CHAPTER LVIII

CURRENT AND PRESSURE LIMITING DEVICES

In any electric installation there must be provided a number of automatic devices to secure proper control. The great multiplicity of devices designed for this purpose may be divided into two general classes, as

1. Current limiting; 2. Pressure limiting.

Because of the heating effect of the current which increases in proportion to the square of the strength of the current, it is necessary to protect circuits with devices which do not allow the current to exceed a predetermined value.

Accordingly fuses, circuit breakers, reactances, etc., are used, each possessing certain characteristics, which render it suitable for particular conditions of service.

For instance, just as in analogy, steam boilers must be protected against abnormal pressures by safety valves, electric circuits must be guarded against excessive voltages by pressure limiting devices, otherwise much damage would occur, such as the burning out of incandescent lamps, grounding of cables, etc.

The control of steam is simple as compared to the electric current, the latter being the more difficult to manage because of its peculiar behaviour in certain respects, especially in the case of alternating current which necessitates numerous devices of more or less delicate construction for safety both to the apparatus and the operator.

~Fuses.~--A fuse is "an electrical safety valve", or more specifically, _the actual wire or strip of metal in a cut out, which may be fused by an excessive current_, that is to say, by a current which exceeds a predetermined value. A fuse, thus serves to protect a circuit from any harm resulting from an undue overload.

Fuses have been treated at such length in Guide No. 2, Chapter XXV, that very little can be said here, without repetition.

~Ques. What effect have the terminals on a fuse?~

Ans. The current at which a fuse melts may be greatly changed by the size and shape of the terminals.

If near together and large, they may conduct considerable heat from the fuse thus increasing the current required to blow the fuse.

~Ques. What is the objection to large fuses?~

Ans. The discharge of molten metal when the fuse blows is a source of danger.

~Ques. What should be used in place of large fuses?~

Ans. Circuit breakers.

~Ques. What are the objections to fuses in general?~

Ans. The uncertainty as to the current required to blow them; the constant expansion and contraction is liable to loosen the terminal screws when screws are used.

~Ques. What is the advantage of fuses?~

Ans. They form an inexpensive means of protecting small circuits.

~Ques. Describe a plug fuse.~

Ans. It is constructed as shown in fig. 2,239, the fuse wire being visible and stretching between the two metal portions of the plug.

~Ques. What is a cut out fuse?~

Ans. One similar to a simple fuse, but provided with clip contacts as used for knife switch contacts.

The fuse wire is usually contained in a china or porcelain tube, which also serves the purpose of a handle for withdrawing the fuse.

~Ques. What is an expulsion fuse?~

Ans. One in which the fuse is placed in an enclosed chamber with a vent hole.

In operation, when the fuse blows, the hot air and molten metal are expelled through the vent.

~Ques. What is a no arc fuse?~

Ans. A cartridge type fuse, in which the space surrounding the fuse wire is filled with powdered material.

The object of the powdered material is to assist in extinguishing the arc formed when the fuse blows.

~Ques. What is a magnetic blow out fuse?~

Ans. An enclosed fuse which is subject to the action of a magnetic field produced by the current, the magnetic field tending to blow out the arc when fusing occurs.

~Ques. What is a quick break fuse?~

Ans. One having a weight suspended from its center, or springs attached to its ends so that the arc formed at fusing is quickly attenuated and extinguished.

~Ques. What is the disadvantage of a fuse as compared to an oil switch circuit breaker?~

Ans. When a fuse blows, the arc causes oscillations in the line, which cause excessive rise of pressure under certain capacity conditions, whereas this disturbance is reduced to a minimum with an oil switch.

~Ques. What metal is used for fuse wires?~

Ans. Various metals. Ordinary fuse wire is made of lead or an alloy of lead and tin.

~Ques. What is the objection to aluminum?~

Ans. It becomes coated with oxide or sulphide, which acts as a tube tending to retain the metal inside and prevent rupture.

~Ques. What is the objection to copper?~

Ans. Its high fusing point.

