Manual / Guide · 1960
AE Training Bulletin 812: Power and Supervisory Equipment
Exchange power plant, ringing machines, tones, alarms.
Manual / Guide · 1960
Exchange power plant, ringing machines, tones, alarms.
POWER AND SUPERVISORY EQUIPMENT Sbbsidiary a: GENERAL TELEPWORE $ EbECTRQNJCS This is one of the helpful booklets i n the AUTOMATIC ELECTRIC TRAINING SERIES S'FROWGER AUTOMATIC TELEPHONE SYSTEMS Electrical Principles of Telephony Mechanical Principle s of Telephony Fundamentals of Apparatus and Trunking Distributing F r a m e s The Plunger Lineswitch and Associated Master-Switch Rotary Lineswitch The Connector The Selector Pulse R e p e a t e r s Power and S u p e r ~ i i s o r yEquipment Party-Line Connectors and Trunk- Hunting Connectors Reverting-Call Methods The T e s t and Verification Switch Train Toll Switch T r a i n Switching Selector -Repeater P - A - X s and P - A - B - X s Manual Switchboards Linefinder Switches IVIay we send you others pertaining to equipment in your exchange ? Copyright 1960 by kutcmatic Electric Company Figure 1 . Powerboal-d for a tltoderntely sized telepholze centrul ofjice. POWER AND SUPERVISORY EQUIPMENT 1. INTRODUCTION The purpose of this bulletin is to explain the equipment in a telephone central office used for power supply, ringing supply, and for supervisory purposes. This includes the equipment used for the production, regulation, control, protection, and distribution of electric a l power (voltages, frequencies, and tones required for operating a common battery telephone central office). It a l s o includes the equipment and c i r c u i t r y used to monitor, to control and regulate i f possible, and to signal the occurrence of nonstandard o r trouble conditions of the switching equipment and power supply equipment and facilities. , The electrical power used in a common battery, automatic, telephone central office includes the following: 50 volts dc (approximately) for voice transmission and for operation of the switching and control equipment; one o r m o r e a - c voltages a t various frequenc i e s f r o m 16-213 c p s (cycles p e r second) to 66-2/3 c p s for operating the audible s i g n a l s a t the s u b s c r i b e r s ' stations; various tones, both interrupted and steady, and pulses (or i n t e r rupted voltages) for audible, and sometimes visual, indication of various conditions of the equipment and c i r c u i t s in the c e n t r a l office. T h i s includes the following equipment: d-c g e n e r a t o r s , batteries, a - c generators, tone g e n e r a t o r s , and interrupting equipment; a powerboard with some control, distribution, . protection, and alarm-indicating features; and individual fuse, fuse a l a r m , and equipmentoperation and failure-indicating panels and c i r c u i t r y on each shelf. 1.1 P r i m a r y - secondary Equipment Most telephone exchanges a r e operated f r o m the commercial a - c power supply in the a r e a . Rectifiers o r r o t a r y c o n v e r t e r s (motor genera t o r s ) convert this a - c power to 50 volts dc to operate the exchange equipment and keep the battery fully chsrged. E a c h exchange must a l s o have one o r m o r e sources o r converters to supply ringing c u r r e n t , tones, and operate the interrupter; these may be separate units, o r , m o r e often, two o r t h r e e may be combined into a single - - unit. These units, either singly o r collectively, may be operated b y a - c power (normally f r o m the commercial a - c s o u r c e ) o r by d-c power i f preferred o r n e c e s s a r y . E a c h unit (ringing generator, tone generator, interrupte r , o r combination unit) usually h a s a n alternate o r stand-by unit which may be used in i t s place i f it should fail. Depending on the type of equipment (and, in part, on the age of the office), the stand-by equipment m a y be operated either by the same type of power (ac o r dc) o r by different power (dc o r a c ) if desirable for any reason. Whenever possible m o r e than one d-c charging unit is installed so that some d e g r e e of s e r v i c e may be maintained in c a s e of failure of one charger unit. The battery a l s o helps maintain service in c a s e of c h a r g e r failure o r overload. The battery is used to maintain s e r v i c e during transfer operations, to cause th.e t r a n s f e r to take place, and, if n e c e s s a r y , to operate the exchange i f a l l other s o u r c e s of power fail. 1.2 Auxiliary Engine-driven Equipment Some installations a l s o include one o r m o r e engine-driven, a-c generators which may be used i n c a s e of failure of the commercial a - c power to the exchange. This m a y be used simply to operate the d-c power equipment (rectifier o r d-c generator) with the d-c power in t u r n used to r u n alternate (d-c operated) ringing tone-interrupting equipment; o r the stand-by a d c power supply may be connected directly to the a-c operated units. An auxiliary engine-driven generator