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Chapter 1 PLC Electrical Safety

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1 Chapter 1 PLC Electrical Safety
Programmable Logic Controllers • PLC Safety • Electrical Properties • Grounding • Force and Disable Safety Considerations • Electrical Noise Suppression • Static Electric Charges • Electrical Safety • Personal Protective Equipment • Lockout/Tagout • Inspecting a PLC System •

2 PLC system programming is viewed by various devices that connect to micro-sized PLCs (8 to 10 inputs and outputs) or to large-sized PLCs that control thousands of inputs and outputs. PLCs are manufactured from nano and micro sizes that control a few input and output terminals (devices and components) to large sizes that control thousands of input and output terminals (devices and components). PLC system programming can be viewed using software with integrated programming devices, handheld programming devices, operator interface panels, and desktop or laptop computers. See Figure 1-1.

3 Improperly installed and/or maintained PLCs can overheat, leading to fire or explosion.
Because PLCs are a part of an electrical system, PLCs can be an electrical hazard to anyone installing, working around, or servicing them. In addition to potentially producing an electrical shock to anyone coming in contact with the electrical parts of a PLC system, improperly installed and/or maintained PLCs can also become a fire hazard or cause an explosion. See Figure 1-2. Technicians must know the proper safety procedures and practices to follow when working on a PLC-controlled (or any) electrical system. Safety procedures and practices include what type of personal protective equipment (PPE) to wear, what types of tools and test instruments to use, and all National Fire Protection Association (NFPA) 70E safety rules and National Electrical Code® (NEC®) installation rules that must be followed.

4 An advantage of a PLC controlling a process is that a PLC can be programmed and reprogrammed as process conditions change. When properly installed and programmed, a PLC can control an electrical system to safely and efficiently operate a process. A major advantage of a PLC controlled process is that a PLC can be programmed and reprogrammed as process conditions change. See Figure 1-3. Another advantage is that when servicing an electrical control system, most PLCs include FORCE and DISABLE commands that, when properly used, aid in process startups and troubleshooting. When misapplied, force and disable commands can present a serious safety problem in any part of the electrical or mechanical systems controlled by a PLC.

5 An electric shock results anytime a body becomes part of an electrical circuit.
An electrical shock results any time a body becomes part of an electrical circuit. See Figure 1-4. Electrical shock varies from a mild shock to fatal current. The severity of an electrical shock depends on the amount of electric current (in mA) that flows through the body, the length of time the body is exposed to the current flow, the path the current takes through the body, the physical size and condition of the body through which the current passes, and the amount of body area exposed to the electric contact.

6 Possible effects of electrical shock include the heart and lungs ceasing to function, and/or severe burns where the electricity (current) enters and exits the body. The amount of current that passes through a body or circuit depends on the voltage and resistance of the completed electrical circuit. During an electrical shock, the body of a person becomes part of an electrical circuit. The resistance a person’s body offers to the flow of current varies. Sweaty hands have less resistance than dry hands. A wet floor has less resistance than a dry floor. The lower the resistance, the greater the current flow and greater the severity of shock. See Figure 1‑5.

7 Current is the amount of electrons flowing through an electrical circuit and is measured in amperes.
Current (I) is the amount of electrons flowing through an electrical circuit and is measured in amperes (A). See Figure 1-6. An ampere is the number of electrons passing a given point in one second. Current may be direct current or alternating current. Direct current (DC) is current that flows in only one direction. Alternating current (AC) is current that reverses direction of flow at regular intervals. PLCs are used to control electrical circuits that have DC, AC, or AC and DC current. Many PLC-controlled systems include DC and AC components.

8 Voltage is the amount of electromotive force in a circuit and is measured in volts.
Voltage (E) is the amount of electromotive force in a circuit and is measured in volts (V). See Figure 1-7. Per NFPA 70E, voltages greater than 50 VAC can cause an electrical shock. To aid in the prevention of electrical shock, the input side of a PLC typically has input terminals that are rated at 24 V or less. The input devices that send low-voltage signals to a PLC include temperature switches, photoelectric/proximity switches, pushbuttons, and pressure switches.

