Documents

07-0801-01.pdf

Categories
Published
of 4
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
Share
Description
History of Personal Protective Grounding © Copyright 2008 & 2010 Hubbell Incorporated Section 1 www.hubbellpowersystems.com E-mail: hpsliterature@hps.hubbell.com 1 Phone: 573-682-5521 Fax: 573-682-8714 210 North Allen Centralia, MO 65240, USA 1-2 History of Personal Protective Grounding CHANCE ANCHOR MAN Worker protection has always been an im- portant activity. Worker safety has become a more important issue than ever before and has received increased attention in recent
Transcript
  History of Personal Protective Grounding © Copyright 2008 & 2010 Hubbell Incorporated Section 1 www.hubbellpowersystems.comE-mail: hpsliterature@hps.hubbell.com  1 Phone: 573-682-5521 Fax: 573-682-8714 210 North Allen Centralia, MO 65240, USA   1-2 History of Personal Protective Grounding CHANCEANCHOR MAN Worker protection has always been an im-portant activity. Worker safety has become a more important issue than ever before and has received increased attention in recent years.  As the country has grown so have the electri-cal needs of the population: More people, more businesses and factories, all using more power. Electric power lines have been upgraded and new ones constructed to supply the increas-ing demand for electric power. Today we are seeing higher voltage lines, with higher levels of both rated and fault current.This growth has increased the difficulty in providing a safe worksite. In many cases the “old” methods are not only inappropriate but are also unsafe. One of the “old timers” at a mid-west rural utility related that they used to cut a “fat green weed” to ground the line. Thankfully, the days of grounding with “fat green weeds” and grounding chains are long gone. Back then, the probability that a worker happened to be in contact at the very instant that the line accidentally became re-energized was very small. In most cases the absence of injuries was more the result of the worker lacking contact at that moment than the protection scheme in use at the time.Now it is important to be aware of fault cur-rent levels, available protective equipment, techniques for establishing safe working ar-eas and the condition of the equipment to be used. New and more appropriate methods of personal protective grounding to meet today’s needs are reviewed in this publication.The growth of the utility industry has been accompanied by an increase in the number of accidents and injuries. This has resulted in an increased awareness for the need of improved safe working conditions within the industry and also from governmental regu-lating agencies. At the federal level rules by the Occupational Safety and Health Admin-istration (OSHA) were published in January 1994. CFR 29 1910.269 Subpart R [7]  regulates a broad scope of utility activities. It puts forth requirements relating to operation and maintenance of generation, transformation, transmission and distribution of lines and equipment and of tree trimming activities. Other rulings by OSHA address other util-ity related topics. Very little is being left to chance. These rules carry the weight of law and violators may face severe penalties and monetary fines. Some states have adopted their own version of the OSHA regulations. This is allowed if the state version is at least as stringent as the federal regulations. Worker protection is the focus of the decade. This publication intends to assist utility per-sonnel at many levels to understand and apply techniques for workers to use during mainte-nance after a line has been de-energized and taken out of service. Each section has been written with a particular reader in mind. The sections are arranged in a sequential manner, and each stands alone on the information it provides. This allows a reader with more ex-perience to skip over the more basic sections that are provided for the lineworker new to the industry.Earlier literature referred to this topic as “grounding” or “jumpering.” However, confu-sion existed with these terms. For example, there are “hot jumpers” used to maintain an energized electrical connection that remain energized during their use. Did grounding mean a connection to earth or could it be a connection to neutral? The terminology was officially changed to personal protective grounding in our national standards in an attempt to eliminate this confusion. A genera-tion of linemen will probably pass before the new terminology is commonly used.  1-3Looking back through the years, a variety of protection schemes followed the use of ground-ing chains. Early methods involved connecting a separate jumper from each conductor to a separate earth connection (13,14) . This is dia-gramed in Figures 1-1.a and 1-1.b. The worker is represented in the following figures by the symbol of resistance, designated as R M . As you can see, this resulted in the worker being placed in series between a possibly energized conductor and ground as a separate or fourth path for current flow to earth if the structure was conductive, e.g., steel tower.  A later modification to this method brought the three connections to a single Earth con-nection point  [13,14] . It was believed to improve worker safety. However, this modification still left the worker as a separate current return path to the power source through the earth if working on a conductive structure. This is diagramed in Figures 1-2.a and 1-2.b. Separate Jumpers To Common Earth ConnectionFig. 1-1.aSeparate Jumpers To Separate Earth ConnectionsFig. 1-1.bFig. 1-2.aFig. 1-2.b  1-4 Another modification used shortened jump-ers between phases and a single jumper to a single Earth connection  [13] , as diagramed in Figures 1-3a and 1-3b. This was another at-tempt to improve worker protection that did not change the basic circuitry. The worker remains a separate current return path. All of these schemes protected the system by indicating a fault, but left the worker in a situ-ation that could prove fatal. As can be seen in the diagrams and the associated schematics, substantial voltage can be developed across the worker. This was not a satisfactory solution. What if the structure is wood? If a pole down Phase to Phase to Single Earth ConnectionFig. 1-3.a wire is present and the worker is near or touch-ing it, the separate current path remains. If there is no pole down wire, the pole may have a resistance high enough to keep the body current flow to a low level but not necessar-ily to a safe level. Each pole is different. Pole resistance depends upon the amount of mois-ture sealed in the wood during the pressure treating, the surface contaminants, and the amount of water present on the surface and the type of wood.Some companies had adopted a policy of plac-ing a full set of grounds on the pole at the worksite and also on each pole on both sides of the worksite. This offered protection but required three full sets of protective grounds. This increased both the cost and the difficulty of the work for the lineman. In 1955 Bonneville Power Administration engineers theorized that a set of grounds on the center worksite pole was adequate, if properly sized and in-stalled. Testing indicated that this was correct.  A paper (17)  of this work was authored by E. J. Harrington and T.M.C. Martin in 1954. This was the beginning of the “worksite” ground-ing movement, but was basically ignored for many years. The low probability of a worker being in contact during the extremely short period the line was re-energized was probably a major factor in the low number of accidents. The prevailing philosophy was that the old methods had kept the number of accidents low before, so why change? Unfortunately, this philosophy exists in some areas today. Additional protection schemes have been devised. “Bracket grounding” became the most accepted and commonly used one. Its use and faults are discussed in detail in a later section of this publication. Temporary protective grounds today offer protection to workers during maintenance on lines believed to be de-energized that are actually energized through induction or that later become en-ergized accidentally. However, they must be installed in a correct manner, which is the focus of this publication. Fig. 1-3.b
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks