Electrical Switching Practices and Precautions
On-Site Circuit Breaker Maintenance Safety Checklists Low voltage (600 V and below) checklist:
• Telephone channels must be made available to summon emergency personnel when needed. The telephone must be close to the work site and functional throughout the period during which the work is to be done.
• The light level in the work area must be sufficient to perform the work safely. (M-G sets, high-powered self-contained lighting systems, and/ or emergency feeders will be used/supplied if the electrical shutdown is complete.)
• No employee should work on-site for more than 12h, and the work period should be preceded and followed by a minimum of 8h off (rest).
• The qualified craftsman should provide technical direction on-site.
• No one shall work alone.
• General workers shall not energize equipment or systems. These activities are to be performed by an assigned qualified person.
• Damp or recently flooded areas shall be worked completely de-energized. Control power also shall be de-energized.
• All stationary (bolted-in) and plug-in type circuit breakers (nondraw out) shall only be worked on when both the line and load sources are de-energized.
• When primary power circuits are energized, no conducting materials, including hardware or tools, shall be inserted in the cubicle.
Examination of equipment/breakers:
Prior to working on equipment/breakers, the following precautions must be observed:
• On solenoid operating mechanisms, trip the breaker “open.” On stored energy mechanisms, trip the breaker “open” and completely discharge “stored energy springs.” (See the appropriate breaker instruction book and determine the exact procedure.)
• Check for proper operation of mechanical or electrical interlocks. (All low-voltage draw out power circuit breakers have either mechanical or electrical interlocks to protect both personnel and equipment while the breaker is being inserted or withdrawn from its cubicle.) Always check these devices to confirm their proper operation. In all cases, consult the manufacturer’s instruction to obtain interlock adjustment data (dimensions and tolerances).
• Check for defeated or bypassed interlocks. (This condition enables the circuit breaker to be withdrawn or inserted in a closed position. This is an extremely hazardous condition.) If defeated interlocks are found, the following steps are to be performed:
Reactivate the interlock (or remove the bypass) and test to verify proper performance.
If equipment or materials are not available to make the repair, notify the appropriate person and do not reinsert the circuit breaker. The responsible person informed of the risk should decide whether to have his men reinsert a breaker with defective interlocks.
To minimize personnel and equipment exposure, de-energize the equipment (including control power). If this is not possible and the responsible person plans to reinsert the breaker, all other personnel should remove themselves from the immediate area.
• Place the keys to interlock on the equipment being worked in the possession of the qualified craftsman or foreman, providing technical and safety direction for the on-site job.
• Check carefully that the spring-loaded contacts of the primary disconnect assemblies on the low-voltage draw out breakers are mounted properly, that the hardware is tight, and none are missing. Also check to determine that springs are in good condition and exert the proper pressure to insure good contact.
• Check the hinge pins and spring clips on the primary disconnect assemblies. (The primary disconnect assemblies on some break- ers may employ hinge pins mounted horizontally and passing through each disconnect cluster. The pins are retained on both ends by spring (or cee) clips. There is one pin per cluster and a total of six clusters—three line and three load. The length of these pins is sufficient to bridge the spacing between the phases, and the pin will still be retained in its original primary cluster. If not properly clipped, these pins will travel with vibration or other external forces. If the pin movement is extreme, a phase-to-phase fault can result.)
• Check each low-voltage breaker with a 1000 V megohm meter phase- to-phase-to-ground to assure adequate dielectric resistance between phases and to ground. These tests will prevent reapplying a breaker which could cause a serious flashover due to the effects of aging and environment.
• Prior to operation of circuit breaker mechanisms, remove all tools, parts, and equipment from the breaker proper. All personnel are to stand clear of the breaker while it is being energized by a qualified person.
Racking in precautions:
• First insure that the breaker is open.
• Inspect the cubicle for foreign objects (such as tools, rags, hipot wire or loose hardware, etc.). Adequate lighting is necessary to thoroughly inspect the cubicle.
