In the previous issue of PowerLineman, I wrote of the dangers of induced voltage and current to linemen working with de-energized transmission lines. The need to exercise caution and use the correct equipment extends to the presence of vehicles and equipment on the worksite as well.
When or when not to connect vehicles and/or equipment to earth (ground) can be a complex issue that requires much thought and consideration. When a piece of equipment is connected to earth in the work area, the possibility of introducing voltage gradients (step, touch, and transferred touch potentials) into that work area exists. These voltage gradients may pose hazards for workers on the ground in the vicinity of the grounded/bonded equipment.
It is important to remember that there is a difference between "grounding" and "bonding":
Grounding means connecting a normally current carrying conductor to earth or something that represents earth. When we talk about grounding for personal protection, we are referring to a system of connecting a conductor or conductive object to earth to provide a low resistant path to earth for the current to flow in situations where the conductor being worked on becomes energized from numerous and varied sources. The purpose is to cause protective relay and circuit breaker operations to "trip" the circuit out of service in a matter of a few cycles. The protective grounding cable therefore has a current carrying requirement: it must carry the fault current for the length of time that it takes the relays and circuit breakers to clear the fault.
Bonding, on the other hand, means connecting two or more conductive objects together with another conductor to place them at the same potential. The goal of bonding is to remove voltage differences between components, therefore reducing current flow between them. The bonding conductor does not have an ampacity/duration requirement.
The terms "grounding" and "bonding" are oftentimes used interchangeably.
Back to vehicles and equipment…
The first questions that need to be asked….and answered….are:
- What purpose does grounding the vehicle or equipment serve?
- Will grounding it make the worksite safer?
- Will grounding it make the worksite less safe?
If grounding provides the only low impedance path to earth and there is a possibility that the equipment could become energized, then it should be grounded with the appropriate size and number of grounding cables. An example of this would be grounding a manlift that is being used in live-line bare-hand work.
If, on the other hand, there is already a low impedance path between the conductor being worked on and earth, i.e., a portable protective ground, then connecting the equipment to that ground may serve no useful purpose. Grounding it in this case may, in fact, create additional problems for workers on the ground in the form of step, touch and transferred touch potentials. This is especially true if there are nearby energized circuits that may cause induced voltages in the work area. (Step potential is the voltage between the feet of a person standing near an energized grounded object, touch potential is the voltage between the energized object and the feet of a person in contact with the object, and transferred touch potential is the voltage difference between the energized object and the feet of a person standing on the ground while making contact with the energized object (the ground electrode) through a conductive medium such as the frame or winch line of a vehicle that has been bonded to the grounding electrode with a length of grounding cable).
The practice of habitually making a piece of equipment an integral part of a protective grounding circuit is never a good idea. Each work site and each work situation should dictate whether or not to ground work equipment that has been introduced into the work area. This last sentence deserves repeating: EACH work site and EACH work situation should dictate whether or not to ground work equipment that has been introduced into the work area.
When equipment or a conductive component of that equipment has the capability of making contact, or, during the course of the work, will make contact with a conductor, the workers have essentially five options: Note: Making contact in this case also means violating the Minimum Approach Distance (MAD) of an energized conductor.
- Ground the equipment.
- Ground the equipment and bond it to the grounding electrode the workers are using as their personal protective ground, and to any other grounded objects in the immediate work area. Doing this brings the equipment into the workers’ equipotential zone and introduces the possible hazards of voltage gradients.
- Isolate the equipment from the immediate work area by keeping it physically remote, thus preventing simultaneous contact between it and the grounding electrode and other grounded objects.
- Insulate the equipment from normally energized parts. This may be accomplished by using insulated boom sections and work platforms/buckets, adding non-conductive slings, live-line tools, or other insulating devices between conductive winch lines and the conductor and/or installing insulating covers over the conductors.
- Do nothing regarding grounding, bonding, isolating, insulating, guarding, or barricading the equipment.
Workers on the ground must understand that equipment that is grounded may, in fact, have a voltage impressed upon it if the possibility of induction from nearby energized lines exists. Do not be lulled into a false sense of security that “if it’s grounded, it must be dead”. Even if it is grounded, it may not be dead--it may be energized due to its connection to a conductor that has a voltage induced upon it.
One method of determining how much, if any, voltage a piece of equipment has is to drive a metallic probe into the earth three feet from the equipment. Install a voltmeter between the probe and the metallic frame of the equipment. Do this prior to grounding the equipment. The magnitude of the voltage reading will indicate the degree of hazard of transferred touch potential for workers on the ground and the necessary precautions they should take, such as a non-conductive means of ingress and egress, rubber gloves, insulating footwear, guarding or barricading the equipment, etc. If the voltage reading is:
- 50 volts or less: No further protective steps are required.
- 51 volts to 99 volts: Workers are discouraged from contacting the equipment. Workers whose duties require contact with the equipment should wear personal protective equipment such as rubber gloves and insulating footwear.
- 100 volts to 499 volts: The equipment should be guarded or barricaded. Workers are to avoid contact with the equipment. If workers must get on or off or otherwise contact the equipment, they must use personal protective equipment of insulating gloves and footwear. In addition, an insulating medium such as rubber blankets, non-conductive pads, steps, ladders, or other insulating devices shall be used to prevent the worker from being exposed to a touch potential or transferred touch potential.
