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Frequently Asked Questions

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.

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.

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 cannotanticipate 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.

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.

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.

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. 

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.

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.

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 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.”

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

Terminate testing with a failure
and one operation is one coil
energized and de-energized




Teminate testing without a failure
and one operation is one coil
energized and de-energized cycle



Teminate testing with a failure
and one operation is a contact
make/break per coil energize and
de-energized cycle



Teminate testing with a failure
and one operation is a contact
make/break per coil energize and
de-energized cycle



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

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.

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
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.

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%.

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:    ________________________
               Min.Rated Life

 Example:             $500                  =  $50

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