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Circuit Protection & Control Devices

Circuit Protection

Circuit Protection Device

Circuit protection devices form an important part of the aircraft power distribution system.  A circuit protection device operates by opening and interrupting current to the circuit. The opening of a protection device shows that something is wrong in the circuit and should be corrected before the current is restored.

When a problem exists and the protection device opens, the device should isolate the faulty circuit from the other unaffected circuits, and should respond in time to protect unaffected components in the faulty circuit.


To do this, a circuit protection device must ALWAYS be connected in series with the circuit it is protecting. If the protection device is connected in parallel, current will simply flow around the protection device and continue in the circuit.

The protection device should NOT open during normal circuit operation.


Fuse is a simplest circuit protection device, it is designed primarily to protect the cables of a circuit against the, flow of short circuit and overload currents.

The earliest type of fuse was simply a bare wire between two connections. In its basic form, a fuse consists of a low melting point fusible element or link, closed in a glass or ceramic casing which not only protects the element, but also localizes any flash which may occur when "fusing".  The element is joined to end caps on the casing, the caps in turn, providing the connection of the element with the circuit it is designed to protect.


Under short-circuit or overload current conditions, heating occurs, but before this can affect the circuit cables or other elements, the fusible element, which has a much lower current carrying capacity, melts and interrupts the circuit, the materials most commonly used for the elements are tin, lead, alloy of tin and bismuth, silver or copper in either the pure or alloyed state.

Fuses are located accessible for replacement, and as close to a power distribution point as possible so as to achieve the minimum of unprotected cable.


Fuses are installed in two types of fuse holders in aircraft. “Plug-in holders” or in-line holders are used for small and low capacity fuses. “Clip” type holders are used for heavy high capacity fuses and current limiters.



Circuit Breaker

Circuit breakers, unlike fuses  isolate faulted circuits and equipment by means of a mechanical trip device actuated by the healing of a bi-metallic element through which the current passes to a switch unit. We may therefore consider them as being a combined fuse and switch device.

While a fuse protects a circuit, it is destroyed in the process of opening the circuit and must be replaced while the circuit breaker can be reset after clearance of a fault.

The mechanism is of the "trip-free'' type, i.e. it will not allow the contacts of the switch unit to be held closed while a fault current exists in the circuit.

The design and construction of circuit breakers varies, but in general they consist of three main assemblies;

  • a bi-metal thermal element,

  • a contact type switch unit and

  • a mechanical latching mechanism.


A push-pull button is also provided for manual resetting after thermal tripping has occurred, and for manual tripping when it is required to switch off the supply to the circuit of a system.

CB Construction.jpg


Circuit breaker Working

Under normal operating conditions current passes through the switch unit contacts and the thermal element, which thus carries the full current supplied to the load being protected. At normal current values heat is produced in the thermal element, but is radiated away fairly quickly.


When the current exceed the normal operating value due to a short circuit, the temperature of the element begins to build up, and since metals comprising the thermal element have different coefficients of expansion, the element becomes distorted.


The distortion eventually becomes sufficient to release the latch mechanism and allows the control spring to open the switch unit contacts, thus isolating the load from the supply, At the same time, the push pull button extends and in many types of circuit breaker a white band on the button is exposed to provide a visual indication of the tripped condition.

Circuit Breaker Working.jpg


Aircraft Circuit breakers

The aircraft circuit breakers are located in the cockpit or near the power distribution panel, the panel are generally number for easy identification of the circuit breaker, if the number of circuit breaker as too many they are individually numbers as well. 

The capacity of the fuse is written on the respective circuit breaker for example 5 amps, 10 amps and 15 amps.



Modern aircraft circuit breakers are colour coded ring for easy identification according to the various aircraft systems.



Current Limiter

The current limiter is very much like the fuse. However, the current limiter link is usually made of copper and will stand a considerable overload for a short period of time. Like the fuse it will open up in an over current condition in heavy current circuits such as 30 amp or greater. These are used primarily to sectionalize an aircraft circuit or bus. Once the limiter is opened, it must be replaced. The schematic symbol for the current limiter is the same as that for the fuse.

Thermal Protector

A thermal protector, or switch, is used to protect a motor. It is designed to open the circuit automatically whenever the temperature of the motor becomes excessively high. It has two positions — open and closed.


