Ind Connectivity Study Guide

8/12/2019 Ind Connectivity Study Guide

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Designers can select from a number ofdifferent connector materials:

Brass Brass has excellent conductivitybut cannot withstand many insertion andwithdrawal cycles. Brass loses flexibility as itages, and under repeated stress experiencescrystallization, which significantly lowersconductivity. It is suited for noncritical,low-contact-force applications, and is easilycrimped, soldered, welded, and brazed.

Beryllium copper.Beryllium copper hasexcellent mechanical, electrical, and thermal

properties and resists corrosion and wear. Itis the best electrical conductor of any springalloy of comparable hardness. Berylliumcopper is stronger, more resistant to fatigue,and withstands more insertion and with-drawal cycles than any other copper-basespring alloy. But it is the most expensive ofthe basic contact materials.

Nickel-silver alloys. Nickel-silver alloysresist oxidation and do not always requireplating. Nickel-silver is susceptible to stress

corrosion, although not to the extent ofbrass.

Gold.Gold is an excellent conductor anda highly stable plating material. It has thelowest contact resistance and provides thebest protection from corrosion. Hard goldplatings are recommended for contacts thatexperience frequent insertion/withdrawalcycles. For even greater cycling, gold can beimpregnated with graphite with a minimalincrease of contact resistance.

Gold-over-silver.Gold-over-silver un-

derplating is good for dry-circuit (millivolt,milliampere range) applications because itprovides low contact resistance. But becauseits corrosion resistance is only moderate, theuse of this combination is limited.

Gold-over-nickel is a widely used platingcombination because it provides the surfacequalities of gold, while the hard underplat-ing of nickel prevents migration of the basemetal and minimizes the amount of goldrequired.

Industrial connectors are divided into four categories based on operating environments:commercial, industrial, military, and hermetic. In commercial applications, outside tem-peratures and atmospheric conditions are the least critical facors affecting performance.Generic connectors merely maintain electrical continuity, allowing use of low-cost mate-

rials. Industrial connectors are devised to handle more rugged environments encompassinghazards that include thermal shock, corrosion, vibration, physical jarring, and sand and dust.

Industrial connectivity includes

any component that is in the path of

delivering power or control signals

to do useful work. Typical connectivitycomponents include connectors and

terminal blocks, motor starters and relays.

Industrial

connectivitybasics

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age and current ratings. And unlikehardware made in North America,all mounting hardware for Europeanterminal blocks follows standard DINformat. Blocks from one vendor can installed on rails from another.

Terminal blocks in the NorthAmerican style generally are vendor

specific. Blocks and rail hardware froone manufacturer usually do not worwith equipment from others.

Another difference between NorthAmerican and European blocks is inconnection techniques. European witerminations follow what is called adead-front configuration. Terminatiohardware is recessed within the plastblock housing, isolating electricallylive parts. Maintenance personnel catouch live blocks with minimal chancof shock or electrocution. North

American blocks employ a more opedesign.

The simplest terminal blocks, callelectrical blocks, merely join wires ocables. These devices accept wire sizranging from about 12 awg to 500 mcwith a maximum voltage rating of 60V. The blocks either snap into a metarail or are screw mounted on the panof the control enclosure.

There are three styles of DIN rails(also called G or asymmetric), flat (al

called symmetric), and mini rail. Flatand C rails are used in systems ratedto 600 V. Terminal blocks on mini raihandle voltages to 300 V.

Special terminal blocks combinesimple terminations with other func-tions found in the control panel, suchas fusing and disconnects. Fusible teminal blocks are widely used and generally contain ferrule fuses rated at 35 A, although some devices contain uto 30-A fuses. Special terminal blockmay also incorporate circuits that pe

form visual annunciation, surge sup-pression, and voltage regulation.Another type of terminal block is

often called an electronic block. Thedevices typically work with smallerwire sizes common to electronic ap-plications, 30 to 14 awg at 300 V orless. The blocks generally mount andsolder onto printed-circuit boards.They transmit power or control signto circuit components through solde

Silver.Silver is a general-purposeplating for power contacts. However,shelf life is poor and silver tarnisheswhen exposed to the atmosphere, in-creasing contact resistance. Althoughthis oxide coating is undesirable in low-level circuits, it does not affect contactscarrying higher currents.

Nickel. Nickel has good corrosionresistance, fair conductivity, and is gen-erally used as an undercoat for high-temperature environments to preventmigration of silver through gold. Nickelhas good wear resistance, but it maycrack during crimping if not properlyplated onto the base metal.

Rhodium.Rhodium is used for itsexceptional wear qualities. It has alower conductivity than gold or silver,but on thin platings this higher resis-tance is acceptable.

Tin.Tin has good conductivity andexcellent solderability. It is a low-costfinish with poor wipe resistance bestsuited for connections requiring veryfew mating cycles. Tin is not a noblemetal and will corrode.

Rhodium-over-nickel.Rhodium-over-nickel provides maximum wearresistance and is suitable for high-tem-perature operation. However, this com-bination has higher contact resistancethan other platings.

Terminal blocks

Terminal blocks require no wirepreparation except stripping and areeasily installed with a screwdriver.They accept a wide range of awg wiresizes, provide ready hookup of wiresfrom different components, and ensurefast disconnection/reconnection dur-ing maintenance and troubleshooting.

Terminal bodies often are madefrom a copper alloy that has the sameexpansion coefficient as wire. This pre-

vents loosening between metals withdifferent thermal expansion rates andlower contact resistance. Corrosion,normally caused by electrolytic actionbetween two different metals, is alsoeliminated.

There are two basic styles of termi-nal blocks, European (DIN) and NorthAmerican. European terminal blocksare physically smaller than NorthAmerican blocks having the same volt-

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Quick-connect contacts consist ofa simple tab or flat blade designed totake a push-on connector installed onthe end of the wire. Such a connector isa force-fit metal sleeve which is pushedover the tab. This type of contact iscommonly used for awg 12 and smallerwires.

Tubular contacts consist of a lengthof metal tubing, rectangular in cross sec-

tion, with a screw threaded through thetop and sides of the tube at each end.Where the flat bottom of the screw is

used to provide the pressure on the in-serted wire, the contact is often called atubular screw contact. Where a flat pres-sure plate is used under the screw, thecontact is called a tubular clamp contact.Preferably, the clamp should be heldcaptive to the screw. Clamp contacts aremainly used with fine stranded wires.

Feed-through contacts have studsthrough the mounting surface. These

contacts are used where the wire leadsmust pass through a wall directly underthe block or as close to the side of it aspossible. Wires are connected as withstrap-screw or stud contacts.

Two-part plug-in dead-front connec-tors can provide position interlockingfor shock and vibration. Socket-matingsprings on quality connectors may pro-vide as many as seven points of contacton posts. Use of beryllium-copper wireprotectors makes the connectors appli-

cable to both single and fine-strandedwire termination. Special flush-mountdesigns are available to minimizethe stress on board solder joints thathappens when screw terminals aretightened.

A strap-clamp contact consists of ascrew through each end of a flat strap,with a wire-clamping element under

the screw head to exert pressure on thewire. Here again, bare wire is inserted inthe pressure contact.

A strap-screw contact consists of ascrew through each end of the con-nector strap. A wire is attached to thecontact either with a ring or spade lug o

by simply looping the wire around thescrew and tightening it down.

A fuse block contains a fuse in serieswith the circuit. A typical unit has acontact as each end of the section, asdo standard blocks. A clip connected toeach contact accepts a cartridge-fuseplug, which facilitates fuse changingand provides each circuit identification

One-piece blocks incorporate one omore circuits, and contacts are mountebetween barriers or in plain rigid insula

ing members. This type may have openbarriers for easy contact accessibility orclosed barriers (dad front) for contactprotection. It is usually available in standard units 2, 4, 6, 8, or 12 circuits in asingle base.

Short-circuit blocks are similar toone-piece blocks. By connecting a shorcircuiting screw into a shorting strip, cu

tion terminals stacked on top of eachother, saving board space. The wire en-tries on the two-tier blocks are offset orstaggered so that screwdriver access isfacilitated to the lower tier, even if theupper tier is fully wired.

Special terminal blocks have been

pins that are mechanically connectedto the clamping body.

