The domain of most electronics engineers these days is confined to voltage levels ranging from microvolts on up to a few hundred volts. But when electronic equipment fails or there’s a problem with facility wiring, these same engineers may, in a pinch, be called upon to do basic troubleshooting of power levels far beyond their comfort level.
Safety measures become particularly important when high voltages and currents are involved. Engineers and technicians in the utility industry pretty much know such safety measures by heart. Technical personnel working outside this industry may find some of these safeguards non obvious.
We are reminded of an EE colleague who had to troubleshoot a control panel, a task outside his usual list of duties. To kill the power, he pulled out the top end of a cartridge fuse feeding power to the panel. Control panels are normally wired so the hot side of the fuse holder is at the top, so pulling out the top fuse contact removes power to the rest of the panel. And of course, the top fuse contact will be dead as well.
This particular panel was miss-wired. The top of the fuse he yanked was sitting at several hundred volts.
You probably already have an idea of what happened next. He touched the hot fuse. Fortunately, the high voltage made the muscles in his legs contract which pulled him away from the panel and broke the connection. Though he was shaken up, his only injury consisted of a dime-sized third-degree burn on the palm of his right hand where he’d touched the end of the fuse.
The moral of this particular story is to make sure the apparatus you’re working on is deenergized, but the bigger lesson is that EEs unaccustomed to working with higher voltages need to think more carefully about their surroundings. With this bit of advice in mind, here are some points to ponder.
If whatever is causing trouble requires you to climb a ladder, remember that a ladder is potentially a conductor. Wooden ladders are largely obsolete and, if still in use, should not be painted because the paint may hide signs of decay. Metal ladders are of course more durable but conduct really, really well. If you touch a live wire, you are solidly grounded and potentially will get nailed. The best solution is to use the electrician’s insulated fiberglass ladder when doing electrical work. Unfortunately the average shop probably doesn’t have one of these, in which case a wooden ladder would be the next best choice.
Always assume equipment is energized until proven otherwise, a point appreciated too late by our EE colleague. In particular, overhead powerlines that are part of an electrical distribution system and must be presumed to be live at all times. Generally, they are energized at over 1,000 V. An extremely hazardous situation exists when a worker carries or attempts to position a metal ladder in the vicinity of a live conductor. If the ladder even brushes against a live wire or conductive object including roof gutter or flashing, all those electrons have a tremendous desire to go to ground, with the hapless worker’s body completing the circuit.
If the ladder touches the energized conductive object and the victim’s body, bystanders must attempt to break open this vicious circuit as quickly as possible. The ideal means of doing so is with a long dry wooden pole or similar insulated object. One easy source of an insulated pole is to dismantle a fiberglass ladder that has reached end-of-life and save the insulated rails. Be sure to strip off any metal brackets and hardware.
One other ladder tip for the occasional user: If a leaning ladder is too steep or not steep enough, it may either tip over backwards or slide back from the building with potentially severe consequences for the user. The ideal angle is roughly 75°, depending upon the ground material. If the ladder leans against a vertical wall, adhere to the 4-to-1 rule, meaning that for every four feet of vertical height, the base of the ladder should be one foot from the building.
If the problem at hand is underground rather than overhead, is ordinarily neither feasible not worthwhile to dig up any great amount of buried cable. Faults in long runs can be located using a time-domain reflectometer. It works like radar, sending out a pulse, timing the interval between transmission and reception of the reflected signal and calculating the distance based on the propagation time to and from the reflecting fault. The procedure is simple but the equipment is costly, though it’s possible to configure a scope/signal generator combo to get rough TDR readings.
When it comes to measuring high voltage, a factor called the CAT rating becomes important. The purpose of the CAT settings is so that the meter doesn’t blow up in your face if there is a voltage/current spike. The severity of damage from the spike has to do with available current and the construction of the meter.
You will notice that electrical measuring instruments invariably have CAT rating printed on the front panel. They are totally separate from the range settings mentioned above. They do not pertain to the amount of voltage the instrument can measure. Instead, they tell you the maximum amount of voltage that can safely be connected to, without endangering the user, for each of four locations.
Here’s how instrument maker Fluke depicts Cat locations.
CAT ratings are based on the location where test instruments will be used and arise from the National Fire Protection Association NFPA 70E electrical safety standard. Test instruments, such as multimeters, are designed to comply with specific NFPA 70E CAT ratings.
The locations defined in NFPA 70E are designated by Roman numerals: I, II, III and IV. CAT I is the least hazardous and CAT IV is the most hazardous. These ratings are based on the idea that while making a routine voltage measurement, the circuit could experience a transitory overvoltage event. The CAT ratings protect the worker from that hazard. Thus instrument users must be able to look at a location and decide on its rating, then consult the meter to find the permitted voltage for that location.
The CAT locations, beginning with the least hazardous, are:
CAT I – Signal-level tools for communications and electronic equipment.
CAT II – Local-level circuits for fixed or non-fixed power devices.
CAT III – Most distribution circuits including fixed primary feeders or branch circuits.
CAT IV – Primary supply sources, including 120 or 240-V overhead or underground lines that power well pumps. The CAT IV rating includes the highest and most dangerous level of transient overvoltage in utility service equipment like exterior transformers.
Each of these CAT locations is accompanied by a maximum permitted voltage specific to the measuring instrument. Within a category, a higher voltage rating denotes a higher transient withstand rating. Thus a CAT III-1,000 V meter has better protection than a CAT III-600 V rated meter. But note: A CAT II-1000 V rated meter is not superior to a CAT III-600 V meter. This requires some explanation.
Transient test values for selected measurement categories.
IEC 1010 test procedures determine a meter’s true voltage withstand value. They take into account three main criteria: steady-state voltage, peak impulse transient voltage, and source impedance. Within a category, a higher “working voltage” (steady state voltage) is associated with a higher transient. For example, a CAT III-600 V meter is tested with 6,000-V transients while a CAT III 1,000-V meter is tested with 8,000-V transients. But there is a difference between the 6,000-V transient for CAT III-600 V and the 6,000-V transient for CAT II-1000 V.
The difference is in the source impedance. Ohm’s Law says the 2-W test source for CAT III has six times the current of the 12-W test source for CAT II. The CAT III-600 V meter has better transient protection compared to the CAT II-1000 V meter, though it voltage rating could be perceived as being lower. It is the combination of the steady-state voltage (called the working voltage) and the category that determines the total voltage withstand rating of the test instrument, including the all-important transient voltage
When dealing with electronic equipment problems, it’s also good to begin by entering an information-gathering mode. Although it may sound obvious, that includes simply talking to equipment operators and other individuals who may have noticed unusual sounds and other clues that something isn’t right. It is surprising how often technical personnel charge off without first having an informal chat with people who noticed the symptoms of the problems.
Finally, physically examine the device or equipment in question. As the noted philosopher Yogi Berra reportedly once said, you can observe a lot just by watching.