THREE IMPORTANT FUNDAMENTALS
In order to accept the theoretical logic of why equipotential worksite grounding works, you need to remember three basic fundamentals from basic electricity.
First fundamental: Most not all electricity (voltage and current) always flows through the path of least resistance.
Second fundamental: Electricity will only flow where there is a path and that path always returns to the source.
Third fundamental: When there is a parallel path, there is always flow through both paths and the greater flow will be in the path of least resistance.
You also need to understand that the human body can, and often does, withstand small or unobjectionable current flows across the body and that the body has significant resistance (reported to be about 1000 ohms) that can resist current flow.
UNDERSTANDING INDUCED CURRENTS
While grounding protects against unintentional energizing of cleared circuits, there should be just as much concern placed on induced voltage (capacitive) or current (inductive) on de-energized lines. Induced voltages are caused by energized parallel circuits or where energized circuits cross other circuits. The hazard of induced voltage is even more important with today's higher distribution voltages and the addition of more transmission circuits to serve expanding loads. Induced current is also known as magnetic induction. Magnetic induction is different than induced voltage in several ways.
• Induced voltage is dependant on the distance of air space between the source circuit and the line on which induced voltage is present, just like induced current.
• Induced voltage, however, is not dependant on the parallel length of the two circuits where induced current is very dependant on the length of the parallel lines.
• Induced voltage is dependant on the level of the voltage in the source circuit (which remains fairly constant over the work day). Induced current, on the other hand, is dependant on the level of the source current and is more likely to become present or to rise as current rises in the source circuit.
• Induced voltage can be harmlessly and easily drained from the circuit with a single ground creating a slight but unobjectionable current. Induced current, on the other hand, will flow to ground in a single ground, but will remain at the highest current level the conditions for magnetic induction will allow.
CIRCULATING INDUCED CURRENT
This brings us to the problem of circulating current that can occur in a two-point bracket grounding arrangement. Keep in mind that unexpected fault current will behave the same as induced current as described below in a two-point ground arrangement.
Induced current will flow in a conductor in the opposite direction of the source circuit if a flow path in the induced line is provided. If one low impedance ground is installed between phase and neutral, a large portion of the full value of induced current will begin to flow into the system neutral or earth. If a second ground is installed phase to neutral on the other side of the work area, a current loop is created. A portion of the full value of the induced current will flow in a circular path down the first ground, across the neutral to the second ground, up the second ground back onto the phase and back again to the first ground in a current loop.
The only way to limit the circulation is to shunt the loop at the center. The shunt, a third ground, will split the current. Part of the current on the neutral will rise on the shunt to the phase. Another part of the current will move from the phase down to the neutral in the opposite direction. This shunt, with current trying to flow in two directions, will tend to cancel one another out, creating two smaller cells of circulation instead of one more powerful one. Without the third shunt ground in place, if an induced current or fault current should arise, a condition could occur where a lineman placed between the phase and pole or ground becomes the shunt. The body, in this case, is not in parallel with either of the outside grounds, but is now in series with two opposing currents. The resistance of the body is not enough to prevent a serious, if not deadly, current contact. Understanding this potential for circulating induced current, it is now time to discuss the equipotential worksite method.
WORKSITE GROUNDING CONNECTIONS
With magnetic or electrical induction, a worksite ground still has current flow, but all of the conductive components bonded together are at the same potential, minimizing the risk of injury. The first connection is to install a grounding bail or grounding cluster bar securely and directly to the pole below the system neutral. The bail provides a common connection point for all the ground connections and makes an electrical bonding connection between the wood pole and grounds. The next connection is from the bail to the system neutral. Use conductor lengths as short as is possible when making connections in the grounding scheme. The third connection is from the bail to the phase and, of course, phase to phases.
As usual it is imperative for the integrity of this protection scheme that all connections be virtually resistance free. This means approved grounding clamps, sufficient conductor size** (no less than #2 copper) and properly wire brushed conductor surfaces, as well as wire brushed ground clamp surfaces.