Several wire entry angles and nu-merous pole and pin configurations areavailable. In addition, these devices canbe obtained in two-tier versions. Two-tier blocks contain two sets of connec-

Terminal block contacts

Two- part plug-in dead front



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holding sections in mounting channetransparent plastic covers for protecting conductive parts while permittininspection of the contacts and markiareas; and sectional fanning strips foconnecting and disconnecting groupof wires.

Relays and contactorsSolid-state relays (SSRs) control

load currents through use of solid-state switches such as triacs, SCRs, opower transistors. These elements arcontrolled by input signals coupled tthe switching devices through isolatimechanisms such as transformers, rerelays, or optoisolators. Some solid-state relays also incorporate snubbercircuits or zero-crossing detectors toreduce spikes and transients gener-ated by load-current interruptions.

Since semiconductor switches can disipate significant amounts of power,solid-state relays must generally beheat sinked to minimize operating te

developed to provide a transition be-tween discrete wiring of power devicesto low-signal cables or controller con-nectors. The most common transitionis to ribbon cables using IDC connec-tions, or to D-subminiature connec-tors, or DIN harness assemblies.

Terminal block manufacturers now

offer special housings that can holdsmall printed-circuit boards. Theseboards, in turn, can be connected tothe block terminations. The configura-tion allows end users to define specialfunctions on the terminal staff.

Creepage and clearance are termi-nal block distances that determinevoltage ratings. Creepage is the pathbetween terminals measured along thesurface of the insulation. Clearance isthe shortest through-air distance be-tween terminals and from terminals to

ground.Accessories for terminal blocks

include jumper straps for connectingadjoining contacts; channel clamps for

rent may be shunted or directed to anydesired circuit.

Section blocks consist of individualmolded units with contacts. When as-sembled together with an end barrier,these units can make up a block withnearly any desired number of circuits.Assembly of sectional terminal blocksconsists of snapping off or addinggroups of contact sections, dependingon the number of circuits in the preas-sembled lengths. The addition of an endpiece completes the terminal block.

Interface blocks typically connect

discrete wiring from power devices tocables from low-signal devices such asmicrocomputers. Dead-front wire termi-nals recess live metal connector parts inthe plastic housing for safety. Two-tier

configurations reduce the block side.Typical cable connections include thosefor flat ribbon, insulation displacement,D-Sub, and DIN connection headers.

PCB blocks contain pins which aresoldered directly to printed-circuitboards. These blocks have up to 12 awgcapacity and typically come with a 10-A,300-V UL/CSA rating. Terminals are de-

signed to protect solder joints andtracks from breaking. These termi-nals are modular, may incorporatefuse block, or double-level blocks,

and provide metric and decimal-pin spacing. Another common fea-ture is a high-precision interlockingmechanism.

Switch blocks incorporate ameans of isolating or disconnectingcircuits. The block contains a knife-blade switch element, operated byan insulating handle attached tothe blade.

Terminal block continued:

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and contact structures are also factors.Third, relay coil inductance can pro-

duce unacceptably high-voltage tran-sients when the device is de-energized.Protective circuits can reduce the tran-sients to an acceptable level, but theydelay relay drop-out as well.

Relays also can be a source of EMI.

Arcs at the contacts, for example, areproduced when a contact bounces onenergization and when contacts openon de-energization. Transients pro-duced by deenergizing the coil are an-other source. EMI can be severe whenswitching inductive loads at high cur-rent and voltage levels.

Reed relays:A reed relay consists ofreed switches within an operating coil.The reeds can be any type of configura-tion, but the quantity is limited by thecoil size. Most manufacturers limit coil

size to handle 12 standard switches,maximum. To obtain additional con-tacts, relay coils are connected in paral-lel. Reed relays are available with con-tact forms from 1A to 12A, 1B to 8B,1C to 4C, and combinations of theseup to the maximum coil size. Coils maybe wound with each magnet-wire sizeto create a large selection of operatingparameters.

Reed-relay contacts typically pro-duce 1 to 3 Vpp at 20 to 30 kHz. The

voltage, which is produced by magne-tostriction, generally decays about 3msec after contact closure. Miniaturereed relays in six-lead DIP and surface-mount packages are used for PC-boardapplications or wherever space is aconstraint. Sensitive relays with coilpickup as low as 1.6 Vdc at 40 mW areavailable.