9 Resistance is the opposition to the flow of electrons and is measured in ohms.
Resistance (R) is the opposition to the flow of electrons and is measured in ohms. See Figure 1-8. The Greek letter omega (Ω) is used to represent ohms. The rule of thumb to remember with resistance is the higher the resistance, the lower the current flow and the lower the resistance, the greater the current flow. Resistance is why technicians must wear insulated electrical gloves and shoes, use rubber insulated matting, and use insulated tools and test instruments when working around a PLC or any energized electrical system. Technicians must remember that the greater the insulation (resistance) between a technician and an electrical circuit, the less the chance of electrical shock.

10 Grounding provides a direct path for unwanted fault current to travel to earth without causing harm to technicians or equipment. Grounding is the connection of all exposed non‑current‑carrying metal parts to earth. Grounding provides a direct path for unwanted fault current to travel to earth without causing harm to technicians or equipment. Grounding is accomplished by connecting devices and components to metal underground pipes, metal frames of buildings, concrete‑encased electrodes, or ground rings. See Figure 1-9.

11 Building grounding, equipment grounding, and electronic equipment grounding are used to create a safe working environment for technicians. The three categories of facility grounding are building grounding, equipment grounding, and electronic equipment grounding. Each grounding category has a different purpose and, when combined, a safe and effective grounding system for technicians and equipment is provided. See Figure 1-10.

12 Building grounding ensures that there is a low impedance (low resistance) grounding path for fault current (electrical short or lightning) to earth ground. Building grounding is the connection of an electrical system to earth ground through a GEC to grounding electrodes, the metal frame of a building, concrete-encased electrodes, or underground metal water pipes. Building grounding ensures that there is a low impedance (low resistance) grounding path for fault current (short or lightning) to earth ground. A low impedance ground is a grounding path that contains very little resistance to the flow of fault current to ground. See Figure 1-11.

13 Equipment grounding prevents electrical shock when a person comes in contact with electrical equipment or exposed metal of machinery. Equipment grounding is the connection of machinery electrical systems to earth ground to reduce the chance of electrical shock by grounding all non-current-carrying exposed metal. The most important reason for equipment grounding is to prevent electrical shock when a person comes in contact with electrical equipment or exposed metal of machinery. See Figure 1-12.

14 Electronic equipment grounding is used to provide a quality ground for electronic systems to enable better communication (less noise) with PLCs, process control equipment, and other facility operations. Electronic equipment grounding is the connection of electronic equipment, such as PLCs, to earth ground to reduce the chance of electrical shock through grounding the equipment and all non-current-carrying exposed metal parts. Although electronic equipment grounding is basically the same as equipment grounding, electronic equipment grounding is used to provide a quality ground for electronic systems to enable better communication (less noise) with PLCs, process control equipment, and other facility operations. See Figure 1-13.

15 Ground resistance measurements are taken on grounding conductors used with service entrances, transformers, utility transmission, and communication (control circuit) grounds. Electronic equipment grounding is primarily used for electronic noise reduction by eliminating noise and other unwanted signal interference. Unwanted current that is removed to ground by electronic grounding systems is typically measured in milliamps (mA) and continues to flow as long as the electronic equipment is connected to a power source. Manufacturers of PLCs and other electronic equipment often specify a grounding system with a resistance of 5 Ω, 3 Ω, 1 Ω, or less. See Figure Low-resistance electronic grounding can be accomplished by connecting a PLC to a large grounding conductor that has a low resistance. Large grounding conductors provide quality electronic grounding and good equipment grounding.

16 PLC force and disable commands are used during system start-up and for troubleshooting.
A major safety issue that must be considered when working with a PLC-controlled electrical system is that the PLC program can include a FORCE command. A force command is a special software override that opens or closes an input device or turns an output component ON or OFF. Force commands are designed for use when troubleshooting a system. Forcing an input or output device ON or OFF allows for checking an electrical circuit with software assistance. See Figure 1-15.

