June – July 2022 Q&A
Q: Why does grounding alone not prevent static discharges, and why don’t we have to ground all flammable dispensing drums and stations?
A: “Flammable” is a relative term, and some of the written standards are detailed to the point that they can be confusing. The best thing any facility can do is to consult a chemist, chemical engineer or fire science specialist to survey and equip your flammable operations.
The simple explanation has to do with volatility, which is how easily a chemical vaporizes and then how flammable that vapor becomes and at what temperature. Every chemical in the workplace should have a safety data sheet that lists the material’s vapor point and relative flammability. The issue then is the material’s vapor flash point. Transferring liquid materials from a drum or container into a receptacle for use is one of the frequent points at which fires begin in industrial operations, and that’s because of static. Bonding of the transfer area controls static. Static is generated when the fluid flows through the nozzle. It will build up on the drum unless it has a path to ground, which collapses the static. The issue is the fluid vapors that become flammable or ignitable at typical ambient temperatures. A full drum is not likely to reach that heat. The lower the drum’s fluid level, the greater the air temperature in the drum since there is no liquid to keep the drum cool. Daily air temperatures are not adequate to assess potential issues. Sun exposure and duration of sun exposure can quickly raise the temperature in a drum, creating hazardous vapors. Every industrial worker has had the experience of opening a valve on a 55-gallon drum and hearing air rush out or air being sucked in. If I were at a threshold or concerned about it, I would ground and install a bonding strap. Transfer station grounds tend to be isolated from a facility electrical system ground and are usually an 8-foot-by-5/8ths ground rod. If the drums are on a steel rack, ground is typically connected to the rack. If the rack has bolted joints, the joints often – but not always – bond across the mechanical joint with a #12 bonding wire, depending on volatility and vapor generation potential of the material. Bonding is the key to ignition of vapors as bonding equalizes static charges so that no static spark can be generated. The ground rod leads and/or bonds are usually flexible #6 or #10 equipped with spring-loaded clamps for attachment to the drum. Your company engineers can guide you through bonding of the transfer area.
Last and most importantly is the transfer or filling area itself. This is where the incident usually begins. The nozzle must be bonded to whatever can the fluid is being dispensed into, including plastic cans. Plastic will create static and be at a different static potential than the nozzle. If the nozzle and the receptacle receiving the fluid touch during or after filling, there will be a static discharge. Bonding the nozzle to the can/receptacle will prevent that static from building up during transfer of fluid.
Static is pretty easy to control, but just in case, don’t forget to put a fire extinguisher near the drums.
Q: Why isn’t an aerial lift’s tested, cleaned fiberglass boom considered insulated?
A: First, being insulated from ground is not the basis of the category classification system detailed in ANSI A92.2, “American National Standard for Vehicle-Mounted Elevating and Rotating Aerial Devices.” Second, we need to qualify the terms “insulating” and “insulated.” The industry technically agrees that there is no pure insulator as all insulating materials do have some leakage. To keep the application of high-electrical-resistance materials pure, we regard all high-resistance materials used to create isolation of the worker as “insulating.” Only those in Category A are considered insulating from ground for the purposes of protecting the worker. In the Category A system, the insulating boom is the primary means of worker protection. These are barehand trucks with leakage monitoring systems. The boom is designed as the primary insulating system for worker protection because it is equipped to constantly monitor leakage and designed to be in contact with uninsulated energized conductors during the work method. All other categories of booms – B, C and D – do not have constant leakage monitoring. Category B booms are equipped with test electrodes for periodic testing. Category B through D insulating booms are considered secondary protection, with the primary protection being the insulating work method, sticks, gloves, coverup and so forth. By the way, the buckets on insulating lifts have no insulating ratings regarding protection of the worker. The categories, except for B, also have voltage limits. Category C is designed for use below 46 kV. Category D is designed for use at a maximum of 20 kV, 5 kV or 1 kV.
Q: I am looking for information about pole tongs in the transmission/distribution industry, such as best practices for use. Can you help?
A: Many pole tongs are not rated or approved for overhead lifting of poles. They are better referred to as “skidding tongs.” Rigging appliances for overhead lifting must meet ASTM standards for safe use. Pole tongs are useful, especially for snagging a pole off a pole pile. The tongs drape themselves over the pole and grip when placed under strain. This keeps us from having to put a worker on those pole piles and the risks associated with that task. There are some pole tongs marked and approved for overhead lifting, and that would be legal as far as the standards require. Most employers limit pole tong use to picking poles off a pole pile or for skidding poles across the ground. Lifting is limited to where no workers will contact the pole or be in any line of fire. Some employers simply don’t allow the use of pole tongs. There are many good reasons to use slings for the lifting of poles. The issue is that tongs can skin a pole if the hook points do not properly cradle the pole, even when the pole tongs are approved for overhead lifting. Lift-rated tongs are rated by strength. Strength does not ensure grip. As with any rigging, users of tongs should assess the hazards of use and train workers to use them in a prescribed manner.
Q: What is ferroresonance, and why is it so dangerous? At our cooperative, we have a fourth switch that temporarily grounds the tie between the transformers on our wye-delta banks. There is very little information out there dealing with this topic, so we thought you might be able to offer some.
