Changes in OSHA requirements have made these safe practices law.
Most nonelectricians can perform the simple task of connecting and disconnecting an electric motor safely. However, certain standards must be maintained to ensure that proper connections are made and the act of connecting the motor is performed in the safest manner possible.
Most motor circuits consist of a combination of starting mechanisms, both automatic and manual. The simple motor control circuit shown in Figure 1 is typical of most motor circuits.
The heavier lines represent the power circuit, which provides line power to the motor. Line voltages are usually 240 or 480 V. The thinner lines represent the control circuit used in a magnetic style starter. The control circuit is used to direct power to a magnetic contactor through the stop/start station, thermal overload relay contacts (normally closed contacts), and holding contacts (labeled 3 and 2).
Control circuits can be connected directly to the line leads (as shown in the diagram), or can be isolated from the power circuit with a transformer. Lower voltages, such as 120 or 240 V, are used if a transformer is employed. The coil on the motor starter will indicate the control voltage in use.
Power is supplied from a three-phase source on the line leads, L1, L2, and L3. Protection against short circuit is required in every motor circuit. This protection, also called “branch circuit protection,” is provided by a circuit breaker or fused disconnect. The circuit breaker contacts and the disconnecting device for the fuses are ganged together mechanically (represented by a dotted line), but are electrically isolated. This allows all three phases to open and close at the same time. Any device used to permanently isolate line power from the circuit is also referred to as the disconnecting means.
The motor contactor is represented by three sets of paralleled lines. The contactor is closed when power is supplied to the contactor coil, which magnetizes an iron core and pulls the three sets of contacts closed. All three contacts are ganged together mechanically (but not electrically), so they close at the same time.
The thermal overload heater elements sense the current flowing to the motor. If the motor is in an overload condition, the normally closed overload contacts will open, stopping current flow in the control circuit. The contactor coil will demagnetize and the motor contactor will open, stopping the motor.
The “T” leads, T1, T2, and T3, are connected to the load side of the thermal overload heater elements. The other ends are connected to the motor.
A combination starter describes the arrangement where the motor contactor, thermal overload device, short-circuit protection, and a disconnecting means are all mounted in the same enclosure. Most modern industrial facilities use motor control centers, which contain a number of combination starters.
A voltage detection device is used to assess the status of exposed electrical components. Using two forms of voltage detection is suggested to check circuit status when work is about to be performed on energized or deenergized equipment. Most voltage testers commonly available for electrical work are designed for 600 V or less. Never use these instruments on higher-voltage circuits.
A voltage sensor, such as a tic tracer or ESP (electric sensing probe), detects an electrostatic field around ac circuits. They are often used as a first check of an electric circuit to assess hazards prior to using other voltage testers.
Voltage sensors cannot be used to detect dc voltage. They also cannot detect voltage through metal enclosures, shielded cables, or metal connector jackets. A voltage will not be sensed if the sensor is held on the ground side of a cable.
Voltmeters are used to determine the level of voltage on energized parts. Direct-indicating digital multimeters are preferred for this check.
Whenever voltage tests are performed, the person making the measurement must come close to energized conductors. Using one hand at a time when making measurements while watching your footing can limit the effects of potential electric shock.
Checks with one hand are the common method used with a voltage sensor. However, many voltmeters are designed for two-hand use. An alligator clip attached to the end of one probe can be used to get around this limitation. The lead can be clipped onto one part of a circuit, such as the ground, and the voltage tests can be performed with the other probe.
The following steps should be followed to check out a motor circuit:
1. Open the disconnecting means for the motor.
2. Using a voltage sensor, check the circuit for power. The voltage sensor should first be checked for operation by placing it near the line leads of the disconnecting means. If a voltage is sensed, the sensor is working properly. If no voltage is sensed, either no voltage is supplied to the line side of the circuit, or the sensor isn’t working. Another known voltage source should be checked with the sensor to determine if the sensor is faulty.
