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.From the service entrance of most industrial facilities power is distributed by way of three-phase busway or wired into distribution panels. Usually voltage at the service entrance is reduced via transformer to three-phase 480VAC. Various fused disconnects or circuit breakers are located to provide protection for branches of the distribution system. Disconnects that can only be reached by long poles with hooks at the end are often located at the point of power drops to individual machines. Wiring is distributed inside rigid conduit or EMT to fixed locations or in flexible Seal-Tite or power cable to more temporary or movable spots. Cable tray is often mounted from the ceiling with multiconductor control or power cable laid in it and dropped to points of use. It is important to consult the National Electric Code or local regulations when planning a power distribution system.
Wire and cable are sized by the allowable amount of current that they are rated to carry for certain conditions, such as temperature or insulation. Wire is rated by gauge and may be sized in terms of American wire gauge (AWG), standard wire gauge (SWG), or imperial wire gauge. Wire is usually rated by Ampacity, another name for the amount of current it can safely carry.
Relays and contactors are a form of switching device that applies or removes power from a circuit based on a remote or external signal.
Timers and counters also switch power or signals based on a delay or set number of pulses.
4.1 Disconnects, Circuit Breakers, and Fusing
An individual line of automation equipment or single machine will typically have one main disconnect to allow power to be removed from a single source. These disconnects usually have a set of fuses or a circuit breaker rated appropriately for the equipment they are supplying. There will often be several levels of branch circuit protection after the main disconnect in the form of fuses or circuit breakers. In most cases these also serve as a manual disconnect for the branch, although some motor circuits will simply have a fuse clip with no disconnecting means upstream. There are regulations concerning disconnects being present within a certain distance from a motor, so disconnects without fusing are sometimes located nearby for quick power removal. Disconnects consist of a set of contacts rated for the amount of current they must break with a manual means of actuation. These may also include a means of remote actuation or control.
A circuit breaker is a circuit protection device that can be reset after detection of an electrical fault. Like all circuit protection devices, its purpose is to remove power from an electrical device or group of devices, protecting the circuit from damage. Circuit breakers are rated by the current at which they are designed to trip, as well as the maximum current they can safely interrupt during a short circuit.
Circuit breakers interrupt a current automatically; this requires some kind of stored mechanical energy, such as a spring or an internal power source, to actuate a trip mechanism. Small breakers such as those used for branch circuit or component protection in a machine are usually self-contained inside a molded plastic case. Larger circuit breakers usually have a pilot device that senses a spike in current and operates a separate trip mechanism. Current is detected in several ways. Magnetic breakers route the current through an electromagnetic circuit. As the current increases, the pulling force on a latch also increases, eventually letting the contacts open by spring action.
Thermal magnetic circuit breakers use a bimetallic strip to detect longer-term over current conditions while using a magnetic circuit to respond instantly to large surges, such as a short circuit.
Circuit breakers usually have a reset lever to manually trip and reset the circuit. This is an advantage over using fuses, which must be replaced after one use. In industrial applications most circuit breakers are used for low-voltage application (under 1000 V). Medium-voltage (1000 to 72 k) and high-voltage (more than 72.5 kV) breakers are used in switchgear applications but are rarely seen in industrial plants, though medium voltage switchgear is used in some process facilities.
Low-voltage breakers may be of the DC or AC variety and generally fall into the categories of miniature circuit breakers (usually DIN rail mounted, up to 100 A) and molded case circuit breakers (self contained, up to 2500 A). An example of a molded case breaker is shown in FIG. 23.
Circuit breakers must carry the designed current load without overheating. They must also be able to withstand the arc that is generated when the electrical contacts are opened. Contacts are usually made of copper or a variety of alloys. Contact erosion occurs every time the contacts are opened under load; usually miniature circuit breakers are discarded when the contacts are worn, but some larger breakers have replaceable contacts.
There are two types of trip units in low-voltage circuit breakers: thermal magnetic and electronic. The thermal magnetic trip units contain a bimetallic thermal device that actuates the opening of the breaker with a delay depending on the overcurrent value. These are used for overload protection. The magnetic trip device has either a fixed or adjustable threshold that actuates the instantaneous trip of the breaker on a predetermined overcurrent value-usually a multiple of the overload current rating.