- 500 volts or more: The equipment must be separated from its ground source by means of a hot-line tool or otherwise disconnected from ground. Steps such as insulating or isolating the equipment from the conductor, or reducing the circulating current at the worksite must then be taken to reduce the voltage to an acceptable value.
An example of a product designed to measure this voltage and provide an alert is the Step and Touch monitor manufactured by Delta Computer Systems, Inc. of Battle Ground, Washington. This tool (see photo) provides audible and visual alarm warnings if the probe voltage is in a dangerous range. Each voltage range has a distinct pattern of bright flashing LEDs and audible 80 dB(A) alert. (The SNT also detects and warns of Lost Probe Connection and Low Battery). More information on this tool is available at stepandtouch.com.
WORKING NEAR ENERGIZED CIRCUITS:
Equipment that has the ability to make contact with an energized circuit or circuit component should be grounded with the appropriate size and number of grounding cables. The purpose of grounding the equipment is to provide a low impedance path to earth and minimize the time that the equipment remains energized by causing the operation of the circuit protective devices (relays and circuit breakers) should contact be made or a flashover occur.
Vehicles and equipment in energized substations may build up a capacitive charge by being in an electric field. If simultaneous contact may be made between such equipment and a grounded object, the equipment should be bonded to that object.
WORKING NEAR NON-ENERGIZED, GROUNDED CIRCUITS:
Equipment that will make contact with a non-energized and grounded circuit or circuit component should be grounded and bonded in a manner depending upon the circumstances at the work site.
If the non-energized circuit or component has been tested and protective grounds of the appropriate size and number have been installed, and the possibility of induced voltage exists, grounding and/or bonding of the equipment may expose workers to differences of potential if induced current is flowing into the earth through the grounding cables. If the equipment is located in such a manner that a worker can simultaneously touch it and another grounded object (ground rod, metallic structure member, guy anchors, etc.), the equipment should be bonded to those grounded objects.
If a worker in a structure or manlift can simultaneously contact a grounded conductor and a conductive component of the equipment, the equipment should be bonded to the same object to which the conductor is grounded. The intent of bonding the equipment to the conductor or grounding electrode in these circumstances is to eliminate any differences of potential between the equipment and the grounded objects (maintaining an equipotential zone). Once the equipment has been bonded to the conductor or to the electrode, workers on the ground must be made aware that if they contact the equipment, they are, in fact, contacting the conductor because the equipment has been connected to the conductor through the bonding and grounding cables. It is at this time that the worker may be exposed to touch and transferred touch potentials. This is especially true on line rights-of-way containing other energized circuits. Any induced current flowing into the earth through the grounding cables causes a voltage gradient in the area around the electrode. In substations, the voltage gradients are kept to a minimum due to the substation ground mat/grid. On line rights-of way workers on the ground should be discouraged from contacting grounded equipment. The fact that the equipment is grounded doesn’t necessarily mean that it is “dead”. It still may be energized by induction. A good rule of thumb is: ‘if you’re on it, stay on it, if you’re not on it, stay away from it”.
WHEN EQUIPMENT GROUNDING IS NOT REQUIRED:
Vehicle/equipment grounding is not required if the equipment is located and positioned in such a manner that all three of the following conditions are met:
- The equipment or any conductive part of it cannot violate the MAD of any nearby energized conductor, and
- The equipment or any conductive part of it will not make contact with a de-energized and properly grounded conductor, and
- A person cannot make simultaneous contact between the equipment or any conductive part of it and another grounded object.
(In other words, if it can't get into an energized conductor, won't get into a de-energized conductor, and you can't get in series with it, it doesn't need grounding.)
Items # 1 and #2 above may require the use of a Safety Watcher to ensure those conditions are met.
Each work situation and work site condition should be the guiding factors of the procedures and methods used to ground and/or bond the equipment.
If the decision is made to ground/bond the equipment, it must be done in a way to eliminate any differences of potential in the work area. Workers must be made aware of the additional potential hazards created by doing this and be advised on how to protect themselves from these possible step, touch, and transferred touch potentials. This should be part of the job briefing and job hazard analysis and noted on the job briefing/job hazard analysis form.
Maintaining a work area that is free from differences of potential (equipotential zone) is vital for worker safety.
|The Step and Touch Kit from Delta Computer Systems, Inc. contains everything you need to monitor step and touch potential, including: monitoring instrument, user guide, carrying case, battery charging adapters, ground rod and magnetic and clamp probe ends.|
STEP POTENTIAL: The voltage between the feet of a person standing near an energized grounded object. It is equal to the difference in voltage between two points at different distances from the grounded object (the ground electrode). For worker protection this distance is usually assumed to be about three feet, or the distance between a worker’s feet while in a walking configuration. A person could be at risk of injury during a fault simply by standing near the grounding point.
TOUCH POTENTIAL: The voltage between the energized object and the feet of a person in contact with the object. It is equal to the difference in voltage between the object (which is at a distance of 0 feet) and a point some distance away, usually assumed to be about three feet, or the distance a worker could touch an object while standing on the ground.
TRANSFERRED TOUCH POTENTIAL: The voltage between the energized object and the feet of a person standing on the ground while making contact with the energized object (the ground electrode) through a conductive medium such as the frame or winch line of a vehicle that has been bonded to the grounding electrode with a length of grounding cable. The same applies to a worker with no worksite protection who is contacting a conductor that is grounded at some remote distance. This voltage may be much higher than the “touch potential” due to the greater distance between the worker and the grounding electrode.