~Current Limiting Inductances.~--The great increase in capacity of power stations, for supplying the demands of densely populated centers and large manufacturing districts, together with the decrease in the reactance of modern alternators and transformers due to improvement in design to obtain better regulation, has presented a problem in apparatus protection not contemplated in the earlier days of alternating current distribution. This problem is entirely separate and distinct from that of eliminating the tendency toward short circuit, incident to the high voltages now common in transmission lines. It accepts that all short circuits must occasionally occur and considers only the protection of the connected apparatus against the mechanical forces due to the magnetic stresses of such enormous currents.

~Ques. What means are employed to limit the value of a short circuit current?~

Ans. A current limiting inductance coil (called a _reactance_) is placed in series with the alternators or transformers.

~Ques. What are its essential features of construction?~

Ans. It consists of bare stranded cable wound around a concrete core and held in place by wooden supports as shown in fig. 2,243.

In order to avoid the prohibitive expense of high voltage insulation, the reactance coil is designed for the low tension circuit. This requirement prohibits the use of a magnetic core which, if economically designed for normal operation, would become saturated at higher densities, or, if designed large enough to avoid saturation at short circuit conditions, would become prohibitive in cost and dimensions.

The elimination of all magnetic material from the construction of the concrete core reactance permits of no saturation, and assures a straight line voltage characteristic at all current loads.

~Ques. Where is the proper location for a current limiting reactance?~

Ans. As near the alternator as possible.

~Ques. Why?~

Ans. To lessen the possibility of a short circuit occurring between the reactance and the alternator.

~Ques. Beside limiting the current, what other service is performed by the reactance?~

Ans. It protects the alternator from high frequency surges coming in from the outside, and limits the current from other machines on the same bus.

~Circuit Breakers.~--The importance of circuit protective devices, commonly called circuit breakers, is fully recognized. The duty of a circuit breaker is to protect the apparatus in an electrical circuit from undesirable effects arising from abnormal conditions, by automatically breaking the circuit. Accordingly a circuit breaker must comprise a switch in combination with electrical control devices designed to act under abnormal conditions in the circuit.

A circuit breaker is a _device which_ ~automatically~ _opens the circuit in event of abnormal conditions, in the circuit_.

In the design of circuit breakers, there are several methods used to effect the rupturing of the arc between contacts when opened on heavy overload, such as:

1. Magnetic blow out; 2. Thermal break; 3. Carbon break.

In the magnetic blow out type, the arc is extinguished between auxiliary contacts confined by a chute in which the arc is rapidly blown out due to a powerful magnetic field from one or more electromagnets. This type may be used in air or watertight boxes and is peculiarly adapted for service where the arc must be confined.

In a carbon break type, the arc is finally ruptured between carbon break contacts. The breaking of the circuit is accomplished progressively, that is to say, it is done in three stages, by several sets of contact, known respectively as

1. The main contacts; 2. The intermediate contacts; 3. The carbon contacts.

_In operation_, as the circuit breaker acts to break the circuit, first the ~main~ _contacts_, separate, then the ~intermediate~ _contacts_, and finally the ~carbon~ _contacts_ between which the arc is ruptured.

~Ques. What is the object of the intermediate contacts?~

Ans. To prevent the forming of an arc on the main contacts.

~Ques. What is the object of the carbon contacts?~

Ans. First to protect the intermediate contacts by providing a path for the current after the intermediate contacts separate, and 2, to "slow down" the current by means of the considerable resistance of the carbon, thus reducing to a minimum the arc which is formed when the carbon contacts separate.

~Ques. How is the automatic operation of a circuit breaker usually accomplished?~

Ans. Usually through the medium of a solenoid, or electromagnet energized by current from the circuit controlled by the breaker.

The essential features of construction and operation of a circuit breaker is shown in the elementary diagrams, figs. 2,250 to 2,253. ~In construction~ as shown in fig. 2,250 it consists essentially of three sets of contacts, a swinging contact arm which is set in the closed position by the handle operating through the toggle joint, the movement of which is limited in the closing direction by the stop. The latter is made adjustable by an eccentric pin or equivalent. Connected to the toggle is the plunger of the solenoid whose winding is energized by current from the circuit which the circuit breaker is to control.

~In operation~, the circuit is closed by hand by turning the handle downward to the position shown in fig. 2,250, that is as far as it will go.

Since the toggle has passed the center line the arm will be held normally in this position because of the spring action of the contacts. Now, if the current rise above a predetermined limit, the pull exerted by the solenoid will overbalance the tendency of the toggle to remain in the closed position, and pull the two toggle links downward below the center line, drawing the contact arm back and breaking the circuit.