Mercury-wetted contact re-lays:Basically, a mercury-wetted con-tact relay consists of one or more glassswitch capsules surrounded by a coil.

These relays maintain their originalresistance to within 1 m throughouttheir lives. When two contacts wettedwith mercury join, the area of contactbetween the surfaces is somewhat largebecause a fillet of mercury surroundsthe mated surfaces. When the twosurfaces are separated, the mercurystretches into a thin filament and thenbreaks at two points that isolate a thinrod of mercury in the middle. The

perature. Typical SSR applications arethose where rapid on/off cycling wouldquickly wear out conventional electro-mechanical relays. General-purposeSSRs have on/off cycle lifetimes as highas 100,000 actuations. SSRs that can beactuated with conventional CMOS andTTL logic-level voltages are available.

The chief failure mechanism of anSSR is mechanical fatigue in the powersemiconductor structure caused bythermal cycling. However, thermal-cycling effects can be controlled bymatching the required load-cyclingqualities to the relay. Heat sinks formost conditions are available or are apart of the SSR.

Relays open and close electrical con-tacts to operate other devices. They areoften used because they cost less thancorresponding electronic switches.

But some inherent relay qualities aresuperior to those of solid-state devices,as well. For example, input and outputcircuits in relays are electrically iso-lated, unlike those in most solid-statedevices. And relays can have numerouscontacts electrically isolated one fromanother. In addition, electromechanicalrelays are becoming smaller, now avail-able in PCB-mount and surface-mountpackages that are suitable for auto-mated soldering.

Another advantage of electrome-chanical relays over solid-state switchesis that relays have much lower contactresistance. Contact capacitance is alsoless, which may benefit high-frequencycircuits. Relays are less likely to beturned on by transients than are solid-state switches. And relays are less easilydamaged by brief shorts or overloads.

Electromechanical relays differ inother important ways from solid-stateswitches. First, relay coils are highlyinductive, and the inductance value is

not constant. Inductance is low imme-diately after energization and rises ascurrent approaches a steady-state leveland the relay armature closes. In con-trast, solid-state switches have mainlyresistive inputs and a constant inputcurrent.

Second, relays have a much lon-ger switching time than solid-stateswitches. Coil inductance is the pri-mary cause, but the mass of armature



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There are four types of analog meterrelays: magnetic contact, locking coil,optical, and solid state. These meterscan be employed with the users controlcircuitry, with a control module option,or with control circuitry contained inthe meter.

Motor starters

Single-speed squirrel-cage induc-tion motors have starters that fall intotwo categories: full-voltage or across-the-line starters; and reduced-voltagestarters.

Full-voltage starters (manual andmagnetic) apply full voltage directlyto motor terminals. Two other types,combination and reversing starters,consist of a starter, usually magnetic,with added functions.

Some machines or loads may requirea gentle start and smooth accelerationup to full speed. In addition to loaddemands, power company regulationsmay limit the current surge or voltagefluctuation that can be imposed onthe power supply during motor start-ing. Variable-speed motor drives canprovide such soft-start capabilities, butin some cases these controls can beoverkill. This is particularly true whenthere is no energy efficiency advantage

to be gained by running the motor atbelow rated speed. Here, basic motorstarters can handle soft starts. Manystarters apply reduced voltage to motorwindings; primary resistor, primary re-actor, autotransformer, and solid state.Part winding and wye-delta starterscan also provide reduced-voltage start-ing, although technically they are notreduced-voltage starters.

Motor windings in multispeedsquirrel-cage motors may requirespecial starters. Starters for separate-

winding two-speed motors consist oftwo standard three-pole starter unitsthat are electrically and mechanicallyinterlocked and mounted in a singleenclosure. Additional units can be usedfor each speed. Although these arealways electrically interlocked, it maynot be practical to provide mechanicalinterlocks on more than two starters.

The starter for a consequent-poletwo-speed motor requires a three-pole

pilot devices. Relays do not controlpower-consuming devices, except mo-tors and solenoids drawing under 2 A.