The most common use for a thermal switch is to keep a motor from overheating. If a malfunction in the motor causes it to overheat, the thermal switch will break the circuit intermittently. The thermal switch contains a bimetallic disk, or strip, which bends and breaks the circuit when it is heated. This occurs because one of the metals expands more than the other when they are subjected to the same temperature. When the strip or disk cools, the metals contract and the strip returns to its original position and closes the circuit scored circus and/o is also included.

Thermal Protector Construction and Worki


Reverse Current Protection

Under normal operating conditions the current flows from the power source to the distribution busbar and then to the respective power consuming equipment with the help of voltage regulator, control units and switches. 


Under fault conditions like generator or engine failure, it is possible for the current flow to reverse direction, since the current would flow from the higher potential to the lower potential, this would cause dame to the generator or possible rotate the generator like a motor and cause damage to the engine drive assembly. It is therefore necessary to provide some automatic means of protection.


The two commonly used methods, used are reverse current relays and reverse current circuit breakers.

Reverse Current Cut Out Relay

A reverse current cut out relay is used principally in a d.c. generating system either as a separate unit or as part of a voltage regulator.


The circuit arrangement varies with the type of aircraft and the power distribution system. The basic form consists of a relay which has two coils wound on a core and a spring-controlled armature and contact assembly.


The voltage coil or shunt winding is made up of many turns of fine wire connected across or parallel to the generator so that same amount of voltage passes through it as the generator.


The current coil or series winding is made up of a few turns of heavy wire, is in series with the main supply line and is designed to carry the entire line current. The winding is also connected to the contact assembly, which under static conditions is held in the open position by means of a spring.

When the generator starts operating and the voltage builds up to a value which exceeds that of the battery, the voltage coil (shunt winding) of the relay produces sufficient magnetism in the core to attract the armature and so close the contacts. Thus the relay acts as an automatic switch to connect the generator to the busbar, and also to the battery so that it is supplied with charging current. The field produced by the current coil (series winding) aids the voltage coil (shunt-winding) field in keeping the contacts firmly closed.

When the generator is being shut down or, say, a failure in its output occurs, then the output falls below the battery voltage and there is a momentary discharge of current from the battery; in other words, a condition of reverse current through the cut-out relay current coil (series winding) is set up.


As this also causes a reversal of its magnetic field, the voltage coil (shunt winding) field will be opposed, thereby reducing core magnetization until the armature spring opens the contacts. The generator is therefore switched to the "off-line" condition to protect it from damaging effects which would otherwise result from ''motoring" current discharging from the battery.

Reverse Current Protection.jpg


Reverse Current Circuit Breaker

Reverse current circuit breakers provide protection against fault currents of a magnitude greater than those at which reverse current cut outs relays normally operate. The remain in "locked-out" condition until the fault has been cleared. 

The application of reverse current circuit breaker depends on the type of aircraft the system discussed here consist of a reverse current circuit breaker operated in conjunction with a cut-out relay, the cut-out relay controls the operation of a line contactor connected in series with the coil of the reverse current circuit breaker.

Under normal current flow conditions closing of the cut-out relay energizes the line contactor, the heavy duty contacts of which connect the generator output to the busbar via the coil and main contacts of the normally closed reverse current circuit breaker. The magnetic field set up by the current flow assists that of the magnet unit, thus maintaining the breaker contacts in the closed position. The generator shunt field circuit is supplied via the auxiliary contacts.

When the generator is being shut down, or a failure of its output occurs, the reverse current resulting from the drop in output to a value below that of the battery flows through the circuit as indicated, and the cut-out relay is operated to de-energize the line contactor which takes the generator "off line". Under these conditions the reverse current circuit breaker will remain closed, since the current magnitude is much lower than that at which a specific type of breaker is normally rated.

Let us consider now what would happen in the event of either the cut-out relay or the line contactor failing to open under the above low magnitude reverse current conditions,e.g. contacts have welded due to wear and excessive arcing. The reverse current would feed back to the generator, and in addition to its motoring effect on the generator, it would also reverse the generator field polarity.


The reverse current passing through the circuit breaker coil would continue to increase in trying to overcome mechanical loads due to the engine and generator coupling, and so the increasing reverse field reduces the strength of the magnet unit. When the reverse current reaches the preset trip value of the circuit breaker, the field of the magnet unit is neutralized and repelled, causing the latch mechanism to release the main and auxiliary contacts to completely isolate the generator from the busbar. The breaker must be reset after the circuit fault has been cleared.