The progressive action which takes place during this operation is shown in figs. 2,250 to 2,253 in which the main contacts separate first, then the intermediate, and finally the carbon contacts as mentioned before.

~Ques. What name is given to this type of circuit breaker?~

Ans. It is called an overload circuit breaker.

~Automatic Features.~--There are three methods of connecting the winding of the solenoid, or _trip coil_ as it is called:

1. In series with the main circuit; 2. In shunt with the main circuit; 3. In shunt with an auxiliary circuit.

The automatic controls arising from these connections give various kinds of protection to the circuit and are known as

1. Overload trip; 2. Underload trip; 3. Low voltage trip; 4. Auxiliary circuit trip.

~Ques. What is the object of the overload trip?~

Ans. It is intended to open the circuit when the current exceeds a predetermined value.

~Ques. What modifications are made in the mechanism shown in the elementary diagrams?~

Ans. Sometimes a latch is used in place of the toggle and a magnet in place of the solenoid as in figs. 2,265 and 2,266.

~Ques. Why is a magnet used in combination with a latch?~

Ans. Because with this arrangement very little movement is required to trip the breaker, and for such conditions, a magnet is more efficient than a solenoid.

~Ques. How does the latch arrangement work?~

Ans. When the proper current is reached, the magnet pulls open the latch and the contact arm of the breaker moves by the force of gravity or other means and opens the circuit.

~Ques. How does the underload trip operate?~

Ans. The same as the overload type except that they operate on a diminution of current instead of an excess.

~Ques. Describe the no voltage trip.~

Ans. The energy for the trip of this breaker is derived from a high resistance or fine wire coil which is arranged to be placed directly across the line, in operation, when the current flowing through the circuit falls below a predetermined value, the energy of the coil is insufficient to counteract the force of a spring, which then trips the breaker.

~Ques. Describe the auxiliary circuit trip.~

Ans. A pressure coil is used which is energized by current from an auxiliary circuit. The coil is only _momentarily_ energized, by push button, relay or other control, as distinguished from the preceding types, in which the coil is _constantly_ energized.

~Ques. What other name is given to the auxiliary circuit trip?~

Ans. It is sometimes called the shunt trip, though ill advisedly so.

~Relays.~--Oil break switches and carbon break circuit breakers are commonly used to open electrical circuits at some given overload and on short circuit. To secure additional protection under a variety of abnormal conditions or to provide for a certain predetermined operation or sequence of operations, relays may be employed.

A relay is defined as: _A device which_ ~opens~ _or_ ~closes~ _an_ ~auxiliary circuit~ _under predetermined electrical conditions in the main circuit_.

The object of a relay is generally to act as a sort of electrical multiplier, that is to say, _it enables a comparatively weak current to bring into operation a much stronger current_.

~Ques. For what service are relays largely used?~

Ans. They are employed in connection with high voltage switches where the small amount of energy derived from an ordinary instrument transformer is insufficient for tripping.

The connections between relays and circuit opening devices are usually electrical. Combinations of this nature are extremely flexible since they permit the use of a number of devices, each having a different function, with a single circuit breaker or oil switch as well as with two or more switches, to secure the desired operation and protection.

~Selection.~--In all electrical installations protection of apparatus is important, but in some large central stations this is secondary to continuity of service.

To combine maximum protection without interruptions of service is not always possible, but these requirements can be approximated very closely by the use of reliable and simple controlling or protecting devices if proper care be taken to select the relays suited to the special conditions of the installation. To do this intelligently, a knowledge of the various types of relay is necessary.

There is a multiplicity of types and a classification to be comprehensive, should, as in numerous other cases, be made from several points of view. Accordingly relays may be classified:

1. With respect to the nature of the service performed, as

_a._ Protective; _b._ Regulative; _c._ Communicative.

2. With respect to the operating current, as

_a._ Alternating current; _b._ Direct current.

3. With respect to the manner of performing their function, as

_a._ Circuit opening; _b._ Circuit closing.

4. With respect to the operating current circuit, as

_a._ Primary; _b._ Secondary.

