High voltage power management ICs need to be handled with utmost care when verifying their electrical characteristics on the testbench. Emerging HEV/EV applications such as Totem-Pole Power Factor Correction (TP-PFC) circuits may operate at voltages reaching 400 VDC (see Totem Pole PFCs with Synchronous FETs). This voltage level, though still ranked as low voltage by the IEC and US voltage classes, is enough to cause human injury like severe burns. It is therefore important to observe safe practices because of this safety hazard.
MFI Polytechnic Institute, Inc. (previously known as Meralco Foundation, Inc.) hosted a 1-day on-site High Voltage Safety Training Seminar to equip engineers with ample knowledge on how to deal with high voltage in the industry. It is ideal and fitting for such an institute to conduct such training because Meralco happens to be a major distributor of household electricity in most parts of Luzon (northern part of the Philippine archipelago). Completion of aforementioned training equips the participant with sufficient know-how to perform work involving high voltage with confidence, being fully prepared against untimely accidents and assured of minimal exposure to risk.
The following is a brief summary of what transpired during the seminar, including important points that were expounded on at length over the entire day. It is not an exhaustive account of all topics covered, but only aims to give an example of what MFI's training is like.
The seminar began at around 08:00 AM (+8:00 UTC/GMT Phil. time) and ended at 17:00 PM. Lunch and break times had free meals, Filipino delicacies home-cooked by the campus cafeteria. All materials and instruments for demonstration purposes were prepared beforehand inside the classroom, attended by approximately 15 participants. The speaker was a registered electrical engineer, with extensive experience in giving talks focused on high voltage safety.
Upon arrival at MFI's campus, the participant must register at the guard station, then proceed to the registrar's office - which was at the 1st floor - room 105 at the time of writing. A pink registration form must be promptly filled up with one's personal details, then handed over to the receptionist at the desk. Room assignments will be provided afterwards. I was personally assigned to room 322 at the 3rd floor. Being my first time at the campus, I had to look for the room, which wasn't that difficult to spot. The staircases were meandering at the center of a seemingly 4-storey building. The structure had signs of being a few decades old, but the interior was well-kempt enough to keep up with aesthetics. There was a small library at the 2nd floor, with the only men's restroom situated at the end of the curved hallway. There was also only one women's restroom placed on the 3rd floor almost adjacent to the room I was assigned in.
Morning Session
Training began with a clear-cut definition of objectives for the seminar, hinging on gaining confidence in handling equipment operating at high voltage, overcoming hazards that may cause accidents and fatalities, and determining how to practice learned concepts onto the participant's actual job since applications and workplace setup vary by niche.
The speaker emphasized the most common factor neglected when working with high voltage is complacency. It is apparent repetitive work without incident nurtures complacency, followed by a tendency to ignore common safety practices. Hence, there's a need to prioritize discipline in enforcing safety procedures unconditionally to prevent unwanted accidents from occurring.
After defining target goals, the seminar moved on to relevant definition of terms. For example, how does one discriminate between low and high voltages? This is important to determine the type of personal protective equipment (PPE) one would need. Higher voltages necessitate thicker gloves. When it comes PPE, it's also important to note until when they can last (or their mean time before failure or MTBF). MTBF needs to be accounted for because rubber deterioration affects resistance. A lot of factors such as environment and chemical exposure, heat, puncture holes through the rubber reducing the effective dielectric constant from ~4.3 (rubber) to ~1.0 (air/vacuum).
ISO 3864 International Safety Symbol for High Voltage
A definition of common terminologies in high voltage setups with examples were discussed too. Standards mentioned include the 2014 National Electrical Code (NEC), NFPA 70e, IEC 60038, and so on. According to the NEC, any equipment rated above 1000VAC or 600VDC falls under the high voltage category. In the US, it is defined to range at 69kV to 230kV by ANSI C84. Safety methods defined by the IEEE 1975 standard were also touched upon.
Grounding was also a key highlight in the discussion. Circuit breakers with built-in ground faults, ground fault connection interrupters GFCIs, as well as other important components and equipment that facilitate proper grounding were examined. This raised a question in my mind too as to what the most efficient way of testing a true earth ground was (aside from the conventional DMM test). I was aware of the lamp test which employed a simple light bulb and two wires, but there may have been a more convenient testing device for it. Aside from grounding, shock and flash hazard analysis under the NFPA 70e was also explored.
Some useful safety practices were also talked about, such as simulation of high voltage scenarios at least twice a year. Doing so prepares and inures those working with high voltage to emergency scenarios. Circuit breakers, being mechanical switches, also mandate annual reset tests to guarantee continued reliability. Rubber gloves, previously mentioned to be susceptible to defects from physical damage or wear and tear, need to be tested every 6 months as well, with the simplest test being inflating the gloves with air by pinching the gloves' entry point. Rubber gloves that fail the test should be cut with scissors. High voltage premises cannot be crowded and must allow enough space for easy movement. Both electrical and mechanical components need to be checked in a high voltage system too.
With regards to live parts, retightening for maintenance necessitates the correct type of screwdrivers. A common mistake was briefly cited by the speaker from hiring experience, where candidates would force an insulated screwdriver to a whole. The correct response would have been to request for the appropriate screwdriver size. Terminal logs, and making good connections at termination points were also addressed.
Performing a hazard or risk analysis when dealing with live parts is also crucial to guaranteeing safety. Participants of the discussion were taught a comprehensive step-by-step process to follow, from evaluating circuit information to determining the correct PPE to use. This also made me curious as to which software was best for demarcation of "Approach Boundaries". There was a brief mention of ETAP electrical transient analyzer program, but delving into it might derail the topic. One area of improvement I can think of that involves coming in contact with live high voltage equipment is replacement of actual personnel with unmanned automation or robotics.
