Every utility and every contractor that works for a utility should have a substation entry training program. These programs are primarily written for non-electrically qualified workers, but there are many line personnel who do not have substation training or who do not understand the risks inherent in a substation. Hazard awareness training for substation entry is necessary for anyone who enters electrical substations to perform work tasks. Following are some recommendations for the type of content that might be appropriate for an entry awareness program. This material may not be all-inclusive and some information may not apply to your stations. Most of this content is necessarily basic, but it is also suitable as pre-entry hazard review and training for experienced electrical workers.
Authorization to Enter a Substation
No person should enter a substation without the authorization of the owner. In most cases, substations are remotely connected to an operator who controls operation and access to all substations on a utility system. In this way the operator is aware that persons are in the station. Knowing people are exposed in the station makes a difference in how the operator responds to conditions displayed on the monitoring consoles or actions that would normally be taken remotely with an unoccupied station. Under the OSHA rules, any person who enters a substation is required to notify the substation operator before they enter and again after they have left the substation. Good entry policies also require that entrants leave phone or other contact information with the remote operator so that they can be contacted if conditions in the station change.
During construction projects, procedures are usually put in place, providing a site contact to take the place of the substation’s operator. That person also authorizes entry and tracks workers entering and leaving the facility.
It is a violation of OSHA rules and substation owner rules to allow an unqualified person to enter a substation. Power company employees who enter substations, just like contractor craftspeople, are especially skilled to safely enter and perform work on the equipment in a substation. After many years of special craft training, they are considered qualified substation workers. Other workers who may have need to enter a substation for non-electrical work are required to participate in hazard awareness training to allow them to enter the non-electrical areas of the substation for the purpose of performing their tasks. If you do not know if you are authorized to enter a substation, you are not authorized. Even if the gate is open – which might be the case during construction or modification of a substation – if you have not been told by a representative of the owner that you are authorized to enter, you must remain outside the gate until you have permission from the substation owner’s representative.
Hazards in Electrical Substations
Electrical substations contain uninsulated electrical equipment. Substation fences and the rules to enter a substation are designed to keep unqualified persons out of substations and to protect those who are authorized to enter. The uninsulated voltages range from the 240-volt substation power supplies to transmission voltages that can be as high as 500,000 volts. The purpose of a substation is to convert the cross-country transmission voltages to local distribution voltages that are further transformed into the customers’ required voltage for homes and businesses. Transmission voltages are usually 46,000 to 500,000 volts or more. Neighborhood distribution voltages are from 4,000 to 34,000 volts. Customer voltages are from 120 to 480 volts.
Most fatalities in substations occur when workers inside neglect basic safety rules or enter the protective spaces established to prevent contact with exposed energized equipment. Today’s substation equipment is almost entirely remotely operated. This can increase the hazards to entrants since switches and equipment can operate without warning. In addition, because the voltages are so high, equipment failures within a substation can cause significant damage to the local space within the substation fence. An entrant in the wrong space can be severely injured or killed in these instances. It is very important that workers entering a substation be aware of these hazards and know what steps to take to avoid unnecessary hazards.
Personal Protective Equipment
PPE is an important part of protecting workers from injury in the work environment. Generally, PPE for entering a substation consists of hard hats and safety glasses.
OSHA considers arc protective wear to be PPE. Employers are required to train employees regarding selection, use and maintenance of PPE. Electrical workers entering a substation must be protected by clothing or equipment from the effects of electrical arcs to which they may be exposed. Qualified workers in close proximity to energized equipment may wear a variety of electrical arc-protective clothing, hoods and face protection.
Outerwear for electrical workers must also be arc rated. Electrical workers should be aware that winter outerwear in non-arc-rated materials may be synthetic. Additionally, workers should be made aware that materials used in undergarments may heat through the arc-rated exterior clothing and melt, burning the wearer even though the outer protective material did not ignite.
Non-electrical workers who enter a substation should be informed of risks from electrical arcs. Although they are not exposed to the effects of an electrical arc, at a minimum they should wear cotton clothing to meet the basic “will not continue to burn” rules of OSHA. When exposed to the heat of an electrical arc, synthetic material used in clothing will melt, burning the wearer. Cotton clothing is harder to ignite, will not melt to the skin and tends to burn away from the body, helping to reduce burn injuries in an electrical arc exposure.
During the pre-task hazard analysis, a person competent to make a determination will establish the minimum protective clothing required. That information will be communicated to all affected workers.
The basic footwear requirement for an industrial worker is leather work boots with a substantial sole. A hardened protective toe is required under some conditions.
Some substation operators require entrants to don dielectric, or insulating, overshoes. Insulating overshoes protect the wearer by creating an insulating barrier between the feet and the earth. The design of substation grounding systems, discussed later in this article, usually precludes the electrical exposure for which dielectric overshoes are designed. Dielectric overshoes are designed by manufacturers to resist passing electrical current up to certain voltages. The insulating value of dielectric overshoes is affected by contamination and conditions of use, and there is no effective method of testing the overshoes’ dielectric value. For this reason, insulating overshoes cannot be depended on as electrical insulation. Dielectric overshoes can be effective in reducing electrical exposure in many conditions, but their use is not considered a primary means of protecting a worker from electrical contact. As with other PPE, the requirements for the use of dielectric overshoes will be established before the work begins and communicated to the worker during the pre-task hazard briefing.