Many manufacturers useMIL-R-5757 as a standard and as aguide for producing government-acceptable relays. This specificationcovers relays with contacts capable of

switching loads up to 10 A. In general,MIL-R-6106 covers and exceeds the re-quirements of MIL-R-5757. It also cov-ers relays capable of switching currentsin excess of 10 A.

Contactors: Contactors are devicesfor repeatedly establishing and inter-rupting electric power circuits. Twotypes of contactors are defined byNEMA electronic and magnetic.Electromagnetic contactors are actu-ated by electromechanical means.They make and break power circuits to

such loads as electric furnaces, lights,transformers, capacitors, heaters, and when overload relays or inherentprotectors are used motors.

The magnet design of an ac contac-tor consists of a stationary core anda movable armature, as in NEMA-Aand B control relays. Some contactorshave a horizontal design; others havea hinged or pivoted clapper magnet.Coils are available in voltages up to 600V, commonly in 110, 220, 240, 380, 440,

480, and 500 V for 25, 50, and 60 Hz.A dc contactor operates like an accontactor. However, while an ac mag-net is laminated steel, a dc magnet ismade of solid steel.

Because copper contacts are usedon some contactors, the current rat-ing for each size is an 8-hr open rating-- the contactor must be operated atleast once every 8 hr to prevent cop-per oxide from forming on the tips andcausing excessive contact heating. Forcontactors with silver to silver-alloy

contacts, the 8-hr rating is equivalentto a continuous rating. This rating alsoapplies to contactors mounted in theopen without enclosures. Contactorsinstalled in enclosures have a ratingequal to 90% of the open rating becauseof reduced contactor cooling.

Meter relays: Meter relays providean analog or digital panel indicationof a measured variable together witha switching function at a preset level.



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the use, location, and control sophis-tication required, three options wereavailable for group or multiple motorinstallations. The first choice was aNEMA-rated starter. It was selected a particular size classification that wsuitable for switching motors and othkinds of loads such as capacitor bank

The second was an IEC (InternationaElectrotechnical CommissionEuropean Standard) starter rated byhorsepower. The third choice was ahorsepower-rated definite-purposestarter.

To add to the complexity, there areover six different classifications available for fuses, including H, J, K, RK,RK-1, and 5. Furthermore, short-circprotection must be coordinated withthe overload relay and the contactorto protect personnel and equipment.

To ensure coordinated protection, anengineer must determine the availabfault current, the corresponding fuseclass, and the need for a single or duaelement device.

Choices for circuit breakers includthermal-magnetic, magnetic-only,solid-state, current-limiting, andstandard or high interrupt capacityunits. Thermal-magnetic devices weoriginally designed to protect wiringbetween circuit breakers and motors

according to code. But they frequenthad to be oversized to handle high inrush current upon start-up. The magnetic-only circuit breaker was designto protect motors, not wire. Thus, thdevice provides protection more in liwith the overload relay and contactoratings.

Solid-state circuit breakers are mocommonly used on motors above 100hp for economy. Special fuses usedfor motor protection have built-intime delays called dual-element time

delay fuses. It is a common practice toversize short-circuit protection forfuses or breakers. This cuts down onfrequent nuisance blowing or trippinEven the NEC allows oversizing withprescribed limits.

Self-protected starters are sized fothe horsepower and full-load currentof the motor. A typical inrush currento start a motor is six to eight timesnormal running current. Some high-

unit and a five-pole unit. The designof the particular motor winding deter-mines whether the fast- or slow-speedconnection is made by the five-poleunit.

For three-speed consequent-polemotors, a three-pole starter is used forthe single-speed winding; a five-pole

starter and a second three-pole starterhandle the reconnectable winding. Afour-speed consequent-pole motor re-quires two sets of three- and five-polestarters.

Different power circuits are neededfor delta-type multispeed motors,because currents circulate within theinactive or unconnected windings. Apair of four-pole starters is requiredfor a two-speed motor with separateopen-delta windings. Another four-pole starter is required for each speed.

Thus, three- and four-speed motorswith open-delta windings require verycomplex starters.

