In the first article we described how electricity is distributed, and how since everything is interconnected other activities on the network can cause issues with the quality of power being delivered to your home or office. We call these problems “power viruses” and this article is intended to describe the causes and effects of these viruses.
The Power Cut, or Black Out
Probably the most obvious power virus, a blackout is a complete loss of power. In some quarters, a blackout is now defined as a low voltage rather than a complete loss. In any event, the net result is that your equipment cannot operate, as it has of course, no power. It is relatively rare for a black out to be due to the National Grid. When it does occur on the grid, this is usually as the result of weather downing pylons or distribution lines, or serious faults or fires within substations.
However, you are more likely to be the victim of accidental cable damage by an unsuspecting JCB driver digging up the road, or more commonly by ciruit breakers or RCDs tripping within your own facility. In our own facility we’ve experienced blackouts as a result of light bulbs blowing and causing circuit breakers to trip, and with lumps of extra toasted bread in the bottom of the toaster causing earth leakage from the element to ground and causing the RCD to trip!
Voltage Fluctuations
Your electricity supply is rated at a nominal 230Vrms. Note the word nominal. In fact, the electricity generating board is obligated to supply you a voltage rated within 10% of this value, and technically another 5% drop is allowed within your building. So the voltage you may obtain at your wall socket may, in fact, be as low as 195.5V or as high as 253V. It could, and generally does, vary like this throughout the day, but perhaps not usually to these extremes.
Voltage variations occur generally due to loading on the network. The network is not ideal, and the more current that is demanded by users, the less the network can cope and as a result the voltage supply falls. Conversely, as the loading on the network reduces the voltage supply rises. Take the network near an industrial park. When the park starts work on a Monday morning, the demand on the network rises and, as a result the voltage supply starts to fall. Then on Friday evening, the park swithces off and then the voltage supply rises. Anybody near these parks will notice a variation in supply voltage due to these loadings.
The electricity company will also raise or lower the supply voltage in order to overcome some of these issues, so you may find the voltage varying because of this.
Facilities that are more rural can notice voltage variations to a more pronounced effect. In these enviroments turning on a high powered load within the facility can cause voltage variations for the rest of the site. For example, you may be able to witness lights dimming as a result of switching on an oven or turning on an electric shower. The lights then return to their brightness when the load is removed.
Now voltage variations are normal, and your equipment should be designed to cope with variations. However performance can be effected. A good example is, as mentioned above. If we have a light bulb that is constantly changing brightness as a result of other euqipment being switched on and off may be annoying, distracting and even shorten it’s working life. A ligght bulb is a visible example, but with other equipment variances may manifest with other problems, particularly if the variations result in prolonged operation at either of the extreme ends of the voltage swing.
An extended low voltage is usually referred to as ”under-voltage” or ”brown out”, whereas a sustained high voltage is referred to as “over-voltage”.
Voltage Surges / Spikes
A surge or spike is a high voltage high energy event that can cause damage to connected equipment. Perhaps the most obvious of these is the effects caused by lightning. A lightning strike packs a lot of punch and if this hits a power line the effects can be particularly damaging for the entire line and everything connected to it. Not only that, but a nearby lightning strike can induce a spike onto supply lines which then dissipate through your connected equipment!
However, lightning is not the only culprit. Inductive loads (anything with a motor for example) can cause voltage spikes when they switch off. The compressor on your fridge for example, the operation of an electric drill, an elevator in an office building all can induce surges onto the supply network.
Surges can be particularly damaging, and sometimes the damage is not obvious, as damage can occur to electronic components internally, without any exterior signs of damage. Devices just no longer work, or are damaged to a point of creating imminent failure.
Voltage Transients
A transient differs from a surge in that its duration tends to be much shorter and its energy far less. Transients are caused by electronic switching components, rotating machines, inductive load switching and lightning effects.
Transients tend to cause equipment malfunction and premature failure.
Harmonic Distortion
Harmonic distortion occurs with loading on the network, particularly with switching and non linear loads and where the supply impedance is relatively high. We will look at harmonics in more detail in a later article.
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