Electronic trip units use a microprocessor to process the current signal. Digital processing provides four different trip functions: the long and short time-delay trip functions per ANSI code 51 (AC time overcurrent), the instantaneous trip function per ANSI code 50 (instantaneous overcurrent), and the ground-fault trip function per ANSI code 51 N (AC time ground fault overcurrent).
Circuit breakers are categorized by their characteristic curves for different applications. Highly inductive loads, such as transformers, can have very high inrush currents of 10 to 20 times the current rating of the device. These are classified as a class "D" curve. Normal inductive loads, including most motors, have a current inrush rating of 5 to 10 times the rating of the device and are classified as a class "C" curve. A class "B" curve is used for most lighter-duty noninductive loads and has a rating of two to five times the circuit breaker rating.
Circuit breakers are also rated for use as branch, supplementary, or feeder devices. Feeder circuit breakers are generally of the molded case variety and are designed for main power feeds. They are typically tested at 20,000A interrupting rating. Branch circuit protection is tested for at least 5000 A interrupting rating and is used for branch circuits under the main breaker. Feeder and branch circuit breakers must be listed devices by Underwriters Laboratories (UL).
Supplementary protection devices are used for equipment protection in a branch circuit. They are classified as "recognized components" by UL rather than listed. They are tested with upstream branch circuit protection and are generally rated for 5000 A or less.
Motor circuit protectors (MCPs) are special application breakers with adjustable magnetic settings. They allow the operator to set the breaker's magnetic protection level just above the inrush level of the motor. Overload protection for the motor is supplied in the starter's overload relay. This combination allows protection of the motor without causing nuisance trips. MCPs are UL-recognized components.
Motor protector circuit breakers (MPCBs) are UL-listed circuit breakers with fixed magnetic protection and built-in motor overload protection. These breakers' trip units are adjustable for Motor FLA ratings and can be set for overload trip class. MPCBs can be used directly with a contactor for a complete motor starting and protection package.
A fuse or fusible link is an overcurrent protection device that is designed to melt (or "blow") when excessive current flows through it. It is composed of a metal strip or wire element rated at a specified current plus a small percentage. This is mounted between two electrical terminals and generally surrounded with a nonflammable insulating housing.
Fuses are placed in series with the current flow to a branch or device. If the current flow through the element becomes too high, enough heat is generated to melt the element itself or a solder joint within the fuse.
Dual element fuses contain a metal strip that melts instantly for a short circuit as well as a low melting solder joint for longer-term overloads. Time delay or "slow blow" fuses allow short periods of overcurrent conditions and are used for motor circuits, which can have a higher current inrush as the motor starts.
Fuses are made in many different shapes, sizes, and materials, depending on the manufacturer and application. While the terminals and fuse element must be made of a metal or alloy for conductivity, the fuse body may be glass, fiberglass, ceramic, or insulating compressed fibers. Fuse sizes and mounting methods generally fall into several standardized formats. FIG. 24 shows several cartridge type fuses; note that the larger fuse on the left has an indented area or groove at the bottom end of the fuse. This is known as a rejection fuse, the feature ensures that the fuse can only be placed into its holder one way.
Most fuses used in industrial applications are cartridge fuses.
These are cylindrical with conducting caps on each end separated by the fusible link covered by the housing. These may be small glass or ceramic fuses for light loads or larger J or R class fuses. Cartridge fuses are also known as ferrule fuses. The caps may have bladed ends for insertion into clips or have a hole in the blade for bolting to a terminal. The most common methods are spring clips or terminal block-style fuse holders.
Fuses for use on printed circuit boards (PCBs) are generally soldered into place. They may have wire leads or solder pads depending on the desired mounting technique.
Fuses Compared with Circuit Breakers
Fuses are less costly than circuit breakers but must be replaced every time an overcurrent event occurs. This is not as convenient as simply resetting a breaker, though, and makes it more difficult to ignore intermittent faults.
Fuses react more quickly than circuit breakers, especially the "current-limiting" variety. This helps minimize the damage to downstream equipment.
4.2 Distribution and Terminal Blocks
Cable and wire is distributed to multiple circuits by terminating the ends into a securing means such as a screw or clamp. Distribution blocks are used for larger gauges of wire. They are usually connected by means of screwing a threaded stud into a block of metal. One side will have one termination point and the other will have multiples to feed branch circuits. Distribution blocks are mounted into insulating carriers with dividers between phases and are available in one to four pole configurations. They can have one or more terminations for each pole on the incoming side and up to 12 terminations each on the outgoing or branch side. FIG. 25 shows a three-phase open-style distribution block; there are six connections for each phase. Wire gauges start at around 14 AWG and go all the way up into large MCM-size cable. MCM is an abbreviation for thousand circular mil (a mil is a thousandth of an inch).
Multiterminal copper or aluminum bus bars with a series of screw terminals are often used for ground or neutral terminations in panels.
These are typically for smaller wire sizes that have no voltage present.
Terminal blocks are used to make wiring and cable connections and manage wiring. They are sized for the ranges of the wire and cables that are to be connected. Screw terminal and spring clamp types are both widely used on smaller conductors, but for large wire sizes screw terminals are usually used.
There are a wide variety of styles and manufacturers of terminal blocks. The main purpose of terminal blocks other than wire management is to insulate the exposed wiring ends when making connections. The National Electrical Manufacturers Association (NEMA) has a number of standards associated with terminal block specifications as does the International Electrotechnical Commission (IEC). Terminal blocks are often categorized as being NEMA or IEC style. NEMA terminal blocks are typically a more open style, while IEC is considered a "finger-safe" style with insulation surrounding the screw or clamp terminals.
Terminal blocks are generally sized to mount on uniform-size metal rails known as DIN rail. DIN is an abbreviation for Deutsches Institut für Normung, a German standard. FIG. 26 shows a selection of various types of labeled IEC terminal blocks mounted on a piece of DIN rail. The larger black block is a cartridge fuse-holding terminal block.
Usually connections are straight through the block from terminal to terminal, but removable jumpers, switches, or fuses are sometimes built into the block. Terminal blocks for fuses are also called fuse blocks. These are made to swing open for fuse removal and also double as a branch or component disconnect. LED indicators are also embedded in some terminal blocks for energy presence or blown fuse indication. Special-purpose terminal blocks with contacts for thermocouples and extremely low or high voltages are also available.
Terminal blocks are commonly available in one-, two-, and three level configurations as a space-saving feature. They are usually mounted to some type of metal rail, the most common being DIN rail.
This allows blocks from different manufacturers to be mounted on a common surface. They are made in a wide variety of colors, which are often used for circuit identification.
A wide range of accessories are available for terminal blocks, including center and side jumpers to form a common bus, DIN rail, labels and labeling kits, and end caps and anchors.
4.3 Transformers and Power Supplies
Transformers are used to isolate or transfer energy in the form of AC current from one circuit to another. This is done by the principle of mutual inductance. If a changing current is passed through a coil of wire, it creates a magnetic field that can be used to create a current in another coil of wire that is electrically isolated from the first coil. This is most often accomplished by wrapping both coils around a common core of iron rich metal.
One of the principles of this induced voltage is that the voltage can be raised or lowered in proportion to the number of turns in the coils.
A formula that can be used to express this relationship is Vp/Vs = Np/Ns, where V is voltage, N is the number of turns in the coil, p is the primary or the coil where the voltage is applied, and s is the secondary where the converted voltage is applied to the load.
A transformer that is used to increase the voltage from the primary to the secondary is known as a "step-up" transformer, and the opposite is a "step-down" transformer. Of course, by Ohm's law, when the voltage is increased, the current will be decreased accordingly and vice versa.
FIG. 27 is a wiring diagram for a single-phase power transformer. Transformers can typically be "tapped" or wired in different ways, as shown in this diagram. This particular transformer can provide 480 to 120, 480 to 240, 240 to 120, and 240 to 240 voltage conversions.
Transformers are also used for isolation purposes, as shown in the 240 to 240 wiring (which could also be 120 to 120). Since a voltage cannot change instantaneously through an inductor, isolation transformers are often used to protect the load from quick spikes in a circuit. They are commonly used in control and drive systems.
Transformers come in a wide range of sizes, from small internally mounted transformers inside devices such as DC power supplies to large three-phase transformers that power an entire production line or section of a plant. Many commercially available transformers have multiple taps, allowing the same transformer to provide a range of voltages depending on how these taps are connected.
Another method of obtaining different voltages from one transformer is to use an autotransformer. This is a transformer that only has one winding with taps on each end and one at an intermediate point. Voltage is applied to two of the terminals. The secondary is then taken from one of the primary terminals and the third terminal.
The location of the intermediate tap determines the windings ratio and therefore the output voltage. If insulation is removed from part of the windings, the intermediate tap can be made movable using a sliding brush, making the output voltage variable similar to a potentiometer.
The purpose of a transformer in an automation system is either to convert an AC voltage to a different voltage for distribution in the system or to isolate a circuit from another. Commonly three-phase 480VAC is applied to the disconnect of a control enclosure. From the disconnect power is distributed through various branches with circuit protection for different purposes. Where a lower voltage (usually 240,208 or 120VAC) is required, a transformer is connected to reduce the voltage level. Transformers may be used between individual phases to develop a single-phase voltage or across all three phases.
Windings are often tapped in the center and grounded to develop two phases 180° out of phase with each other. This is similar to a common residential service entrance, where 240VAC is wired to a distribution or breaker panel along with a neutral (the grounded center tap). The 240 V can then be used for higher-power appliances and two rows of breakers supply 120VAC to branch circuits.
Transformers typically have circuit protection such as fuses or power supplies on both the primary and secondary side.
DC power supplies are used to provide lower-voltage DC power for I/O devices such as sensors and solenoid valves. Power supplies usually have regulated outputs to prevent current or voltage fluctuations. They are usually protected on the AC and DC sides by fuses or circuit breakers.
The most common voltage used for industrial machinery is 24VDC. This is a low enough voltage to prevent most injuries but high enough to minimize noise interference and allow distribution over a reasonable distance. 12VDC is also sometimes used, while higher DC levels of 48 or more volts may be used for DC motors such as steppers. Servos and DC motors do not usually use separate power supplies, but generate their own DC power in the drive.
4.4 Relays, Contactors, and Starters
A relay is a device that allows switching of a circuit by electrical means. There are various types of relays, including electromechanical and solid-state coils, reed or mercury wetted contact, but the purpose is generally the same: to control a circuit with one voltage with a signal from another or to use one signal to switch multiple circuits, as in FIG. 28.
Electromechanical relays use an electromagnetic coil to physically pull a set (or sets) of contacts either from an open to a closed position or from closed to open. AC or DC may be used to switch the coil; this is one of the specifications of a relay along with the number of poles and amount of current that can be handled by the contacts. Contacts are specified as NO or NC, referring to their deenergized state. Relays may have multiple poles of both NO and NC contacts. FIG. 29 shows a variety of different relay types; the relay to the far left is a tube-base electromechanical relay and socket, and the next two are often referred to as "ice-cube" relays. The relay on the lower right is a heavier-duty DIN rail mount electromechanical relay, while the one at the upper right is an adjustable timing relay.
Solid-state relays use transistor technology to switch current flow.
Voltage is applied to a solid-state "coil" that may switch current directly through a transistor or CMOS device or energize an LED to optically isolate the circuits. Solid-state relays have no moving parts, which gives them greater longevity than electromechanical relays; however, they are rated at a lower current-switching capability.
Some relays have a coil to latch the relay on and a separate coil to reset it. These are used when a circuit state needs to be maintained even if power is lost. These are known as latching or set-reset (SR) relays.
Safety circuits often use relays that have force-guided contacts.
This means that the contacts are mechanically linked together so that all of them switch together. This ensures that if a set of contacts weld together because of arcing, then one set of contacts can be used to reliably monitor the state of the relay. These safety relays also use redundant sets of contacts for each circuit for the same reason.
Relays come in a variety of form factors also. Large relays generally are screw or bolt mounted directly to a panel or backplane, while many standard industrial relays have round pins or blades that can be plugged into a DIN rail-mounted socket. Sockets are available in tube base for round pins and bladed base or pin sockets for small relays. Small relays may also be soldered to a circuit board.
A type of relay that can handle the high power required to directly control an electric motor is called a contactor. Continuous current ratings for common contactors range from 10 A to several hundred amps. Contactors are an element of motor starters; a motor starter is simply a contactor with overload protection devices attached. The overload sensing devices are a form of heat operated relay where a coil heats a bimetal strip, or where a solder pot melts, releasing a spring to operate an auxiliary set of contacts. These auxiliary contacts are in series with the coil. If the overload senses excess current in the load, the coil is deenergized.
Motor starters are generally categorized as NEMA or IEC style.
NEMA starters are generally larger and have replaceable overload elements. They can generally be rebuilt if necessary; however, they are physically larger and more expensive than an IEC motor starter of the same rating. IEC starters are not usually rebuilt and are simply discarded when the contacts wear out. FIG. 30 is an IEC motor starter in a manual motor control enclosure.
4.5 Timers and Counters
A timer reacts to an applied signal or power feed and switches a set of contacts based on a delay. It may also create a repetitive series of pulses. Timers may be purely mechanical, such as with a pneumatic timer; electromechanical with a motor and clutch; or entirely electronic. They are available in both analog and digital formats.
Timers generally fall into the following categories:
On Delay-Timer changes state after a specified period of • time and remains in that state until the signal is removed.
Off Delay-Timer changes state immediately and reverts to • its original state after a specified period of time.
One Shot-Timer creates a single pulse of specified length. •
Pulse or Repeat-Cycle-Timer creates a series of on and off • pulses with configurable on and off times until signal is removed.
As with temperature controllers, timers and counters are often sized using the DIN system, ensuring that they will fit a certain-size panel cutouts. They are usually available in 1/16 DIN, 1/8 DIN, or 1/4 DIN sizes. FIG. 31 shows a 1/16 DIN digital timer.
Electromechanical timers such as the Eagle Signal Cycle Flex timer shown in FIG. 32 are often used in applications where electronic timers may not be appropriate. Mechanically switched contacts may still be less expensive than the semiconductor devices needed to control powerful lights, motors, and heaters. An electromechanical cam timer uses a small synchronous AC motor turning a cam against a bank of switch contacts. The AC motor is turned at an accurate rate by the applied frequency, which is regulated very accurately by the power companies. Gears drive a shaft at the desired rate and turn the cam. These timers are still in use in many industrial facilities because they are easily rebuilt, rugged, and switch high-current loads; however, they are often replaced with less expensive and more reliable electronic timers.
FIG. 32. Eagle Signal Cycle Flex timer
The most common application of electromechanical timers now is in washers, driers, and dishwashers. This type of timer often has a friction clutch between the gear train and the cam, so that the cam can be turned to reset the time. This is a less expensive method of performing multiple timing segments with high-current load switching than with an electronic version.
A counter also reacts to input signals, totalizing them and changing the state of a signal when the specified count has been reached. Counters are generally classified as up counters, where state changes increment the value until the set point is reached, or down counters, where the counter starts at the set point and counts down to 0. Counters may also be combinational with both up and down signals. Counters also have a reset input to set the counter back to its starting point. They may be mechanical in nature, such as with a totalizer, or be incremented electronically.
4.6 Push Buttons, Pilot Lights, and Discrete Controls
Before the advent of touch screens, signaling and machine control had to be done with push buttons, switches, and indicator lights. A large variety of components are still used as discrete interfaces between an operator and a machine today.
Push Buttons and Switches
A push button is a manually operated spring-loaded method of opening or closing a set of electrical contacts. Industrial push buttons generally come in several standard sizes; 30 mm, 22 mm, and 16 mm diameters. There are larger and smaller sizes also available, but the vast majority of push buttons, switches, and pilot lights fall within these standard sizes.
Larger push buttons (22 and 30 mm) are often modular in nature, having an actuator to which contact blocks can be mounted and removable mounting rings and bezels. Contact blocks are available in NO and NC configurations and may be mixed and matched as necessary. These blocks can also be stacked on top of each other for up to four sets of contacts. Push buttons may also have an internal light that may be illuminated from a control output or through one of the sets of contacts; these are usually LED or incandescent bayonet base bulbs.
Push buttons come in various colors, generally black, red, yellow, green, blue, or white, although other colors are also sometimes seen.
The actuator may be mushroom head, extended, or flush with the bezel. They also may be of the momentary (spring return) or maintained (toggling) variety. FIG. 33 is a 30 mm flush push button.
Selector switches have many of the same characteristics as push buttons; they use the same types of contact blocks and come in the same 16, 22, and 30 mm diameters. The color is usually black, although inserts may be of various colors. Lights are not commonly used in selector switches.
As with push buttons, switches may be of the maintained or spring return variety. Most switches have two or three positions, although four positions or even more are sometimes seen. Unlike push buttons, however, all of the contacts do not switch at the same time. For a three-position switch the contacts on one side will switch in the left position and the opposite side will switch in the right position. Both sides usually remain unswitched in the center.