• Exercise care when cleaning and inspecting cubicles. Use only insulated nonconducting tools (brushes, vacuum hoses, screwdrivers, etc.) to clean or adjust elements within the cubicle. Handles or grips are to be sufficiently long to avoid the necessity of major extensions of the arm into the cubicle. Long sleeves and rubber gloves/gauntlets are mandatory for all interior cubicle adjustments when the system is energized. Technicians are to wear long sleeved shirts and remove all jewelry such as watches and rings. If the stationary line side or load side stabs or bars require maintenance which involves other than vacuuming, the system is to be de-energized.
• Check to determine that the control circuits (24 to 250 V DC, 120 to 550 V AC) are de-energized. Pulling the fuses or disconnect plug on control circuits will ensure that these circuits are de-energized because they are not necessarily de-energized by opening the circuit breaker.
• Inspect the circuit breaker on the lift table or overhead crane just prior to insertion. This is to insure that all parts are tight and in their proper positions. It is also intended to insure that all foreign materials (such as rags, tools, hipot wire or loose hardware) are removed.
• Perform a 1000 V megohm meter test with the circuit breaker in the open position (the last step prior to racking in). Perform megohm meter test phase-to-phase and phase-to-ground on all the circuit breaker primary disconnects.
• The steps above shall be verified by the qualified craftsman. On those competitive units using cee clip retainers, a count is to be made to insure that all clips required are in position and all hardware is prop- erly mounted.
• Before racking in (or racking out) the circuit breaker, communicate audibly your intention to the other members of the work crew.
• Be certain to wear the required protective equipment (PPE) and position yourself to either side of the cubicle.
Medium–voltage (601 through 15,000 V) checklist:
• Telephone channels must be made available to summon emergency per- sonnel when needed. The telephone must be close to the work site and functional throughout the period during which the work is to be done.
• The light level in the work area must be sufficient to perform the work safely. (M-G sets, high-powered self-contained lighting systems and/or emergency feeders will be used/supplied if the electrical shut down is complete.)
• No employee should work on-site for more than 12 h, and the work period should be preceded and followed by a minimum of 8 h off (rest).
• The qualified craftsman should provide technical direction on-site.
• No one shall work alone.
• General workers shall not de-energize and/or energize equipment or systems. These activities are to be performed by an assigned qualified person.
• Damp or recently flooded areas shall be worked completely de- energized. Control power also shall be de-energized.
• All stationary (bolted-in) and plug-in type circuit breakers (nondraw out) shall only be worked on when both the primary and secondary (control power) sources are de-energized.
• When primary power circuits are energized, no conducting materials, including hardware and tools shall be inserted into the cubicle.
Examination of equipment breakers:
• Prior to working on equipment breakers, the following precautions must be observed:
On solenoid operating mechanisms, trip the breaker “open.”
On stored energy mechanisms, trip the breaker “open” and com- pletely discharge “stored energy springs.” (See the appropriate breaker instruction book and determine the exact procedure.)
• Check for proper operation of mechanical or electrical interlocks. (All medium-voltage draw out power circuit breakers have either mechanical or electrical interlocks to protect both personnel and equipment while the breaker is being inserted or withdrawn from its cubicle.) Always check these devices to confirm their proper operation. In all cases, consult the manufacturer’s instruction to obtain interlock adjustment data (dimensions and tolerances).
• Check for defeated or bypassed interlocks. (This condition enables the circuit breaker to be withdrawn or inserted in a closed position. This is an extremely hazardous condition.) When defeated interlocks are found, the following steps are to be performed:
Reactivate the interlock (or remove the bypass) and test to verify proper performance.
If equipment or materials are not available to make the repair, notify the appropriate person and do not reinsert the circuit breaker.
The responsible person informed of the risk should decide whether to have his men reinsert a breaker with defective interlocks.
To minimize personnel and equipment exposure, the equipment (including control power) should be de-energized. If this is not pos- sible and the responsible person plans to reinsert the breaker all other personnel shall remove themselves from the immediate area.