5. With respect to the abnormal conditions which caused them to operate, as

_a._ Overload; _b._ Underload; _c._ Over voltage; _d._ Low voltage; _e._ Reverse energy; _f._ Reverse phase.

6. With respect to the time consumed in performing their function, as

_a._ Instantaneous (so called); _b._ Definite time limit; _c._ Inverse time limit.

7. With respect to the character of its action, as

_a._ Selective; _b._ Differential.

8. With respect to whether it acts directly or indirectly on the circuit breaker, as

_a._ Main; _b._ Auxiliary.

~Protective Relays.~--These are used to protect circuits from abnormal conditions of voltage, or current, which would be undesirable or dangerous to the circuit and apparatus contained therein.

~Ques. How do protective relays operate?~

Ans. They act in combination with automatic circuit breakers, operating when their predetermined setting has been reached, energizing the trip coil of the circuit breaker and opening the circuit.

Fig. 2,279 shows the principles of relay operation. When the current or pressure in the main circuit reaches the predetermined value at which the protective system should operate, the relay magnet attracts the pivoted contact arm and closes the auxiliary circuit; this permits current to flow from the current source in that circuit and energize the trip coil thus opening the main circuit.

~Regulative Relays.~--This class of relay is used to control the condition of a main circuit through control devices operated by a secondary circuit.

~Ques. For what service are relays of this class employed?~

Ans. They are used as feeder circuit or generator regulators.

~Ques. How do they differ from protective relays?~

Ans. They have differentially arranged contacts, that is to say, arranged for contact on either side of a central or normal position.

~Communicative Relays.~--These are used for signalling in a great variety of ways for indicating the position of switching apparatus or predetermining the condition of electric circuits.

~A. C. and D. C. Relays.~--As here used, the classification refers to the kind of current used on the auxiliary circuit. In some cases direct current is used to energize the trip gear of the circuit breaker or oil switch, and in others, alternating current.

A. C. and D. C. relays are respectively known as _circuit opening_ and _circuit closing_ relays, being later fully described.

~Circuit Opening Relays.~--The duty of a circuit opening relay is _to open the_ ~auxiliary circuit~, _usually alternating current, nd thereby cause the oil switch or circuit breaker to be opened by the use of a trip coil in the secondary of a current transformer, or by low voltage release coil_.

The trip coil of the breaker is generally shunted by the relay contacts and when the moving contact of the relay disengages from the stationary contact, the current from the transformer which supplies the relay, flows through the trip coil thus opening the breaker. These features of operation are shown in fig. 2,282.

~Ques. Where are circuit opening relays chiefly employed?~

Ans. In places where direct current is not available for energizing the trip coil.

~Ques. What is the objection to alternating current trip coils?~

Ans. They have relatively high impedance and impose a heavy volt ampere load on the transformers.

~Circuit Closing Relays.~--The duty of a circuit closing relay is to close the auxiliary circuit at the time when the predetermined abnormal condition is reached in the primary circuit. The closing of the auxiliary circuit energizes the trip coil and opens the breaker.

~Ques. What kind of current is generally used for the auxiliary circuit of a circuit closing relay?~

Ans. Direct current.

~Ques. At what pressure?~

Ans. From 125 to 250 volts.

~Ques. Where is this current usually obtained?~

Ans. From a storage battery, or from the exciter.

~Ques. For what current are the contacts ordinarily designed?~

Ans. About 10 amperes.

~Primary and Secondary Relays.~--Primary relays are sometimes called series relays as they have the current coils connected directly in series with the line, both on high and low tension circuits.

Secondary relays receive their current supply from the secondary circuits of current transformers. Alternating current relays connected to secondary of pressure transformers and relays with both current and pressure windings are included in this class.

~Ques. What is the usual winding of the coils?~

Ans. The current coils are usually wound for 5 amperes and the pressure coils for 110 volts.

~Ques. What refinement is made in the design of relays and why?~

Ans. Care is exercised to reduce to a minimum the volt ampere load imposed by the relay on the current transformer to permit the use of un-stranded meters and relays upon the same transformer.

The use of circuit opening relays to cut out the trip coil of an oil switch during normal operation, has been described, and in the short time that the trip coil is in circuit, it does not affect the accuracy of the instrument readings. This practice, however, does not apply in the case of curve drawing meters, voltage compensators or other devices which have in themselves sufficient load for separate current transformers. In this connection it should be noted that to obtain accurate instrument and meter readings; the current transformers should not be loaded beyond certain limits which depend upon the volt ampere load and power factor of each of the connected devices.

