December 2015 Q&A
Q: I’ve been reading ASTM 855, IEEE 1048 and the National Electrical Code, and I’m a little confused by the practice of grounding through a switch. Can you help me better understand this?
A: In transmission/distribution applications, there is no issue with grounding through a switch. To explain, we always have to ask whether the issue is grounding through (in the path) a switch or grounding (by way of closing) a switch. The application may sound the same, but it depends on which standard you read. Our subject matter experts think the confusion lies in the well-known NEC rules, which require permanent installations to have a connection-free path for the ground electrode conductor at the service entrance of an electrical system. According to the code, grounds – except in some specialty connections – cannot be disconnected through operation of a switch or breaker contact. ASTM 855 is an equipment manufacturer’s standard that has no application to utility practices in the field other than being used as a guide for shop construction, sizing, rating and assembly of personal protective grounds. IEEE 1048 does address the value of having the grounding switches closed when de-energizing a system for work; that ground switch is a very low-resistance path to earth at the feeder or transmission bus source that will lower fault current in an accidental or inadvertent energizing of the source. The ground switch in the station is also a path to ground that will divide and help reduce the amount of induction current on a circuit. Closing the switch can help reduce induction current at a work location, depending on how far apart the work location and the ground switch are.
There are many benefits to using station ground switches in parallel with personal protective grounds. We are not aware of any guidance in IEEE 1048 or other consensus standards that prohibits grounding through a switch. However, here is an important concept related to personal protective grounding that should be recognized: We always want to limit the available fault current, and we are not telling you to seek to limit fault current as part of your protective system, but the ground switch itself is not the real factor in the efficacy of the personal protective grounding system. OSHA’s intent is clearly defined in the standard. Grounding must be arranged to prevent any potential differences across a worker. That being the case, any questions regarding switches between workers and a remote ground are pretty much rendered moot since it’s the local arrangement – equipotential bonding – that protects the worker, not the remote ground.
Q: Our meter department is concerned about suiting up for detailed meter installations when working Category 3 and 4 exposures. The workers usually end up in hooded moon suits. Is this the expectation of the new standard?
A: I assume the employer has done an analysis and is sure there are Category 4 exposures. Some employers have made pretty conservative assumptions concerning service equipment exposures, adding a safety factor in order to cover all possibilities and not always using the information derived from valid computations. Doing so can put workers in the dreaded moon suit when they may be perfectly well-protected in Category 3 clothing. The trade-off is not good. It can be much harder to handle meter base parts when blanketed in overweight, dense protective equipment, and even potentially result in flash incidents or mistakes that would not occur with better dexterity and vision.
You can layer for body protection. That usually ends up as overalls worn over the standard Category 2 daily wear. Most arc-protective clothing suppliers can tell you the lab-certified ratings of layers for their offerings. Under OSHA, clothing ratings cannot be arbitrarily added up to assume a new layered protection rating. In other words, 8 cal and 8 cal don’t make 16 cal. You do have to use a certain procedure to test layers. Some manufacturers have performed layered testing for their garments and have approved “layering addition” for garments of 8 cal/cm2 or less. That’s OK as long as the manufacturer’s recommendations are based on approved testing.
To get back to Category 4, besides body protection, the tinted hood that usually accompanies a Category 4 suit really compromises visual acuity. It is possible to achieve the appropriate level of face protection with an arc-rated balaclava and face shield. Appendix E to OSHA 29 CFR 1910.269 has lots of good information regarding application of rated clothing and principles of protection.
Q: I am curious to know your expert opinion regarding arc-rated gloves as it pertains to the new OSHA standards. Does any leather glove manufactured using the material weight specified meet the standard?
A: Throughout the preamble and in the rules, OSHA permits heavy leather
It’s tough to make a decision about rated gloves because you could test a heavier leather weight and show it to protect from
Manufacturers of arc-rated gloves design their products specifically for such exposures. The manufactured products we’ve sampled are designed with attention to comfort, mechanical performance, arc-rated performance and dexterity. Most leather gloves are designed for price point, comfort and dexterity only. They are often reinforced at the palm but not so much on the back of the hand. More expensive, higher-quality leather gloves may meet all of the criteria; however, that’s not a given. Employers have to reasonably ensure the equipment they provide employees meets the protective criteria of the standard.
Q: I have a question regarding placement of the grounded end of a ground lead to perform work on a transmission circuit inside of a substation. Can the grounded end of the lead be installed on the structure, close to the phases of the transmission circuit versus placing it on the ground cable at the base of the structure? Does one way or the other buy you anything? These grounds are in addition to mechanical ground switches that were closed prior to personal protective grounding installation, and they are provided only as additional grounds for personal protection.
A: We could just say “no issues,” but it will help you in your decision if we cover several points that make a difference. Remember that Incident Prevention works to make technical issues understandable from a practical standpoint, so it’s worthwhile to discuss the benefits that could be derived. We also have to say here that we have written this without the benefit of detailed knowledge or measurements. The advice we are offering is in no way universal for applications, but it is based on sound theory. Every situation has its own peculiarities.
