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Most of what we said about residential wiring also applies to commercial and industrial work, but the non-dwelling setting is typically much larger and contains an abundance of highly specialized electrical equipment and types of circuitry that are not seen in the home. Higher voltage and power levels are common, and where there are flammable gases or explosive dusts, many areas are classified as hazardous and require special wiring methods. A large manufacturing facility will have an in-house electrical department made up of professional specialists who possess considerable knowledge and expertise going way beyond what is required to wire a house. This is not to disparage residential electricians. Their work, though narrower in scope, requires impeccable attention to detail and the responsibility cannot be overstated. Moreover, the majority of electricians wear both hats in the course of their life work.
The greatest single difference between these two environments is in the dominant wiring method. For branch circuits, residential wiring is almost exclusively done in type NM, which stands for nonmetallic-sheathed cable. It is usually known by the trade name Romex.
Each type of cable and raceway is covered in National Electrical Code (NEC) Section 3, Wiring Methods and Materials, in Articles 320 through 398. Article 334 covers type NM and, as in all of these articles, there are subsections titled Uses Permitted and Uses Not Permitted.
Using Type NM It turns out that type NM is permitted to be used in one- and two-family dwellings and their attached or detached garages and storage buildings. Type NM is permitted in other locations as well, depending upon the fire rating of the building. In actual practice, most commercial and industrial electrical work is done using type MC (metal clad) cable and EMT (electrical metallic tubing) and these are permitted in all locations except for the most restrictive of hazardous areas.
In a large facility where it is frequently necessary to test incandescent or self-ballasted fluorescent bulbs, you can make a quick tester mounted on the wall above your bench in the maintenance shop. Using a hacksaw, split the screw shell of a porcelain base fixture.
Then, you can quickly stick in the bulb to be tested without screwing it in.
MC and EMT work well in commercial and industrial settings, and it is a simple matter to transition from one to the other-just terminate them through separate knockouts in a 4 × 4 box and wire nut the conductors. EMT is preferable for most long runs with few bends. In tight places where there are numerous bends, or through drilled holes in studs, it is easier to snake MC as needed. Then you can transition back to EMT.
EMT is technically not a conduit. It is tubing, although in using it most electricians talk about "putting the wire in conduit." EMT has a thinner wall than rigid metallic conduit (RMC), which resembles galvanized water pipe. EMT is not threaded in the field. It slides easily into fittings, where it is held in place with setscrews or, for wet locations, compression fittings.
For small jobs, EMT can be cut with a hacksaw. If there is a lot of work to be done or for larger sizes, a portable bandsaw is more efficient. All cut ends must be reamed to remove sharp burrs. Bending conduit starts very simply and quickly becomes complex. The basic premise are that bends must be gradual and uniform, so that the internal diameter is not materially reduced. To get a good bend, it is necessary to use a hand or hydraulic bender.
These are easy to use, but it is essential to visualize the product and how it will interact with the building layout to get a good finish appearance. A fundamental principle in all conduit work is that the raceway must closely follow the wall or ceiling surface, and not take shortcuts at odd angles that would clutter the available space in the building (see Figs. 4-1, 4-2, and 4-3).
The Code, of course, regulates many aspects of raceway installation, and it should be consulted for information regarding minimum spacing of supports, maximum number of bends, and similar requirements. One basic rule is that the entire raceway system is to be assembled and terminated at both ends before pulling the wire.
Figure 4-1 Bending electrical metallic tubing.
Simple Conduit Bends
To begin with a simple exercise, suppose we are coming out of the top of a load center that is already mounted on the wall. We want to bring the EMT up to the ceiling, whereupon it will make a 90-degree bend and run along the ceiling to a fluorescent light fixture. You will ...
Figure 4-2 Four-point saddle bend.
Figure 4-3 Three-point saddle bend.
... notice that where the raceway exits the enclosure, it is spaced some distance from the wall.
To follow the wall, the pipe needs to make two bends. In combination, they are called an "offset bend." This consists of two bends at the same angle, in the same plane, but in opposite directions, so that the pipe runs that are on either side of the sloping segment are parallel but at different elevations. The amount of offset depends upon the angles and the distance between them, and this is what you have to decide to make the offset. Center-to center, the length of the raceway between the two bends is equal to the depth of the offset multiplied by the cosecant of the two angles. (Cosecants for various angles are given in trigonometric tables.) The most commonly used offset angle is 30 degrees, which is preferred by electricians because the cosecant, 2, is easy to remember and to calculate. For offsets less than 4 in., however, 30 degrees is too large an angle. Frequently used angles are 22.5 degrees, with a cosecant of 2.6, and 10 degrees, with a cosecant of 5.76.
