generation transmission & distribution

classification of Relays

No.		Name			Definition
Electromagnetic relays					Relays that operate “on”、“off” or changeover by the magnetism produced by the control current putted to the coil.
1		Electro- magnetic relay		DC	The control current is direct current. According to the contact load, it can be classified into four kinds of micro power, low power, intermediate power and high power.
2				AC	The control current is alternating current. It can be classified into two kinds of 50Hz and 400Hz according to the frequency of the coil power source.
3				Magnetic latching relay	Using permanent magnet or parts with high remanence to keep the armature at the same position as the coil is electrified when the coil power source is taken away.
4		Solid state relay (SSR)			SSR is a kind of merely solid-state component that can make the whole circuit “on” or “off” as electromagnetic relays do, and the I/O isolation resistance is equivalent to electromagnetic relays.
5		Hybrid relay			The relay is composed of electronic components and electromagnetic relays. Generally, the input part is composed of electronic circuit playing the role of enlargement and rectification; the output part is electromagnetic relay.
6		High frequency relay			The relay is used in AC circuit whose switching frequency exceeds 10kHz.
7		Coaxial relay			The relay is used together with coaxial cable to switch high frequency and RF circuits. But the power consumption is minimum.
8		Vacuum relay			The contact of the relay is sealed in vacuum container. It is used to fleetly switch on, off, or over high voltage, high frequency and RF circuits.
Thermal relay					The relay acts on the effect of heat.
9	Thermal relay		Temperature relay		The relay will act when the ambient temperature rise to a certain value.
10			Electrothermal relay		The relay will act when the heat energy that is from electricity energy in the controlling circuit rise to a certain value.
11	Photoelectrical relay				The relay acts on photoelectric effect.
12	Polarized relay				The relay acts on the effect of the polarized magnetic field and thecontrolling current through the coil. The operating direct depends on the direct of the coil current.
13	Time relay				When input signal is putted on or taken away, output parts won’t turn the controlled circuit on or off until the time is delayed or limited to a specified spot.
14	Reed relay				The relay opens, closes, or changeovers the circuit depending on the action of the reed sealed in the tube. The reed acts as both contact blade and armature.

2.2 Classification on the contact load. Showing in table 2.(Table 2)

Name	Definition
Micro power relay	When the off state voltage of contact is 27VDC, the contact rated load current (resistive load) is 0.1A, 0.2A.
Low power relay	When the off state voltage of contact is 27VDC, the contact rated load current (resistive load) is 0.5A, 1A.
Intermediate power relay	When the off state voltage of contact is 27VDC, the contact rated load current (resistive load) is 2A, 5A.
High power relay	When the off state voltage of contact is 27VDC, the contact rated load current (resistive load) is 10A, 15A, 20A, 25A, 40A, etc.

Note: There is only a kind of DC resistive load showed in the table; other load forms can be calculated according to the product technical conditions.  2.3 Classification on the outline dimension. Showing in table 3. (Table 3)

Name	Definition
Microminiature relay	The dimension of the longest side doesn’t exceed 10mm.
Subminiature relay	The dimension of the longest side exceeds 10mm, but less than 25mm.
Miniature relay	The dimension of the longest side exceeds 25mm, but less than 50mm.

Note: For hermetic or sealed relay, the outline dimension refers to the largest dimension of L, W and H of the relay body itself, not including the dimension of mounting parts, terminals, pushed strip, pushed side, upturned side and sealed welding dot. 2.4 Classification on the protective construction. Showing in table 4.(Table 4)

Name	Definition
Hermetic relay	The contact, coil and other parts are all hermetically sealed in a cover by welding or other means to insulate the relay with ambient media to lessen the leakage.
Sealed relay	The contact, coil and other parts are all sealed (not hermetically) in a cover to protect the relay.
Open relay	There is no protective cover to protect the contact, coil and other parts.

ABOUT SWITCHGEAR

ABOUT SWITCHGEAR The importance of electric supply is well known. It is very much necessary to protect the power system, equipment, motors, generators, etc from the dangerous fault condition in an electric supply. Hence it is necessary to have the arrangements with which all these equipments can be switched ON or OFF under no load or load conditions or even under fault condition. The collection of various equipments used for the switching and protecting purposes in a power system is called switchgear. Low voltage switchgears have wide range of applications in today’s market. Some of the switching devices are mentioned here under.  Air circuit breaker  Moulded case circuit breaker  Miniature circuit breaker  Motor protection circuit breaker  Earth leakage circuit breaker  Isolator / Switch disconnector  Contactor  Relay  Timer  Fuse and So on… With the help of these switching devices, various types of Switchgear Panels are manufactured to distribute and protect the power and appliances. We offer you high quality training on the complete range of switchgear devices and panels by means of practical approach. To join us please click here.