17 Electrical noise enters a PLC system through input devices, output components, and power supply lines. Electrical noise is any unwanted signal present on power lines. See Figure Electrical noise enters a PLC system through input devices, output devices, and power supply lines. Placing a PLC away from noise-generating equipment such as motors, motor starters, welding machines, and electric motor drives reduces unwanted noise pickup.

18 To prevent false signals from entering a PLC, input and output lines must cross at right angles (90°) and not run parallel to each other. Even when PLCs are placed away from noise-generating devices, noise can reach a PLC through the conductors used to connect the system input devices (switches and sensors) and output components (lights and motors). To prevent noise and false signals from entering PLCs through the routing of power lines and control lines, technicians must not route low-voltage DC signal conductors near high-voltage (115 V) AC signal conductors. When different types of signals must cross, the cables should cross at 90° to minimize noise interference. See Figure 1-17.

19 A shielded cable uses an outer conductive jacket (shield) to block electromagnetic interference from the inner, signal-carrying conductors. A shielded cable is a type of cable that uses an outer conductive jacket (shield) to surround the inner conductors that carry the signals. The shield blocks electromagnetic interference. The shield of a cable must be properly grounded to be effective. Proper grounding includes grounding the shield at one point only. A shield grounded at two points tends to conduct current between the two ground locations. See Figure 1-18.

20 Snubber circuits are used to suppress voltage spikes in PLCs.
A high-voltage spike is a type of electrical noise that is produced when inductive loads such as motors, solenoids, and coils are turned OFF. High-voltage spikes typically cause problems for PLCs. High-voltage spikes must be suppressed with snubber circuits to prevent PLC and other problems. A snubber circuit is an electrical circuit designed to suppress voltage spikes. See Figure Typical snubber circuits use metal oxide varistors (MOV), resistors and capacitors (RC), or diodes, depending on the specific type of load being protected.

21 Depending on the PLC application, an enclosure with a cooling unit can be required.
PLCs are typically placed in enclosures (cabinets) that include additional electrical components such as control transformers, fuses, or circuit breakers. An electrical enclosure is a housing that protects wires and equipment and prevents personal injury by accidental contact with energized circuits. An enclosure also provides the main protection for a PLC from atmospheric conditions. Using the proper enclosure prevents problems caused by contamination, moisture, and physical damage. Enclosures are categorized by the protection provided. An enclosure is selected based on the location of the equipment and NEC® requirements. See Figure 1-20.

22 Article 500 of the NEC® classifies hazardous locations according to the properties and quantities of the hazardous material that may be present. The NEC® classifies hazardous locations according to the properties and quantities of the hazardous material that can be present. Hazardous locations are divided into three classes, two divisions, and seven groups. See Figure 1-21.

23 Electrical safety rules aid in the prevention of injuries from electrical energy sources.
Technicians must work safely at all times around electrical systems. Basic safety rules must be followed when working with energized electrical equipment. Electrical safety rules aid in the prevention of injuries from electrical energy sources. See Figure 1‑22.

24 PLC safety begins with a sufficient number of emergency stops and a master control relay that removes power to the inputs and outputs of the PLC and stops all motion of the machine(s) or process. PLC safety begins with a sufficient number of emergency stops and a master control relay that removes power to the inputs and outputs of a PLC and stops all motion of the machine(s) or process. See Figure When working with energized PLCs and functioning PLC systems, proper personal protective equipment must be worn.

25 Personal protective equipment includes items that protect a technician from electrical and other hazards. Per NFPA 70E, only qualified persons shall perform work on or near energized equipment operating at 50 V or more. All personal protective equipment and tools are selected for at least the operating voltage of the equipment or circuits to be worked on or equipment that is near to the place of work. All PPE, tools, and test equipment must be suited for the work to be performed. Personal protective equipment includes protective clothing, head protection, eye protection, ear protection, hand protection, foot protection, back protection, knee protection, and rubber insulated matting. See Figure 1-24.