For a superior finish appearance, tighten screws in wall plates so that the slots are perfectly vertical.
For a standard box offset, a very useful tool is the box offset bender, which makes both bends at the precise angle and in the same plane with a simple push of the lever (see Fig. 4-4).
After making the offset bend that is required so that the EMT conforms to the wall, we have to bend the pipe at a 90-degree angle so that it can travel along the ceiling. This configuration is called (by me) a "brick wall," by which is meant that the start of the ...
Figure 4-4 Box offset bender.
... raceway is fixed (because the box was already mounted on the wall, and the adjacent building surface, a ceiling in this example, is fixed). To make this bend, measure the distance to the brick wall and deduct a certain number, depending on the size of the EMT. For ½-in. pipe, it is 5 in., and for ¾-in. pipe, it is 6 in. The number is stamped on the bender.
Measure from the end of the pipe, deduct the correct number, and mark the EMT using a fine felt-tip pen. Line up the mark with a dedicated symbol on the bender, and make your 90-degree bend. The piece will conform to the wall-ceiling corner.
An easier method, if a little less elegant, is to start from the box with a short length of EMT, the one with the offset. Then make the 90-degree bend in a long piece, hold it in place to mark the length of the leg going down the wall to a coupling, and cut to fit. Another shortcut, where the exact height of the box is not critical, is to install the raceway first and then mount the box at a height conforming to the pipe installation.
What remains is to fit the piece needed to reach the light fixture using the box offset bender. Install an EMT connector with a locknut on the inside. Drive the locknuts at both ends very tight so that they scratch through the paint and dig into the metal, making a good grounding connection.
Now that the raceway assembly is complete, the time has arrived to pull the wires. For this indoor installation, 12 AWG THHN will probably be used. For a short, simple installation like this, a pull rope will not be necessary. Set up the three spools of wire- white, black, and green-on a rod suspended at both ends. A pair of stepladders works well, but for big jobs, you will want a portable wire caddy. Tape the wire ends together and push the wire through the raceway from the box to the fixture. When the wire comes to a dead stop, it has probably arrived at the light fixture. Trim the wires at both ends and terminate them. If there is a switch loop, follow the same piping and wiring procedure from the fixture or panel to the wall box.
This is a typical small EMT job. Some bending projects will be more complex, but industrial electricians do this work day after day, and it becomes familiar. EMT, as the word tubing implies, is the lightweight of metal raceways. However, it provides ample protection and shielding for common commercial and industrial applications. Intermediate metal conduit (IMC) and RMC are needed for hazardous locations and particularly rough environments. You are free to use them anywhere EMT is permitted, but the material cost is much higher and the labor-lifting, cutting, threading, and screwing together-for these heavier pipes is a very big factor, especially in larger sizes.
One benefit of any metal raceway, including EMT, is that conductors may be easily replaced or extra circuits added, provided the pipe is not overfilled, merely by pulling out old and installing new. Another advantage of metal raceway is that when grounded (which is always), it provides excellent electrostatic and radio frequency shielding for data and communications circuits. This shielding eliminates crosstalk, hum, and signal corruption.
For ordinary Ethernet lines, although not mandatory, it is an excellent practice to install them in EMT between terminations.
In an excavation or other damp place, use cordless power tools only. If this is not possible, make sure that the power supply is GFCI protected.
Regarding troubleshooting and repair, EMT offers some advantages but also presents unique challenges, particularly concerning tracing and identifying individual conductors.
If it is a Romex installation, it is often possible to follow individual cables in the basement of a residence where they are not concealed behind wall or ceiling finish. Where many wires of the same color are installed in EMT, identification can be difficult, especially if the original installer did not leave a good directory. There can be many 12 AWG circuits in a single large raceway, and there will be no way of telling which white (neutral) goes with which black (hot). This information is needed when doing certain types of upgrades such as adding arc-fault protection.
Identifying Wires
For short EMT runs, have a helper tug on the conductors at one end of the run and see if you can pin down the correct wire at the other end. Alternately, with a heavy load connected at the downstream end, remove the whites from the neutral bar one at a time and do ohmmeter tests.