WHAT IS POWER CIRCUIT BREAKER?

WHAT IS POWER CIRCUIT BREAKER?
A Tutorial on Power Circuit Breakers


A circuit breaker is defined as “a mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying and breaking for a specified time, and breaking currents under specified abnormal circuit conditions such as a short circuit” (IEEE Std. C37.100-1992).


Circuit breakers are generally classified according to the interrupting medium used to cool and elongate the electrical arc permitting interruption. The types are:
• Air magnetic
• Oil
• Air blast
• Vacuum
• SF6 gas


Air magnetic circuit breakers are limited to older switchgear and have generally been replaced by
vacuum or SF6 for switchgear applications. Vacuum is used for switchgear applications and some outdoor breakers, generally 38 kV class and below. Air blast breakers, used for high voltages ( ≥ 765 kV), are no longer manufactured and have been replaced by breakers using SF6 technology.


Oil circuit breakers have been widely used in the utility industry in the past but have been replaced by other breaker technologies for newer installations. Two designs exist — bulk oil (dead-tank designs) dominant in the U.S.; and oil minimum breaker technology (live-tank design).


Bulk oil circuit breakers were designed as single-tank or three-tank mechanisms; generally, at higher voltages, three-tank designs were dominant. Oil circuit breakers were large and required significant foundations to support the weight and impact loads occurring during operation.


Environmental concerns forcing the necessity of oil retention systems, maintenance costs, and the development of the SF6 gas circuit breaker have led to the gradual replacement of the oil circuit breaker for new installations.


Oil circuit breaker development has been relatively static for many years. The design of the interrupter employs the arc caused when the contacts are parted and the breaker starts to operate. The electrical arc generates hydrogen gas due to the decomposition of the insulating mineral oil.


The interrupter is designed to use the gas as a cooling mechanism to cool the arc and to use the pressure to elongate the arc through a grid (arc chutes), allowing extinguishing of the arc when the current passes through zero.


Vacuum circuit breakers use an interrupter that is a small cylinder enclosing the moving contacts under a high vacuum. When the contacts part, an arc is formed from contact erosion. The arc products are immediately forced to and deposited on a metallic shield surrounding the contacts. Without anything to sustain the arc, it is quickly extinguished.


Vacuum circuit breakers are widely employed for metal-clad switchgear up to 38 kV class. The small size of the breaker allows vertically stacked installations of breakers in a two-high configuration within one vertical section of switchgear, permitting significant savings in space and material compared to earlier designs employing air magnetic technology.
When used in outdoor circuit breaker designs, the vacuum cylinder is housed in a metal cabinet or oil-filled tank for dead tank construction popular in the U.S. Market.


Gas circuit breakers generally employ SF6 (sulfur hexaflouride) as an interrupting and sometimes as an insulating medium. In “single puffer” mechanisms, the interrupter is designed to compress the gas during the opening stroke and use the compressed gas as a transfer mechanism to cool the arc and to elongate the arc through a grid (arc chutes), allowing extinguishing of the arc when the current passes through zero.


In other designs, the arc heats the SF6 gas and the resulting pressure is used for elongating and interrupting the arc. Some older two-pressure SF6 breakers employed a pump to provide the highpressure SF6 gas for arc interruption.


Gas circuit breakers typically operate at pressures between six and seven atmospheres. The dielectric strength of SF6 gas reduces significantly at lower pressures, normally as a result of lower ambient temperatures. Monitoring of the density of the SF6 gas is critical and some designs will block operation of the circuit breaker in the event of low gas density.

Circuit Breakers

Circuit Breaker:A circuit breaker's function is, like a fuse, to break a circuit path when a predetermined amount of current is passed. In my opinion, circuit breakers should never be used to protect electronic devices such as radios, amplifiers or crossovers. Most common circuit breakers (thermal snap action) take far too long to open the circuit path. This does not mean that they are not useful. When they are properly selected they do a good job of protecting wiring and devices such as electric motors. Some breakers are self resetting. Others require manual resetting. I strongly recommend using a manual reset type. This will allow you to watch for any problems when the circuit path is restored.