3. If the voltage sensor is working properly and a voltage is sensed on the line leads, check for voltage on all three connections on the line side of the motor starter. If a voltage is sensed, power must be reaching the starter. A faulty disconnect or circuit breaker, or improperly connected control wiring can let power leak to the starter. Isolate all sources of power to the starter cubicle and retest the circuit.
4. Check for voltage on all three T leads leading to the motor from the starter cubicle. If a voltage is sensed, power must be reaching the T leads. Faulty starter contacts or improperly connected control wiring can let power leak to the T lead connections. Isolate all sources of power to the starter cubicle and retest the circuit.
5. If no voltage is sensed on the motor T leads, recheck the sensor on a known power source to be sure it is still working properly.
6. Using a voltmeter set for ac volts, connect one lead to ground and the other lead to L1 of the line side of the disconnecting means. Repeat this step for L2 and L3. A voltage should be detected on all three phases. If no voltage is detected, either the voltmeter is faulty or no power is present on the line side of the disconnect. Another known voltage source should be checked to determine if the voltmeter is faulty.
7. If the voltmeter is working properly and voltage is measured on the line leads, check for voltage on all three connections to the line side of the motor starter from phase to phase and from phase to ground. If a voltage is measured, power must be reaching the starter. A faulty disconnect or circuit breaker or improperly connected control wiring can let power leak to the starter. Isolate all sources of power to the starter cubicle and retest the circuit.
8. If no voltage is measured on the motor T leads, recheck the voltmeter on a known power source to be sure it is still working properly.
9. Check the voltage to ground on all three T leads leading to the motor from the starter cubicle from phase to phase and from phase to ground. If a voltage is measured, power must be reaching the T leads. Faulty starter contacts or improperly connected control wiring can let power leak to the T lead connections. Isolate all sources of power to the starter cubicle and retest the circuit.
10. Switch the voltmeter to dc volts and repeat steps 7 and 8. External dc sources can leak through a control circuitry or from a power factor correction capacitor.
11. Close the door to the disconnecting means and lock the switch in the open position.
12. Open the motor conduit box. Using a voltage sensor, check for the presence of voltage at the motor. If no voltage is sensed, the motor can be safely disconnected.
The lockout and tagout rules discussed in 1910.147 do not cover exposure to electrical hazards, although some of the lockout activities described are related to electrical energy. This requirement is mostly geared toward mechanical or fluid discharge hazards.
A lockout and tagout procedure that complies with 1910.147 can apply to electrical work as well if it includes the requirements spelled out in Subpart S — Electrical, 1910.333 (b)(2). A typical job covered by this procedure would be the disconnecting and reconnecting of an electric motor.
It is not sufficient to just disconnect and lockout a motor starter. The circuit must be checked with voltage testers before electrical work can proceed.
Assuming that a facility’s lockout and tagout procedure satisfies the requirements of 1910.147, the following steps should be amended to the procedure for electrical work on deenergized parts. The part of a lockout procedure that pertains to work on or near exposed deenergized electrical parts should have the following as a minimum:
1. The safest method for deenergizing circuits must be determined before circuits or equipment are deenergized.
2. Conductors and parts of electrical equipment that have been deenergized but have not been locked out or tagged must be treated as energized until proven otherwise. While any employee is exposed to contact with parts of electrical equipment or circuits which have been deenergized, the circuits energizing the parts must be locked out and tagged.
3. A “positive disconnecting means,” such as a fused disconnect switch or circuit breaker, should be used to disconnect circuits and equipment from all electric energy sources.
4. Unacceptable means of disconnect include control circuit devices such as pushbuttons, selector switches, and interlocks.
5. Stored electrical energy which might endanger personnel must be released before work can begin. Capacitors are the most common devices that can contain stored electrical energy.
Power factor correction capacitors are sometimes used in a motor circuit. These capacitors usually have a resistor that discharges the capacitor quickly. Ac voltage testers cannot detect the remaining charge on a capacitor. A dc voltage check is required.
High-capacitance elements, such as very long, shielded cables, must be short-circuited and grounded. A capacitor can be discharged by connecting all the phases together and to ground. A suitable grounding device must be used.
6. Block or relieve stored nonelectrical energy in devices that could reenergize electric circuit parts or that can injure an employee in any other way.
7. A lock and tag must be placed on each disconnecting means used to deenergize circuits and equipment on which work is to be performed. The lock must be attached so as to prevent persons from operating the disconnecting means unless they resort to undue force or the use of tools.
Each tag must contain a statement prohibiting unauthorized operation of the disconnecting means and removal of the tag.
8. A person qualified in the operation of the equipment, such as a plant operator, must attempt to start the equipment once it has been locked out and tagged to verify that the equipment cannot be restarted.
9. A person qualified in electrical testing methods must test the circuit elements and electrical parts of the equipment to which employees will be exposed. The circuit elements and equipment parts must be verified as deenergized.
Two forms of testing must be used:
The test equipment must be checked for proper operation immediately before and immediately after this test. (This is an OSHA requirement for above 600 V only, but is a good practice at any voltage.) This is best accomplished by testing the device on a known energized source, such as the line side of the breaker or disconnect.
Any sensed voltage may feed back from a source on the load side of the circuit. Faulty insulation in a disconnect can allow current to “leak” through to the load side. If the current leaks through the disconnect to generate a significant voltage on the load side, the main disconnect that feeds the system must be turned off and locked out. The faulty disconnect must be repaired before other work can be performed.
Load-carrying conductors can be grounded. If the protective ground connection is inadvertently energized, all current will flow through the path of least resistance — the ground conductor. A circuit breaker may trip or a fuse may blow if this occurs, but personnel are protected from the effects of the voltage.
Fuses should be removed as an extra measure if there is any remaining question about the positive disconnecting means.
The following requirements must be met in the order given before circuits or equipment are reenergized, even temporarily.
10. A qualified person must conduct tests and visual inspections, as necessary, to verify that all tools, electrical jumpers, shorts, grounds, and other such devices have been removed, so that the circuits and equipment can be safely energized.
11. Employees exposed to the hazards associated with reenergizing the circuit or equipment must be warned to stay clear of circuits and equipment.
12. Each lock and tag must be removed by the employee who applied it or by someone else under that employee’s direct supervision.
13. A visual determination that all employees are clear of the circuits and equipment must be made.
Wire metals: The metal of the conductor may be listed on the cable insulation. The abbreviations for the metals are:
AL — Aluminum;
CU — Copper; and
CU-AL — Aluminum, copper clad.
Copper wire is the most common used in industry. Aluminum wire was popular for a short while in the 1960s, but numerous application problems made it fall from favor.
Aluminum wire is making a comeback lately and may be found as large branch feeders in industrial systems. Aluminum wire is not as conductive as copper, so larger-diameter cable is required to provide the same current-carrying capacity. Aluminum wire with the same resistance as copper is about 28% thicker than the copper wire.
The larger cable diameter usually means a larger-diameter conduit will be required to handle the aluminum cable. This adds to the cost of an installation.
Another problem with aluminum wire occurs when terminations are made. Aluminum begins to oxidize as soon as it is exposed to air. The oxide is like an insulator. This oxide must be removed before a connection is made. Copper, on the other hand, oxidizes over a long period of time, so exposed cable will most likely not build up an insulated film before use.
Insulation color: Electrical insulation color codes have changed through the years which makes matching old wiring with new difficult. The new standard for cable colors is shown below.
Hot wires — Black, red, blue, yellow, or any color other than white, gray, or green;
Neutral wires — White (also referred to as the grounded wire);
Grounding wires — Green insulation, bare copper, or bare aluminum.
A distinction is made between intentionally grounded and grounding wires. A grounded wire will carry current during normal operation. A grounding wire will not carry current under normal conditions. The grounding wire is connected to ground which is a non-current carrying metal part of the system. The neutral wire in a 120/240-V, three-wire circuit found in most homes is grounded at the service entrance. For this reason it is called the grounded wire in the circuit. This wire is usually white.
Terminal color codes: The following color codes apply to the terminating point of wires.