A: Ferroresonance is a runaway voltage in the core that occurs in transformers under three very particular conditions. Grounding the floating tie on a wye-delta bank helps to change the very particular conditions required to cause ferroresonance. Rural electric associations (REAs) have a fourth switch to temporarily ground the tie to limit the possibility of faulting one or more of the transformers should ferroresonance occur. Ferroresonance is more common in REAs because there are often very long feeders to remote three-phase banks on farms and with some small industrial customers. The three conditions required to cause ferroresonance in a transformer bank are (1) a capacitance created by the long parallel conductors in the feeder, (2) in series with the inductive reactance of the transformer bank, (3) in a no-load condition. On the high side of the bank, each primary coil is in series with the adjacent transformer through the floating tie. When the tie is grounded, the coils are still in series, but they are also in parallel with each other from phase to neutral. The second condition necessary is equal capacitance and reactance in the series path. If the feeder KVAR in capacitance (by percentage) is the same or very close to the reactance of the transformer coil in percentage, the second condition is now present. If there is no load on the bank, the third and final condition is present – and boom. The result is superheating of the transformer core and oil-blistering exterior paint within seconds. A runaway voltage, usually in the primary voltage range, appears in the laminated core, typically showing up on the secondary terminals. Sudden heating in the laminated core results in magnetostriction (i.e., sliding of the laminated plates, breaking the bindings), creating a sound similar to shaking a coffee can full of bolts. The transformer can explode, but typically the damage is related to the secondary. Ferroresonance is characterized by high voltage, not current, so transformer fuses rarely blow when this occurs.
The way to prevent the damage caused by the ferroresonance is to change any of the three conditions. You can create an imbalance by changing either of the reactances that are in the series, or add load to the bank before you close it in. Most REAs don’t want to add load as the customer is single-phasing in the process of energizing the bank, risking damage to the customer’s three-phase motors. It’s not easy to change capacitance, and it’s impossible to change the reactive impedance of the transformer coils. A solution is to temporarily ground the tie, interrupting the series connection between the phase capacitance and coil inductive reactance. Grounding the tie splits the current, creating a low-resistance parallel path through the coils and to the neutral. Grounding the tie does not entirely remove the possibility of ferroresonance, but it considerably lowers the level of resonance and the damage that can be done.
Closing the switch or opening the switch to the neutral does create quite a current shift for a cycle or two, which is why we load-break the opening of the switch. If you left the switch closed, the bank would run, just at a little lower power rating.
Q: We ground trucks in a substation. Recently we had a pumper truck in the fence to pump concrete for a new pad. It wasn’t grounded, and the question came up: Why not? Should the pumper have been grounded?
A: Ultimately, the utility engineering staff can estimate voltage differences that may occur in a substation. The answer to your question is multifaceted and conditional. Let’s start with grounding. Grounding the pumper truck, assuming it has an articulating boom, makes sense in an energized substation, but it may not actually be required. The purpose of grounding the truck is to trip protective devices in the substation, eliminating the continued risk of an electrical exposure created by an energized truck. If the boom cannot get in the substation energized bus, there is no reason to ground it. But that isn’t the only issue. Grounding the pumper does not ensure any protection for any worker in contact with the truck when it becomes energized. The only way to protect a worker in contact with the truck is to insulate the worker (rubber gloves) or use equipotential mats. However, the design of the substation ground grid is precisely to create an equipotential mat for workers and equipment in the substation.
The truck is not subject to voltage rise from a fault since the substation ground grid creates an equipotential plane across the substation. The pumper boom would be subject to contact with the bus if an errant move occurred, and that would be a reason to ground that truck under the OSHA rules. However, you must keep in mind that grounding the pumper does not necessarily provide protection to a pumper operator who is not standing at the operator’s station on the chassis of the pumper truck. A person who is in contact with the hopper, or a chute in contact with the hopper, or while wet concrete is flowing from the chute into the hopper, may be exposed to electrical contact if the pumper boom were to become energized. In each of these cases, the contact injury would be between the pumper truck and the earth, except where the substation ground grid provides the plane of equipotential protection for the worker as explained below.
In a properly functioning substation, there will be a voltage difference between the grounded pumper and the substation floor. That voltage difference should be very low, but it depends on the impedance of the grounding cable and the distance from the grounding point. The design intention of a substation grid is to create an equipotential plane for workers walking in the substation and any touch potential between columns or frames bonded to the substation grid. Those bonding calculations for worker protection from touch potential are planned for an arm’s reach away from the column or structure because the farther you are from the column, the greater the potential difference will be. The same applies for grounding of the pumper. There is some protection provided, but it is limited by the impedance of the path from the pumper to the grounding grid connection and back to the pumper underground along the substation grid. This is where the substation ground cover plays a role in protection. The substation cover is crushed granite that provides an insulating barrier above the substation grid to further aid in protection by creating an insulating buffer above the equipotential plane. The opinion of many experts is that a properly functioning grid, covered by an effective rock layer and with workers wearing boots rated for electrical hazards, would receive sufficient protection from electrical hazards. In terms of the big picture, we cannot ignore that with the properly functioning grid, the current associated with the fault flows downward through the ground rod connection and grid, limiting the risk related to voltage flow across the grid.
Now, back to the pumper truck. The equipotential plane and rock barrier provide the same protection to the truck and the worker at the truck. They are both isolated and protected to the same degree. The differences in potential that may occur would be caused by the wheels of the pumper truck cutting through the rock, making contact with the earth and grid below. In that case, there might be a rise in potential differences between the truck and the worker in contact with the truck. If the rock bed is intact and the grid is properly designed and functioning, there would be little risk.
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