Thermal Circuit Breakers:The diagram below shows the simplified version of a self resetting circuit breaker. In this device, the current flows from the battery terminal, through the bi-metal strip and then to the other terminal. The bi-metal strip is made of two different types of metal which have different coefficients of expansion. This means that one will expand more than the other when the rise in temperature is the same for both pieces. In this case, the two metals are bonded to each other. (now keep in mind that this is a simplified diagram) When the strip heats up from the current flow through it, one type of metal expands more than the other. In this case, the black metal expands more than the red and the strip tends to bend upward and disconnect the contacts. You can see that the metal starts to bend as the current increases. When the temperature reaches a given point, the piece will snap into the open position and the current flow will stop. The bi-metal strip is stamped into a special shape which causes the 'snap' action. This will assure that there is EITHER a solid connection OR a complete disconnect. You can see a similar snap action in the top of some soda cans. If you push down on the top it starts to bend downward. After the pressure reaches a certain point, the top will snap down. If you release the pressure slowly, the top will snap into it's original position. This is what happens when the bi-metal strip cools in the breaker.Below is one example of a self resettig thermal circuit breaker.

Magnetic Circuit Breakers:Some circuit breakers use a magnetic actuator to trip the circuit. In this type of breaker, the current flow through the electrical device (amp, fog lights...) passes through an electromagnetic actuator. When the current flow reaches a preset level (determined by the current rating of the breaker), the magnetic field in the electromagnet is strong enough to trip the breaker and allow the contacts to open. This type of breaker generally has to be manually reset. A well designed 'magnetically' actuated circuit breaker can operate very quickly (possibly as fast as or faster than a fuse of equal current rating).

In the following diagram you can push the 'overcurrent trip' button to simulate too much current flow and trip the breaker. Then press the 'reset' button and watch the breaker reset. The 'show legend' button will show you a legend of the parts. Keep in mind that this is just a generic diagram and doesn't depict any particular breaker.



Thermal/Magnetic Breakers:Some breakers use both thermal and magnetic trip functions. The magnetic function works the same as the previous explanation. The thermal part functions a little differently than the previous example. In the combination breaker, the bi-metal strip is more likely to be used to trip the breaker internally (by tripping the latch) instead of pulling the contacts apart when heated.

Reliability:In my opinion and from my experience, circuit breakers are less reliable than fuses (especially when the breaker is mounted in the harsh environment under the hood). Quality fuses like ANL and Maxi fuses have a solid element (no solder connections) and will almost never have an intermittent or poor electrical connection. A circuit breaker will eventually have higher contact resistance than when it was new. This is especially true if the breaker has been tripped (by overcurrent) more than a few times. If you're going to compete and can't take a chance of having a problem like a bad connection in the power line, you should use a good quality fuse. Now I know that people have had fuses blow in competition but it was because the fuse was not properly rated, not because the fuse was defective. For those who have had trouble with glass fuses, read the fuses page of this site.

Circuit Breaker Selection

A typical circuit breaker operating time is given in Fig. 6.11. Once the fault occurs, the protective devices get activated. A certain amount of time elapses before the protective relays determine that there is overcurrent in the circuit and initiate trip command. This time is called the detection time. The contacts of the circuit breakers are held together by spring mechanism and, with the trip command, the spring mechanism releases the contacts. When two current carrying contacts part, a voltage instantly appears at the contacts and a large voltage gradient appears in the medium between the two contacts. This voltage gradient ionizes the medium thereby maintaining the flow of current. This current generates extreme heat and light that is called electric arc. Different mechanisms are used for elongating the arc such that it can be cooled and extinguished. Therefore the circuit breaker has to withstand fault current from the instant of initiation of the fault to the time the arc is extinguished.

Fig. 6.11 Typical circuit breaker operating time.

Two factors are of utmost importance for the selection of circuit breakers. These are:

• The maximum instantaneous current that a breaker must withstand and
• The total current when the breaker contacts part.

In this chapter we have discussed the calculation of symmetrical subtransient fault current in a network. However the instantaneous current following a fault will also contain the dc component. In a high power circuit breaker selection, the subtransient current is multiplied by a factor of 1.6 to determine the rms value of the current the circuit breaker must withstand. This current is called themomentary current . The interrupting current of a circuit breaker is lower than the momentary current and will depend upon the speed of the circuit breaker. The interrupting current may be asymmetrical since some dc component may still continue to decay.