Copper or brass — for hot wires;
Nickel, tin, or zinc-plated — for grounded wires (white only); and
Green terminals — for grounding wires (green only) or bare.
The wire nut can connect two or more conductors just by screwing it over the bare wire ends. The set screw type requires that the wires be placed in the nut and then are locked together with a set screw. The cap is then screwed on to the nut.
Solderless pressure connectors (Figure 3, page 106) are used when larger connections are made. The split bolt and set screw type are common.
The split bolt type provides the best electrical connection of all the solderless type. Both pressure connectors must be covered with electrical tape to insulate them once the connection is made.
Final terminations or splices can be made with solderless lug connectors (Figure 4, page 107). The open-ring, compression type is also called by its trade name, Staycon.
All of the above electrical connectors and terminating devices are used for motor connection. Wire nuts are not suggested for motor applications over 0.25 hp. Wire nuts provide minimal metal contact and create a higher resistance to current flow. Setscrew type, split-bolt, pressure washers, and ring type solderless lugs are acceptable.
All bare connections must be taped with a thick layer of electrical tape. It is also suggested that even insulated connections should be covered with a layer or two of tape to keep out moisture.
The tape covering the existing connection should be removed. A knife works best for this, but caution is advised not to nick or strip the wire insulation. Also, cuts on hand from knifes are common when disconnecting motors. Therefore, watch yourself.
Disconnect the electrical connections. All matching wires should be marked if the motor will not be replaced and will only be temporarily disconnected. Commercially available wire markers are the preferred method of marking wires. If the motor will mot be reconnected within the same day, the bare wires for the power source should be insulated with electrical tape.
Reconnect the motor connections the same way they were connected. Match or improve the wire size if the cable is replaced. Be sure the connections are tight and try to avoid breaking or cutting wire strands as the wire is tightened into the connector.
Tape all connections with a thick layer of insulating tape. The tape should be slightly stretched as it is wrapped around the connection. This tension will limit air gaps and points for entrance of moisture. Be sure to cover all bare connections. The tape cover should also be extended down each wire to prevent moisture from entering the connection.
Push the connections back into the conduit box and reinstall the cover. Be sure to replace the gasket on the cover to limit entrance of moisture. (Explosion-proof motors intentionally do not have gaskets and no gaskets should be added.)
The phase sequence requirements of the motor is usually unknown as well. It is often necessary to change the direction of a motor after it is connected. This is easily accomplished by reversing any two of the power supply leads. The best place to make this reversal is at the motor starter, rather than breaking the connections made in the conduit box.
Checking the direction of rotation of a motor can be accomplished by starting and quickly stopping the motor. This is euphemistically called “bumping” the motor. If the direction is correct, no change is required. If the direction is incorrect, the motor disconnect must be opened again, the circuit must be tested to be sure it’s dead again, and any two leads must be reversed.
Some equipment cannot be run in reverse under any circumstance. When this is the case, either the direction of the motor should be checked before coupling the motor or the proper connections must be ascertained without bumping the motor. Phase and direction instruments are commercially to aid in the latter option. These instruments connect up to the motor leads and the power leads. The shaft is turned by hand in the proper direction and the instrument indicates if the direction is proper.
Brown is a principal at New Standard Institute Inc., Milford, CT. In addition to maintenance consulting services, the company offers training courses covering technical skills and maintenance management. For more information, call the author at 203-783-1582, or visit www.newstandard institute.com.
Sidebar: Safe Electrical Work Practices The following work practices are required as a minimum when working around energized parts.
Alertness (NFPA 70E): Be alert at all times when working near exposed energized parts. Ill, fatigued, or otherwise impaired employees should not stay on the job. Stop all work if you are distracted by unrelated activity.
Illumination: Do not enter any area of exposed energized parts unless adequate illumination is provided to work safely. If adequate lighting is not available, an employee must not disconnect power lines or cords and must not actuate any breakers or switches. An employee may not perform tasks near exposed energized parts if illumination or an obstruction prevents observation of the work to be performed. Employees may not reach blindly into areas which may contain energized parts.
