Low Level Transient Voltages: Should We Care?
Modern electronics have brought about many desired conveniences and benefits to mankind. These same electronics are both susceptible to and creative of low-level transient voltages. We accept failed electronic equipment as a state of the product’s age or the fact that a storm came by with too much lightning activity – sometimes combining the two. There is some element of truth in both scenarios, lightning activity could be the catalyst to cause an already weakened product or even new product to fail, or a product’s age causes it to fail. When it comes to electronics (non-moving parts), the product’s age often is never the issue.
In the reliability engineering world there is a concept known as “infant mortality” or early/premature failure – this is often associated with what is known as the “bathtub curve” which plots the failure rate of a product vs. time (Fig.1).
This curve and reliability engineering is a statistical science which is more involved than this article can adequately address. Nevertheless, one of the factors to consider in this science is the operating voltage of an electrical product or component. Voltage should be considered as the voltage levels (amplitude) that the equipment was designed to normally operate; however, not only the amplitude but the “shape” which is the waveform and its symmetry. The typical power delivered to our homes and businesses has a normal “shape” (waveform & symmetry) known as a sine wave (Fig. 2); we may remember that term from trigonometry, tech school or college.
Any aberration from this voltage level or shape presents a voltage that is beyond or outside of the intended normal operating level or shape the equipment was designed to operate. This abnormal voltage level or shape represents phenomena known as electrical stress that can shorten the useful life of equipment and cause equipment “upset”. A common form of abnormal voltage can be seen in what is called “notching” which is especially prevalent from the use of modern electronics like variable frequency drives and switch mode power supplies. This type of voltage abnormality, noise or “dirty power” can be seen in an oscillograph like the one in Fig. 3.
Figure 3 shows a little more than one cycle of AC power, there are 60 of these cycles every second in a typical USA power system. There are roughly 6 low-level transient events occurring every cycle; therefore, simple calculations show that in one hour of this transient activity, the electrical system experiences some 1,296,000 transient voltage events per hour! It is amazing that some electrical equipment will function when the AC power sine wave that looks like Figure 3 or 4. Some equipment, like audio or radio equipment might experience audible noise at their outputs when exposed to such transient activity. Regardless of the nuisance, the equipment was not designed to operate with such poor power quality and is at risk for premature failure or upset. The longer the equipment operates with this type of “dirty power” the greater the failure risk which becomes a financial risk too.
The Institute of Electrical & Electronic Engineers (IEEE) has several technical standards, guide and recommended practices that concern power quality and the application of surge protective devices (SPD) and filters to mitigate these low-level transients as well as the stronger transient voltages originating, as an example, from lightning. The standard IEEE 1100TM – IEEE Recommended Practice for Powering and Grounding Electronic Equipment mentions throughout the use of filtering networks to help equipment continue to run and have a long useful life. Section 220.127.116.11 of the IEEE 1100TM, mentions that an SPD with the addition of a filtering network could be advantageous to mitigating power line noise. Further, section 8.6.1 of the same IEEE practice mentions that an SPD may also be specified with a high-frequency filter to help mitigate problems caused by low-level transient voltages (dirty power). Further work is being done by the IEEE Power & Energy Society Section 3.6 in PC62.72 to create an application guide for SPDs. Also, recent and pending changes to the National Electric Code (NFPA 70) have added and are adding surge protective devices as requirements to certain installations like emergency operations electrical panels.
So, the world is quickly realizing the need to mitigate problems caused by transient voltages –including the low-level transient voltages. Cost effective ANSI/UL 1449 Listed SPD technology exists today that mitigate both high-amplitude transient voltages and the ever present low-level transient voltages. This technology can be installed in the circuit electrical panels that feed numerous pieces of equipment and luminaries (Fig. 6). Proper selection and installation are the keys to any successful SPD implementation. A poor installation could render the SPD useless, so it is very important to follow the manufacturer’s installation instructions and the National Electric Code Article 285.
What is the potential benefit with a properly selected and installed SPD? Figure 5 shows the oscillograph after installing an SPD with Frequency Responsive Circuitry (FRC) filtering (aka, sine-wave tracking). The result restores the AC power to the proper “shape” and amplitude. Figure 6 is a photograph showing a good installation of several SPDs (Black boxes with green lights).
NFPA 70-2014; National Electric Code
IEEE 1100TM - 2005 - IEEE Recommended Practice for Powering and Grounding Electronic Equipment (The Emerald Book)
IEEE PC62.72 – Draft Guide for the Application of Surge-Protective Devices for Use on the Load Side of the Service Equipment in Low-Voltage (1000 V or Less, 50 or 60 Hz) AC Power Circuits
ANSI/UL 1449-2014 – Safety Standard for Surge Protective Devices
Practical Grounding, Bonding, Shielding and Surge Protection; Vijayaraghavan, G.; Brown, M.; Barnes, Malcolm; 2004