Breakers are usually classified by their nominal voltage, continuous current rating, rated maximum voltage, K -factor which is the voltage range factor, rated short circuit current at maximum voltage and operating time. The K -factor is the ratio of rated maximum voltage to the lower limit of the range of the operating voltage. The maximum symmetrical interrupting current of a circuit breaker is given by K times the rated short circuit current.

Basic of protection system

Introduction
The purpose of an electrical power generation system is to distribute energy to a multiplicity of points for diverse applications.

The system should be designed and managed to deliver this energy to the utilization points with both reliability and economy.

 As these two requirements are largely opposed, it is instructive to look at the relationship between the reliability of a system and its cost and value to the consumer, which is shown in Figure 1.
 

Figure 1 Relationship between reliability of supply, its cost and value to the consumer.

         It is important to realize that the system is viable only between the cross-over points A and B. The diagram illustrates the significance of reliability in system design, and the necessity of achieving sufficient reliability.

On the other hand, high reliability should not be pursued as an end in itself, regardless of cost, but should rather be balanced against economy, taking all factors into account.

Security of supply can be bettered by improving plant design, increasing the spare capacity margin and arranging alternative circuits to supply loads. Sub-division of the system into zones, each controlled by switchgear in association with pro­tective gear, provides flexibility during normal operation and ensures a minimum of dislocation following a breakdown.

The greatest threat to a secure supply is the shunt fault or short circuit, which imposes a sudden and sometimes violent change on system operation.

The large current which then flows, accompanied by the localized release of a considerable quantity of energy, can cause fire at the fault location, and mechanical damage throughout the system, particularly to machine and transformer windings. Rapid isolation of the fault by the nearest switch-gear will minimize the damage and disruption caused to the system.
A power system represents a very large capital investment. To maximize the return on this outlay, the system must be loaded as much as possible. For this reason it is necessary not only to provide a supply of energy which is attractive to prospective users by operating the system within the range AB (Figure 1.1), but also to keep the system in full operation as far as possible continuously, so that it may give the best service to the consumer, and earn the most.

Revenue for the supply authority. Absolute freedom from failure of the plant and system network cannot be guaranteed.

The risk of a fault occurring, however slight for each item, is multiplied by the number of such items which are closely associated in an extensive system, as any fault produces repercussions throughout the net-work. When the system is large, the chance of a fault occurring and the disturbance that a fault would bring are both so great that without equip­ment to remove faults the system will become, in practical terms, inoperable.

The object of the system will be defeated if adequate provision for fault clearance is not made. Nor is the installation of switchgear alone sufficient; discriminative protective gear, designed according to the characteristics and requirements of the power system, must be provided to control the switchgear.

A system is not properly designed and managed if it is not adequately protected. This is the measure of the importance of protective systems in modern practice and of the responsibility vested in the protection engineer.

Fundamentals of protection practice

This is a collective term which covers all the equipment used for detecting, locating and initiating the removal of a fault from the power system. Relays are extensively used for major protective functions, But the term also covers direct-acting A.C. trips and fuses.

In addition to relays the term includes all accessories such as current and voltage transformers, shunts, D.C. and A.C. wiring and any other devices relating to the protective relays.

In general, the main switchgear, although funda­mentally protective in its function, is excluded from the term 'protective gear', as are also common services, such as the station battery and any other equipment required to secure operation of the circuit breaker.

In order to fulfil the requirements of discriminative protection with the optimum speed for the many different configurations, operating conditions and construction features of power systems, it has been necessary to develop many types of relay which respond to various functions of the power system quantities.

For example, observation simply of the magnitude of the fault current suffices in some cases but measurement of power or impedance may be necessary in others. Relays frequently measure complex functions of the system quantities, which are only readily expressible by mathematical or graphical means.

In many cases it is not feasible to protect against all hazards with any one relay. Use is then made of a combination of different types of relay which individually protect against different risks. Each individual protective arrangement is known as a 'protection system^'; while the whole coordinated combination of relays is called a 'protection scheme'. ·     ReliabilityThe need for a high degree of reliability is discussed in Section 1. Incorrect operation can be attributed to one of the following classifications:

a.        Incorrect design.b.       Incorrect installation.c.        Deterioration.d.       Protection performance
 

 1. DesignThis is of the highest importance. The nature of the power system condition which is being guarded against must be thoroughly understood in order to make an adequate design. Comprehensive testing is just as important, and this testing should cover all aspects of the protection, as well as reproducing operational and environmental conditions as closely as possible. For many protective systems, it is necessary to test the complete assembly of relays, current transformers and other ancillary items, and the tests must simulate fault conditions realistically.
 2.     Installation.     The need for correct installation of protective equipment is obvious, but the complexity of the interconnections of many systems and their relation-ship to the remainder of the station may make.