• Place the keys to interlock on the equipment being worked on in the possession of the qualified craftsman or foreman, providing technical and safety direction for the on-site job.
• Examine the condition of the ball-type contacts on the medium draw out breakers. This examination is to be performed with the breaker removed from the cubicle. Spring-loaded clusters from the mating contact to the primary bus. These clusters are protected by a sliding safety shutter that moves (open or closed) with the breaker elevating mechanism, which is part of the cubicle construction. The clusters must not be exposed by sliding open the shutter when the breaker is not in the cubicle or until the cubicle is completely de- energized, tested, and grounded.
• Check the hinge pins and spring clips on the primary disconnect assemblies. (Some breakers may employ hinge pins mounted horizon- tally and passing through each primary disconnect cluster. The pins are retained on both ends by spring (or cee) clips. If not properly clipped, these pins will travel with vibration or other external forces. If the pin movement is extreme, a phase-to-phase fault can result.)
• Check each medium-voltage breaker with a 2500 V or higher voltage megohm meter phase-to-phase-to-ground to assure adequate dielec- tric resistance between phases and to ground. These tests will prevent reapplying a breaker which could cause a serious flashover due to the effects of aging and environment.
• Prior to operation of circuit breaker mechanisms, remove all tools, parts and equipment from the breaker proper. All personnel are to be away from the breaker and to keep hands off the breaker.
Racking in precautions:
• First insure that the breaker is open.
• Inspect the cubicle for foreign objects (such as tools, rags, hipot wire or loose hardware, etc.). Adequate lighting is necessary to thoroughly inspect the cubicle.
• Exercise care when cleaning and inspecting cubicles. Cubicle heaters, powered from a CPT source, are a potential problem. When cleaning, the cubicle heaters should be turned off. Care must be exercised not to damage the heaters or their wiring. The stationary secondary coupler (control power connections) are mounted vertically and are recessed.
Care must be exercised when working close to the bottom of the cou- pler since potentially dangerous voltages could exist on several of the contact points.
• Check that the control circuits are de-energized (24 to 250 V DC, 120 to 550 V AC). Pulling the fuses or disconnect plug on control circuits will ensure that these circuits are de-energized because they are not necessarily de-energized by opening the circuit breaker.
• Inspect the circuit breaker just prior to inspection. This is to insure that all parts are tight and in their proper positions. It is also intended to insure that all foreign materials (such as rags, tools, hipot wire, or loose hardware) are removed.
• Perform a 2500 V or a higher voltage megohm meter test with the circuit breaker in the open position (the last step prior to racking in). Megohm meter phase-to-phase and phase-to-ground on all the circuit breaker primary disconnects.
• The steps above shall be verified by the qualified craftsman. On competitive units using cee clip retainers, a count is to be made to insure that all clips required are in position and all hardware is properly mounted.
• Before racking in or racking out the circuit breaker, communicate audibly your intention to the other members of the work crew.
• Close the cubicle door prior to closing the circuit breaker.
Confined Spaces—Procedure for Entering
General: A confined space is an enclosed structure or space with restricted means of entry (such as a manhole, transformer vault, transformer tank, elevator pits, motor basements, etc.). The confined space is so enclosed and of such volume that natural ventilation through openings provided does not prevent the accumulation of dangerous air contaminants nor supply suffi- cient oxygen to protect the life, health, and safety of any person occupying such structure or space.
General workers are not to enter confined spaces (see definition above) where dangerous air contaminants have been present, are present, or could be introduced from potential sources. Workers may enter these confined spaces only after the atmosphere has been tested and found free of dangerous air contaminants.
Any such confined space shall be continuously maintained free of danger- ous air contaminants by mechanical ventilation or equivalent means during any period of occupancy. If, however, due to emergency conditions, any such confined space cannot be cleared of dangerous air contaminants by mechan- ical ventilation or equivalent means, any person entering such confined
space shall be provided with and shall use an approved air line respirator, or approved self-contained breathing apparatus.