Reverse Current Curcuit Breaker.jpg


Overvoltage Protection

Overvoltage is a condition which could arise in a generating system in the event of a fault in the field excitation circuit, e.g. internal grounding of the field windings or an open-circuit in the voltage regulator sensing lines. Devices are therefore necessary to protect consumer equipment against voltages higher than those at which they are normally designed to operate.


The methods adopted vary between aircraft systems and also on whether they supply d.c. or a.c.


DC System Overvoltage Protection

The relay consists of a number of contacts connected in all essential circuits of the generator system, and mechanically coupled to a latching mechanism. This mechanism is electromagnetically controlled by a sensing coil and armature assembly, the coil being connected in the generator shunt-field circuit and in series with a resistor, the resistance of which decreases as the current through it is increased.


Under normal regulated voltage conditions, the sensing coil circuit resistance is high enough to prevent generator shunt-field current from releasing the relay latch mechanism, and so the contacts remain closed and the generator remains connected to the busbar.


lf, however, an open circuit occurs in the regulator voltage coil sensing line, shunt-field current increases and, because of the inverse characteristics of the relay sensing coil resistor, the electromagnetic field set up by the coil causes the latch mechanism to release all the relay contacts to the open position, thereby isolating the system from the busbar. After the fault has been cleared, the contacts are reset by depressing the push button .

Overvoltage Protection DC System.jpg


AC System Overvoltage Protection

There are two types of ac systems used in the aircraft one is frequency wild system and other constant frequency system.

In a frequency-wild a.c. generating system, the full control of which is provided by magnetic amplifiers. The output of the overvoltage protection magnetic amplifier is fed to a bridge rectifier and to the coil of a relay, via a feedback winding. The main contacts of the relay are connected in the normal d.c supply switching circuit to the line contactor.

Under normal voltage output conditions the impedance of the magnetic amplifier is such that its a.c. output, and the rectified a.c. through the relay coil, maintain the relay in the de-energized condition. When an overvoltage condition is produced the current through the relay coil increases to a predetermined energizing value, and the opening of the relay contacts interrupts the d.c. supply to the line contactor, which then disconnects the generator from the busbar. At the same time, the main control unit interrupts the supply of self-excitation current to the generator, causing its a.c. output to collapse to zero. The relay resets itself and after the fault has been cleared the generator output may be restored and connected to the busbar by carrying out the normal starting cycle

An overvoltage protection system adopted in a constant frequency (non-paralleled) a.c. generating system consist of a detector which utilizes solid-state circuit elements which sense all three phases of the generator output, and is set to operate at a level greater than 130 ± 3 volts.


An overvoltage condition is an excitation type fault probably resulting from loss of sensing to, or control of, the voltage regulator such that excessive field excitation of a generator is provided.

The signal resulting from an overvoltage is supplied through an inverse time delay to two solid-state switches.


When switch S1 is made it completes a circuit through the coil of the generator control relay, one contact of which opens to interrupt the generator excitation field circuit. The other contact closes and completes a circuit to the generator breaker trip relay , this in turn, de-energizing the generator breaker to disconnect the generator from the busbar.


The making of solid-state switch S2 energizes the light relay causing it to illuminate the annunciator light. The purpose of the inverse time delay is to prevent nuisance tripping under transient conditions.

Overvoltage Protection AC system.jpg


Undervoltage Protection

Undervoltage occurs in the course of operation when a generator is being shut down, and the flow of reverse current from the system to the generator is a normal indication of this condition .


In a single d.c . generator system undervoltage protection is not essential since the reverse current is sensed and checked by the reverse current cut-out.


It is, however, essential in a multi,generator system with an equalizing method of load-sharing, and since a load-sharing circuit always acts to raise the voltage of a lagging generator, then an undervoltage protection circuit is integrated with that of load-sharing.


A typical circuit normally comprises a polarized relay which disconnects the load sharing circuit and then allows the reverse current cut-out to disconnect the generator from the busbar.

Differential Current Protection

This circuit detects short-circuits in AC generator feeder lines or busbars; it is a method of protecting the generator from overheating and burning out.

Assuming a three-phase AC generator is installed, each phase has its own protection circuit. For illustration purposes, the circuit for a single phase is described. Two control transformers (CTs) are located at either end of the distribution system. CT1 is located in the negative (earthed) connection of the generator’s output. CT2 is located at the output from the busbar performing a monitoring function in a generator control unit (GCU).