26 Arc-flash protective clothing made of Nomex®, Basofil®, and/or Kevlar® fibers must be used when working with live high-voltage electrical circuits. Arc-flash protective clothing must be worn when working with energized high-voltage electrical circuits. See Figure Arc-flash protective clothing is made of materials such as Nomex®, Basofil®, and/or Kevlar® fibers. Arc-resistant fibers can also be coated with PVC to increase arc resistance and offer weather resistance. Arc-flash protective clothing must meet the following three requirements: Clothing must not ignite and continue to burn. Clothing must have an insulating value high enough to allow heat to dissipate through clothing and away from the skin. Clothing must provide resistance to the break-open forces generated by the shock wave of the arc.

27 The National Fire Protection Association (NFPA) specifies boundary distances that vary depending on voltage. The NFPA specifies boundary distances where arc protection is required. All personnel working within specified boundary distances require arc-flash protective clothing and equipment. Boundary distances vary depending on the voltage involved. See Figure 1-26.

28 Protective helmets are identified by class of protection
Protective helmets are identified by class of protection. For example, Class E protective helmets protect against high-voltage shock and burns. Head protection requires using a protective helmet. A protective helmet is a hard hat that is used in the workplace to prevent injury from the impact of falling and flying objects, and from electrical shock. Protective helmets resist penetration and absorb impact force. Protective helmet shells are made of durable, lightweight materials. A shock-absorbing lining keeps the shell away from the head to provide ventilation. Protective helmets are identified by class of protection for specific hazardous conditions. See Figure 1-27.

29 Eye protection must be worn to prevent eye or face injuries caused by contact arcing, radiant energy, or flying particles. Eye protection must be worn to prevent eye or face injuries caused by flying particles, contact arcing, and radiant energy. Eye protection must comply with OSHA 29 CFR , Eye and Face Protection. Eye protection standards are specified in ANSI Z87.1, Occupational and Educational Eye and Face Protection. Eye protection includes safety glasses or goggles, face shields, and arc blast hoods. See Figure 1-28.

30 Ear protection is worn to prevent technician hearing loss caused by electrical systems, machinery, power tools, and HVAC equipment. Ear protection is any device worn to limit the noise entering the ear, such as earplugs and earmuffs. See Figure An earplug is an ear protection device made of moldable rubber, foam, or plastic and inserted into the ear canal. An earmuff is an ear protection device worn over the ears. A tight seal around the ear by the earmuff is required for proper protection.

31 Rubber insulating gloves have color-coded labels that represent voltage ratings for specific applications. Hand protection is a set of hand coverings (gloves) that are worn to prevent injuries to hands caused by cuts or electrical shock. The hand protection required is determined by the duration, frequency, and degree of the hazard to the hands. Rubber insulating gloves are gloves made of latex rubber and are used to provide maximum insulation from electrical shock. Rubber insulating gloves are stamped with a working voltage range, such as 500 V to 26,500 V. Leather protector gloves are gloves worn over rubber insulating gloves to prevent penetration of the rubber insulating gloves and provide added protection against electrical shock. Safety procedures for the use of rubber insulating gloves and leather protector gloves must be followed at all times. See Figure 1-30.

32 Rubber insulating gloves must be air tested before each use and when there is cause to suspect damage. Rubber insulating gloves must also be air tested when there is cause to suspect damage. The entire surface of an insulating glove is inspected by rolling the cuff tightly toward the palm in such a manner that air is trapped inside the glove or by using a mechanical inflation device. See Figure When using a mechanical inflation device, care must be taken to avoid overinflation. Puncture detection is made easier by listening for escaping air while holding the glove to the face or ear. Gloves failing an air test must be tagged as unsafe and returned to a supervisor.