Dangerous contaminants that may be found in confined spaces may be grouped as follows:
• Fuel gases (e.g., manufactured gas, natural gas, or liquefied petro- leum gases)
• Vapors of liquid fuels and solvents (e.g., gasoline, kerosene, naphtha, benzene, and other hydrocarbons)
• Products of combustion (e.g., carbon monoxide-engine exhaust or carbon dioxide)
• Nitrogen and/or carbon dioxide used for testing or burning gases and volatile substances within industrial drainage
• Gases from fermentation of organic matter (e.g., hydrogen, hydrogen sulfide, methane, carbon dioxide, and mixtures deficient in oxygen)
• Gases generated by the customers’ processes
The hazards of explosion, fire, and asphyxiation may all be encountered in the preceding contaminants because mixtures of these classes of contaminants are not uncommon.
Preparing to enter a confined space: All confined spaces shall be considered hazardous until proven safe by tests. General workers shall not enter a confined space, even momentarily, until it has been tested for oxygen and combustible gas content and then power ventilated for a minimum of 5 min or four complete air volume changes, whichever is the greater.
Smoking, or any device which produces a spark, shall not be allowed in a confined space. In addition, smoking is not permitted within 10 ft of an open confined space.
Every employee that is to enter a confined work area should be properly trained in the procedures for detecting hazardous conditions and must be provided with the proper equipment to make this determination. Before a confined space is entered, the foreman/qualified employee must also review with the general workers the work to be performed and the hazards that may be encountered.
Testing a confined space: Every confined space that has been closed for any period of time should be tested to determine if sufficient life-supporting oxygen is present and if combustible gases are present. In addition, any instruments that are used to sample a confined space environment must first be tested for proper working operation before they are used. Periodic calibration of the test instrumentation as recommended by the manufac- turer of the instrument is a mandatory requirement, and such calibrations must utilize the type of equipment suitable for the air contaminants involved:
1. If doors or covers contain vents, the preentry test is made with the doors or covers in place in order to test conditions of confined space before it has been disturbed. If the cover or door is unvented, it is opened enough only to admit the test hose or the instrument to be inserted.
2. When a test indicates hazardous gases, the cover or door must be very carefully opened or removed in order not to create sparks. If the test indicates that air contaminants are in excess of safe concentrations or that explosive hazards are present in the confined space, the space must be purged by forced ventilation until another test indicates that the air contaminant concentration is safe (see ventilation).
3. If initial tests indicate that the atmosphere is safe, the confined space must be force ventilated for a minimum of 5 min, using a blower of 500 cubic feet per minute (CFM) or more, or four complete air volume changes before it is entered. The blower must be operated during the entire time period when an employee is occupying the confined space.
4. After the confined space is entered and the blower hose is positioned, initial testing for gas is accomplished by sampling in the areas of possible gas entrance and then generally throughout the confined space. If the test results are satisfactory, the work may be performed.
5. If an unsatisfactory atmosphere is found as the result of the preceding test, employees must immediately leave the confined space. The blower must be operated for 10 additional minutes and then a second test is performed. The second test must be performed away from the direct output of the blower.
6. If the second test indicates that the atmosphere is safe, the confined space may be entered. The blower shall be operated during the entire time period that personnel are occupying the confined space. Again, a test is made for gas by sampling or testing in the area of possible gas entrance and then generally throughout the confined space.
7. If the confined space is still contaminated and cannot be cleared, after venting and retesting, the cause must be determined. If the area where the contaminant is entering can be plugged, sealed, or capped to render it safe, these procedures shall be performed by personnel wearing an approved air line respirator or approved self- contained breathing apparatus. The area is then to be retested, continuously vented, and monitored.
Ventilation procedures: Confined spaces containing air contaminants and/or explosion hazards must be purged by mechanical ventilation until tests indicate that the concentration of air contaminants in the confined space is not more than 10% of the lower explosive level of such air contaminants, and that there is sufficient oxygen to support life available in the confined space.
