FAQs

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 many relays just to demonstrate a failure rate. Most likely, the accepted method was not used to calculate the relay failure rate.

By definition :

RELAY : An electromagnetic device for remote or automatic control that is actuated by variations in conditions of an electric circuit and wich, 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.

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 circult 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.

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.

Relay fallures are primarily due to functional operation (cycling) rather than the amount of time accumulated while operating. A relay manufacturer can not 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.

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.

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 reliablity of each unit.

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 coll, 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.

Latch relays are usually 2-coils, 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.

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.

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 manufacturers “Low-Rel Relay.” There is no accepted definition of the term “Hi-Rel”.

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 this table.

ACCEPTED

Method : Terminate testing with a failure, and one operation is one coil energized and de-energized cycle.

Total Operations : 10 000 000

Failures : 6

Failure Rate* : 0,600

SOMETIMES USED

Method : Terminate testing without a failure, and one operation is one coil energized and de-energized cycle.

Total Operations : 10 000 000

Failures : 5

Failure Rate* : 0,500

Method : Terminate testing with a failure: one operation is a contact make/break per coil energized and de-energized cycle.

Total Operations : 80 000 000

Failures : 6

Failure Rate* : 0,075

Method : Terminate testing with a failure: one operation is a contact make/break per coil energized and de-energized cycle.

Total Operations : 80 000 000

Failures : 5

Failure Rate* : 0,063

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

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.

A relay is an electromagnetic device, within wich 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.

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

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.

Yes, Two relays with a 90% reliability can be used in parallel to achieve a combined reliability of 99%.

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 fallure over the number of relays to be used, you will know the “add-on” cost per relay.

Formula :

Add on Cost/Relay = Cost of Field Failure ÷ MCBF ÷ Min.Rated Life

Example :

$500 ÷ 1 000 000 ÷ 100,000 = $50

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: 115VAC 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 115VAC 10, 115/200 30, 220/400 VAC. In some instances heavy industry will use 440/660VAC 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 service aircraft. 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.