor element. The
military specifications and standards are listed in Table 1, and other information can be found in MIL-STD-1346. The following lists the major categories
a. A reed relay is operated
by an electromagnetic coil or solenoid which, when energized, causes two flat
magnetic strips to move laterally to each other. The magnetic reeds serve both as magnetic
circuit paths and as contacts. Because
of the critical spacing and the frailty of the arrangement, the reeds are
usually sealed in a glass tube.
b. A solid state relay
incorporates semiconductor or passive circuit devices. As the name implies, it
contains no moving parts, and therefore has low switching noise and essentially
no bounce or chatter. Solid state relays
also have long life and fast response times.
Their main disadvantage is a limited number of applications for which
they can be used.
c. A latching or magnetic
latching relay is a bistable polarized relay having contacts that latch in
either position. A signal of the correct
polarity and magnitude will reset or transfer the contacts from one position to
Note: In addition
to the categories listed above, mercury, bimetal (thermal), and contractor relay
technologies are also available.
Assessment and Quality
Quality and reliability levels of relays may be
expressed as the number of switch cycles before wear-out rather than the more
traditional failure rate. Vendors
consider rated number of switch cycles to be the guaranteed minimum number of
cycles the relay can withstand under normal operating conditions before failure
(intermittent or constant). Relays
manufactured to the military standards have a failure rate level designator as
shown in Table 4
at 90% confidence level for qualification and 60% confidence level for
maintenance of qualification). Quality
is further dependent on the ruggedness of the package and how well the internal
switching elements are sealed against influences of the outside
environment. Commercial grade relays and
relays found in COTS equipment are not routinely acceptable for use in Military
able 4: Failure Rate Level Designators for Relays
Failure Rate Letter Designator
Failure Rate Level (%failures per 10,000 Operations)
Some relay vendors will advertise ISO 9000 quality
systems or state they are ISO 9000 certified.
Many manufacturers will then give a higher vendor rating (or increased
preference) to the ISO 9000 certified vendor.
While acceptable, care must be taken to also account for the fundamental
design aspects of the relay. For
example, a commercial-grade relay designed to withstand a sufficient number of
switch cycles to operate 3‑5 years in a particular application, should not be
used in a system with an anticipated life of 15 years, even if the vendor for
the commercial relay is ISO 9000 certified.
The trend in Military procurements is to specify end
item performance requirements rather than specifying specific sampling
plans. Sampling plans differ between
vendors, and the particular part vendor must be consulted if test and inspection
sampling rates are needed.
If process controls or SPC requirements are needed
for a particular application, consult the individual vendor detailed
Relays typically do not require additional screening
or testing by the user at the piece part level.
Handling and Storage
No special handling precautions are necessary for
relays. Mechanical relays are not
considered ESD sensitive, and solid state relays are normally rated at Class 3
ESD sensitive. Care should be exercised
when handling hermetically sealed relays to retain the hermetic seal.
Solid stage relays are generally preferred over
electromechanical relays due to decreased chatter noise, increased reliability,
and more consistent performance with age.
Exceptions are relays used in high temperature environments.
Special mounting consideration are necessary for
mechanical relays in high or vibration environments because relays are typically
high mass parts and can switch unintentionally when subjected to shock. Particular care is needed in airborne
applications. Relays should not
unintentionally switch even during absolute worse case operating
conditions. In addition, the designer
should take into account the wear of springs in long life
Arc suppression techniques should be used to protect
relay contacts of intermediate and power level devices to increase long term
reliability. Arc suppression usually
consists of external circuitry (e.g., diodes) to limit current surge.
To increase reliability, relays can be designed
into circuits with parallel redundancy.
The relative probably of a relay failing in the open position is
substantially higher than failure in a closed position (see Table 2), thereby
improving reliability in parallel redundant configurations. However, parallel redundancy should only be
used to increase reliability, not to increase the current handling capabilities
of a relay circuit.
For relays used over a wide temperature rate,
account for increased switching current demand at higher temperatures. As a general rule of thumb, coil resistance
increases with temperature at a rate of 0.004 W/W/oC.
If a relay is rated under grounded case
conditions, the relay should only be used in applications where the case will be
grounded. Use in an ungrounded
application may cause a personnel hazard.
When using relays to reverse motor loads while
running, use only relays specifically rated to reverse switch motor