Personnel performing the ventilation procedure must be familiar with the operating instructions for the particular equipment being used, and must also perform the following general procedures:
• Place the blower so it will not be subject to damage, obstruct traffic, or present a hazard to pedestrians.
• On sloping surfaces, avoid placing the blower on the upgrade side of the confined space opening. If it is necessary to place the blower on the upgrade side of a manhole or vault, block the unit so that vibration will not cause it to move toward the manhole opening.
• Do not operate or store blower in a confined space.
• Always remove the blower hose from a confined space before the blower is turned off.
• Place blower on a firm level base at least 10 ft from a manhole or vault opening and in accordance with above.
• Attach the blower hose to the air outlet of the blower by slipping the end of the hose, which is equipped with a strap-type clamp, over the air outlet and then pull the strap tight to hold the hose in place.
• Connect the power cord to the power source to start the blower. (Only grounded electrical equipment shall be used.)
• Let the blower run for 1 min with the hose out of the confined space.
Check the end of the hose to see that the hose is securely attached to the air outlet.
• Place the blower hose in the confined space and adjust the position of the blower so the hose will run directly into the confined space without unnecessary bends. The optimum position of the output end of the blower hose is with the hose opening directed toward an end wall.
• If the ventilating blower stops, leave the confined space immediately.
Remove the hose from the confined space. Do not replace the hose in the confined space until the blower is operating. When the blower is again operating, purge the hose and test the atmosphere before replacing the hose in confined space.
Equipment necessary for entering a confined space: Any person entering a confined space should be provided with and should use the following additional safety equipment:
• Either an approved life belt, approved safety harness, approved wrist straps, or approved noose-type wristlets should be worn.
• A lifeline should be attached to such life belt, approved safely harness, approved wrist straps or approved noose-type wristlets with the other end securely anchored outside the confined space.
• A safe means of entering and leaving the confined space (such as a portable ladder) should be provided. Such means must not obstruct the access opening.
• An explosion-proof battery-operated portable light in good working order.
• Nonsparking striking, chipping, hammering, or cut tools and equipment where the confined space may contain explosive or flammable air contaminants.
Safety monitors: If the confined space is found to be contaminated, a person designated as a safety monitor should be stationed at the access opening of any confined space while such space is occupied for any reason. The safety monitor is responsible for performing the following:
• Maintaining visual contact with every person in the confined space where the construction of the space permits
• Having continuous knowledge of the activities and well-being of every person in the confined space either through verbal communi- cation or other positive means at all times
• Assisting a person in a confined space with such tasks as handling tools or supplies or removing containers of refuse or debris, provided that these tasks do not interfere with his primary duty as a safety monitor
The safety monitor selected should have the following characteristics:
• Be an alert, competent person, and fully capable of quickly sum- moning assistance for the administration of emergency first aid when required
• Be physically able to assist in the removal of a person from a confined space under emergency conditions The following should be available to the safety monitor or rescue personnel for use if required.
• Approved air line respirator, approved hose mask, or approved self-contained breathing apparatus
• Explosion-proof battery-operated portable light in good working order The emergency equipment should be located at the access opening of the confined space or not more than 15 ft. from such opening. In the case of a manhole or in-the-ground enclosure, a universal tripod should be set up before the confined space is entered.
Emergency conditions: The safety monitor should not enter a confined space until he or she is relieved at his or her post. An additional employee or another person should be available to summon aid immediately. The monitor will
attempt to remove the victim by the use of the lifeline and to perform all other necessary rescue functions from the outside. Upon arrival of help, the monitor may enter the confined space for rescue work only when he is assured that his outside assistance is adequate. Rescuers entering confined space should be protected with the approved safety equipment required by the situation such as lifeline and harness and proper personal protection equipment.
There are many types and designs of disconnecting switches, commonly known as disconnects, which are used to sectionalize a line or feeder, make connections, and isolate equipment on electrical systems. The type used depends upon the kind of service, voltage, current-carrying capacity, and the equipment design. This section discusses only the most common types, which use air as an insulating medium, and the hazards involved and what measures should be taken to avoid them.
