Ecmweb 8327 Electrical Forensics Pr
Ecmweb 8327 Electrical Forensics Pr
Ecmweb 8327 Electrical Forensics Pr
Ecmweb 8327 Electrical Forensics Pr
Ecmweb 8327 Electrical Forensics Pr

The Case of the Test Load Tragedy

July 19, 2016
A charged high-voltage capacitor leads to electrocution of unsuspecting technician at power supply manufacturing facility.

When a technician reported for work shortly before 7 a.m., he picked up exactly where he’d left off the previous day — continuing the task of testing a water-cooled power supply at the unit’s manufacturing facility. An employee of the company that made high-voltage power supplies, he had left the test setup intact with power off overnight. On this particular morning, he resumed his regular testing activities, but it wasn’t long before this seemingly routine job would take a tragic turn.

Photo 1. This is an interior view of the cabinet housing the test load equipment.
The scene

The power supply unit being tested was capable of sustained operation, generating electrical pulses of 21,000V and 1.5A at the rate of 300 pulses per second (total power output of 15,000W). Such a power supply would typically be used in industry to power a high-energy laser or other specialized scientific device.

The incident involved a test load that was not a commercial product but had been assembled specially for or by the power supply manufacturer for internal use of testing its products (Photo 1). That is, the company (and/or its predecessor companies) had hand-built a small number of special apparatuses that would appear electrically (to the power supply) the same as apparatuses to which the power supply would be connected in use elsewhere.

As noted in the power supply manufacturer’s testing procedure, the test load is denoted “Appropriate H.V. [high-voltage] load.” The test load apparatus is housed in a padlocked metal cabinet standing about 6 ft tall and equipped with casters. When either cabinet door is opened, the test load exhibits exposed electrical components, including high-voltage terminals. This apparatus simulates the load on the power supply by triggering (or actuating) a thyratron to close a circuit connected to its resistor-capacitor bank (Photo 2).

Photo 2. The resistor stack at the top of the cabinet simulates a load during testing.

The thyratron is a gas-filled “vacuum tube” that acts as a switch. When the thyratron conducts in response to one or more trigger pulses, the energy of the power supply is dissipated by the resistor-capacitor bank as heat. Therefore, the test load is an “active load” in that trigger pulses can be run with a single pulse or repetitively (under control of an operator), thereby exercising the range of capability of the power supply under test.

The accident

At the time of the incident, the power supply’s high-voltage output had been set to the test level of 18,700V, while the power supply’s output had been switched OFF.  Although the test load was not being triggered (i.e., not drawing current from the supply), the anode of the thyratron would remain at — or slowly decay from — 18,700V.

Photo 3. Interior of the test load cabinet showing high-voltage circuitry.

Pausing the test sequence, the technician reached into the test load cabinet to effect an adjustment to the bias network, which is an electronic circuit associated with (and adjacent to) the thyratron (Photo 3). Reconstruction reveals that the technician’s right hand contacted the thyratron anode and his left leg made contact with the grounded cabinet door. The resulting current flow through his body resulted in burns and electrocution. The victim was found on the floor beside the test load cabinet a few minutes later and was unable to be revived.

Immediately after the accident, the right cabinet door was observed open, and the left cabinet door was closed. The thyratron was inside the right door but mounted low — reachable only from a stooping or crouched position. Damage to the victim’s right hand included burned flesh, of which residue was later found on the thyratron anode. Similarly, damage to his left leg matched a burn scar low on the edge of the left door.

The lawsuit

I was hired as a forensic engineering expert by the plaintiff in this case (the deceased victim’s estate) to determine the cause of the incident.

The facts

After beginning the initial investigation, I determined there were several undisputed facts that would be important in this case.

• A sign indicating “Danger — High Voltage” was posted on the outside of the test load cabinet. It was not visible when the cabinet was open. A small sign was also placed inside the cabinet near the accessible high-voltage components. The existence of high voltage in the area was an everyday hazard, since this was the high-voltage power supply test area. As such, workers would be expected to use available test equipment and would not need to contact high-voltage terminals during power supply testing.

• The victim, who was working alone in the area, was a tester — untrained in electrical theory. He was not wearing gloves or any other personal protective equipment (PPE).