consists of a n a - c generator driven by either a gasoline o r Diesel engine. Exchanges usually use engine-driven auxiliary generator equipment when the a-c power is not v e r y r e l i a b l e and/or they either prefer to o r must u s e a small capacity battery (one which can c a r r y the exchange load for only a relatively short period of time). An exchange which uses engine generators may have one o r m o r e units, depending upon the size of the exchange and the load. The number and total capacity of the units used is sufficient, normally, to supply the maximum busy-hour load plus any other emergency equipment required. The fuel tank 1 COMMERCIAL A.C. CHARGING L r-+- f I b t EQUIPMENT POWER I BATTERY P ENGINE - DRIVEN A C GENERATOR (WHEN USED) I I I 0 DC-POWERED UNITS RlNG G EN. TONE I N T. .' GEN. I w POWER E TONES R AND 6 0 I I 1 I \ I I C t / RING I L-, GEN. COMMERCIAL b - A & SIGNALS T0 EXCHANGE b R D b INT. A. C. P O W E R I TONE GEN. \ B , Fi,:rlr~.c2. Block d z ~ r - n iq~fi cenh-nl qtfice pocl3eJ*srcpply eqztipti~e~zt slzoic~i~rg az~xilinzyuppal-cihcs, nrzd ~ ~ e l a h o ~ mzdl s irzterco~l~zectiorzs bet~oeeuzcrzils. m a y be a s l a r g e a s n e c e s s a r y to operate the engine and exchange for whatever period of time desired. When a n exchange uses engine g e n e r a t o r s , the b a t t e r y capacity i s usually onlygreat enough to supply the exchange load until the engine units a r e s t a r t e d . Engine g e n e r a t o r s a r e normally furnished with automatic starting; they a r e available with manual starting only i f desired. 2 . . D-C POWER G e n e r a t o r s and r e c t i f i e r s provide most of the 50-volt d-c power required by telephone exchange equipment. The operating range of m o s t exchange equipment is about 46 to 52 volts. When the power supply equipment cannot m e e t the load c u r r e n t demands of the exchange, a s during peak loads o r commercial power failure, the exchange battery furnishes the r e q u i r e d power. The power supply equipment a l s o functions to keep the battery fully charged f o r emergency use. The powerboard is the control center of the power supply equipment. This board holds the voltage-control equipment for the c h a r g e r units and the battery. The board also h a s fuse panels f o r distributing the power to the exchange equipment and for protection against s h o r t circuits. 2.1 The Exchange Battery The exchange battery i s a group of electric storage c e l l s connected in the proper combination ( s e r i e s o r series-parallel) to furnish the voltage and c u r r e n t required by the exchange equipment. The battery s e r v e s t h r e e main purposes to the exchange: (1) provides the additional power which might be needed for pe2.k loads, ( 2 ) supplies stand-by emergency power in c a s e the a-c power fails, and ( 3 ) f i l t e r s any a c which is not completely rectified in the d-c power supply. If there is a commercial a - c power f a i l m e , the battery normally has a sufficient capacity to supply the average busy-hour (maximum normal) load continuously for a period of about 5 hours. The s i z e of the battery and the type of control equipment used i s dependent upon the s i z e of the exchange, the reliability of the c o m m e r c i a l power, and the presence of enginedriven power supply equipment in the exchange. 2.1.1 The electric cell. The electric cell is a device which s t o r e s e l e c t r i c a l energy in the f r o m of potential chemical energy. E a c h cell h a s t h r e e essent i a l parts: a container, a n electrolyte, and electrode plates. The container holds the electrolyte which s t o r e s the chemical energy; the electrode plates a r e i m m e r s e d in the electrolyte and c a r r y the e l e c t r i c a l energy f r o m the electrolyte to the two terminals on top of the container. When the cell is charged, the stored energy in the electrolyte c r e a t e s a potential difference ( a voltage) between the cell terminals. F o r example, the potential of a fully charged leadplate cell is about 2.05 volts; whena conducting path is connected between the terminals, the voltage c a u s e s the cell to discharge o r deliver a n electric current. During discharge the c e l l l o s e s potential energy, and the voltage gradually d e c r e a s e s . When the voltage falls below a c e r t a i n value, such a s 1.75 volts for a lead-plate cell, the r e s e r v e power of the c e l l i s considered exhausted, since any further discharge c a u s e s the voltage to rapidly drop to zero. The complete discharge of a cell often causes permanent damage to the electrode plates. If there is a commercial power failure, the 2 3 regular cells will supply the exchange power immediately; when the potential of the 23 cells d r o p s from the fully charged value of 47.2 volts to some p r e s e t value between 46 and 47 volts, the three end c e l l s a r e automatically connected in s e r i e s with the 23 cells. The r e s u l t is a 26-cell battery with a potential of about 52 volts. 