Management should thoroughly define who has the authority to operate disconnect switches or electrical controls or apparatus that will in any manner affect the safety of personnel or interrupt electrical service. Switching should be done by persons who are fully qualified and authorized to do this work and by other individuals only when they are under the direct supervision of such qualified and authorized persons.
All apparatus should be legibly marked for every identification. This mark- ing should not be placed on a removable part.
Switching orders should be in written form, with every step in the switching sequence spelled out in detail. Telephone or radio orders should be written down and then repeated. These procedures are particularly important for long or complicated operations. Every manual switching operation exposes the operator to some degree of hazard. Therefore, for his own safety he must understand the switching job to be done and be completely familiar with every detail of his part of the operation. An operator should not start a switching sequence until he has carefully checked the written order and is satisfied that it is correct in every respect. Once he has begun the operation, he must keep his mind on what he is doing, ignoring distractions, until the job is completed. If his attention is diverted to another task while he is executing a switching operation, he should not continue the operation before carefully checking what has already been done.
Loads and Currents
Ordinary disconnects should not be used to interrupt loads and magnetizing currents or to energize lines, cables, or equipment unless all the following conditions are met:
• The amount of current should be small.
• The kVA capacity of the equipment being interrupted should be relatively low.
• The location and design of the disconnect assure that it can be operated without danger of flashover.
• Experience has shown that the disconnect can be used successfully for the particular purpose. Therefore, disconnects should be properly connected and installed before proceeding with an operation.
Disconnecting switches are frequently used to break parallel circuits. As the blade leaves the clip, a relatively light or weak arc is drawn, which is quickly broken as the arc resistance increases. This operation is safe provided that the impedance of the circuit is low enough to permit the arc to break. Here again, experience is the best guide as to which parallels can be broken. Energizing or magnetizing current is the most difficult to break because of its low power factor.
Disconnects should never be used to de-energize lines, cables, capacitors, transformers, and other equipment unless specific approval is given and then with full knowledge that the disconnects will interrupt the current.
Underhung disconnects are mounted horizontally, and careful consideration must be given before they are used to break a parallel or to interrupt load current. The heat and ionized gas of even a small arc may be enough to cause a flashover.
Switch sticks or hook sticks are insulated tools designed for the manual operation of disconnecting switches and should be used for no other purpose. A switch stick is made up of several parts. The head or hook is either metal or plastic. The insulating section may be wood, plastic, laminated wood, or other effective insulating material, or a combination of several such materials. Glass fiber and epoxy resin materials are being used instead of wood by some manufacturers, and although they may cost somewhat more than switch sticks made of wood, the extra expense can be justified by longer life and reduced maintenance. Some manufacturers make a switch stick with a thin extruded plastic coating. This type of stick requires less maintenance than other types because the coating is tough and can be easily repaired.
A stick of the correct type and size for the application should be selected. Standard switch sticks are made in lengths up to 24 ft with proportional diameters. Special or telescoping sticks are available in longer lengths.
Switch sticks with insulated heads should be used to operate disconnects mounted indoors or on structures where the metal head of the stick might be shorted out when inserted into the eye of the switch.
The parts of a switch stick are pinned together and are therefore subject to wear. Consequently, they should be examined frequently. Varnish or a similar
nonconductive coating used to seal the wooden parts and prevent their absorbing moisture, should be in good condition at all times.
Storage of switch sticks is important. When stored indoors, a stick should be hung vertically on a wall to minimize the accumulation of dust. (It should be located in a convenient place but not where it might be sub- ject to damage.)
If a switch stick must be stored outdoors, it should be protected from sun and moisture. The varnish or insulating coating on a stick exposed to direct sunlight or excessive heat may soften and run. A long pipe capped at both ends, ventilated, and shielded or insulated from direct sun rays makes a good storage place.
