When an electrical accident occurs, hindsight is always 20/20. More often than not, upon investigation and re-evaluation, investigators find multiple alternatives to the procedures that led to the incident, particularly when serious accidents are involved. Why does it take such accidents to make us rethink a safer means of performing a certain task?
Ironically, the phrase I hear most often after an electrical accident is “I thought.” Truth be told, it doesn't really matter what comes next. The point is, when you are working with a force that can cause your immediate demise as a result of any accidental contact, any explanation that begins with “I thought” is simply unacceptable.
To avoid an electrical mishap or tragedy, you must be certain that your work procedure is the safest procedure to accomplish a given task. For this reason, employees need to be trained to recognize potential hazards specific to the equipment they're working on as well as the task they'll be performing under specific circumstances. The fact is that most electrically related accidents are preventable — in most cases, the employee must commit an unsafe act for an injury to occur in the first place.
When assessing a work situation and determining the best course of action, it's often difficult to realize that sometimes, when the existing procedures are hazardous, it is often better to find a safer alternative to complete the work. Following are a handful of examples that illustrate safer alternatives to some industry procedures commonly used in the electrical industry.
Example No. 1: Using line voltage to test a fuse
Following is the textbook procedure for the simple task of determining which fuse is blown in a 480V, 3-phase disconnect switch.
Turn off the disconnect.
Defeat the door interlock, and open the disconnect door.
Turn the switch back on.
Take voltage readings across the fuse or in a 3-phase application from the line side of the disconnect switch to the load side of the fuse on another phase (click here to see Fig. 1).
Note: The technician is trying to determine what, if anything, is wrong with this equipment. Electrical equipment can be most dangerous when a defect is suspected. That's why it's important to spend as little time as possible in close proximity to energized electrical equipment with the door open and /or cover removed.
This test is dangerous for a number of reasons:
It exposes the employee to two major hazards: electrocution and arc flash.
Applying a meter across an open fuse could cause the equipment connected to start unexpectedly.
If the problem is not a blown fuse, the employee just closed the disconnect switch with the door open, which is hazardous.
If the fuse is blown, there is the possibility of installing a fuse in an energized circuit, which is also hazardous.
A safer procedure to check fuses would be:
First, open the disconnect switch and leave it open. Then, verify there is no voltage present at the line or load of the fuses. Use an ohmmeter or continuity tester to check the fuses.
Why use 480V to test a fuse when 9V (meter battery) will accomplish the same thing? Some years ago, an electrician and four other people died in a fire when the electrician installed a fuse in an energized disconnect. By all indications, the fuses in the disconnect switch were tested using the line voltage procedure. Apparently, the worker either did not realize or forgot that it was energized when he attempted to install the new fuse. One sure way to prevent this from happening at your facility is to establish a policy that no fuse shall ever be tested with a line voltage. This could be supplemented by a policy that any violation would be grounds for termination.
Example No. 2: Voltage drop/watts loss test
An electrician suspects that high-contact resistance in a starter connection is responsible for nuisance tripping of the overload relay or fuse blowing. One procedure involves using a voltmeter across the contacts to measure the voltage drop. The higher the voltage measured, the higher the resistance at the point of connection. This high resistance generates heat, eventually resulting in blown fuses and/or overload relay operations. Once again, we find the employee standing in front of equipment that is energized and under load.
A substantially safer way to evaluate connections in electrical equipment is to use a low-resistance ohmmeter on de-energized equipment. For approximately $100, you can purchase a non-contact temperature probe and — when used with the proper personal protective equipment (PPE) — detect abnormal heat generated by high-contact resistance at fuse, breaker, or starter connections and/or contacts. This test is not as safe as using a low-resistance ohmmeter on a de-energized circuit, but it is an alternative that increases the distance between you and the equipment — plus it doesn't require you to disturb energized parts.
Example No. 3: Locating a ground fault in a power distribution system
A 4,000A, 480V bolted pressure switch has tripped on ground fault. Because this is the main for the facility, production has come to a halt. Except for emergency lighting, there are no lights. The electrician closes the switch, and it immediately re-opens. The decision is made to open the down stream feeder disconnects, close the main, and then close the feeder disconnects one at a time until the 4,000A main trips. Leading to a switchboard fault and fire, this procedure resulted in extensive equipment damage, loss of production, and a very close call for the electrician.