Contact blocks are actuated with a cam rotating with the switch body.
Part of the push button or selector switch assembly is usually an antirotation ring. This is a ring with a tab that fits into a slot in the device and also a tab that fits into a slot cut into the punched hole in the cabinet or enclosure.
Pilot Lights and Stack Lights
Pilot lights are available in the same standard sizes as push buttons and switches, although much larger lights are sometimes used for greater visibility and small 8 mm and 10 mm lights are common for higher-density display. Lamps for pilot lights are generally of the incandescent or LED type. Most lamps are white and a plastic cover is used to change the color of the light. They are available for "full voltage" applications of 120 to 240VAC, which use a small transformer, as well as 12 and 24VDC. For low-voltage or computer card outputs, 5 and 6VDC lamps are often used. Some pilot lights also have a spring-loaded "push to test" feature that will illuminate the lamp, although there are no external contacts for these as there would be on a lighted push button. If pilot lights are connected to controlled outputs, a separate "push to test" push button is sometimes used to illuminate all of the lamps on a panel at once.
Stack lights, also called light stacks or tower lights, are columnar sets of lights that usually indicate the state of a whole machine or control system. They are also modular, usually beginning with a base unit that may or may not include a horn or buzzer. The base may be connected using a quick disconnect cable or terminals with a strain relief entry. Light units are then stacked onto the base in the required order, generally up to five units in height. A common combination of lights would be red, yellow, and green (from top to bottom), red generally signifying a fault or alarm, yellow signifying either caution or manual/maintenance mode, and green signifying auto mode or machine running. There is no universal stand for these colors, and each company or plant may have their own specifications. Blinking the lights to signify auto/not started or cycle stop versus immediate stop conditions is an example of how a stack light might be used to deliver additional information. Blue or white lights are sometimes added for signals like low bin or hopper or other special functions as defined by the designer. FIG. 34 shows several arrangements of four-color stack lights; the two rightmost stacks have a buzzer or audible alert in the top position.
As with pilot lights, they may be energized by 24VDC, 120VAC, or various other voltages as required. Other modules for playing a recorded voice or music are also available. A feature of some stack lights that may be beneficial is a flexible base to reduce the chance of the stack breaking off a low-mounted cabinet or machine. Stack lights may also be pole mounted and side mounted, depending on the application.
Other Panel-Mounted Devices
Some items grouped under the heading "discrete controls" may not be discrete at all. An example is a panel-mounted potentiometer for analog speed control of a motor drive. These are often available in the same form factors as selector switches in 22 mm or 30 mm sizes.
Temperature controllers, timers, and counters are also devices you might find mounted on the front of a controls enclosure.
Horns and buzzers are other discrete devices that may be panel mounted. Buzzers are generally piezoelectric and susceptible to moisture since they cannot be easily sealed.
Along with all of the devices that might be mounted on a controls enclosure come labels for these devices. Most commonly engraved plastic tags or painted metal tags that have the appropriate-size hole are used with push buttons, switches, and pilot lights. Engraved plastic or metal tags are of two colors, an inside and outside color. An example is Gravoply plastic, which may be black or red on the outside with a white inside color. When characters are cut into the plastic, the inside color shows through. Tags may be premade with common terms like stop, start, and so on, or be sold as blanks for the user to engrave.
Tags are not only used for devices on the outside an enclosure; they are also common inside the cabinet or mounted next to a sensor on a machine. These may be engraved or printed and contain schematic, I/O, or descriptive text for components. Safety warnings also fall into the premade or purchased label category.
4.7 Cabling and Wiring
An important part of the distribution of power and signals throughout a system is cable and wire. Individual wires and multiconductor cables are used to connect the various control devices and distribution components within a machine or system. Wire sizes or gauges are specified as described in the appendixes of this book. Wire may be made of any conductive metal but is usually copper or aluminum.
Usually it is covered with a thermoplastic insulation available in a wide range of colors. Wire is manufactured in solid or stranded forms, depending on the application.
Multiconductor cables consist of a collection of insulated wires inside a protective jacket. The wires may be twisted together in pairs for noise immunity or simply run in parallel. Multiconductor cables often carry a noninsulated shield or drain wire to help carry away unwanted stray signals. This wire should be grounded at one end only to prevent a ground loop. An additional foil covering is often wrapped around the bundle inside the jacket but in contact with the shield wire.