Opening Disconnects by Using the “Inching” Method
The “inching” method of opening manually operated disconnects should be used wherever the opening operation can be controlled. The inching method should never be used for load break disconnects, air break switches, or other switching devices designed to break load or magnetizing currents.
In the inching method, the operator opens the disconnect gradually until he is sure that there is no load current. He then opens the disconnect fully. If a small static arc develops, but no more than is expected, the disconnect may be opened further, with caution, until the arc breaks. The opening can then be completed. If an arc develops that is greater than the normal charging current warrants or, in the case of breaking a parallel, greater than expected, the disconnect should be quickly closed.
Using these techniques, an operator can open disconnects by the inching method nearly as fast as he can by other methods and with maximum safety.
A selector disconnect has three phases with a double blade in each phase. The blade may be placed in either of two positions. Each blade is operated separately. The three operations—to open, to open and close, and to transfer from one position to the other under load conditions—must be done in the right sequence for proper functioning.
To open a set of selector disconnects, first one blade of each phase is fully opened. Then the second blade of each phase is fully opened.
To open a set of selector disconnects from one position and close them to the other position, first one blade of each phase is fully opened, and then the second blade of each phase is fully opened. All six blades are then open. One blade of each phase is then closed to its selected position; and to com- plete the operation, the second blade of each phase is closed to this position. To transfer selector disconnects from one position to the other under load conditions, first one blade of each phase is fully opened. After these blades have been opened, they are then closed one at a time to the selected position. The two sources of power for the circuit are thus paralleled. In a like manner, the second blade of each phase is opened from the original position, breaking the parallel, and then closed to the selected position. Now all six blades are closed to the selected position, and the transfer is completed without interruption to load.
When operating selector disconnecting switches, the operator should never open a blade of one phase from one position and swing it closed to the selected position in one operation. The corresponding blades in each of the phases should be opened successively, and only one step should be taken at a time.
Circuit Breaker Disconnects
The bus and line side disconnects of a circuit breaker must not be operated until the operator has made certain by observation of the circuit breaker indicating target or mechanism that the breaker is in the open position. (The exception to this rule is that the line side disconnects may be operated to make or break a parallel, for example, to shunt out feeder voltage regulators.)
Checking the position of the breaker is a routine part of operation that must never be neglected. Sometimes a breaker operated by remote control may not open because the control contact has failed or the operator has not held the opening control long enough.
Even if a mechanical failure has occurred or a control fuse has blown, it is still possible for a breaker to operate partially, reaching a semiclosed position. Wherever possible, all three phases of a breaker should be checked for failure of a lift rod or other mechanical failure that could cause a phase to remain closed. (An operator should always be on the alert for such conditions.)
Before the circuit breaker is restored to service after maintenance work has been completed, the operator must check to make sure that it has been left in the open position.
The need to interrupt load currents and to de-energize regulators and similar equipment has led to the development of the interrupter switch. There are many different designs, and the type used depends upon the voltage and the current interrupting capacity required.
Generally, there is an auxiliary blade or contact in addition to the regular load contacts. Before the switch is opened, it is important to check this aux- iliary contact, where possible, to make sure that it is fully engaged. When the switch is opened, the load contact breaks first and then the auxiliary contact is opened. The arc is extinguished by an arc chamber, arcing horns, or other means. No attempt should ever be made to inch an interrupter switch.
Closing Disconnects under Load Conditions
While disconnects are not designed to be used as load-pickup devices or to energize lines, cables, or apparatus, they may be used for these purposes where all the following conditions are met:
• Length of line or cable should be limited.
• Load and the capacity of the apparatus should be small.
• Voltage should be low.
Approval for these operations should be obtained from the person in charge only after all conditions have been studied.
In all cases, the procedure to be followed is the same. An operator must be aware that he is closing a disconnect under load conditions; he should select a switch stick of the correct length and then take a comfortable stance in direct line with the disconnect; and he should first move the disconnect to about the three-fourths closed position. After checking to see that the blade is in line with the clip, he should then use a firm direct stroke to seat the disconnect completely.