An investigation revealed the switch failed to open due to a damaged trip coil caused by the electrician repeatedly closing into a system fault. Breakers and switches are not designed to be closed into faults. The fault should be located, isolated, and corrected prior to re-energizing the service. OSHA specifically addresses this in the Code of Federal Regulation - 29CFR 1910.334(b) (2). Because of the potential for serious injury or death, “After a circuit is de-energized by a circuit protective device, the circuit may NOT be manually re-energized until it has been determined that the equipment and the circuit can be safely re-energized.” The repetitive manual closing of circuit breakers or re-energizing of circuits through replaced fuses is strictly prohibited.
Unfortunately, the procedure used by the electrician is a common practice in the electrical industry. But when you think about it, using a 4,000A, 480V switch or circuit breaker for locating any fault is extremely dangerous.
Once again, a safer procedure involves the use of a 9V battery in an ohmmeter to locate the feeder with the fault without exposure to the hazards associated with having line voltage present during the test (Fig. 2).
The first step is to leave the main switch or breaker open, and open all the feeder breakers. The next step is the most important one. While wearing the required PPE and using a properly rated voltmeter, test for both voltage phase-to-phase and phase-to-ground at the load side of the switch or breaker. Keep in mind that clearing a fault can stress and damage the insulation in a protective device. As a result, voltage can be present at the load side with the breaker open. If you measure voltage on the load of the breaker or switch, do not continue with the test. The breaker or switch may have to be replaced. Using the ohmmeter on the load side of the open switch or breaker, place one lead on ground and the other lead on the A phase terminal. The meter should indicate very high resistance (i.e., an open circuit). Check B and C phase-to-ground for the same result. Close one feeder at a time and check A, B, and C phase-to-ground.
You are looking for very low resistance (i.e., a short or closed circuit on one or more phases to ground). Some single-phase loads will give you a low-resistance reading, but the circuit that caused the trip will give you a reading close to zero resistance. If the fault is in a motor, the starter will be open, and you will need to check individual motor circuits at the load side of the starter to find the fault. After you locate the fault, continue to check all the feeders to assure there is only one faulted circuit.
Prior to re-energizing the main, you must first verify there is no damage to the switch or breaker caused by the original fault condition. Where applicable, test the device to assure it is suitable to be returned to service, consulting the manufacturer for specific instructions as necessary. While wearing the required PPE, close the main and one feeder breaker at a time, leaving the faulted circuit off until you locate and clear the fault on this feeder. Note: If a main protective device trips, the trip unit or ground-fault relay may be set too low for the system. Typically, the manufacturer sets the ground-fault pick-up to minimum 100A, ±20%. A coordination study by an engineer is required to determine the proper setting after the equipment is installed and put into service. Do not set the ground-fault pickup to maximum — this could create additional system coordination issues.
Example No. 4: Loss of ground in a portable device or appliance
One of the most dangerous conditions at work or at home is the loss of ground to electrical equipment. Because losing the connection to ground does not affect normal operation, this dangerous condition can exist for years. Proper grounding of equipment allows the upstream circuit breaker to trip open to clear a fault that might occur internally within the equipment. If a portable tool or piece of equipment loses its connection to ground and later develops an internal short, the exterior can become energized — and anyone who touches it and ground will get shocked. For example, the washing machine loses its ground connection in 2003 and develops an internal fault four years later. You touch the washer and dryer (which is solidly connected to ground). If that happens, you or a family member will receive a potentially lethal shock.
A simple, inexpensive test can alert you to this dangerous condition. What's needed in this situation is a simple non-contact voltage tester. To demonstrate how this works, you will need a non-contact tester and a two- to three-prong outlet adapter. First, test the tester in an outlet. Then hold the tester close to the metal case of a piece of equipment that is connected to the outlet with a three-prong plug. If properly grounded, the voltage probe will not light or sound. Remove the appliance plug from the wall and connect to a three-prong to two-prong adapter. Do not use the outlet cover screw. Plug it into an outlet, and then hold the voltage probe close to the metal case of the appliance. The probe will sound and/or light, indicating the presence of voltage. In this test, there is no voltage present. This test simply indicates the loss of ground to electrical equipment, typically for less than $20.
Generic safety programs do not address specific hazards. Employees must be trained to recognize safety deficiencies in established procedures and take immediate corrective action. A qualified electrician who has specific training on the power distribution and control equipment in the facility should conduct all testing and repairs. Whenever possible, choose a testing method that does not require line voltage as part of the test.
Kennedy is a licensed electrical contractor in Illinois and field service system representative for Square D/Schneider Electric, Crown Point, Ind. He also serves as an OSHA authorized general-industry outreach safety trainer and instructor/curriculum developer for Prairie State College, Joliet Junior College.