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As said all along, the fact that mains power is fed as an unbalanced signal would (if he gave it any thought) make an audio engineer shudder! Simply having one leg tied to mother earth is asking for currents to find their way back to the source through routes not intended for this purpose. Such routes are often made up of the Protective Earth, but has been found to be building metalwork, water pipes and, for the real horror story, gas pipes! Because this 'leakage' is mainly caused through capacitance, the current is very dependent on the frequency of the applied voltages. The frequency also plays a major factor in the type of faults being experienced. Earth Leakage Current is usually referred to as frequencies in the range from the fundamental through to about 1 to 2kHz. These currents would be the primary cause of nuisance tripping, screen shudders, etc. Frequencies above 1kHz can easily bypass Earth Leakage Breakers, but can also easily manifest as currents in telecoms lines etc. Therefore, as frequency is a major determining factor, different tools and methods need to be employed to trace the culprits. In determining Earth Leakage Currents, one needs to emulate the way a Residual Current Device operates as closely as possible. The RCD simply measures the current flowing simultaneously in the Live and Neutral and if they are not equal but opposite, then current must be returning ('leaking') back to the source via another path. Domestic 30mA RCDs can trip on as low as 15mA imbalance and thus makes measurement of such currents a fair challenge. This is further aggravated by the cable sizes used in mains distribution boards being rather large and making it impossible to feed through a current clamp that is sensitive enough to determine such low currents (remember, one needs to feed both the Live and Neutral through the clamp at one time). Finally, someone has produced a clamp that can measure as low as 1mA, and has a 50mm jaw aperture! This is more than enough to fit around two commonly used 16mm² cables. This must for ELB/RCD nuisance tripping cases is the Chauvin Arnoux C-173 clamp on CT. A full review of this is given here. Just the device needed in the PQ bag. The section "Connecting Up" covers the methods used to measure earth leakage currents. Unfortunately most current clamps are too small to wrap around items like water pipes, and also too bulky to fit into areas where many pipes are in close proximity. Even if a clamp could fit around the pipe in question, the response is more made for the fundamental frequency (50/60Hz) and the higher frequency noise could be lost - and these could be the very currents that are causing damage. There is, however, one 'current clamp' that is perfect for this type of work and is known as the Rogowski Coil. Yip, you've read about this before under Current Clamps but what was not stated is its frequency response is very much the same as a capacitor's, and this is exactly what we want. The fact such coils are flexible allow them to be wrapped around the most awkwardly fitted pipes and structures. In fact, if long enough, they could even be wrapped around building pillars if required! If fed into an audio amplifier and/or oscilloscope one can very easily hear/see any potentially damaging currents that may be flowing in the path being tested. If there is evidence that a large fundamental current is flowing then all that is required is to insert the integrator and do the measurement.
© 06.04.02 |
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