Ungrounded conductors (hot wires) can be identified by using any colors except those reserved for the grounded neutral and equipment grounding conductor. For single-phase, 120-volt circuits, black is generally used. As an alternate hot wire for three-way switches or a two-speed motor, red is most common. When pulling multiple circuits in a single raceway, blue, yellow, pink, orange, brown, and so on are used for the hot wire, in addition to black and red.
Though not Code mandated, it is trade practice to denote three-phase legs as follows:
• Lower voltage, typically 208Y/120V: black, red, blue
• Higher voltage, typically 480Y/277V: brown, orange, yellow In tracing circuits, there are some specialized tools that work well. In an industrial or commercial facility, there is often a substantial amount of telephone work to be done. New lines are needed from time to time, and maintenance is needed when something goes awry.
The relevant tools are a toner, which injects a pulsed audio tone at one end of a line; a test set, which clips onto lines to listen for the pulses, talk to coworkers, or dial an outside line; and a wand, which when brought near a line to which the toner has been connected will pick up the signal. There is no reason why these tools cannot be used to trace de-energized power lines.
Type MC, as mentioned, is permitted in many of the same locations as EMT, and the two are often intermingled as part of the same job. The main difference is that for EMT, the metal tubing, properly terminated, qualifies as an equipment grounding conductor, while the metal armor of type MC does not. This is mostly a nonissue, however, because MC contains a green wire that is the equipment-grounding conductor. The only grounding advantage possessed by EMT is when redundant grounding is required, as in the patient bed area of a healthcare facility.
An essential tool for working with Type MC cable is the Roto-Split. It cuts through the metal armor without nicking the enclosed conductors. To make a termination, cut off approximately 8 in. of the metal sheath and slide it off. Cut back and remove the plastic filler strip. Put on the antishort bushing ("redhead") and attach the connector. Snap-on connectors are easy to use and do not require the bushing. Install the cable through a knockout in the box and wire nut the conductors. Use a grounding pigtail to ground the box.
Details concerning uses permitted, uses not permitted, spacing of securing hardware, minimum bending radius, and so on are found in NEC Article 330. Eventually, many industrial electricians have occasion to work on programmable logic controllers (PLCs). The advantages of this technology in assembly-line manufacturing operations are numerous and there are millions of these installations worldwide. There are specific setup and programming protocols that are similar, although not exactly the same, among the various makes. You may find at first that there is a somewhat steep learning curve, but the background information is readily available from manufacturers, online forums, and print-format textbooks. After a little experience, most technicians agree that these systems are remarkably user-friendly and efficient.
PLC Anatomy
The brain of a PLC system is a modular, rack-based, central processing unit (CPU) that resembles your desktop computer in that it has memory and software. Systems may be as simple as a $200 "brick" that fits in the palm of your hand, to large, fan-cooled, floor-to ceiling enclosures that orchestrate complex machinery. For initial installation and to make changes, the technician goes to the PLC control panel, plugs in a laptop computer, and programs the unit, using an on-screen ladder diagram that can be constructed and manipulated using the computer mouse and keyboard. The control panel is wired by means of low-voltage circuitry to actuators, motor controllers, sensors, and so on that are attached to the assembly lines and production equipment throughout the facility, sometimes in different buildings. The PLC gathers information and, based on its internal timer and programming, issues commands that result in an efficient workflow.
Most repairs involve the motors, sensing apparatus, and actuators rather than the actual PLC, although when first entering programming a certain amount of debugging may be required. PLCs surfaced in 1968 when General Motors embraced the technology in order to improve upon antiquated systems of switches and relays that had to be rebuilt once a year when new car and truck models appeared. Instead of hardware elements, PLCs have programmed instructions that reside in microchips on printed circuit boards within the CPU.
What quickly evolved was a rack-based system that accepts input and output modules.
They may be digital or analog and plug into slots in order to accommodate whatever sensors and actuators are required by the installation.
The PLC consists of a very rugged housing that can withstand vibration, dust, moisture, and other environmental stresses. Within the housing is the solid-state circuitry that initiates actions as needed to perform the tasks. Disc-based software permits the technician to insert virtual components onto the on-screen ladder diagram to program the PLC. The two ladder rails are positive pole (left rail) and ground bus (right rail). Programming begins at the top of the ladder and each rung is made up of an input and an output, one complete executable command. Each of these has a numerical address, such as 0500 or 1000. An example would be a rung with a single-pole switch on the left and a lighting load on the right.
