Page 7 - EngineerIt January 2021
P. 7


        Stay Grounded!

        By John Wilson, Accreditation & Metrology Services (Pty) Ltd.

                       he 2020 year has certainly     In the electronic instrumentation world where small values are often
                       been up in the air and it is time  required to be measured, these can be seriously distorted by earth loops or
                 Tfor us to come back down and        small currents flowing between different “earths” or grounds and being
                 be grounded in more ways than one.   superimposed on the values being measured. This is very common in
                 Let us explore being grounded. I am  industrial plant applications where the signal wires coming from a remote
                 not referring to the times I misbehaved  part of the plant back to the control room are long and as a result, the earth
                 and was "grounded" and not allowed   potentials of the two systems are likely to be different. Invariably when
                 out as a child, but often wondered   there is an unstable measurement on a plant control system, it is due to
                 where that phrase came from. It      earth loops.
                 supposedly comes from the term used
                 in the aircraft and shipping industry,  Audio and video equipment earth loops cause all sorts of audio and visual
                 meaning that the vessel cannot go    distortion, along with the classical hum that is so annoying. It is generally
                 anywhere. In the more classical usage,  caused by different grounds and has resulted in the development of optical
                 someone who is grounded is perceived  or wireless connection between the various components to eliminate the
                 to be sensible or “down to earth”. Even  possibility of ground loops.
                 those in the more esoteric fields of
                 Yoga, Reiki, Polarity, TRE and other  Even in the metrology or calibration world, where ideal conditions are
                 complimentary healing sciences       supposed to exist, earth loops creep in and distort the measurements.
                 frequently refer to the need to be   These come about when two or more instruments are at different earth
                 grounded. So, what do they mean by   potentials and they try to equalise through the signal wires. These
                 the term? Well, they literally mean for  potential differences are known as “Common Mode” (CM) voltages. You
                 you to go outside and put your feet in  may wonder how it is possible to have CM voltages when the instruments
                 the sand and come down to earth, or  are in such a close proximity but incorrectly wired power distribution, poor
                 be “discharged”. If you don’t believe it,  earth connections, wrong power cords and particularly, the wrong
                 then think about it when you next get a  connection of signal leads can lead to perfectly good instruments being
                 shock from a static discharge - you will  defined as faulty or out of specification or worse, faulty equipment
                 just have had a rapid and            classified as being serviceable.
                 uncomfortable grounding experience.
                                                      So, let us go into the New Year
                 In the electrical world, grounding is  with a well-grounded approach
                 ultra-important for our safety. Without  and be sensible about the way we
                 proper grounding, appliances and     live and work. When we feel the
                 installations can “float” up to high  stress levels rising, let us go outside
                 voltages and cause a shock or        and put our feet firmly in
                 damage when you least expect it.     the sand, remember where
                 Similarly, we are not in control of when  we are and be grateful
                 or where lightning may strike and    that we have come
                 without proper grounding, the results  through the waves of
                 can be disastrous. The correct design  the pandemic and
                 and regular checking of an electrical  are ready for
                 installation and even your home, is  whatever may
                 important for your safety.           come next. n

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