An operator should never reopen the disconnect to make a second attempt at closing it. If it is not seated completely, he should use added pressure to finish the closing. If the alignment is wrong, the lines or apparatus should be de-energized before the disconnect is opened.
Air Break Switches
An air break switch is a gang-operated disconnect designed with arcing horns and with sufficient clearance to energize and de-energize load currents, magnetizing current of power transformers, charging current of transmission lines, and to make and break transmission line parallels.
Air breaks, like other types of gang-operated disconnects, are connected so that operation of all three phases is controlled by means of a hand lever.
Some of the procedures governing stick-operated disconnects apply also to air breaks. The inching procedure should not be applied to air breaks designed to interrupt load or magnetizing currents.
When air break switches are installed, their use should be specifically stated and any limitation must be made known to all operating personnel concerned.
Weather conditions can affect the successful breaking of current by an air break—a strong wind can blow the arc across phases or a heavy rain can change the normal insulating air clearances.
Air breaks should be firmly opened and closed. When closing an air break to pick up a load, an operator should be careful not to open it after the load circuit has been completed by either the arcing horns or the main contacts, regardless of whether or not the air break has been closed properly.
Before air breaks are used to break transmission line parallels, the load current should be checked to assure that it is within the capacity of the air break to interrupt.
Operators should never depend upon the position of the operating lever to determine whether the air break is open or closed. They should check the air break visually before and after operation and make sure that each operating blade is in the selected position. One blade may fail to operate and remain either closed or open.
To prevent inadvertent operation when maintenance, repair, or other work is to be done, all air breaks should be locked in an open position and tagged.
Protection against Air Break Flashover
During the operation of air breaks, a flashover causing a flow of fault current may occur. Several measures have been commonly used to protect the opera- tor against electric shock. They include using an insulating section in the handle of the operating rod, grounding the handle of the rod, and providing ground mats or insulating stools at the operating position. (Opinions vary concerning the effectiveness of these measures and there are no widely accepted standards.) Rubber gloves should always be worn by the operator.
The handle of the operating rod may be insulated against possible contact with energized parts of the air break by a section of nonconductive wood or porcelain, in order to effectively protect an operator against the hazards of an air break failure. However, this does not protect against the shunting effect of the pole mounting and attachments. The ground voltage gradient is reduced but not eliminated by an insulating section.
Whether or not an insulating section is provided, the handle of the operating rod should be grounded with the lowest possible resistance. (A large majority of air break installations have a continuous metal operating rod that is grounded.)
Ground mats should be provided for operators to stand on; they will give him or her maximum protection against touch voltage and ground gradient voltage and prevent any dangerous potential gradient from occurring across the body in case of an insulation failure or flashover. Some companies provide portable ground mats of small iron mesh. Other companies install fixed ground mats, and specifications for installation vary widely. (Whether a ground mat is portable or fixed, however, it must be electrically connected to the operating rod and to ground to equalize the ground gradient in the area where an operator stands.)
Operators should keep both feet on ground mats. Regardless of the type of installation or the protection provided, they should always stand with their feet as close together as is comfortable.
Motor-Controlled Disconnects and Air Breaks
Many of the routine procedures and practices previously discussed for manually operated disconnects and air breaks also apply to motor-controlled disconnects and air breaks. In addition, the practices and precautions outlined in the following paragraphs should be observed.
The operators should hold the remote control contact long enough for the operating relay to seal in and ensure operation of the disconnect.
To determine whether the switch is open or closed, operators must never rely upon the indicating lights or upon the position of the operating handle. Instead, they must always visually check the position of the blades at the switch, making sure that each blade is in the selected position. (This check is particularly important for switches with high-pressure contacts.)
When a motor-operated switch is used as a tagging point for work clear- ance, the motor drive should be uncoupled from the operating rod. If this precaution cannot be taken, a heavy pin, lock, or blocking device must be used on the switch to prevent inadvertent operation. In addition, the switch for the motor control circuit should be tagged and locked in an open position to prevent operation of the motor in case of an accidental ground.