Clearance Space for Safety
Clearance space – the space between the worker and the exposed electrical hazard – is the primary means of protection for a worker in a substation. When qualified workers will be near uninsulated conductors or equipment, the clearance to be maintained between workers and the uninsulated conductors or equipment is established before the work begins and is communicated to all workers during the pre-task briefing. This clearance space, known as the minimum approach distance, has been established by consensus standards, adopted by OSHA and is based on several calculations, most importantly voltage. The minimum approach distance can be entered by a qualified worker under specific conditions using special procedures to accomplish a task.
For unqualified workers, the minimum approach still applies, but that does not mean an unqualified worker can use the minimum approach distance as a safe distance unless they have been specifically trained on the nature of the hazard and procedures to be used to ensure the safety of the worker. The clearance space between unqualified workers and uninsulated equipment is generally much greater than the minimum approach distance to increase safety, and it is established by a person competent to make the determination.
Exposed Electrical Hazards
Conductors in a substation are often referred to as the “electrical buss.” Electrical systems in a substation are three-phase, meaning every transmission or distribution circuit has three energized conductors. Electrical conductors or buss in a substation can be bare cable, aluminum or copper pipe, or aluminum or copper flat plate. The uninsulated electrical conductors in a modern substation are mounted at a specified distance above the ground to reduce the chances that a worker in a substation could come in contact with them.
Even when the electrical buss is mounted high, the tools or materials a worker carries can reach these conductors. The worker’s reach is extended by the tools or equipment that will be handled during the work task. The worker, the worker’s reach, and the extended reach posed by the tools or equipment handled by the worker cannot extend into the minimum approach distance established in the pre-task briefing.
Workers carrying tools or materials through a substation must be able to identify the energized electrical buss as well as the relative voltage and minimum approach distances required for the buss that will be in proximity to their work area. When moving to and from the work area, pathways should be planned to avoid unnecessary proximity to electrical buss. When carrying tools or equipment, the load being carried must be kept low. If the load is long or unwieldy, two workers should carry the load to keep it controlled. In many companies the rule is that any load longer than 4 feet is to be carried by two persons. Four feet is a good rule because the conductors that connect equipment to a substation’s electrical buss come down to within 10 feet of the ground in some cases.
Minimum Approach for Equipment
Qualified workers use specific work procedures and techniques to keep them safe. Cranes and lifts working in a substation also have minimum approach distances established by OSHA that must be maintained to electrical conductors and equipment. Only highly qualified operators, specifically trained to operate lifts in an electrical environment, may operate equipment in a substation. As with any work task, the minimum approach distances will be established in the pre-task briefing and communicated to all affected workers.
Clearance Space Between Workers and Substation Equipment
Electrical connections of equipment were mentioned in a previous section. The yard of a substation is divided into two parts, the high side and the low side. The high side is the transmission side, characterized by higher-mounted buss and larger insulators. The low side is the distribution side. The distribution buss may be closer to the ground, but it is characterized by shorter insulators. The space between the buss conductors is only 2 to 3 feet while the high side buss is usually spaced 4 to 6 feet apart or more.
Like the buss and insulators, the equipment in the high side is usually larger with larger insulators. The dividing line is usually the power transformer, which is typically the largest piece of equipment in the substation. Smaller stations have one power transformer while large stations may have several.
In addition, the substation high side may have reactors directly connected to the incoming transmission line and often has racks of capacitors connected by switches to the high-side buss. On the distribution low side, the equipment is usually lined up side by side in bays. Conventional substation bays have a three-phase recloser, sometimes referred to as the breaker, that is the beginning of a distribution circuit that leaves the station either by underground or overhead busses. There is almost always a regulator bank associated with each outgoing feeder. The regulators automatically adjust the voltage, leaving the substation to accommodate voltage drops caused by load.
All of these pieces of equipment are subject to failure. Substation equipment failures are rare, but they do happen. Equipment failures are characterized by electrical arcs of several thousand degrees, high-pressure explosions that spray burning insulating oils, splattering melted conductor and fragments of metal and/or porcelain. Even though they are highly tested and maintained, they do fail. Because the substation is the origin of the electrical circuits, there are no fuses or protective devices that can simply turn them off if they go bad. Equipment failures in substations are sometimes spectacular displays that can be deadly to workers standing too close. For this reason, it is always a practical safety rule to not go near equipment unnecessarily. There is no distance that can be said to be safe from an exploding breaker. If you are not a technician working on or with substation equipment, a practical clearance to equipment adopted by many utilities is no closer than 15 feet but never closer than you need to be.
We previously discussed the rare occurrence of equipment failure in a substation and the need to maintain safe space between worker and equipment. Hazards also exist when substation equipment does what it is intended to do, act to interrupt and clear faults.
During a fault condition, current on the affected circuit rises quickly and circuit voltage spikes upward to dangerous levels. The substation breaker controls the current rise and surge arrestors control voltage spikes.