2.1.4 Counter -electromotive force (CEMF) cells. The CEMF cell is a special type of cell used to regulate battery potential. The function of the CEMF cell differs f r o m that of t h e r e g u l a r battery cells; instead of contributing to the battery potential, the CEMF cell lowers i t by developing a potential equal to and opposing that of one regular cell. The CEMF cell c r e a t e s this counter-potential only while c u r r e n t flows through it. The most widely used wet CEMF cell is the alkaline CEMF cell, which h a s a sodium hydroxide electrolyte and pure nickelor stainl e s s steel electrodes. The counter-potential developed by this cell v a r i e s from 1.70 to 2.25 volts, depending upon the amount of exchange load current i t c a r r i e s ; the g r e a t e r the c u r r e n t , the higher the counter-potential. Another type of wet CEMF cell is the lead-plate CEMF cell; the use of this cell is not a s extensive a s that of the alkaline CEMF cell. T h e r e a r e two types of cells, the d r y cell and the wet cell. The d r y cell contains a paste electrolyte, and i t cannot be recharged. The wet cell u s e s a liquid electrolyte; it can be recharged. The magneto telephone, which has a very light demand for power, u s e s the d r y cell. The telephone c e n t r a l office battery, which has a r a t h e r heavy demand for power, u s e s the wet cell. The d r y CEMF cell, made of special plates which maintain a fairly constant counterpotential within a given range of current, is a m o r e recent development. This type of CEMF cell comes in current capacities ranging from l e s s than one ampere to over several thousand a m p e r e s . Section 2.1.5 d e s c r i b e s the specific u s e of the CEMF cell i n a n exchange battery, 2.1.2 2.1.5 The 23-cell battery. A 23-cell battery consists of 23 wet electricstorage c e l l s connected in s e r i e s . E a c h cell h a s a 2.05 -volt fully charged potential, giving the battery a total potential of about 47.2 volts. T h i s battery is both sufficient and economical f o r up to approximately a 1000-line exchange where the c o m m e r c i a l a - c power supply is reliable, o r where all operations c e a s e i f the commercial power fails. A minimum of control equipment is required for the 23-cell battery, since all 23 cells a r e always used together. 2 -1.3 The 23-cell battery with three end cells: In exchanges of approximately 1,200 lines and l a r g e r , a 23-cell battery having three additional wet electric-storage c e l l s , called "end cells", is generally used. Figure 3 illustrates the connection of the end c e l l s in the battery circuit. The 24-cell battery with one CEMF cell. Except f o r the simple 23 -cell battery which u s e s little control equipment, a 24-cell battery in s e r i e s with one CEMF cell is the next most economical type of stand-by power source to u s e in exchanges of approximately 1,200 lines o r l e s s . Figure 4 illustrates the connectionof a CEMF cell in the battery circuit. If there is a commercial power failure, the 24 cells in s e r i e s with the CEMF cell will supply the exchange power immediately. When the resultant potential of the 24 cells and the CEMF cell drops from the fully charged value of 47.2 volts to some p r e s e t value between 46 and 47 volts, the CEMF cell is automatically shorted out of the battery circuit. This removal of the CEMF cell i n c r e a s e s the resultant potential to about 49 volts. CORRECTION CApf lQN6 QF FIGURES 3 AND 4 ARE TRANSPOSED (AUTOMATIC) 88 EXCHANGE L O A D F~gro-e3. Schcnlatzc diagra~nof 23-cell exchalzge battery ztlith thypeeend cells, a11d ztzclr((~i17g chai-ging appa~,atz~s and szc-~tckiugcont~eclzorts. I CHARGING I ( F O R 23 C E L L S ) CHARGER T O EXCHANGE L O A D Figure 4. Scke~uaticdiagram of 24-cell excha?tge battei'y zaith one CEMF (Coztn&e~--electl*o-vzotitle force) cell, and including clta~~gzng uppal-ah~sand atcto?izatic switch. 2.1.6 I Exchange battery capacity. "Capacity" is the t e r m used to denote the power storage o r r e s e r v e of a fully charged battery. Battery capacity is defined a s the amount of current in a m p e r e s which a batt e r y will continuously deliver for a given number of hours. The unit of capacity is the ampere -hour. Many exchanges use a battery of sufficient capacity to provide five continuous hours of the average busy-hour current. If the commercial a-c power is not too reliable, a battery with the capacity to deliver the average busy-hour load for eight continuous hours might be used. If engine-driven charger units a r e available, a battery with the capacity to deliver the average busy-hour load for only three continuous hours might be used. There a r e several f a c t o r s which affect battery cell capacity. An increase in the a r e a , thickn e s s a r number of the electrode plates, o r a r i s e in the temperature of the electrolyte, a l l increase the cell capacity. However, if this temperature gets too high, reactions may occur within the cell causing it to discharge, even though the battery is not delivering any power. The most efficient temperature for the leadplate cell is about 70°F. A s the battery discharges, the concentration (specific gravity) of the electrolyte decreases. The concentration of a fully charged lead-plate cell is about 1.2 15; i t is considered discharged when the concentration drops to about 1.150. The charge o r available capacity of a battery c e l l is determined by taking hydrometer readings of the electrolyte concentration. 