So great is the variety of combination used and the variations of these factors in their several combinations at different loads and settings, that special consideration of each arrangement is advisable.

~Overload Relays.~--Series relays are connected directly in series with the line and are chiefly used with high pressure oil break switches for overload protection. If current transformers are to be used on the same circuits for other purposes, and have sufficient capacity to admit of adding a relay coil, secondary relays would be more economical; otherwise, the series relays are less expensive.

By means of a specially treated wooden rod, the relay operates a tripping switch, closing a separate tripping circuit, usually 125 or 250 volts direct current. Series relays are essentially the same as secondary relays except in the coil winding and insulation.

~Underload Relays.~--These are similar in construction to low voltage relays but have current instead of pressure windings.

~Over Voltage Relays.~--These are usually of the circuit closing type and are similar to secondary overload relays, but have pressure instead of current windings.

~Low Voltage Relays.~--Relays of this class are in most cases used

~Reverse Phase Relays.~--This type of relay is used chiefly to prevent damage in case of reversal of leads in reconnecting wiring to two or three phase motors.

~Time Element.~--It is often inconvenient that a circuit breaker should be opened immediately on the occurrence of what may prove to be merely a momentary overload, so that time lag attachments are frequently provided, particularly with relays. These devices, which may form part of the relay or may be quite distinct from it, retard its action until the overload has lasted for a predetermined time--several seconds or more.

~Ques. What should preferably govern the time lag?~

Ans. It should depend on the extent to which the overload is reduced as the time elapses.

~Instantaneous Relays.~--The so called instantaneous relays operate almost instantly on the occurrence of the abnormal condition that they are to control.

There is of course a slight time element comparable with that of an overload circuit breaker, but for practical purposes, the operation may be considered as instantaneous.

~Time Limit Relays.~--Under this classification there are two sub-divisions.

1. Definite time limit; 2. Inverse time limit.

~Ques. Describe the time mechanism of a definite time limit relay.~

Ans. It consists of an air dash pot, and an air diaphragm or equivalent retarding device connected to the contact mechanism.

~Ques. How does it operate?~

Ans. In some designs, when the contacts are released, they descend by gravity against the action of the retarding device thereby making contact a definite interval after the occurrence of the abnormal condition.

~Ques. How does the inverse time limit type operate?~

Ans. The actuating and contact mechanism is attached directly to an air bellows and in operation tends to compress the bellows against the action of a specially constructed escape valve in the latter.

~Ques. Why is the arrangement called _inverse_ time limit?~

Ans. Because the retardation varies inversely with the pressure on the bellows, and therefore inversely with the magnitude of the abnormal condition.

~Ques. What other device may be used to retard the operation?~

Ans. A damping magnet is sometimes used which acts on a disc or drum and which may be adjustable.

~Ques. How is the inverse time element introduced by this arrangement?~

Ans. The retardation is due to eddy currents induced by moving the disc or drum through the magnetic field. The reaction thus induced varies inversely with the magnitude of the force with which the disc or drum is urged through the field and hence inversely with the abnormal condition.

~Ques. What are the ordinary limits of adjustment for inverse time limit relays?~

Ans. From one-half second to 30 seconds, depending upon the time setting and magnitude of the overload current.

A setting of from two to six seconds is ordinarily used, depending upon the requirements. Where selective operation is desired a minimum setting of two seconds is recommended.

~Differential Relays.~--In this type of relay there are two electromagnets. In normal working these oppose and neutralize each other. Should, however, either winding become stronger or weaker than the other, the balance is upset, the magnet energized, and the relay comes into operation.

A modification of such a relay for alternating current is shown in fig. 2,322, from which it will be seen that when the currents are as indicated, the circuit A has the larger pressure induced in it, whereas, should the main current reverse with reference to the shunt current, the circuit B would have the larger induced pressure. [Illustration: FIG. 2,323.--Diagram of modern power house wiring and busses showing location of relays.]

~[1]How to Select Relays.~--The following general information on relays, together with reference to the one line diagram, fig. 2,323, will be of interest and assistance in making a selection from the various relays previously described to meet the requirements of modern power house and sub-station layouts.