33 Insulated rubber-soled shoes are typically worn during electrical work to aid in the prevention of electrical shock. Foot protection is shoes worn to prevent foot injuries typically caused by objects falling less than 4′ and having an average weight of less than 65 lb. Safety shoes with reinforced steel toes protect against injuries caused by compression and impact. Insulated rubber-soled shoes are commonly worn during electrical work to prevent electrical shock. See Figure Protective footwear must comply with ANSI Z41, Personal Protection—Protective Footwear. Thick-soled work shoes are worn for protection against sharp objects such as nails. Insulated rubber-soled boots are used when working in damp locations.

34 Lifting an object with the legs reduces the possibility of a back injury.
One of the most common injuries resulting in lost time in the workplace is the back injury. Back injuries are the result of improper lifting procedures and are prevented through proper planning. Individuals should seek assistance when moving heavy objects. When lifting objects from the ground, the path should be clear of obstacles and free of hazards. See Figure When lifting objects, the knees should be bent and the object should be grasped firmly. The object is lifted by straightening the legs and keeping the back as straight as possible. The load should be kept close to the body and held steady.

35 When carried on the shoulder by one person, objects such as conduit must be transported with the front end down. Long objects such as conduit may not be heavy, but the weight might not be balanced. Long objects should be carried by two or more people whenever possible. When carried on the shoulder by one person, objects such as conduit must be transported with the front end pointing downward to minimize the possibility of injury to others when walking around corners or through doorways. See Figure 1-34.

36 Rubber insulating matting aids in protecting a technician from electrical shock when working on energized electrical circuits. Rubber insulating matting is a floor covering that provides technicians added protection from electrical shock when working on energized electrical circuits. Dielectric black fluted rubber matting is specifically designed for use in front of open cabinets or high-voltage equipment. A rubber mat should be placed in front of a PLC enclosure or cabinet before opening the cabinet to add shock protection when working on the PLC system. Matting is used to protect technicians when voltages are over 50 V. Two types of matting that differ in chemical and physical characteristics are designated as Type I natural rubber and Type II elastomeric compound matting. See Figure 1-35.

37 Lockout/tagout kits contain reusable danger tags, tag ties, multiple lockout hasps, magnetic signs, and information on lockout/tagout procedures. Electrical power must be removed when a PLC or any electrical equipment is inspected, serviced, repaired, or replaced. Power is removed and all equipment must be locked out and tagged out to ensure the safety of personnel working on the equipment. See Figure 1‑36.

38 Lockout devices resist chemicals, cracking, abrasion, and temperature changes and are available in colors to match American National Standards Institute (ANSI) pipe colors. Lockout devices are sized to fit standard industry equipment. The procedure for lockout/tagout per OSHA is as follows: Prepare for machinery shutdown. Shut down machinery or equipment. Isolate machinery or equipment. Lock out and tag out electrical disconnect and other energy supply devices (valves). See Figure 1-37. Release all stored energy (capacitators, pneumatic, and hydraulic). Verify that machinery or equipment is isolated.

39 A proper inspection of a PLC ensures safe control of an electrical system.
An inspection of all parts of a PLC system is required before any power is applied to the system and any time service is performed. Inspection is performed to ensure that each module is in the correct location, securely mounted, correctly wired, and properly programmed and that no burning or discoloration of any conductors or terminals has occurred. See Figure 1-38.

40 Proper grounding is important in PLC applications
Proper grounding is important in PLC applications. Improper grounding can lead to interference (noise) being induced into PLCs, which can cause output devices to be falsely turned ON and put personnel and equipment at risk. Proper grounding is an important safety precaution in any electrical installation. See Figure Proper grounding is especially important in PLC applications because improper grounding can lead to interference being induced into PLCs. Induced interference causes output devices to be falsely turned ON, which can cause personnel injury and/or equipment damage. Refer to equipment manufacturer grounding recommendations. To prevent problems in a PLC system, the grounding path must be permanent, continuous and uninterrupted, of minimum resistance, and of sufficient size to carry any potential fault current.


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