• The test load cabinet had solidly closable doors with a hasp for a padlock. The interior of the cabinet was not normally accessible, and access would not normally be required for a worker during a test.

• The cabinet door had an electrical interlock switch, but the interlock function had been disabled. Nevertheless, the interlock did not affect the presence of high voltage generated by the power supply under test. When the power supply was energized, the voltage was maintained irrespective of the door being open.

• The front of the test load cabinet was turned away from the test bench so that a worker could not simultaneously view the power supply under test and the interior of the cabinet. There would be no need for a tester to gain access to the inside of the test load cabinet while a unit was under test.

• The manufacturer’s employee handbook listed general safety regulations, such as “Report all hazards or unsafe conditions to your Supervisor” and “Maintain good housekeeping in your area.” The only association with electrical hazard was the admonition: “Do not perform any electrical repairs unless authorized to do so.”

Investigation and analysis

Several findings came out of the forensic investigation, including the following.

• Although the power supply generated the high voltage that electrocuted the technician, it exhibited no product defect or malfunction. Instead, it was performing correctly, generating the specified 18,700V into the load being used for test purposes.

• Discussion on design or construction of the power supply itself was not appropriate to the accident; the test load apparatus was the main focus.

• The test load was an apparatus assembled strictly for the purpose of testing the company’s power supplies. It had no provisions for adjustments to be made by power supply testing personnel, and its hazardous interior was guarded by lockable doors. It is unknown why the technician accessed the test load cabinet to adjust the bias network.

Photo 4. The test technician was unaware of the hazard associated with this high-voltage capacitor.

• The testers were occasionally required to access the interiors of the test loads in order to change capacitors (Photo 4). Capacitors retain an electrical charge over a period of time. The charge bleeds off through the internal resistance of the capacitor or decays through resistors, such as those in the bias network. No instructions were given to the testers about the electrical hazard posed by the capacitors inside the test load.

• The test load was wired so that the power supply under test charged the capacitor(s). When the thyratron fired, it conducted electricity and instantly discharged the capacitor(s). If the thyratron does not fire, the charge decays gradually. Mathematically, the process is known as “exponential decay.” The decay is characterized by a “time constant.” This is the time required for voltage to decay from some starting level to 36.8% of that level.

• After the incident, the power supply was found set at the “logic off” mode. This mode provides for the power supply to be activated but with no output: High voltage is instantly available when logically enabled. When disabled, the power supply generates no further output. However, the test load retains the electrical charge it was given. This is termed “residual” charge because the electricity remains stored in the capacitor irrespective of the power supply that generated it.

Depending on the capacitor and resistor values comprising the circuit of the test load, the decay takes seconds or minutes for the voltage to drop to a safe level. One expert witness for the employer believed that the time constant of the test load used by the victim was 10 seconds. If this were the case, the voltage dropped from 18,700V to 36.8% of its initial level (or 6,622V); to 2,437V in 20 seconds; and to about 900V in 30 seconds.

Nevertheless, the victim was probing the interior when he came in contact with high voltage. While it is not known whether the voltage was still at the test level of 18,700V or had begun to decay, it was sufficiently high to burn flesh and to cause his death.

• The power supply manufacturer provided handbooks and safety training to employees covering a variety of subjects, such as evacuation procedures and hard-toed shoes for protection from falling objects. However, the company exhibited no recognition of the danger posed to power supply test personnel from residual charge within the test loads.

The outcome

My client intended to pursue a claim of gross negligence against the employer.  The last I heard was that the employer was found to NOT have exceeded the threshold of “wanton disregard for employee safety.”  I don’t know whether an appeal was/is being filed.

Conclusions

Based on the forensic investigation results, I formulated several conclusions that can serve as lessons learned.

• First, the time required for the victim to switch the power supply under test to logic off, crouch in front of the test load cabinet, and reach into the area of the thyratron anode could have been much less than the time required for the residual charge on the capacitor(s) in the test load to decay to a safe level.

• Since it is well recognized that even residential power line voltage in the order of 120V is hazardous and possibly lethal, the employer (the power supply manufacturer) would know that workers operating equipment generating or using 18,700V could be exposed to serious electrical injury or electrocution.