The surge arrestor exterior is insulated, conducting the faults internally within its porcelain body under normal conditions. During a fault condition, the surge arrestor lets these over-voltage conditions through the arrestor and shunts them to earth by way of the arrestor’s ground connection. The arrestor also shunts all lightning currents to earth through the arrestor. Sometimes, usually as the result of a failure, dangerous pressures rise within the arrestor. To prevent rupture, a vent system is designed into the base of the arrestor to release excess pressure during failure, reducing the likelihood of a fragmenting explosion of the arrestor housing. Arrestors vent without warning and can vent when there is no fault condition on the line. Arrestors are oriented in the station to reduce exposure of workers to the vents, but that is not always possible. Workers should be aware of the locations of arrestors and arrestor vents.
Fuses and Exhausts
Fuses and some types of breakers have venting systems that expel debris during faults. Workers in a substation should be aware of the locations and vent paths of these devices to avoid being showered by debris in the vent pathways.
Fault Current Control
Since the substation is the origin point of circuits, faults on the circuit return to the substation. By design, these high-current faults are forced into the earth at the substation. This is possible because the substation is built over an interconnected mat of copper conductors. Fault currents returning to the station by way of the system neutrals are given a path into earth through the mat. The mat is buried in the earth and then covered with rock. The purpose of the rock is to act as an additional barrier between workers above the mat and the electrical charge on the mat and earth during the fault.
Protection by Equalizing Potentials
Every piece of equipment, structural steel columns and substation equipment mounting frames are electrically connected, or bonded, to the mat. The overall design is to create a plane of equal potentials during the transmission of the fault to the earth.
In order to receive an electrical shock, a person has to be in contact with at least two conductive surfaces at different levels of electrical charge. The electrical charge is known as “electrical potential” or just “potential.” A conductor connected to a ground would be at a zero potential. A conductor connected to a column during an electrical fault might be at a potential of several thousand volts. A person who gets between these potentials receives an electrical shock. However, the difference in potentials is not always between two separate conductors.
If a clamp between two conductors is poorly installed, there may be resistance to electric current flow through the connection. In that case, there will be a potential difference across the clamp. A person who touches the conductor on both sides of the clamp will receive a shock because of the difference between the electrical potential across the clamp.
In the same way, a poor connection between structural steel and the ground mat will create a difference in potential at the base of the column. The closer a worker stands to a metal column, equipment mounting frame or piece of equipment when a fault occurs, the higher the risk of electrocution if there are resistances in the connections between column and ground mat.
Substation Fencing and Gates
Unequal potentials resulting in electrical hazards are particularly troublesome with substation fences. Because metal fences continuously encircle substations, they magnetically become electrically charged by the lines crossing over them going into and out of the substation. If the fence is properly grounded, the electrical charge is reduced to a safe level. Occasionally thieves or sometimes corrosion will compromise the bond connections so that the fence becomes an electrical hazard. In the same way, if a fence section is removed, an electrical potential may be created across the ends of the unjoined fence, creating an electrocution hazard.
At the substation’s entry gates, the swinging gates are bonded with flexible jumpers to keep the gates connected to the grounded system. Below the ground, the substation’s ground mat is extended past the swing radius of the gates to keep the earth and the gate at equal potentials to reduce the risk of electrical shock to workers opening the gates.
Bonding and Safety
When all electrically conductive components in a substation are connected together and all of the connections are tight and resistance free, there is an equal plane of electrical potentials. A person standing out in the open in a substation during a fault would be protected from an exposure to the electrical fault by the equipotential mat of the substation buried below their feet. Ideally, the interconnection of the conductive surfaces to the mat creates a condition of electrical equipotential, protecting workers near the conductive surface.
Without testing, there is no way of knowing if the equipment in a substation is properly bonded to the substation’s grounding mat. Over time and conditions of exposure to the elements, what once was a resistance-free connection to earth might degrade, creating areas of potential difference. A person in contact with the earth and a poorly bonded equipment frame during a fault would receive an electrical shock that could result in severe injury or death. Even though electrical injury through contact in the station is a rare event, substations do clear faults every day. With every fault, the opportunity for exposure to electrocution rises for nearby workers. For this reason, when it is not necessary because of the work being done, the rule is to stay clear of structural steel and equipment, reducing risk to the worker.
What to Do in an Emergency
Your work project will have specific procedures to follow during an emergency. Those procedures are communicated to workers before the work begins through the pre-task briefing. As previously discussed, the types of emergencies that can occur in a substation can involve high-power electrical failures. Fires following equipment ruptures can be hot and continue to grow as oil from equipment escapes into the flames. Following are general principles every substation entrant should know and follow in case of an electrical explosion:
• Be familiar with the types of failures prone to the equipment in the area in which you will be working.
• Do not fight fires in substations – get out!
• The best place to be in a substation failure is outside the gate. Get there as soon as practical.
• Know the best emergency route away from your work area to the substation gate.
• If the way to the gate is blocked, the next-safest places to be are the open areas between the equipment and the fence, particularly on the low side or distribution side of the station.
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