[1] NOTE.--As suggested by the General Electric Co.

~Single pole relays~ are used on single phase and on balanced three phase circuits.

~Double pole relays~ are used on ungrounded three phase and on quarter phase.

~Triple pole relays~ are used on three phase grounded neutral and interconnected quarter-phase.

~Circuit closing relays~ are recommended in all cases where a constant source of direct current is available for operating trip coils.

The conditions for which relays have been designed for power circuits may perhaps be best described, by considering a one line diagram from the generator end to the sub-station auxiliary machines and feeders.

Considering first alternating current circuits, the prevailing practice is to make the circuit breakers by which the alternators are connected to the low tension bus _non-automatic_, in order to insure minimum interruption of alternator service. The chance of trouble in this part of the circuit is remote, but should it occur, the station attendant could generally open the circuit breaker before the machines would be injured.

~Reverse current relays~ of instantaneous or time limit types are often connected to the secondaries of current and of pressure transformers to indicate by lamp or bell any trouble that may occur in the generator circuit.

These relays operate with a low current reversal at full pressure and conversely with a proportionally greater current at voltages less than normal. At zero pressure, the relay would act as an overload one, set for high overload. At zero current, a voltage considerably in excess of normal would be required to operate it.

Specifications sometimes call for automatic generator circuit breakers: in this case _definite time limit overload relays_ are used. They are connected in the secondaries of current transformers and are designed to give the same time delay for all trouble conditions; they allow the defective circuit to be opened, if possible, at a point more remote from the generator than the generator circuit breaker.

When the total generator capacity exceeds the rated rupturing capacity of the circuit breakers, one or more sectionalizing circuit breakers are placed in each bus.

If operating conditions admit, these devices are made non-automatic and are left disconnected except in case of emergency; but if it be necessary for them to be continually in service, they may be made automatic by _means of instantaneous overload relays_ connected to current transformers in the low voltage bus; the relays being adjusted to trip the circuit breaker under short circuit conditions, confining the trouble to one section and preventing the circuit breakers rupturing more than their rated capacity.

Installations with but _one bank of power transformers_, and without high voltage bus, are provided with automatic circuit breakers operated by an _inverse time limit relay_.

The relay is connected to the secondaries of current transformers, which in turn are connected in the low voltage side of the power transformer.

Stations with _more than one bank of power transformers_, a high voltage bus, and high and low voltage circuit breakers, may have both circuit breakers arranged to trip at the same time or one after the other. As in the former case, they are operated from the inverse time limit relay connected in the low voltage side.

In plants in which two or more banks of transformers are operated in parallel between high and low voltage busses, it is desirable to have for each transformer bank, an automatic circuit breaker equipment which will act selectively and disconnect only the bank in which trouble may occur. With a circuit breaker on each side of transformer bank, selective action may be secured in two ways as follows:

1. By means of an instantaneous differential relay connected in the secondaries of current transformers installed on both the high and low voltage sides of each transformer bank.

The relay operates on a low current, reversal on either side of the bank.

2. By means of one inverse time limit, secondary or series relay installed on that side of the transformer bank which is opposite the source of power, the relay being arranged to trip both the high and low voltage circuit breakers.

_The first method_ has the disadvantage of high first cost due to the high voltage current transformers required, but is more positive than the second method and is independent of the number of transformer banks in parallel.

_The second method_ is the less expensive of the two and protects against overloads as well as short circuits in the transformers, but it is less positive and introduces delay in the disconnection of the transformer when trouble occurs. Furthermore, it is not selective when less than three banks are operating in parallel.

The automatic circuit breakers in the outgoing line may be operated from inverse time limit relays connected in the secondaries of current transformers; or in case transformers are not necessary for use with instruments, series high voltage inverse time limit relays connected directly in the line may be used.

Whether to select current transformers with relays insulated for low voltage, or to choose series relays, is a question of first cost and adaptability to service conditions. Below 33,000 volts, the commercial advantages in favor of the series relay are slight, and since it is somewhat difficult to design this device for the large current capacities met with at the lower voltage, it is generally the practice to use the relay with current transformer, because of its operating advantage. This practice, however, is not entirely followed, since some service conditions (described later) make the use of series relays very desirable and practical.