An operating PLC is continuously scanning. The PLC looks at the input to ascertain whether it is ON. Then, the PLC executes the programming, beginning at the top of the ladder. Finally, the PLC updates the output status. There are two modes-programming mode and operating mode.
To make it all work, input and output modules are inserted into the slots. Examples of inputs are limit switches, pressure transducers, voltage sensors, flow meters, and thermocouples. The output modules are wired to low-voltage control circuits to motors and other actuators. An internal oscillator-based clock permits the PLC to scan the inputs.
Latching or momentary outputs permit the machinery to function in a harmonious and productive fashion.
Maintenance electricians who work in a facility having PLCs have an easy way to get into the field. Manufacturers, such as Allen Bradley and Siemens, have free online material including machine-specific documentation, so there is a clear shot at gaining on-the-job expertise in this interesting field. Two good introductory websites are www.plcs.net and www.plcdev.com.
To effectively troubleshoot and repair PLC systems, the technician will need good electrical, computer, and mechanical skills. The undertaking is somewhat more advanced than most electrical work encountered on a routine basis, but then that is what makes it interesting.
The maintenance electrician is usually the first responder when PLC-controlled equipment fails to perform as expected. Often it is an incredibly simple fix, but there is always the potential for problems to spread and multiply, particularly if a deranged system has caused mechanical jamming or worse on the assembly line.
As always at the outset, conduct a thorough interview with the operators. Is the equipment completely dead, is it tripping out instantly if power is applied, or does it trip out after a period of time? The PLC output voltages are low control voltages that activate, let us say, a motor controller, so it is important to see if there is line voltage at the input and, with contacts pulled in, at the output. If so, is there voltage at the motor? Do the usual motor and driven load checks, exercising extreme caution that you do not start the motor where it could endanger workers who are within range of driven machinery.
If the problem is not in that area, you can work back toward the PLC, or start at the upstream end, depending in your judgment which of these approaches is more likely to take you to the fault. From the alphanumeric display and the power light, it will be evident if the PLC is receiving power. If not, check the breaker and branch circuit wiring. If the supply power is 240 volts, make sure that both legs are intact.
If the schematic is available or from inspection, note whether the 24-volt dc power supply is inside of the unit or supplied externally. There may be primary fuses that should show no voltage across them if they are not blown.
There should be internal diagnostics, depending on the make and model of the PLC. If an error code is given on the alphanumeric display, look it up in the PLC documentation or consult an Internet search engine, as these resources usually provide the answer.
Look at the input LEDs and manually activate the switches. For this operation, if there is a conceivable hazard, be sure to have any motors or actuators powered down and locked out.
If the inputs are all good, put your meter on the outputs, and this will reveal the status of the PLC. Check if the proper voltages are coming out of the PLC so that all solenoids will turn on as needed. If there are relays that make the outputs 120 volts, one of these may be stuck or have an open coil. See if the relay pulls in as required.
These are the basic diagnostics for a PLC, but details will differ, depending on the make.
There is usually good technical assistance available from the manufacturer, and sometimes the distributor will have someone with a wealth of information to share.
In industrial wiring, the overriding issues that confront electricians every day are complexity and integration, and these considerations make it markedly different from residential electrical work. An industrial or large commercial facility will have a great many systems and subsystems, and they are frequently connected in one way or another, so to competently maintain, diagnose, and repair the equipment, workers need to understand how these elements work together. They need an extensive knowledge of each piece as well as an understanding of how it relates to the whole.
A manufacturing facility, to take one example, typically has elevators, sprinklers, a fire-alarm system, a telephone system with in-house private branch exchange (PBX), and other related systems.
If a sprinkler head ruptures due to heat from below, the water flow is sensed and this activates the fire alarm system. The fire alarm control console initiates a call to the fire department or monitoring agency on one of two dedicated telephone lines, and it causes elevator cars to proceed immediately to the ground floor or, if fire is detected there, to some other predetermined floor. Additionally, LP or natural gas, exhaust ducts, and other systems can be shut off or redirected as mandated by local codes and building requirements.
In the remainder of this guide, we shall look at some of these systems as discrete entities and also as parts of the larger structure, particularly as troubleshooting and repair procedures relate to them.