If a fault were to develop between the generator and busbar, a current IF flows to ground. The net current received at the busbar is therefore the total generator output current IT minus the fault current (IF). The fault current flows back through the earth return system through CT1 and back into the generator; the remaining current (IT-IF) flows through CT2 and into the loads.


Current transformer CT1 therefore detects (IT-IF) 􏰀 IF which is the total generator current. Current transformer CT2 detects (IT-IF); the difference between control transformer outputs is therefore IF.


At predetermined differential current, the generator control relay (GCR) is automatically tripped by the GCU and this opens the generator field.

Differential Current Protection.jpg


Phase protection (Merz Price circuit)

This circuit protects against faults between phases, or from individual phase to ground faults. Connections are as follows; a three-phase system would require the same circuit per phase. Two current transformers (CTs) are located at each end of the feeder distribution line:

  • CT1 monitors the current output from the generator

  • CT2 monitors the current into the distribution system.


Secondary windings of each current transformer are connected via two relay coils; these windings are formed in the opposite direction. When current flows through the feeder, there is equal current in both coils; the induced EMF is balanced, so no current flows. If a fault develops in the feeder line, current CT1 flows (but not CT2), thereby creating an unbalanced condition.

Current flow in either of the coils opens the contacts and disconnects the feeder line at both ends.

Merz Price Circuit.jpg


Circuit Controllling Device

Circuit Controlling Devices

The circuit controlling device form an important part of the aircraft power distribution system since the function of initiating, and subsequently controlling the operating sequences of constituent circuits is important.


These are generally performed principally by switches and relays.


A switch is the on/off device used to isolate circuits. In its simplest form, a switch consists of two contacting surfaces which can be isolated from each other or brought together as required by a movable connecting link. This connecting link is referred to as a pole. The term throw thow indicates the number of circuits each pole can complete through the switch. Switch contacts can be normally open or closed and this is normally marked on the switch (NO/NC).


Switches are characterized by:

  • Number of poles

  • Number of switched positions

  • Type of switched contacts (permanent or momentary).

Based on the number of poles there are the following types of switches 

SPST (Single Pole Single Through)

This is a simple ON/OFF switch. When a user presses the button of the switch, then the plates of the switch connect with each other and the current starts to flow in the circuit and vice versa. It is also called as a “One Way” or “Single Way” Switch.

SPDT (Single Pole Double Throw)

SPDT switch has three pins (terminals). One pin among three is used as common and called a Two-Way Switch. We can send two different signals to the same pin by using this switch. Because of this functionality, this switch is also known as selector switch or two way switch.

DPST (Double Pole, Single Throw)

This switch is basically two SPST switches in one package and can be operated by a single lever. This switch is mostly used, where we have to break both ground and lines at the same time.

DPDT (Double Pole Double Throw)

This switch is equivalent to two SPDT switches packaged in one pack. This switch has two common pins and four signal pins. Total four different combinations of singles can be applied to the input pins of this switch.

Types of Switches.jpg


In addition to the number of poles and throws, switches (toggle types in particular) are also designated by the number of positions they have. Thus, a toggle switch which is spring-loaded to one position and must be held at the second to complete a circuit, is called a single-position switch. lf the switch can be set at either of two positions, e.g. opening the circuit in one position and completing it in another, it is then called a two-position switch. A switch which can be sot at any one of three positions, e.g. a centre "off' and two "on" positions, is a three-position switch, also known as a selector switch.

Toggle Switch / Tumbler Switch

Toggle or tumbler type switches, are the most common types of switches used. They are sometimes called as "general-purpose" switch.

The toggle switch consist of a operating lever or arm called as Toggle, which moves about a ball pivot and maker or breaks a pair of contacts which are connected to the terminals. 

In some applications it may be necessary for the switches in several independent circuits to be actuate, simultaneously. This is accomplished by ''ganging" n switches together by means of a bar linking each toggle.

Switch Construction.jpg


Some switches are designed with instinctive tactile features so that the risk of selecting the wrong system is minimized, e.g. the flap up/down switch-operating lever would be shaped in the form of an aerofoil; the undercarriage selection switch-operating lever would be shaped in the form of a wheel. 

Aircraft System Switches.jpg


Push Switches

ln basic form a push-switch consists of a button-operated spring loaded plunger carrying one or more contact plates which serve to establish electrical connection between fixed contact surfaces.