Difficult the checking of such correctness. Testing is therefore necessary; since it will be difficult to reproduce all fault conditions correctly, these tests must be directed to proving the installation. This is the function of site testing, which should be limited to such simple and direct tests as will prove the correctness of the connections and freedom from damage of the equipment.

No attempt should be made to 'type test' the equipment or to establish complex aspects of its technical performance;
  3. Deterioration in service.  After a piece of equipment has been installed in perfect condition, deterioration may take place which, in time, could interfere with correct function­ing. For example, contacts may become rough or burnt owing to frequent operation, or tarnished owing to atmospheric contamination; coils and other circuits may be open-circuited, auxiliary components may fail, and mechanical parts may become clogged with dirt or corroded to an extent that may interfere with movement.

One of the particular difficulties of protective relays is that the time between operations may be measured in years, during which period defects may have developed unnoticed until revealed by the failure of the protection to respond to a power system fault. For this reason, relays should be given simple basic tests at suitable intervals in order to check that their ability to operate has not deteriorated.

Testing should be carried out without disturbing permanent connections. This can be achieved by the provision of test blocks or switches.

 Draw-out relays inherently provide this facility; a test plug can be inserted between the relay and case contacts giving access to all relay input circuits for injection. When temporary disconnection of panel wiring is necessary, mistakes in correct restoration of con­nections can be avoided by using identity tags on leads and terminals, clip-on leads for injection supplies, and easily visible double-ended clip-on leads where 'jumper connections' are required.

The quality of testing personnel is an essential feature when assessing reliability and considering means for improvement. Staff must be technically competent and adequately trained, as well as self-disciplined to proceed in a deliberate manner, in which each step taken and quantity measured is checked before final acceptance.

Important circuits which are especially vulnerable can be provided with continuous electrical super-vision; such arrangements are commonly applied to circuit breaker trip circuits and to pilot circuits.
4. Protection performance

The performance of the protection applied to large power systems is frequently assessed numerically. For this purpose each system fault is classed as an incident and those which are cleared by the tripping of the correct circuit breakers and only those are classed as 'correct'.

The percentage of correct clearances can then be determined.

This principle of assessment gives an accurate evaluation of the protection of the system as a whole, but it is severe in its judgment of relay performance, in that many relays are called into operation for each system fault, and all must behave correctly for a correct clearance to be recorded.

On this basis, a performance of 94 % is obtainable by standard techniques.

Complete reliability is unlikely ever to be achieved by further improvements in construction. A very big step, however, can be taken by providing duplication of equipment or 'redundancy^'. Two complete sets of equipment are provided, and arranged so that either by itself can carry out the required function. If the risk of an equipment failing is x/unit, the resultant risk, allowing for redundancy, is x². Where x is small the resultant risk (x²) may be negligible.

It has long been the practice to apply duplicate protective systems to bus-bars, both being required to operate to complete a tripping operation, that is, a 'two-out-of-two' arrangement. In other cases, important circuits have been provided with duplicate main protection schemes, either being able to trip independently, that is, a 'one-out-of-two^' arrange­ment. The former arrangement guards against un­wanted operation, the latter against failure to operate.

These two features can be obtained together by adopting a 'two-out-of-three' arrangement in which three basic systems are used and are interconnected so that the operation of any two will complete the tripping function.

Such schemes have already been used to a limited extent and application of the principle will undoubtedly increase. Probability theory suggests that if a power network were protected throughout on this basis, a protection performance of 99.98 % should be attainable.

This performance figure requires that the separate protection systems be completely independent; any common factors, such as, for instance, common current transformers or tripping batteries, will reduce the overall performance to a certain extent.
·       Selectivity.Protection is arranged in zones, which should cover the power system completely, leaving no part unprotected. When a fault occurs the protection is required to select and trip only the nearest circuit breakers. This property of selective tripping is also called 'discrimination' and is achieved by two general methods:

1.  Time graded systems.

Protective systems in successive zones are arranged to operate in times which are graded through the sequence of equipments so that upon the occurrence of a fault, although a number of protective equip­ments respond, only those relevant to the faulty zone complete the tripping function. The others make incomplete operations and then reset.
  2.     Unit systems.It is possible to design protective systems which respond only to fault conditions lying within a clearly defined zone. This 'unit protection' or 'restrictedProtection' can be applied throughout a power system and, since it does not involve time grading, can be relatively fast in operation.