2.1.7 The battery eliminator. In areas where the commercial power is very reliable, an exchange which is too small to warrant the cost of a battery installation may use a rectifier power supply unit called a "battery eliminator". This unit has very limited use since i t has no provision for stand-by power in case the commercial power fails. 2.2 f Charging Equipment and Methods Charging equipment converts a - c power into d-c power to s e r v e the telephone exchange in the following ways: (1) provides the normal exchange power requirements, ( 2 ) charges the battery when the battery potential is low, (3) keeps the battery fully charged for emergency use, and (4) extends battery life by eliminating ontinual "cycling" (discharging and rechargSome of the types of ing) of the battery. charging equipment a r e motor generators, and dry-disc rectifiers. The charging equipment i s connected in parallel with the battery to operate the exchange equipment and to charge the battery (if the exchange has one). The equipment normally maintains a constant potential that is slightly greater than the battery potential. The potential difference between the charger and the battery causes a slight current to flow through the battery in a direction opposite to the current flow during discharge; this "float-charging" current maintains the battery charge a t maximum value a t a l l times to be ready for maximum emergency use. The amount of charging current and thus the r a t e of charge a r e limited by the capacity of the charging equipment, which i s selected in relationship to the capacity of the battery. The capacity of the charging equipment i s normally chosen so that it will deliver to a fully discharged battery, in an 8-hour period, enough charging current to recharge it to about 85'7'0 of i t s fully charged condition. When the battery finally reaches a fully charged potential, the charger supplies just enough current to the battery to keep it fully charged. This function of maintaining a full charge, i s termed "float-charging". Since the capacity of the charging equipment is lower than the maximum allowable battery charging current, a safe charging r a t e is a s s u r e d a t all times. Cost of the charger units, type and size of the exchange and i t s battery, and the reliability of the commercial power, a r e some of the factors considered when charging equipment i s to be installed, Normally, an exchange uses a t least two (or more) charger units to a s s u r e that the power supply is sufficient for any period of the day and yet i s still economical to use. If one unit cannot supply the entire load, a second unit can be operated to s h a r e the load; the charger units usually have equal capacities so that one can replace another in c a s e of a breakdown. 2.2.1 Motor generator charging unit. A motor generator charging unit consists of a d-c generator driven by an a - c motor; it i s usually found in older exchanges. The type of generator most often used is the compoundwound, diverter-pole type, which generates a constant voltage from no-load up to about 110% of the rated capacity of the unit. If the load current from such a generator exceeds this value, the diverter pole and diverter-pole windings will cause the output voltage to decrease in order to prevent overloading of the charging unit. If the voltage level of the generator falls below that of the battery (due to excessive load), the battery will begin to deliver c u r r e n t to the load in parallel with the generator. If the g e n e r a t o r voltage should fall below that of the battery due to some fault o r failure condition, and a r e v e r s e c u r r e n t begins to flow from the b a t t e r y back into the generator, a r e v e r s e c u r r e n t r e l a y in a generator output circuit (but located on the powerboard) will release and r e s t o r e , opening the generator circuit and preventing any further back flow of current which might cause the generator to operate a s a motor (with considerable harmful effects to the whole exchange power plant). 2.2.2 The usual trickle-charger unit has a capacityr of only a few a m p e r e s and is not used f o r charging the end cells; if the end c e l l s become discharged, a s a f t e r a power failure, they a r e recharged by the main charging equipment i n s e r i e s with the 23 regular cells. 