• The employer had failed to apply or enforce safety rules regarding wearing protective equipment, not working alone, following test procedures, and taking other precautions into account to avoid electric shock and its effects. For example, test personnel were not warned about residual charge. In addition, the company provided inadequate or no training to the technician about the electrical hazards presented by the test load he was using to test the power supply. The common practice of placing one hand in a pocket to prevent completing an electrical circuit across the body is not applicable because it was the victim’s leg that completed the circuit.

• Probing the interior of the cabinet to make an adjustment or to correct a defect or malfunction, such as an inoperative or intermittent trigger generator, required technical skills beyond the technician’s expected duties as a tester. Potential exposure to lethal voltage within the cabinet was outside of the normal hazards presented by his workplace and was not an everyday hazard.

Photo 5. Note the small “Danger High Voltage” sign located inside the cabinet.

• Although the employer posted generic warning signs about the presence of high voltage (Photo 5), nothing indicated the particular danger presented by the thyratron anode and other live parts contained within the cabinet. In particular, the company provided no instructions or warning about residual charge on the high-voltage capacitor(s).

• By permitting the technician to open the doors and access the interior of the test load cabinet, the employer effectively allowed him to remove safety guards.

Had the employer provided appropriate training, supervision, and/or physical barrier to the victim’s access to hazardous voltage within the test load cabinet, the incident would not have occurred.

Boyell, P.E., is a forensic analyst, consultant, and expert witness in Moorestown, N.J. with 40 years of experience in the electrical and electronics industry. He can be reached at [email protected].

SIDEBAR: Electrical Hazard Identification and Recognition

One example of potential hidden electrical hazards deals with television receivers and video displays that rely on cathode-ray tubes, which require internal voltages in the order of 18,700V. Technicians working with this type of equipment recognize the electrical hazard of high voltage. Television service manuals and equipment access panels provide prominent warnings.

The “Standard Handbook of Video and Television Engineering” (J.C. Whitaker and K.B. Benson, eds., McGraw Hill, 2000) devotes Chapter 19.3 to “Safety Issues.” In particular, Table 19.3.2 is a checklist of “High Voltage Precautions.” These include:

• Discharge all capacitors using the discharge stick provided.

• Do not remove, short circuit, or tamper with interlock switches.

• Keep away from live circuits.

Irrespective of whether the high voltage is generated to operate a cathode-ray tube inside a TV receiver (or by a power supply), the hazard is recognized by the electrical/electronic industry:

“When working around high voltages, always look for grounded surfaces. Keep hands, feet, and other parts of the body away from any grounded surface. Even concrete can act as a ground if the voltage is sufficiently high. If work must be performed in ‘live’ cabinets, then consider using, in addition to rubber gloves, a rubber floor mat, rubber vest, and rubber sleeves. Although this may seem to be a lot of trouble, consider the consequences of making a mistake. Of course, the best troubleshooting methodology is never to work on any circuit with being certain that no hazardous voltages are present. In addition, any circuits or contactors that normally contain hazardous voltages should be firmly grounded before work begins.”

Here are three additional references that are worth reviewing:

The NESC [National Electrical Safety Code] Handbook, Fifth Edition, p. 484, states: “The NESC recognizes the responsibility of employers of persons engaged in the installation, operation, and maintenance of electric supply and communication lines to provide those employees with training, equipment, and tools, and supervision that is appropriate for the intended work under the expected conditions…. Rule 410 requires employers to use positive procedures to secure compliance of employees with the rules. Generally, this takes the form of training, supervision, and verification of employee knowledge.”

The National Electrical Code [NEC] Handbook, Eighth Edition, p. 52 states: “Live parts of electrical equipment should be covered, shielded, enclosed, or otherwise protected by covers, barriers, mats, or platforms to prevent the likelihood of contact by persons or objects.”

OSHA regulations (in particular subsections of 29 CFR 1910.147 and of 29 CFR 1910.269) expand these principles of workplace safety with respect to electrical hazards and their mitigation.

About the Author

Roger Boyell, P.E. | Forensic Analyst

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