In this arrangement, an unplanned current, whether from fault or induction, will be passed to ground around the protected area between the limits of the phase and ground connections. A lineman on the pole, feet planted above the bail with hands on the phase, will be in parallel with the protection scheme. But, since induced voltages or currents constantly flow through the common connected scheme, there is no "potential" difference to create a hazard. If there is a sudden rise in voltage because of an unplanned energizing of the circuit, the voltage rises on all connected parts of the zone protecting the worker. A hazard still exists for workers on the ground. Around the pole or equipment connected to ground, there will still be high potential differences between each piece of grounded equipment AND the equipment and earth. There will also still be ground surface voltage gradients radiating out from the earth connections. Persons at ground level are not protected by worksite or bracket grounding and must take appropriate measures for personal protection from those hazards. (See image for further information on this topic.)
TOUCH POTENTIAL ABOVE THE NEUTRAL
A frequently asked question concerning worksite grounding is about touch potential on the wood pole between the lineman's feet and hands if he is touching the pole during a fault. For touch potential to exist there must be current flow and resistance across the touch surface to produce a difference in potential. From neutral to pole top, all of the components including the wood pole are bonded together so the voltage on all surfaces rises at the exact same instant. As a result, there is no voltage gradient to create a touch potential hazard.
BONDING WOOD POLES
OSHA requires that for the purpose of fault and protective ground calculations, a wood pole must be considered a conductor because the resistance of the pole changes from day to day depending on atmospheric conditions. Poles used to be routinely bonded from neutral to pole top pin. That is not always the case today for high BIL purposes. Pole top bonding helps to reduce the resistance of the pole, making it more conductive. Bonding of a wood pole to bring it to equal potential with all other conductive surfaces is one of the keys to effective worksite grounding. The pole bond plays an important role in the scheme and, if installed, should not be removed.
THE LAST CONSIDERATION
Any protective grounding arrangement jeopardizes the safety of persons on the ground. Grounding, whether bracket or worksite, is placed to protect the employee aloft. All of the above mentioned configurations expose the employee on the ground, particularly when the truck is bonded to the system neutral. The system neutral in a wye system is a current-carrying conductor. The voltage on the neutral is limited by the grounding of the conductor. Normally, the neutral carries the expected system current with only insignificant amounts of current flowing down the higher resistance pole grounds. A truck connected to a system neutral, with or without the outriggers in contact with the earth, is a high resistance path to ground from the system neutral in parallel with the pole bond. It is estimated that 79% of all phase faults are phase-to-ground or neutral faults and the average fault current is in the neighborhood of 1500 amps lasting between 2 cycles and 2 seconds (Hard To Find Information About Distribution Systems, Synergetic Design Engineering Consultants, 2003). That amperage would overload the current carrying capacity of a neutral and fault current would flow through the grounds or any conductive equipment connected in parallel with the neutral and ground—like a truck.
If a remote fault impresses current on a truck connected to the neutral, any person touching that equipment is exposed to contact. In addition, since that current is impressed into the earth through the ground rods or the truck, the ground in the vicinity of the rods or truck will be electrified. Because the earth has varying degrees of conductivity, even over the course of a couple of feet, there exist differences in potential that can cause the electrocution of an individual who has been exposed to the earth gradient (difference) between his feet—known as step potential. Any contact between an employee standing on the ground and touching a piece of equipment grounded to the system is subject to injury by touch potential. The only way to ensure against these conditions on the ground is for all grounded equipment to be surrounded by equipotential mats (required by OSHA 1910.269 (p)(4)(iii)[C]) where access to and egress from the mat is done by hopping, as opposed to stepping, so that the feet are never touching the mat and the earth at the same time, or by all ground personnel wearing dielectric overshoes.
A FINAL POINT
The fundamental that makes any protective grounding work against unplanned energizing of the line is the potential difference in the connected components. It is imperative that the grounding connections be clean and resistance free. Always remember this: The more resistance you build into your protective ground, the more current there will be across your body in case of a fault. Ground it, but ground it right and ground it well! ip
*This presentation attempts to explain in a clear fashion the best information available about the practice of protective grounding for personal protection. It is necessary that the reader understand that while the author believes the information represented is accurate and reliable, the author and Incident Prevention Magazine makes no claims as to the appropriateness of any particular method of grounding for personal protection. The Occupational Safety and Health Administration requires the worker's employer to define and administer the appropriate practices for their employee's protection according to the conditions that exist on the respective electrical system.
**The utility is responsible for assessing the maximum current that may be encountered in any condition and specifying the appropriate ground conductor size.video
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