2.2.4 Rectifier c h a r g e r units. The main type of rectifier charger used in telephone exchanges is the dry-disc rectifier. Through r e c e n t developments, the dry-disc r e c t i f i e r has been made m o r e efficient and economical than the vacuum tube type. The dry-disc type has a capacity ranging from a few a m p e r e s to 400 a m p e r e s . Rectifier charger units u s e commercial a-c input power, usually 3-phase for l a r g e r units and single-phase for s m a l l e r units. The d-c output passes through a filter before going to battery and exchange. T h i s filtering helps to smooth out voltage fluctuations and c u r r e n t s u r g e s which might c a u s e noise in talking circuits. T h e s e r e c t i f i e r s put out a constant voltage f r o m no-load up to a maximum which is between 110 and 120% of i t s rated capacity. If the load on a r e c t i f i e r exceeds this maximum, an overload protection circuit usually will cause the r e c t i f i e r voltage to decrease, limiting i t s maximum load to a safe value. Ifthe rectifier voltage s t a r t s to drop, the exchange battery will provide the additional c u r r e n t necessary to o p e r a t e the exchange. The inherent high resistance of rectifying devices to r e v e r s e !potentials o r reverse flows of c u r r e n t , prevents any back flow of current f r o m the battery through a rectifier. Reversec u r r e n t r e l a y s a r e not required. However, when t h e r e is an a - c power failure, o r a r e c t i f i e r is otherwise de-energized, a relay o r a hand switch usually disconnects the rectifier f r o m the battery circuit; this action removes a n undesirable r e v e r s e potential from a c r o s s the r e c t i f i e r . R ecti'fier units may supplement motor gener - a t o r kharging equipment when the latter is not adequate to provide the increased load r e q u i r e m e n t s of a n exchange; however, adapting a r e c t i f i e r to function pr6perly with motor g e n e r a t o r charging equipment requires special control apparatus. 2.2.3 6.45 volts to "float-charge" t h r e e end c e l l s o f a 23126-cell battery. Figure 3 shows a tricklec h a r g e r unit connected to three end c e l l s . End-cell trickle-charger. An end-cell trickle-charger unit is a small r e c t i f i e r which maintains a potential of about Constant-voltage battery ' 'float-charging" . The battery is normally kept fully charged. The charging units maintain the potential of the battery a t about 0.10 volts p e r cell above the fully charged value of 2.05 volts p e r cell. T h i s potential, about 2.15 volts per cell, c h a r g e s the c e l l s when they a r e below full charge, and keeps them fully charged once they have reached that state. This float-charge potential is about 49.5 volts for a 23-cell battery, 51.6 volts for a 24-cell battery, and 6.45 volts for the three end cells. 2.2.5 Maximum c u r r e n t self-limiting feature. Most r e c t i f i e r s and d-c generators a r e constructed to limit the maximum c u r r e n t they will produce. If the load exceeds the maximum safe value which a charger unit is designed to c a r r y , the unit output voltage is controlledand d e c r e a s e d to prevent the current from exceed- i ing the maximum safe value. This c u r r e n t limiting feature protects r e c t i f i e r s and d-c g e n e r a t o r s against overloads without unnecess a r y interruptions of service. In general, the maximum safe load ranges from about 115 to 13570 above the rated capacity of the units, depending upon their type, s i z e and make. 2.2.6 Equalizing charge. At r e g u l a r intervals a n d / o r whenever one o r m o r e individual c e l l s of a "fully charged" battery has, for any reason, a lower potential than the other cells, the battery is given a n "equalizing" charge. This is done by r a i s i n g the rectifier o r d-c generator output voltage level about 70Jo o r 0.15 volts per c e l l (to approximately 2.30 volts per cell). The exact value of the equalizing charge voltage depends upon the type of cells and the local exchange practice. The change in output voltage level of the d-c generator unit ( o r units) is accomplished by increasing the shunt field c u r r e n t . Usually the variable resistance rheostat, located on the powerboard and connected in s e r i e s with the shunt field windings, is adjusted to do this. In r e c t i f i e r s , the change in the output voltage level is accomplished by changing thd. reference-voltage or control voltage to a " higher voltage tap (usually by operating a s m a l l toggle switch on the rectifier control panel). 