Frequently Asked Questions

Frequently Asked Questions

How does a relay work?
A relay is an electromagnetic device, within which an electro magnet (sometimes
called a motor) is fixtured to cause controlled movement either by magnetic
attraction or magnetic repulsion. Other hardware attached to the moving magnetic
portion of the motor such as relay contacts will cause switching of electrical circuits.
Why use a relay vs a circuit breaker or switch?
By definition:

RELAY: An electromagnetic device for remote or automatic control that is
actuated by variations in conditions of an electric circuit and which, in turn,
operates other devices (such as switches) in the same or a different circuit.

CIRCUIT BREAKER: A switch that automatically interrupts an electric circuit
under an infrequent abnormal condition; e.g., a fault condition such as an overload
or rupture of either high voltage or high current or both.

SWITCH: A device for making, breaking, or changing the connections in an
electrical circuit. Usually mechanical and operated by hand.
What does normally open, normally closed mean?
The word “normally” refers to deenergized condition of the relay (no power on coil).
The second words “open” and “closed” refer to the position of the contacts at the deenergized condition.
What is the difference between 50, 60, and 400 HZ?
Most AC (alternating current) relays operate on standard frequencies and voltages.

50 HZ FREQUENCY: This is a standard alternating current frequency used
mostly in Europe, Africa, Australia and Asia. Voltages commonly used with this
frequency are: 11 5VAC 10, 115/220 30, 220VAC 10, 220/440 30.

60 HZ FREQUENCY: This is a standard frequency used primarily in the USA and
Canada, but it is also used in many other countries. The typical voltages used in
households and industries are 11 5VAC 10, 115/200 30, 220/400 VAC. In some
instances heavy industry will use 440/66OVAC 30.

400HZ FREQUENCY: This is a standard frequency used in most commercial/military aircraft.
It is also used on Navy aircraft carriers, but primarily to serviceaircraft.
Most aircraft utilize the 115/208 VAC 400 HZ WYE system, except some
newer aircraft, which use 230/400 VAC 400 HZ because of its lighter weight generator system.

The 400 HZ frequency is used in aircraft systems primarily because the
generators are much lighter in weight. 50/60 HZ frequencies are used because
that was the chosen frequency by the federal government years ago, and no one
has upgraded the frequency because of the associated cost.
What is a latch relay, and where is it used?
Latch relays are usually 2-coil, 2-position relays. The coil activation of one coil will
transfer the armature and contacts to the other position. Conversely, activation of
the other coil will transfer the armature and contacts back to its original position.

The purpose of a latching relay is to conserve energy by pulsing the coil with a
short pulse and removing the power once the relay has transferred.

The latch relay lends itself easily to space applications because of minimal power
drain on batteries.
Explain center-off relays. When and where should they be used?
A center-off relay such as a HC or ZC contactor (relay) has its contacts in a null
position, when deenergized. This means that the moving contact is not making
contact with either stationary contact. The relay contactor has two coils, and each
coil operates the moving contact to one or the other stationary contact position.

Center-off relays/contactors can do the following:
- Transfer a power source to two different positions.
- Transfer one circuit to another circuit.
What is the difference between resistive, inductive, motor, and lamp loads?
We must express the load as a contact rating, which is the electrical load-handling
capability of relay contacts under specified conditions and for a prescribed number
of operations or life cycles.

RESISTIVE LOAD: A resistive load usually consists of some sort of resistance in
the circuit; e.g., heaters, resistors, etc.

INDUCTIVE LOAD: An inductive load consists of a load created by a wire wound
coil, such as in a relay or solenoid, a transformer, or any load which uses a
winding over a magnetic iron core. Breaking an inductive load is usually more
severe than breaking a resistive load and will generally produce heavy arcing.

MOTOR LOAD: A motor load can be referred to as a rotating inductive load,
generally with a high inrush of six times the normal load. The breaking of the load
is much the same as a resistive load.

LAMP LOAD: There are many types of lamp loads such as tungsten filament,
fluorescent, mercury-vapor, and other exotic gas lamps. The loads we normally
concern ourselves with are tungsten filament. Tungsten filament lamps, when first
turned on, will draw an inrush current of 10-15 times of the steady-state current.
The inrush is similar to a motor load inrush and is caused by the cold filament in
the lamp. After the lamp filament has heated up, the current will drop to its normal
level. Most tungsten filament lamp load ratings are 20% of a resistive load.
Why do we have 3-phase ratings available?
3-phase power is used over 1-phase power primarily for weight reduction/saving
and efficiency in smoother running accessories. The 3-phase circuits will deliver
both 208 VAC as well as 115 VAC with more torqueing power.

Delta Systems are used almost exclusively aboard military naval vessels. This is
done because there is no ground in the system. This method is used to protect
personnel on board. Should any phase come in contact with ground, the person
shorting himself to any one of the three phases can never short to ground and,
therefore, will never be electrocuted.
Why can’t a power contactor switch low current?
First, we must establish how low is low current, and do we mean dry circuit or low level?

Power contactors can switch low current, but the question arises why would we
want to use a heavy-duty contactor or switch low level or dry circuit if we didn’t
have to?

Also power contactors (contact rating of 25 amperes and upwards) have a
tendency to falter or become intermittent when switching currents of one ampere
or less.

Under normal conditions, one would choose a relay, which is rated for low level
current switching capabilities.
What does “Hi-Rel Relay” really mean?
It is a term that is used to achieve a psychological stimulation and has no relation
to the degree of demonstrated reliability. One manufacturer’s “Hi-Rel Relay” may
actually be equivalent to another manufacturer’s “Low-Rel Relay.” There is no
accepted definition of the term “Hi-Rel.”
When a relay has demonstrated a given failure rate, is the failure rate
applicable for all rated load conditions?
No. In order for a failure rate to be significant, the test conditions and the failure
criteria must be stated. Without including these important factors, a failure rate is
meaningless.
Why is relay reliability expressed in MCBF rather than MTBF like other
electronic components?
Relay failures are primarily due to functional operation (cycling) rather than the
amount of time accumulated while operating. A relay manufacturer cannot
anticipate the number of cycles per hour the relay will be operated when installed
in equipment. If the MTBF is required, the relay user can convert MCBF into
MTBF by dividing the cycling rate (operations per hour) into the MCBF.
Is MCBF a guaranteed period of failure-free service?
No. An MCBF has absolutely nothing to do with the minimum rated life of a relay.
An MCBF is derived from live testing a large number of relays for their minimum
rated life. The number of failures that occur during the testing are then divided
into the total number of completed operations.
Do initial screening tests have any effect on the reliability of relays during the use life?
Yes. Screening tests detect those failures, which would otherwise occur during
the early life of a relay. This is why it is important to know what screening tests
are performed by the manufacturer.

Is there more than one method used to calculate relay reliability failure rates?
Yes. There are several methods used to calculate relay failure rates. The
accepted method and three other methods that are sometimes used to enhance a
particular failure rate are shown in Figure 1.

	Method	Total Operations	Failures	*Failures Rate
A C C E P T E D	Terminate testing with a failure and one operation is one coil energized and de-energized cycle.	10,000,000	6	0.600
S O M E T I M E S U S E D	Teminate testing without a failure and one operation is one coil energized and de-energized cycle	10,000,000	5	0.500
	Teminate testing with a failure and one operation is a contact make/break per coil energize and de-energized cycle	80,000,000	6	0.075
	Teminate testing with a failure and one operation is a contact make/break per coil energize and de-energized cycle	80,000,000	5	0.063

*Failure rate is expressed in % per 10,000 operations.
Figure 1

Would a demonstrated failure rate of 0.001% per 10,000 operations
(one failure per billion operations) at the 90% confidence level be a
reasonable failure rate to expect from a Hi-Rel relay?
No. A 0.01% per 10,000 operations failure rate at the 90% confidence level is
considered to be about the lowest failure rate possible with the present state-of-
the art. . . As a point of information, 23,100 relays, rated for 100,000 operations
each, would have to be tested with no failures in order to demonstrate the 0.001 %
failure rate. It is very unlikely that a relay manufacturer would test and destroy that
many relays just to demonstrate a failure rate. Most likely, the accepted method
was not used to calculate the relay failure rate.

Are there any latent costs that should be considered in the procurement of relays?
Yes. Although relays appear relatively inexpensive when compared to the cost of
an entire system, there is one big cost factor that is almost always overlooked.
That is the cost of a field failure caused by a relay malfunction.

There is a simple method to determine the “add-on” failure cost of a relay. By
dividing the minimum rated life cycles of a relay into its demonstrated MOBE, you
can determine how many relays will be used before a failure will occur. Now, by
amortizing the estimated cost of a field failure over the number of relays to be
used, you will know the “add-on” cost per relay.


               Cost of Field Failure       =Add on Cost/Relay
 Formula:    ________________________
                      MCBF
             ________________________
               Min.Rated Life




 Example:             $500                  =  $50
             ________________________
                  1,000,000
             ________________________
                  100,000

Can a 99% reliability for a particular mission time be achieved when a relay is only capable of 90%?
Yes. Two relays with a 90% reliability can be used in parallel to achieve a
combined reliability of 99%.
What is the effect on reliability by using relays in series to reduce high current arcs?
First of all, operate and release times of identical relays are not always the same
therefore, the last relay to “make” and the first relay to “break” are the only
contacts which would see the high current arc. Second, relays should never be
used in series since the total reliability is reduced by the following relationship.

Total Reliability = R x R,
Where R is the reliability of each unit.