20 KJ Marx generator at DNV GL facility
Power metrology instruments are assumed to be non-invasive observers of the system they monitor, just sitting there and accurately doing their assigned tasks. Unfortunately, where power metrology instruments often do the most good is in heavy industrial or electric utility service, wired into circuits with capability to deliver high currents, high voltages or both. Besides that it’s often not practical to turn power off to install -or worse- modify or repair them.
Safety assurance is a big deal and requires thoughtful engineering at every corner. For example, if the instrument is a standard form watthour meter (for 240V) with a remote disconnect switch one had better be absolutely sure the dissipation of the switch contacts stays low enough over time that the high currents flowing next summer don’t melt anything internally and start a fire. By the way, that switch could have sat open for years before the meter is put into service.
If the instrument has current transducers, can they be safely installed where they are needed with power on? Let’s say that a clamshell design is required to get the transducers installed around big bus bars – can they be maneuvered into place and properly closed by someone wearing the required personal protection equipment? The only way to be sure is to understand what’s required and demonstrate it in advance.
Another significant dimension of safety is dealing with transient voltages. What makes the world go around is basically inductive so transients are continually being generated as elements switch on and off. As one moves up the grid segments from distribution to transmission, exposure to lightning also becomes something that must be routinely tolerated.
An aside about high voltages is worth mentioning. Inadvertent contact with the neon sign transformer in the window of the local pizza shop can kill you just as dead as contact with the medium voltage distribution lines that run along above (or sometimes under) the street. The difference is the available energy and the ability to sustain an ionized plasma arc. Safety assurance in the electric utility environment is about controlling the formation of or at least managing plasma arcs potentially dissipating megawatts of power. Preventing them from getting started is the most common goal.
Instruments intended for service on the medium voltage (or above) segments of the grid will be required to demonstrate tolerance to basic impulse level (BIL) pulses that have proven effective in assuring safe operation in the lightning-rich outdoors. Demonstrating compliance with BIL requirements is strictly just testing but doing it is woven together with a wealth of general safety practice, hence the title of this section as “Safety and Testing” as opposed to simply, “Safety Testing.”
The impressive looking thing in the image is a Marx generator used for creating megavolt level BIL pulses to test high voltage gear and instruments. This particular one is capable of 20 KJ and is located at the DNV-GL facility (formerly KEMA Powertest) in Chalfont, PA. One might correctly guess that equipment like this is not common, inexpensive or easy-to-use and that would be correct. Getting this sort of testing work done efficiently at external labs is another way DJA Engineering brings hands-on “know how” to the table for your product.