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"A true case of the wheel turning" Imagine this; Arriving on site to trace an earth leakage fault and the only means left to do it is completely isolate the circuit involved. However, you fall foul of the very thing you have just done at the client you have just left - wire in a circuit Neutral to the nearest available spot on the Neutral bar without any form of identification whatsoever. You vow to never do this again, especially in light of your now impending pain (but let's face it, memories are short!), but this does not help your present situation. Many are left with the pretty nasty situation of having to down the whole site as all Neutrals need to be disconnected and then reconnected one-by-one to trace the matching Neutral of the offending circuit. If this is a domestic property, then usually the owner is relatively compliant as there is little "productivity loss" while this work is being carried out. It's when it is a business premises, then the tune changes for the significant worse! And the bigger the distribution board, the bigger the problem. Lives are typically taken to the circuit breakers at one end of the cabinet, With Neutrals bundled together at the other (with some bright spark often deciding to lace them together!). There is one other problem and that is safety, both for humans and machinery. By disconnecting the Neutrals, the circuit is deemed by the human to be dead yet is still very much live. It's just that no current is flowing. And then reconnecting the Neutrals could cause inrush currents that damage devices still left connected. But tracing matching Lives and Neutrals is exceedingly easy! And, what's more, can be done without having to remove power or disconnect a single thing. Two possible methods are shown below, each having advantages and disadvantages.
In "the basics of current flow" we showed that everything that does out on the Live is intended to return on the Neutral. To our advantage, there is usually one Neutral per circuit per set of phases i.e. one Neutral per Live on single phase and one Neutral per three Lives on three phase, etc. One small curve ball to remember is power point ring circuits have two Lives, and as these are typically single phase will have one Neutral per Live therefore two Neutrals! For this method you will need two identical (matched) current clamps (e.g. Northern Design's M1.U type). Please do not be tempted to use two "like" clamps with the same range. You'll only be opening yourself to heartache. They must be the same make, model, and range. The clamps are fed into one meter. The one clamp is then clipped around the required Live and a suitable load placed on the circuit at a convenient point and a reading taken (adjusting the meter to read a suitable reading and confirming the load is what is being read). The second clamp is then clipped to each Neutral in turn, ensuring the clamp is in the same direction as that on the Live, until the reading falls to zero (nothing is perfect, so the reading may fall to very close to zero - this is good enough). If doing ring circuits, do ensure all Neutrals involved with the circuit are clipped by the second clamp. The final step is to remove the load and confirm the reading stays at zero (there is the exceedingly slim chance you may have two circuits with the same load thus causing a perfect cancel). If the reading changes, you'll have to find the real Neutral. If the Neutral is not traced through this method (which occurs with a Neutral-Earth short), then one needs to switch to a "process of elimination" i.e. trace each Live and Neutral pair of the other circuits. The Neutral left over is the one belonging to the faulty circuit (and, if you were really good, you numbered everything as you went along!). One major disadvantage of this system is on 3-phase circuits as all three Lives must be clamped in the 'Live' clamp in order to recognize the fully cancelled current via the Neutral.
This is for the person who wants to become a real PQ guy and has read the section on instruments for the PQ bag and gone out and sourced an audio test amplifier. A little bit of assembling is required, but, believe me, is not rocket science! You need some form of an on-off load control (thermal cooker control, lamp flasher, etc.) and a dimmer unit, and install these two items in a suitable box. The dimmer is relatively vital. The on-off mechanism just makes tracing so simple as the noise is turned on and off thus removing any doubt as to the circuit it is connected to. This combination then drives a reasonable load. I love using a 300W tungsten lamp element (make sure the dimmer can handle this level of load). Connect your "noise generator" to the circuit under test. Adjust the dimmer till the lamp is just glowing red (I find this creates the most noise on a circuit but you may have to experiment), and then adjust the on-off controller to a reasonable, recognisable speed. Feed a suitable current clamp into the test audio amplifier and then clip to each Live in sequence till the Live is located by the on-off sound heard (try not short-circuit this step as it gives you a mental picture of what it is you're listening for). Now move to the Neutrals and locate the Neutral giving the same output. Should it be a ring circuit, there may be two separate Neutrals each giving a similar sound (if not, then the ring circuit has either been broken somewhere or it never was a ring in the first place!). This method's advantages are the ease with which Neutral-Earth faults are indicated as the relevant Earth will have the same noise (the levels may vary with that of the Live and Neutral with the current shared between Neutral and Earth). A disadvantage is the pulsing of the dimmer can cause noise on an unrelated circuit that has capacitors (e.g. power factor correction) strapped across it. The dimmer will use the capacitor as a low impedance source to energise the lamp at switch-on. An oscilloscope used in conjunction with the audio amplifier will assist greatly in identifying the true circuit (if the oscilloscope has a meter style readout with the waveform, then even better!).
Of course, there is nothing prohibiting the use of both methods thus attracting the best of both worlds. The noise injection used to quickly identify the circuits, and the cancellation method used to test if the circuit is whole or faulty. Using an amplifier-oscilloscope combination to hear the noise and the oscilloscope to then identify the waveform and levels (especially in industrial sites where capacitors are the order of the day) is just simply really doing the job properly!
© 20.07.04 |
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