_Inverse time limit_ relays are satisfactory for one, or more than two outgoing lines in parallel as they act selectively to disconnect the defective line only, but installations with only two outgoing lines in parallel have the same load conditions in both lines and selective tripping of the circuit breakers in the defective line is obtained by means of a selective relay acting instantaneously under short circuit conditions only.

The relay design and action is similar to the reverse current relay previously mentioned, and is connected to the secondaries of current transformers in each high voltage line and pressure transformers in the low voltage bus.

In the sub-station, the conditions are the reverse of those in the main station, the incoming lines becoming the source of power.

If there be only one incoming line and no high voltage bus, the line circuit breaker is generally non-automatic. With one incoming line and high voltage bus, the circuits from the service side of the bus are equipped with automatic circuit breakers and relays. These relays and those used for other arrangements of two or more incoming lines in parallel, as well as high and low voltage circuit breakers, are of the same design and are applied in the same manner as for the generating station.

Regarding the relay equipments for auxiliary machines, the same practice is recommended with the generator end of alternating current motor generator sets as with the main generators, the outgoing feeder circuit breakers being tripped from inverse time limit or instantaneous relays.

With several synchronous machines in parallel, the relays are arranged to operate with the least time delay with which it is possible to get selective action, in order to prevent the machines being thrown out of step in event of trouble conditions causing a decrease of voltage.

The various types of _induction motor_ and various conditions under which they are employed, have brought about the development of several types of relay to protect the motors and the apparatus with which they are used.

It is desirable to disconnect a _large motor_ in case of voltage failure, and with conditions requiring either a motor operated, or a solenoid operated circuit breaker, a _low voltage relay_ is used to close the tripping circuit whenever the voltage decreases to, approximately, 50 per cent. below normal.

Up to 550 volts, these relays may be connected across the line, but for higher voltages they are connected to secondaries of pressure transformers. _Smaller motors_ with which hand operated circuit breakers are used, are generally provided with low voltage release attachments that perform the same function as the relay.

Induction motors are sometimes subjected to _high voltage conditions_ and to protect them from injury, high or excess voltage relays are employed to trip the automatic circuit breaker. These relays are of similar design and wired in the same manner as the low voltage relays.

_Reverse phase relays_ have been developed for operating conditions under which a _reversal of phase_ would cause trouble, as for example, in the case of _elevator motors_.

These are so designed that any phase reversal that would reverse an induction motor, would operate the relay and disconnect the automatic circuit breaker.

The design is based on the principle of the induction motor, and in the case of low voltage motors of limited capacity, the relay may be connected in series in the motor leads. If the voltage or capacity of the motor make this arrangement inexpedient, the relay may be placed in the secondaries of current or pressure transformers connected in the motor leads.

_Underload relays_ are often used to trip the automatic circuit breaker that is placed in the primaries of _arc lighting circuits_ to prevent an abnormal rise of secondary voltage in case of a break in the secondary circuit.

The underload relay is similar in design to the low voltage relay excepting that it acts on a decrease of current.

The problem of _protecting induction motors_, from injury, that may result from running on single phase, or from an overload, and at the same time permit the motor to be started with the necessarily high starting current that may be greatly in excess of the overload current, has caused the development of the _series relay_.

This device may be connected in series with the motor leads for voltages up to 2,500; it is designed with an inverse time limit device which may be adjusted to give the desired protection.

The field for relays is more extensive for alternating current than for direct current power circuits, the latter being generally confined to much smaller and simpler systems and areas of distribution, and generally sufficient selective action can be obtained by the use of fuses or circuit breakers arranged with instantaneous trip.

Operating conditions sometimes make it advisable for the generator circuit breakers to open only after the auxiliary and feeder circuit breakers have failed to isolate the trouble.

This is accomplished by using direct current _series inverse time limit relays_ to trip the generator circuit breakers.

_Instantaneous reverse current relays_ are used to trip the machine circuit breaker of battery charging sets, rotaries and motor generator sets to prevent their running as a motor on the charging or direct current end. These relays can act only in case of current reversal.

To prevent serious unbalancing of voltages in Edison three-wire systems causing trouble, _differential balance relays_ are used to trip the circuit breakers on a small percentage of unbalancing.