High power measurement with a micro-ohmmeter (apparatus that uses a Wheatstone bridge) was also examined. There may be a need to check contact resistance in circuit breakers, and micro-ohmmeters are the best measuring instruments for the job. It uses the common 4-wire connection setup, where barely any current goes through the sense lines, making measurements immune to voltage drops along the main lines. (Please see my column about 4-wire measurements in Electronic Design here). It is common practice to stop using circuit breakers with contact resistances that have exceeded the mΩ range.
Another key highlight was the consequence of misinformation and failure to corroborate, commonly rooted in complacency. Such misinformation naturally leads to accidents. Validating information before taking any actions can therefore prevent high voltage mishaps from occurring.
A description of "Qualified" high voltage workers was also examined in the discussion. It was further elaborated that some workers can be qualified for some safety methods or high voltage equipment use, but not for others. Sequence of tasks is of utmost importance in this scenario and when there are more workers, everyone should be qualified. Unqualified persons are disallowed from working with exposed high voltage circuits/conductors without special training under the NFPA 70e provisions and without direct supervision from a "qualified" worker.
Also mentioned in the morning part of the seminar were cases of ventricular fibrillation caused by unwanted exposure to high voltage. Knowing how to do cardiopulmonary resuscitation (CPR) can be life-saving when the need arises.
Afternoon Session
Lunch was delightful and the speaker was open to some chit-chat. After the participants had their fill, discussion resumed on ideal conditions of an electrically safe work environment.
Points covered included consideration of all involved energized circuits and power supplies, checking of drawings and diagrams, examining potential hazards, LOTO (lock out tag out) system, voltage testing with pertinent equipment, and so on. Most demonstrations used equipment from Salisbury, Lotus and other PPE brands. Examples are the Salisbury 4454 non-contact type voltage detector which detects presence of voltage similar in operation to ElectroBoom's video below:
and ILP Leather protector which protects rubber gloves by preventing lacerations - important note that leather gloves are not substitutes for rubber gloves!
Proper use of a ground cluster (commonly called a spider) was also demonstrated, where it was repeatedly emphasized not to connect it in a loop, and to first establish earth ground prior to connecting with other equipment. A shotgun-type hot stick was also presented, which helps move and whack objects without fearing electrocution.
Different kinds of circuit breakers were also exhibited, such as the molded case circuit breaker MCCB and miniature circuit breaker MCB. Participants were advised to also use appropriate thickness in matting, similar to rating of gloves used.
Contents of an energized electrical work permit were also scrutinized, where important work details must be listed, from a justification why work must be done to results of various risk assessments. This document must be authorized with an energized work approval signature - with signatories notified in advance.
Employee and employer responsibilities were also reviewed, from proper training and information awareness to providing appropriate job briefings.
There was also a lengthy discussion on high voltage PPE price points, where implementing high voltage safety measures can be quite pricey. Thus, there must be balance between reasonability in risk assessment and PPE purchases.
Principles of arc flash and arc blast were also delved into. An arc flash is the sudden release of heat, and can reach temperatures of 35,000 °F - almost 4 times hotter than the sun's surface! An arc blast is the pressure wave resulting from an arc flash, much like a bomb which is strong enough to propel workers many feet away. It can vaporize copper and expand 67,000 times its mass (necessitating ear muffs or ear plugs as pressure waves can reach 160 dB - Note OSHA requires hearing protection for ambient sounds greater than 85 dB). Incident energy (heat released from arc flashes) is measured in cal/cm^2. Over-all suits are labelled with a clear threshold of up to how many cal/cm^2 incident energy it can protect you from. Different values of cal/cm^2 were compared, including incident energy categories.
A recording of an electrolytic capacitor exploding due to overvoltage in the training
The speaker also gave an important reminder that any form of metal is not allowed on the body of workers when dealing with high voltage.
Current faults such as overcurrent protection (OCP), short circuit protection (SCP), and overload current were talked about. Some OCP device triggers were examined, with timing trigger ranges from 8.3ms to 100ms. It is worth noting that faster circuit breakers are more expensive. Common causes of high voltage accidents were also discussed, from carelessness, broken insulation, exposed live part and high voltage cables, static electricity, damaged tools/equipment, nearby water/liquids, obstructed disconnect panels, loose wire connections, and improperly maintained PPE to name a few.
To augment insulation, Super 33 scotch 3M vinyl and rubber tape samples were passed around to participants.
Different layers of an "Approach Boundary" (where 10ft. from the source is a good rule of thumb) were delineated by the speaker. Such layers are the arc flash boundary and a limited and restricted approach boundary.
PPE categories were also discussed. Higher cal/cm need higher category PPE. For most purposes, CAT 1 PPE will suffice like hard hat, face shield, safety goggles, hearing protection, safety gloves rated at the correct system voltage, long-sleeved shirt and arc-rated pants made of polyester and leather footwear. Energies involving 8cal/cm^2 need CAT 2 PPE, 25 cal/cm^2 for CAT 3 PPE, and 40 cal/cm^2 above requiring CAT 4 PPE such as arc flash hoods. Based on PPE category, it is up to the participants to properly assess which PPE is applicable to their practice.
Finally, the seminar closed with a short recap on all safety measures discussed with real-life cases of high voltage effects on the human body, possible sources of electrical hazards and control procedures that address them, as well as some important pointers and rules to follow including common rescue protocols to be practiced.
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