2.2.7 26-cell charge. Since the end-cell trickle-charger normally used does not have sufficient capacity to r e c h a r g e the three end cells when theybecome discharged, they a r e recharged by the main charging equipment in s e r i e s with the 23 r e g u l a r cells. To do this the main c h a r g e r potential is raised to 55.9 volts s o that i t can c h a r g e the full 26 cells; this is accomplished by manually operating a switch on the r e c t i f i e r o r by adjusting the d-c generator fieldr h e o s t a t on the powerboard. When a l l c e l l s a r e fully charged, the main c h a r g e r is returned to i t s normal 23-cell float-charge potential of 49.5 volts; the end-cell trickle-charger then maintains the float-charge of the t h r e e end cells. 1 @ 2.3 Charger control switch. F o r each d-c generator and rectifier unit i n a n exchange, there is one c h a r g e r control switch on the powerboard. This is a threeposition switch with HAND, AUTOMATIC, and O F F positions. Normally a t l e a s t one charger unit will have i t s control switch in the HAND position; this unit is maintained energized, supplying power to the exchange equipment and the battery. Usually a t l e a s t one, o r m o r e , of the remaining units will have i t s control switch in the AUTOMATIC position. With i t s switch in the AUTOMATIC position, a c h a r g e r unit will s t a r t and operate in parallel with the unit already in operation, i f the load exceeds 100% of the rated capacity of the unit already i n operation. With i t s switch in the O F F position, a charger unit is disconnected f r o m the power supply circuit and cannot operate. 2.3.2 8 i: - If there a r e any additional c h a r g e r s available and s e t for AUTOMATIC operation, they will be started and operated in parallel in a predetermined sequence a s the load c u r r e n t exceeds 95-100% of the rated capacity of the operating units. This will continue until a l l available units a r e started. If the maximum r a t e d capacity and maximum safe overload capacity of the combined charge r s is exceeded, each individual rectifier o r d-c generator will, in turn, begin to d e c r e a s e i t s output voltage level in o r d e r to limit i t s c u r r e n t output to a safe value. The exchange battery will furnish any additional c u r r e n t required a s outlined in. previous sections. Control Equipment The efficient operation of the exchange under a l l conditions is dependent upon the proper functioning of i t s control equipment. This control equipment m a y include voltage-sensitive r e l a y s , knife switches, multi-position s e l e c t o r switches, metering devices, and fuses. T h i s equipment will be located a s is m o s t suitable for the s i z e and type of exchange, power equipment, and facilities. 2.3.1 operate in parallel with the already energized HAND selected unit. Automatic parallel charger operation. When a rectifier o r d-c generator unit h a s i t s control switch in the AUTOMATIC position, i t s s t a r t control circuit is connected to a n autom a t i c c u r r e n t monitoring circuit, which is , essentially a current- sensitive relay, v e r y much like an a m m e t e r with a n external shunt and with contacts instead of a calibrated scale. When the load on the f i r s t unit (the one already energized with i t s switch in the HAND position) i n c r e a s e s above 95-1000J0 of i t s rated capacity, the f i r s t AUTOMATIC unit will s t a r t and A s the load c u r r e n t d e c r e a s e s and d r o p s below 90% of the combined rated capacity of a l l operating units except the one s t a r t e d last, the c u r r e n t monitoring and s t a r t control c i r cuits will function to de-energize the l a s t unit. A s the load continues to drop, any other automatically s t a r t e d units will be disconnected a s the load becomes s m a l l enough to be handled by other units alone; this will continue until a l l automatically started units have been de-energized (in r e v e r s e of the sequence i n which they were started), and only HAND s t a r t e d units will r e m a i n energized. 2.3.3 CEMF c e l l cut-out control. When a 24-cell battery in s e r i e s with a CEMF c e l l is supplying exchange power, and the exchange voltage level d r o p s to some p r e s e t value between 46 and 47 volts, a voltagesensitive r e l a y functions to operate a contactor which s h o r t s out the CEMF cell, increasing the exchange voltage to about 49 volts. When the low voltage condition is corrected, and the battery power is no longer required, the contactor is released, automatically reconnecting the CEMF cell in s e r i e s with the 24-cell battery and reducing the voltage to i t s c o r r e c t , normal value. 