One consideration is a slight increased resistance in steel compared to copper cable right at the ground grid. However, if the steel is hefty, the added mass in the steel helps to increase conductivity, just like the difference in mass of aluminum over copper and current-carrying capacity. Assuming your made connections are low resistance, most station steel is bonded to the ground mat, so you shouldn’t have any issues with sensing and clearing a fault or voltage drop across the mechanical foundation due to resistance. Additionally, you have the grounding switches closed and that parallel path further reduces fault current at the grounds you make to the steel. Still, check with your relay engineers for their input. One of our subject matter experts asked about circulating induction current in the parallel paths. There will be some, but they will be no more dangerous than induction current already flowing in the ground path. The very low resistance of the station ground mat will conduct a larger percentage of the induction current available, so the circulating current should be negligible.
As far as those voltage drops, current and voltage drop across gaps in the protective system have to reach risk levels before they are a real issue. There have been fatalities associated with bonding or grounding where it was assumed that the substation ground system was equal at all points. Above ground or below, the length of cables or number of connections can equate to potential differences in the work area. Grounding to steel in the immediate work area can help equalize potential between phases and steel in the local work area. That may help reduce risk of potential differences across the insulators in the work area for any worker who may get between phase and steel. The shorter ground lead is yet another benefit to making the ground connection to steel close to the bus. Long cable creates additional whipping and mechanical stress. The risk of mechanical injury from a whipping 4/0 ground is a real but often overlooked issue.
Q: Please clarify rubber glove classes. If I have a Class 4 rubber glove, can I work on a system voltage (phase to phase) of 41,600 volts?
A: To clarify, the maximum-use voltage is not intended to promote a phase-to-phase application, meaning a worker using gloving methods should never have hands on two energized conductors at different potentials. Class 4 gloves are proof-tested at 40,000 volts AC and have a maximum electrical rating of 36,000 volts.
Some state plan rules and some contract agreements vary, but by OSHA, you can actually use Class 3 gloves for phase-to-ground voltage of 24,000 volts. The voltage rating on gloves is not qualified as phase to phase or phase to ground like some types of insulation cover equipment. Glove insulating ratings are expressed as maximum voltage use. In anticipation of your next question, OSHA has clearly established that working conditions – not system voltage – determine what class of gloves you use. The system may be 36 kV, but if you limit your exposure to phase to ground, the exposure is 24 kV and you can use Class 3 gloves. Limiting exposure is accomplished by insulating the phases not being worked on and planning the path or approach to those phases you will be covering. While covering, you cannot enter the minimum approach distance of the other phases.
Frankly, Class 3 gloves are hard enough to work in without dropping things and hurting your hands. It is widely believed that repeated use of heavy gloves results in carpal tunnel syndrome or repetitive-use injury to the hands. If you can use a lighter-weight glove, your long-term hand health issues will be diminished.
Q: There are quite a few situations in which you can’t use equipotential grounding on poles you have to work, like when the poles are broken by vehicles or there is a tree on a pole. My question is, if you are wearing high-voltage protective rubber gloves and rubber sleeves, and you install grounds above and below your primary conductors, how does the OSHA rule for equipotential grounding apply?
A: Your scenario demonstrates that you recognize the lifesaving value of equipotential grounding. As you are aware, the OSHA rule for personal protective grounding requires that grounds be placed in such a manner that all workers are protected from differences in potential. The standard also requires that if grounds cannot be placed in a manner that protects all workers from differences in potential, isolation, insulation or guarding must be used to protect those workers. You will find that guidance in Appendix C of 1910.269. By wearing sleeves and gloves, your workers are doing exactly what they should be doing when they cannot protect themselves using equipotential grounding.
Q: If someone is using a digger that has the capability of setting a pole, does it come under the digger derrick exemption? It is also my understanding after reading about the OSHA final rule that all digger derrick work is exempt from the crane rules but not from 1910.269(p). OSHA refers to 1926 Subpart V in the notice and to 1910.269, but they are all the same, correct?
A: Just because you use a boom to set a pole does not enable the exemption for pole-setting trucks. OSHA standards in the U.S. are pretty well settled regarding the digger derrick exemptions. Our experts in Canada tell us that digger derrick exemptions vary for a number of reasons, and exemptions under Canadian standards are still in a state of flux. In the U.S., except where state plans require otherwise, a digger derrick is exempt from 1926 Subpart CC, “Cranes & Derricks in Construction,” if it is designed for the purpose of setting poles, in particular being equipped with an auger and pole claws. Even though you may use it to lift equipment, as long as the equipment is associated with the pole – like moving phases and handing pole-mounted gear as the standard describes it – it is still exempt. It is important to understand the limitations of the exemption. That same digger derrick truck is not exempt if it is lifting steel in a substation. In such a case, even if it is a digger derrick, the operation will fall under Subpart CC. In addition, although the regulation requires digger derrick exempt operations to meet the rules of 1910.269 and 1926 Subpart V, if an employer is found in violation of those digger derrick operation rules, they will also be cited for any related violations of the crane and derrick standard. Finally, in your federal jurisdiction, digger derricks are exempt from the crane rules, but state plans or federal agencies in other countries may not exempt digger derricks from their crane operating standards.
Do you have a question regarding best practices, work procedures or other utility safety-related topics? If so, please send your inquiries directly to firstname.lastname@example.org. Questions submitted are reviewed and answered by the iP editorial advisory board and other subject matter experts.