Switches may be designed as independent units for either ''push-to-make" or "push-to-break'' operation, or designed to be double-acting.

Modern aircraft panels utilize a combined switch and light display; the display is engraved with a legend or caption indicating system status. These can be used in a variety of ways e.g. to show system on/off. The switch portion of the device can be momentary or continuous; small level signals are sent to a computer or high-impedance device.


Internal backlighting is from two lights per legend for redundancy; these are projected via colored filters. The two captions provide such information as press to test (P/TEST). 

Modern Aicraft Swithes


Switches are sometimes guarded or wire-locked with fuse wire to prevent inadvertent operation. Some switch designs have to be pulled out of a detent position before the position can be changed.

Aircraft Switch Guards .jpg


Rocker Switches

Rocker switches combine the action of toggle and push/pull devices.

Rocker Switch.jpg


Rotary Switch

These are manually operated; and for certain operating requirements they offer an advantage over toggle switches in that they are less prone to accidental operation, Furthermore, the rotary principle and positive engagement of contacts made possible by the constructional features make these switches more adaptable to multi-circuit selection than toggle type switches.


A typical application is the selection of a single voltmeter to read the voltages at several busbars.


In the basic form a rotary switch consists of a central spindle carrying one or more contact plates or blades which engage with corresponding fixed contacts mounted on the switch base. The movement is usually spring-loaded and equipped with some form of eccentric device to give a snap action and positive engagement of the contact surfaces.

Rotary Switch.jpg


Modern Aircraft Panel consist of a combination of all the different type of switches.

Modern Airccraft Panel Switches.jpg


Micro Switches

These are used to sense if a device has moved or has reached its limit of travel, e.g. flap drive or undercarriage mechanisms. Figure illustrates the internal schematic and electrical contacts of a typical micro switch product.


The contacts open and closes with a very small movement of the plunger. The distance travelled by the armature between make/break is measured in thousandths of an inch, hence the name ‘micro’. A snap action is achieved with a contact mechanism that has a pre-tensioned spring.


Micro-switches are attached to the structure and the wiring is connected into a control circuit.



An example of micro-switch application is to sense when the aircraft is on the ground; this is achieved by mounting a micro-switch on the oleo leg. When the aircraft is on the ground, the oleo leg compresses and the switch is operated.


Micro-switches are used to sense the mechanical displacement of a variety of devices, including:

  • Control Surfaces

  • Undercarriage

  • Pressure Capsules

  • Bimetallic Temperature Sensors

  • Mechanical Timers.

Aircraft Micro Switch Locations.jpg


Proximity Switches

A proximity switch is one detecting the proximity (closeness) of some object. By definition, these switches are non-contact sensors, using magnetic, electric, or optical means to sense the proximity of objects.

Proximity switches perform the same function as micro-switches; they sense the presence of an object by the interruption of a magnetic circuit.

These switches are used in several types of aircraft as part of circuits required to give warning of whether or not passenger entrance doors, freight doors, etc. are fully closed and locked. Since they have no moving parts they offer certain advantages over micro-switches which are also applied to such warning circuits.

It consist of two main components, one is a target and the other a switch unit comprising of detectors connected to the warning circuit.

There are two types of proximity switch:

  • Reed Type

  • Solid State Type

Proximity Switch.jpg


Reed Type Proximity Switch

The reed switch device comprises two hermetically sealed sections. One section (the actuator) contains a magnet; the other section (the sensor) contains a reed armature with rhodium-plated contacts.


The usual arrangement is for the sensor unit to be fixed to the aircraft structure; the actuator is attached to the item being monitored, e.g. a door.


When the gap between the actuator and the sensor reaches a predetermined distance, the reed contacts close thereby completing the circuit. They open again when the actuator and sensor are moved apart.

Reed Switch.jpg


Solid State Type Proximity Switch

The solid state proximity switch is based on an inductance loop and steel target in this inductance loop is the input stage of an electronic switch unit incorporated as part of the actuator. As the target moves closer to the coil, the inductance of the coil changes.


An electronic circuit determines when the inductance has reached a predetermined level. The obvious advantage of this type of switch is that there are no switch contacts, hence higher reliability.

Solid State Switch.jpg


Time Switch

Certain consumer services are required to operate on a predetermined controlled time sequence basis and as this involves the switching on and off of various components or sections of circuit, switches automatically operated by liming mechanisms are necessary.


The principle of time switch operation varies, but in general it is based on the one in which a contact assembly is actuated by a cam driven at constant speed by either a speed-controlled electric motor or a spring-driven escapement mechanism.


In some specialized consumer services, switches which operate on a thermal principle are used, in these the contact assembly is operated by the distortion of a thermal element when the latter has been carrying a designed current for a predetermined period.

Mercury Switch

Mercury switches are glass tubes into which stationary contacts, or electrodes, and a pool of loose mercury are hermetically sealed.


Tilting the tube causes the mercury to flow in a direction to close or open a gap between the electrodes to "make" or ''break" the circuit in which the switch is connected.


The rapidity of "make" and "break'' depends on the surface tension of the mercury rather than on externally applied forces. Thus, mercury switches are applied to systems in which the angular position of a component must be controlled with in a narrow band of operation, and in which the mechanical force required to tilt a switch is very low.


A typical application is in torque motor circuits of gyro horizons in which the gyros must be precessed to, and maintained in, the vertical position.

Pressure Switch

In many of the aircraft systems in which pressure measurement is involved, it is necessary that a warning be given of either low or high pressures which might constitute hazardous operating conditions.


In some systems also, the frequency or operation may be such that the use of a pressure-measuring instrument is not justified since it is only necessary for some indication that an operating pressure has been attained for the period during which the system is in operation. To meet this requirement, pressure switches are installed in the relevant systems and are connected to warning or indicator lights located on the cockpit panels.

It consists of a metal diaphragm bolted between the flanges of the two sections of the switch body.


A chamber is formed on one side of the diaphragm and is open to the pressure source . On the other side of the diaphragm a push rod, working through a sealed guide, bears against contacts fitted in a terminal block connected to the warning or indicator light assembly.


The contacts may be arranged to "make'' on either decreasing or increasing pressure, and their gap settings may be pre adjusted in accordance with the pressures at which warning or indication is required.

Pressure switches may also be applied to systems requiring that warning or indication be given of changes in pressure with respect to a certain datum pressure; in other words, as additional pressure warning device.


The construction and operation are basically the same as the standard type, with the exception that the diaphragm is subjected to a pressure on each side

Thermal Switch

Thermal switches are applied to systems in which a visual warning of excessive temperature conditions, automatic temperature control and automatic operation of protection devices are required. Examples of such applications are, respectively, overheating of a generator, control of valves in a thermal de-icing system and the automatic operation of fire extinguishers,

A principle commonly adopted for thermal switch operation is based on the effects of differences of expansion between two metals, usually invar and steel. In some cases mercury contact switches may be employed.

An example of a differential expansion switch is employed in as a fire detecting device. The heat-sensitive element is an alloy steel barrel containing a spring bow assembly of low coefficient of expansion. Each limb of the bow carries a silver-rhodium contact connected by fire.resistant cable to a terminal block located within a steel case.

ln the event of a fire or sufficient rise in temperature at the switch location (a typical temperature is 300°C) the barrel will expand and remove the compressive force from the bow assembly, pennitting the contacts to close the circuit to its relevant warning lamp. When the temperature drops, the barrel contracts, thus compressing the bow assembly and re· opening the contacts.


Relays are in effect, electromagnetic switching devices by means of which one electrical circuit can be indirectly controlled by a change in the same or another electrical circuit.

In the basic form a relay is made up of two principal elements, one for sensing the electrical changes and for operating the relay mechanism, and the other for controlling the changes. The sensing and operating element is a solenoid and armature; and the controlling element is one or more pairs or contacts.

The relays are classified by the order of making and breaking of contacts, whether normally open ("NO") or normally closed ("NC")in the de-energized position.

Relay Construction.jpg


As in the case of switches, relays are also designated by their "pole" and "throw" arrangements and these can range from the simple single-pole, single-throw type to complex multiple contact assemblies controlling a variety of circuits and operated by the one solenoid.

Type of Relays.jpg


Working of relay

The working of the replay depends on the type of relay used and also on configuration in which is is used i.e. Normally opened or Normally Closed.


The solenoid is energized either directly from the aircraft power supply, or by signals from an automatic device such as an amplifier in a cabin temperature-control system, or a fuel detector unit.


When the solenoid coil is energized a magnetic field is set up and at a predetermined voltage level (called the "pull in" voltage) the armature is attracted to a pole piece against spring restraint, and actuates the contact assembly, this in turn either completing or interrupting the circuit being controlled.