Examples of several multiconductor cables are shown in FIG. 35.
For high-flexing and repetitive movement applications, multiconductor cable is often made with fine stranding to improve its bend radius and increase usable lifetime. Specifications for expected number of cycles and minimum bend radius are often listed in wiring catalogs.
Connecting individual wires or multiconductor cables together may be done with terminal blocks, but in some cases they must be spliced. This may be done using crimp on malleable metal pieces called butt splices or wires may be soldered. After soldering, the wire junction must be insulated using electrical tape or shrinkable tubing, also known as heat shrink.
To prevent pulling wire and cabling out of terminations a strain relief-type fitting is placed at enclosure entry points and built into cable plugs. These may be of a screw clamp type or a rubber "donut" shape that clamps down on the cable when a fitting is tightened.
Another type of strain relief has a series of ridges at the point where the cable meets the enclosure or junction box. The main purpose of a strain relief is to reduce wear or stress at the point of entry when a cable is pulled. Strain reliefs often also provide ingress protection from liquids. Strain reliefs often come in standard hole sizes like electrical fittings; 3/8 in, 1/2 in, and 3/4 in. One inch and larger are all standard sizes. They may be made of a galvanized metal or plastic.
FIG. 36 illustrates a 1/2-in cord grip strain relief installed in the side of an enclosure.
A ferrule is a circular clamp or sleeve used to hold together and attach fibers or wires by crimping the ferrule to permanently tighten it onto the wire end. Wiring ferrules often have a color-coded piece of plastic molded around one end both to allow easy wire entry and for identification of wire gauge. Ferrules prevent smaller stranded wires from splaying and provide a solid electrical connection for terminal block clamps or screws. Special crimping tools with selectable dies are used to crimp the ferrule firmly onto the exposed wire end. FIG. 37 shows several different sizes of insulated ferrules.
A common method of attaching wires to each other or to pins in plugs is soldering. It is used in electronics, where it is used to connect electrical wiring and to connect electronic components to PCBs.
Soldering is also used in plumbing to connect metal piping together with a water and gas tight bond.
Solder is a metal filler material that melts at a low temperature.
For electrical connections it is usually composed of tin and lead in various proportions, the most common being 63 percent tin and 37 percent lead. This proportion also has the benefit of being eutectic, meaning it passes directly from a liquid phase to a solid phase. This is important because metals that pass through an intermediate "plastic" phase are subject to cracking if disturbed while cooling.
Other alloys used for electrical connections are lead-silver for higher strength, tin-zinc or zinc aluminum for joining aluminum, and tin-silver and tin bismuth for other electronics. These alloys all melt at a lower temperature than the materials they are joining. This is the major difference between soldering and welding, which melts part of the workpiece. All of these alloys are known as soft solder, although silver solders are sometimes excepted from this classification.
The process of soldering involves melting the solder and flowing it into the joined wires or components. This process can be assisted by using a rosin water-based or "no-clean" type of "flux" to coat the joined pieces; solder flows to wherever the flux is applied. Many solders have a flux core that helps in this process. In addition to assisting the solder to flow, flux also helps clean the materials and prevent oxidation. When soldering stranded wires, solder is usually applied to the wire ends individually first; this is known as tinning the wires. If flux is applied to the strands beforehand or as part of the solder core, solder is drawn up into the strands by capillary action, called wicking.
Soldering by hand is done with a soldering iron, which is an electrically heated tool with an insulated handle and various different sizes of tip. Many of these have a temperature adjustment for different-size work. FIG. 38 shows a soldering iron and roll of rosin-core solder. Often when soldering solid-state components, a clip-on "heat sink" is used between the wire lead and the component to prevent damage; proper temperature and tip size are important here also.
Soldering of components to PCBs on a production line is done by a process known as wave soldering. Components are adhesively attached to the board with leads extending through holes in the board and touching contact pads. The boards are then passed over pools of molten solder, which are vibrated, creating waves. This allows solder to contact the pads and leads without immersing the entire underside of the circuit board.
Another method of production soldering is to apply a solder powder and flux mixture in little clumps to the solder joint. This can then be melted with a heat lamp, hot air pencil, or most commonly in an oven. This method is called reflow soldering.
Often a combination of wave, reflow, and hand soldering will be used on the same PCB.