Unit protection is usually achieved by means of a comparison of quantities at the boundaries of the zone. Certain protective systems derive their 'restricted^' property from the configuration of the power system and may also be classed as unit protection.

Whichever method is used, it must be kept in mind that selectivity is not merely a matter of relay design.

It is a function of the correct co-ordination of current transformers and relays with a suitable choice of relay settings, taking into account the possible range of such variables as fault currents, maximum load current, system impedances and so on, where appropriate.

 ·     Zones of protectionIdeally, the zones of protection should overlap across the circuit breaker as shown in Figure 2, the circuit breaker being included in both zones.

Figure 2. Location of current transformers on both sides of the circuit breaker.

For practical physical reasons, this ideal is not always achieved, accommodation for current trans-formers being in some cases available only on one side of the circuit breakers, as in Figure 3. This leaves a section between the current transformers and the circuit breaker A within which a fault is not cleared by the operation of the protection that responds. In Figure 3 a fault at F would cause the bus-bar protection to operate and open the circuit breaker but the fault would continue to be fed through the feeder.
 

Figure 3 Location of current transformers on circuit side of the circuit breaker.

The feeder protection, if of the unit type, would not operate, since the fault is outside its zone. This problem is dealt. With by some form of zone exten­sion, to operate when opening the circuit breaker does not fully interrupt the flow of fault current. A time delay is incurred in fault clearance, although by restricting this operation to occasions when the bus-bar protection is operated the time delay can be reduced.

Figure 4 Overlapping zones of protection systems.

 
The point of connection of the protection with the power system usually defines the zone and cor­responds to the location of the current transformers. The protection may be of the unit type, in which case the boundary will be a clearly defined and closed loop. Figure 4 illustrates a typical arrange­ment of overlapping zones.

Alternatively, the zone may be unrestricted; the start will be defined but the extent will depend on measurement of the system quantities and will therefore be subject to variation, owing to changes in system conditions and measurement errors.

·     Stability.

This term, applied to protection as distinct from power networks, refers to the ability of the system to remain inert to all load conditions and faults external to the relevant zone. It is essentially a term which is applicable to unit systems; the term 'discrimination' is the equivalent expression applicable to non-unit systems.
·     Speed.The function of automatic protection is to isolate faults from the power system in a very much shorter time than could be achieved manually, even with a great deal of personal supervision. The object is to safeguard continuity of supply by removing each disturbance before it leads to widespread loss of synchronism, which would necessitate the shutting down of plant.

Loading the system produces phase displacements between the voltages at different points and therefore increases the probability that synchronism will be lost when the system is disturbed by a fault. The shorter the time a fault is allowed to remain in the system, the greater can be the loading of the system. Figure 1.5 shows typical relations between system loading and fault clearance times for various types of fault.

It will be noted that phase faults have a more marked effect on the stability of the system than does a simple earth fault and therefore require faster clearance.

It is not enough to maintain stability; unnecessary consequential damage must also be avoided. The destructive power of a fault arc carrying a high current is very great; it can burn through copper conductors or weld together core laminations in a transformer or machine in a very short time. Even away from the fault arc itself, heavy fault currents can cause damage to plant if they continue for more than a few seconds

 Figure 5 Typical values of power that can be
 transmitted as a function of fault clearance time.
It will be seen that protective gear must operate as quickly as possible; speed, however, must be weighed against economy.

For this reason, distribu­tion circuits for which the requirements for fast operation are not very severe are usually protected by time-graded systems, but generating plant and EHV systems require protective gear of the highest attainable speed; the only limiting factor will be the necessity for correct operation.·     SensitivitySensitivity is a term frequently used when referring to the minimum operating current of a complete protective system. A protective system is said to be sensitive if the primary operating current is low.

When the term is applied to an individual relay, it does not refer to a current or voltage setting but to the volt-ampere consumption at the minimum operating current.

A given type of relay element can usually be wound for a wide range of setting currents; the coil will have an impedance which is inversely proportional to the square of the setting current value, so that the volt-ampere product at any setting is constant.          

This is the true measure of the input requirements of the relay, and so also of the sensitivity. Relay power factor has some significance in the matter of transient performance.

For D.C. relays the VA input also represents power consumption, and the burden is therefore frequently quoted in watts.