Conductive apparel: Keep conductive apparel away from exposed energized parts. Conductive apparel includes jewelry, watch bands, bracelets, rings, key chains, necklaces, metalized aprons, cloth with conductive thread, or metal headgear. Wear suitable insulating gloves if the conductive articles can not be removed.
Insulated tools and equipment (NFPA 70E): The use of insulated tools is required when working with energized or exposed parts. The voltage present must not exceed the rated voltage of the tool under use.
Do not loosen or tighten screwed or bolted connections that still have electrical current flowing through them. Some employees have wrongly felt that any energized conductor can be removed with insulated tools or equipment. This is only true if no electrical current can flow through that equipment. A dangerous flashing arc will be drawn as the connection comes apart. This arc will damage the connection and create a hazard from vaporized metal.
PROTECTIVE SHIELDS An employee should take additional precautions when there is a possibility that they may come in contact with exposed energized parts.
Protective shields, protective barriers, or insulating materials must be used to protect each employee from shock burns, or other electrically related injuries while that employee is working near exposed energized parts. This must be done whether they are energized or not.
When normally enclosed live parts are exposed for maintenance or repair, they must be guarded to protect unqualified persons from contact with the live parts.
Interlocks: Only a qualified person can defeat an electrical safety interlock. This work must be performed in accordance with the preceding safe work practices. Disconnecting interlocks is only allowed temporarily while an employee is working on the equipment. The interlock system must be returned to its operable condition when this work is completed.
OTHER PRACTICS TO ENSURE SAFETY Never open or perform any voltage tests on energized equipment located in areas classified as hazardous. Areas of a facility may be have been predetermined as hazardous due to the presence of flammable gases and vapors, ignitable dusts, or combustible fibers.
These gloves should be used with leather protectors to protect the gloves from mechanical damage. The gloves should be inspected for punctures, tears, or abrasions prior to every use. Independent electrical glove testing services are available to verify glove voltage ratings.
The actual area of a circle with one mil diameter is 0.000,000,785 sq in. The area in circular mils is simply 1 CM. If a wire has a diameter of 5 mils, it is said to have an area of 25 CM. This is simply 5 mils squared.
Smaller-diameter wires are assigned size numbers determined by the American Wire Gauge (AWG), depending on the circular mils of the wire. The AWG system should not be confused with the steel wire numbering system which as signs the same number as the AWG standard to a larger diameter steel wire. The smaller the AWG number, the larger the wire. Sizes 1/0, 2/0, 3/0, and 4/0 are pronounced one-naught, two-naught, etc. These sizes can also be written 0, 00, 000, and 0000, respectively. Sizes over 4/0 are list by MCM, which stands for thousand circular mils.
Multiple strand wire, called stranded wire, is used when a more flexible cord is required. Stranded groups of 3, 7, 12, 19, 37, 61, 91, 127, and 169 are commercially available.
Stranded wire is available in all sizes listed but is more common for #8 AWG sizes and larger. The diameter of stranded wire is slightly larger than the solid wire diameters. However, the number of circular mils is the same for the same size stranded or solid wire. For example, #8 AWG solid wire has a diameter of 128.5 mils (0.1285 in.) and an area of 16,510 CM. A seven strand #8 AWG wire has a diameter of 0.146 in. Each of the seven strands has a diameter of 48.6 mils, which is an area of 2359 CM (48.6 squared). Seven times 2359 equals 16,510 CM, which is the same as the solid wire.
Size 14 AWG and higher are used for industrial equipment wiring. Smaller sizes are used for some instrument and electronic circuits.
Whether an employee is considered to be a “qualified person” or not will depend upon various circumstances in the workplace. It is possible and, in fact, likely for an individual to be considered “qualified” with regard to certain equipment in the workplace, but unqualified as to other equipment. For example, an employee can be qualified to work on low-voltage motor circuits but not on higher-voltage circuits.
An employee who is undergoing on-the-job training and who, in the course of such training, has demonstrated the ability to perform duties safely at his or her level of training and who is under the direct supervision of a qualified person is considered to be a qualified person for the performance of those duties.
Publication date: 01/29/2001