Specific winding information is usedto select the motor controls. Torquecharacteristics also deserve specialattention to ensure selection of theproper control. Constant-horsepowermotors require larger starters than ei-ther constant-torque or variable-torquemotors of equal horsepower. Reversingand reduced-voltage operations can

be incorporated in a multispeed motorstarter.Self-protecting starters (SPSs) were

first introduced to the U.S. in the1980s. A self-protected starter com-bines contactor, overload, and short-circuit protection in one package. It issized according to motor load currentand horsepower. Generally, a small in-terchangeable module protects againstboth thermal and magnetic overload.The self-protected starter can be usedin single or multiple installations and

satisfies Article 430 of the NationalElectric Code (NEC), which addressesthe safe installation of motors, circuits,and controllers.

Power-control equipment com-prises motor control, overload andshort-circuit protection, and isolation.Before SPSs, these functions werehandled by motor starter (contactorplus overload relay) wired to eitherfuses or circuit breakers. Depending on



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contacts. If field upgrades and expan-sions are expected later, it is also wiseto make sure that spare parts will beavailable and that installed units canaccept accessories. For example, it iseasier to add auxiliary contacts to afusible switch in the field than to add acircuit breaker which must be factory

installed.

PLC power interfacing

Communication or dialog withautomated systems is an importantfeature of any control device. In mostsystems, this responsibility has typi-cally been left to a programmable logiccontroller (PLC). Screw-in modulesenable self-protected starters to bothreceive controls signals from PLCsand transmit status on trip conditionsor signal normal operation. The SPS

can also receive direct control signalsfrom proximity, photoelectric, limit,or pushbutton switches.

In fully automated systems, an ad-ditional communication functionenables the device to be reset from aremote panel or central control loca-tion. A shunt-trip enables tripping byexternal commands.

SPS starters can cut installationtime by 33% compared with the timeneeded to mount NEMA-style start-

ers. Self-protected starters are DINrail mounted, which eliminates in-stallation of additional protectioncomponents such as fuses and circuitbreakers. And using SPSs in multiplemotor-starter applications can reduceinstallation time as much as 80%.

The operational design of SPSs pro-vides a clear visual indication of con-tact operation and trip status. A visualindicator on the face of the deviceconfirms that the unit is operating. Inoverloads, the handle rotates to a trip

position. In a short circuit, the deviceprovides an additional optical indica-tion of the trip.

Auxiliary contacts can provide PLCswith feedback from three independentsignals: contactor status , short-circuitstatus, and overload status. PLCs can thentrigger alarms in the event of trips.

ficiency motors have inrush currents ofeight to 10 times their running current.SPS devices eliminate nuisance trip-ping with adjustable protection of twotypes. First, adjustable overload protec-tion for full-load current is provided.In any case, the overload setting shouldnot be greater than the actual full-load

current. Second, a magnetic-only tripcoil, which is adjustable from six to 12times full load, compensates for a vari-ety of inrush currents.

When sizing motors, service fac-tor rating is a necessary consideration.Most U.S.-made motors have a servicelife of 1.15, meaning they will handle115% of normal running current indefi-nitely without damage. A service factorof 1.0 indicates the motor will toleratenameplate running current only. Thistype of motor is being used more fre-

quently, and needs overload protectionthat trips faster than the traditionalClass 20 overload relay.

Class 10 overloads (trips within 10sec at six times full-load motor current)for small motors work well with a 1.0 or1.15 service factor. Some special appli-cations with over 5 sec start-up timescan cause early tripping.

At full locked-rotor current, theClass 10 overload relay trips in 10 secor less, protecting motors without cre-

ating nuisance tripping. Relays do nottrip because most motors reach fulloperating speed in less than 5 sec. Thisis particularly important with the trendtoward smaller motors with short start-up times. Another advantage is thatoverload adjustments can be sealed orlocked to prevent tampering.

Another factor to consider is phaseimbalance during brownouts or phaseloss. In the event one phase dropsout in IEC devices like SPSs, there isa 57% increase in current across the

other phases. Time is a critical factorin protecting motors from thermaldamage, so these devices contain a dif-ferential bar that increases the speed atwhich the overload trips. This reducesthe likelihood of motor damage orburnout.

For complicated installations usingPLCs or similar controls, it is best tomake sure the starter can handle acces-sories such as shunt trips and auxiliary


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Ind Connectivity Study Guide