When the solenoid coil circuit is interrupted at what is termed the "drop- out'' voltage, the spring returns the armature and contact assembly to the inoperative condition.

Working of Relay.jpg


Polarised Armature Relay

In certain specialized applications, the value of control circuit currents and voltages may be only a few milliamps and millivolts, and therefore relays of exceptional sensitivity are required.


This requirement cannot always be met by relays which employ spring controlled armatures, for although loading may be decreased to permit operation at a lower ''pull-in" voltage, effective control of the contacts is decreased and there is a risk of contact flutter .


A practical solution to this problem resulted in a relay in which the attraction and repulsion effects of magnetic forces are substituted for the conventional spring control of the armature and contact assembly.

It consist of a armature which is a permanent magnet and is pivoted between two sets of pole faces formed by a frame of high permeability material (usually mu- metal), lt is slightly biased to one side to bring the contact assembly into the static condition as shown in Fig. A

The centre limb of the frame carries a low inductance low-current winding which exerts a small magnetizing force on the frame when it is energized from a suitable source of direct current. With the armature in the static condition, the frame pole-faces acquire, by Induction from the armature, the polarities shown, and the resulting forces of magnetic attraction retain the armature firmly in position.

When a d.c. voltage is applied to the coil the frame becomes, in effect, the core of an electromagnet. The flux established in the core opposes and exceeds the flux due to the permanent magnet armature, and the frame pole-faces acquire the polarilies shown in Fig. B. As the armature poles and frame pole-faces are now of like polarity, the armature is driven to the position shown in Fig. C by the forces of repulsion.


ln this position it will be noted that poles and pole-faces are now of unlike polarity, and strong forces of attraction hold the armature and contact assembly in the operating condition.


The fluxes derived from the coil and the armature act in the same direction to give a flux distribution as shown in.Fig. C. When the coll circuit supply is interrupted, the permanent magnet flux remains, but the force due to it is weaker than the armature bias force and so the armature and contacts are returned to the static condition Fig. A.

Polarised Armature Relay.jpg


Slugged Relay

For some applications requirements arise for the use of relays which are slow to operate the contact assembly either at the stage when the armature is being attracted, or when it is being released. 

Some relays are therefore designed to meet these requirements, and they use a simple principle whereby the build-up or collapse of the main electromagnet flux is slowed down by a second and opposing magnetizing force. This procedure is known as ''slugging" and a relay to which it is applied is called a "slug" relay.


The relay usually incorporates a ring of copper or other non-magnetic conducting material (the "slug") in the magnetic circuit of the relay, in such a way that changes in the operating flux which is linked with the slug originate the required opposing magnetic force. In some slug relays the required result is obtained by fitting an additional winding over the relay core and making provision for short-circuiting the winding, as required, by means of independent contacts provided in the main contact assemblies.


These devices sometimes referred the as contactors, are commonly used in power generation systems for the connection of feeder lines to busbars, and also for interconnecting or "tying" of busbars. Unlike conventional circuit-breakers, these devices can be tripped on or off remotely.

It consists of main heavy-duty contacts for connecting the a.c. feeder lines, and a number of smaller auxiliary contacts which carry d.c. for the control of other breakers, relays, indicating lights as appropriate to the overall system.


All contacts are closed and/or tripped by a d.c .·operated electromagnetic coil; a permanent magnet serves to assist the coil in closing, and also to latch the breaker in the closed position. The coil Is also assisted in tripping by means of a spring. Two zener diodes are connected across the coil lo suppress arcing of the coil circuit contacts during closing and tripping.

When say, a main generator switch is placed in its "on" position, a d.c. "closing" signal will now through the relaxed contacts ''A" and then through the coil to ground via relaxed contacts "B". With the coil energized, the main and other auxiliary contacts will therefore be closed and the spring will be.compressed. The changeover of the coil contacts ''A" completes a hold-in circuit to ground, and with the assistance of the permanent magnet the breaker remains latched.

A tripping signal resulting from either the generat, switch being placed to "off', or from a fault condition sensed by a protection unit, will flow to ground in the opposite direction to that when closing, and via the second set o f the " close" contacts. The spring assists the reversed electromagnetic field of the coil in breaking the permanent magnetic latch.

Breakers of this type are installed with their opening-closing axis in the horizontal position.

Aircraft Breakers or Contactors.jpg


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