2.3.4 End cell cut-in control. Whenever a 23-cell battery, with t h r e e additional end cells, is furnishing exchange power, and the 23-cell voltage d r o p s to some p r e s e t value between 46 and 47 volts, a voltagesensitive r e l a y o p e r a t e s a double-throw switch which connects the t h r e e end c e l l s in s e r i e s with the 23 regular cells, increasing the exchange voltage to about 52 volts. When the low voltage condition is c o r r e c t e d , and the battery power is no longer required, the double -throw switch r e s t o r e s , disconnecting the three end c e l l s f r o m the 23-cell battery and exchange load. 2.3.5 F u s e s with ratings a s h i g h a s 400-600 a m p e r e s m a y be used for the main fuse. This fuse p r o t e c t s the main power supply leads connecting the charging equipment and the battery to the power distribution panels. These main l e a d s m a y be either rubber-insulated copper cable o r rigid copper bus b a r s . End c e l l recharge control. If the t h r e e end c e l l s of a 23/26-cell battery a r e discharged for any reason, they a r e r e c h a r g e d by the main charging equipment i n s e r i e s with the 23 regular cells. This connection is changed manually by moving the charging control switch from the 23 CELLS position to the 26 CELLS position. It is a l s o n e c e s s a r y to i n c r e a s e the c h a r g e r potential f r o m 49.5 volts to 55.9 volts. T h i s is done i n a manner s i m i l a r to that for obtaining the higher potential for equalizing charges ( s e e section 2.2.6). On most r e c t i f i e r s a separate 23 CELL 26 CELL switch is provided for changing the r e f e r e n c e voltage; on motor generator (dc), the same rheostat is used to i n c r e a s e the voltage to the required value. When a l l the c e l l s a r e fullycharged, the charging control switches a r e manually r e s t o r e d to the 23 C E L L positions to float-charge the 23 cells, and the end cell trickle-charger is energized to float-charge the t h r e e end cells. In s m a l l e r exchanges usually only one fuse is used f o r a main fuse; i f this fuse blows, the c i r c u i t through the main fuse holder is opened. After the blown fuse h a s been removed f r o m the holder, a new fuse can be inserted. - 2.3.6 In l a r g e r exchanges, sometimes two o r m o r e f u s e s in parallel a r e required for the main fuse function. When this is the c a s e , a double-throw switch and two s e t s of paralleled fuses a r e usually used, a s shown in figure 5. If one s e t of f u s e s blows, the second s e t can be placed in s e r v i c e immediately by operating the switch f r o m one position to the other. The blown f u s e s m a y then be removed and replaced by new ones, while the second s e t c a r r i e s the load. D-c a m m e t e r and d-c voltmeter. + F u s e s with a 30 o r 60 a m p e r e rating a r e usually used for distribution. These protect the distribution leads, extending f r o m the distribution panel to individual fuse panels on the equipment shelves. E a c h distribution fuse connects one distribution lead to the main negative bus b a r of the distribution panel. The The d-c voltmeter (when provided) is used to m e a s u r e the potential of the battery (23, 24, o r 26 c e l l s ) , and the potential a c r o s s the main distribution panel. The desired reading is selected by means of a multi-position s e l e c t o r switch o r lever key. FROM SOURCE Distribution and Protection Equipment F u s e s a r e used in the exchange mainly to protect the equipment and leads against damage due to short circuits. The fuses used a r e the main fuse, the distribution fuse, and the a l a r m f u s e . The rating of each fuse depends on the s i z e of the leads which i t protects. As the load is divided and distributed, from the main power supply leads through to the switching equipment shelves, both the size of the leads and the rating of the fuses decrease. t In v e r y l a r g e exchanges, two o r m o r e doublethrow switches and s e t s of paralleledfuses a r e s o m e t i m e s needed, each feeding power to a l a r g e p a r t of the exchange. No single main fuse for the whole load is practical (due to the s i z e of the exchange load); the power supply units a r e normally a l l individually fused. In t h i s c a s e t h e r e will be a s e t of main leads for each switch, and each s e t will extend to a different distribution panel. The d-c a m m e t e r (when provided) is used to m e a s u r e the exchange load c u r r e n t and the c u r r e n t f r o m the charging equipment. The d e s i r e d reading is selected by m e a n s of a multi-position selector switch o r l e v e r key which is mounted below the a m m e t e r . By using external shunts which a r e permanently connected in the power supply c i r c u i t s and special calibrated leads to connect the a m m e t e r movement to the m e t e r terminal posts on the shunts, the c u r r e n t readings of s e v e r a l different capacity c i r c u i t s can be made rapidly and with ease, and without interrupting any power circuit. E a c h shunt is designed to have the s a m e potential drop for the maximum c u r r e n t of the circuit in which it isused; thus, only one a m m e t e r movement is needed for s e v e r a l circuits. 2.4 ji - 1 - LOAD L I ALARM FUSE - A L A R M STUD/' ro ALARM CIRCUITS Figure 5. Mzclti-positiorr fuse mzd s~oitcha?-rungement, shotuiirg alui'i?zfxse coiarectiorzs. i . audible signals, usually ringers, at each telephone station. The generators used for this purpose deliver continuous power of the r e quired frequencies; this must be fused and then broken into ringing and silent periods by an interrupter mechanism. The interrupter distributes the ringing power to ringing group leads. F r o m the powerboard each lead extends to a group of connectors to provide them with ringing power. 3.1 Figitre 6. Alav?n fztse sholr~rin ope?-atin(:condition (A), a12d i(~lte?t"blot(.ll" (B). positive (ground potential) distribution leads a r e not fused; these a r e connected directly to the positive-ground bus bar of the powerboard. An additional lead is used to connect the positive-ground bus to a grounding b a r on the Main Distributing F r a m e (MDF); this MDF grounding b a r , in turn, is connected to e a r t h by a heavy cable and several buried copper plates which establish a stable r e f e r e n c e level f o r the exchange voltage. Alarm fuses, having ratings of one to five a m p e r e s , a r e used on the shelf fuse panels and in parallel with a l l main and distribution cartridge-type fuses. When used alone, a l a r m f u s e s protect the shelf power wiring and , equipment. When used either alone o r in parallel with a l a r g e r fuse, they a l s o provide a m e a n s for signaling the failure (or blowing) of any fuse. When used in parallel with a l a r g e r non-indicating fuse, if the l a r g e r fuse blows, the full current in the circuit tends to flow through the a l a r m fuse, causing i t to blow a t once. Whenever a n a l a r m fuse blows, two pretensioned springs on the a l a r m fuse, which a r e normally held in place by the fuse element, a r e f r e e d and allowed to bend outward f r o m the fuse body. The back spring of this pair touches an a l a r m bar o r stud, completing a c i r c u i t to a signal lamp and audible a l a r m . A r e d lamp on the fuse panel is usually used to indicate the panel on which the blown fuse is located; normally a 500 ohm r e s i s t o r is connected in parallel with each such lamp to provide a circuit path for the audible signal (connected in s e r i e s with the lamp) even if the lamp should burn out. The front spring of the a l a r m fuse, bending forward, gives a visual indication, within a group, of the particular fuse that h a s blown. 3. - A-C RINGING POWER Alternating c u r r e n t of a specific frequency, o r of five different frequenctes, is generated in the exchange for the purpose of operating Requirements and Applications The frequency, o r frequencies, of the ringing power required by a n exchange i s determined by the ringing scheme used in the exchange. Single-frequency ringing power is used for straight-line, coded, and superimposed ringing, and combinations of these. Multi-frequency ringing power is used for selective (frequency) ringing on party lines. 3.1 -1 Ringing frequencies. Single-party lines usually use 20 o r 25 cps ringing power for straight line ringing; also, coded and superimposed ringing on multiparty lines usually use 20 o r 25 cps. Multiparty lines using frequency selective ringing r e q u i r e one of the t h r e e s e t s o r groups of five ringing frequencies; the three usual s e t s o r groups of frequencies used for selective ringing a r e : a. Multiple (also known a s harmonic), using the following frequencies: 16-213, 25, 33-1/3, 50, and 66-213 cps. b. Non-multiple (also known a s synchromonic), using 20, 30, 42, 54, and 66 cps. c, Decimonic, using 20, 30, 40,50,and 60 cps. 3.1.2 Ringing frequency voltages. The voltages a t which ringing power is generated varies f r o m 75 volts a t the lower frequenc i e s to about 150 volts a t the higher frequencies. The higher voltages a r e used with the higher frequencies in o r d e r to overcome increased line and apparatus impedances encountered at higher frequencies and maintain sufficient power to the r i n g e r s . 3.2 Ringing-frequency Generating Equipment The principle types of frequency generating units used to supply exchange ringing power a r e the vibrating r e e d converter, rotarygenera t o r , magnetic (static) generator, and electroni c (vacuum-tube o r t r a n s i s t o r ) generator. The method of operation, power output, shape of the output wave, initial cost, efficiency, amount of adjustment required, size, and power supply r e q u i r e d a r e the principle f a c t o r s involved i n determining which type of unit to use in a p a r t i c u l a r exchange. In m o s t exchanges the ringing equipment is usually located a t o r n e a r the powerboard. Frequency generating equipment normally ope r a t e s continuously, except in some s m a l l exchanges where the ringing g e n e r a t o r s opera t e only when a c a l l is made. 3.2.1 Vibrating r e e d frequency generator unit. T h e vibrator unit consists primarily of two magnet coil assemblies, a n a r m a t u r e , a r e e d a s s e m b l y , and a f r a m e upon which a l l of these a r e mounted. This device convei-ts 48v dc t o square-wave a c of the c o r r e c t voltage f o r the out