|
With flicker, it must be remembered that this is a perception issue. A flicker of 1 Pst is the level of flicker that 50% of the population will deem "on the border of annoyance". That means there are some that will not be bothered by high levels of flicker, and there are some that are driven insane by a change so small that the majority may not even notice it. The symptom of flicker has one direct cause; sudden changes in voltage. The symptom of sudden changes in voltage has one cause; the sudden change in impedance. The symptom of sudden change in impedance, however, can have numerous causes. And this is where the fun begins! Having deduced that flicker is directly attributable to sudden changes in impedance assists in narrowing the field as to what measurements are required.
Before continuing on, it may be helpful to cover why it can prove important to trace the cause of flicker. Actually, the importance can be summed up in one simple word… FIRE! It may sound dramatic, but it is the most common indicator of a pending electrical fire - and yet it is so often ignored, with dire consequences. The sudden onset of flicker (over, say, a matter of weeks) usually calls for concern. If the cause of the flicker is a loose connection (hot-spot) or an intermittent short (high energy releases), then knowing about it sooner rather than later will prove fortuitous. It must also be said that flicker does not say an electrical fire is pending. Flicker could simply be the result of a large number of changing loads on a less than perfect supply (e.g. thin cable feeding a row of houses with immersion heaters that click on and off many times a day). It could even be more simple than that! Unaware a backdoor neighbour started a small home-business of welding up security gates had one poor chap chasing his tail for some time! But it still remains a wise idea to find out which scenario is behind the annoyance!
External source and load impedances are usually out of grasp so one has to rely on the common factor being incoming voltage. Internal load impedance is, thankfully, available as it helps to track things down. As with all measurement techniques, determining the root cause of a problem in the shortest possible time is always prudent. The method outlined below will help determine whether the cause is external or internal influences, and if internal whether it is a loose connection or intermittent short or intensely varying load. Pre-measurement: Before coupling up any instrumentation, I usually run through a small test to see if there are any obvious reasons for flicker. If flicker is intermittent or sparse, then this phase may prove a waste of time, but it may also prove to bring the cause to the fore! Most loose connections are found within the distribution board. Testing for these is relatively simple. Create the highest possible load on as many circuits as possible (ensure the immersion heater is on, switch on the oven and put a large pot of water on the highest powered cooker element, boil a full kettle, etc.). Switch on all lights and watch for flicker during this high-load period. Do this for about 10 minutes then remove all load. Without wasting time, trip the mains breaker (don't ignore this step!). Then feel all parts of the distribution board for any signs of very local heat (a prime target is where wires enter circuit breakers, fuses, and connection blocks). While you have the distribution board open, you may as well check all connections are tight (be careful not to over-tighten). Re-energise the board and only have the lights powered and watch if the intensity of flicker has reduced or stayed the same. Reduce the lamp circuit load to only one lamp and again watch for flicker. If the same, then it is almost going to prove the cause is external. Measurement: As with all measurements, having the right kit always proves essential. Having a power quality digital storage oscilloscope (PQ-DSO) that can capture waveform events is the best method of determining flicker causes, but that does not mean a multimeter and bit of lateral thinking cannot achieve the same results (it just takes a lot longer!). My favourite tool in association with the PQ-DSO is an audio amplifier attached to a current clamp. This is placed around the lamp circuit (you'd be amazed how quickly you hear a loose connection!). The Rules:
While monitoring a lamp for flicker When flicker has occurred, review the waveforms on the PQ-DSO. If external then any drop in voltage will also show a clear drop in the main current. Depending on the amount of voltage drop you may even see a bit of current being exported from the premises (especially if fluorescent lamps are used with power factor correction capacitors). Investigate the N-E and E-G voltages to ascertain if the fault is a loose Neutral/Earth system. If so, involve the local electrical supply company immediately. A high load on the other phases will push yours above the limit with possible damage to equipment. If internal, then the voltage and current have opposing characteristics (the voltage drops as current goes up). An intense increase in current indicates an intermittent short. Drops in current with high levels of transients shows a loose connection (and is also clearly heard on the amplifier if fitted). Ensure these were not normal operations (someone switching an appliance on or off). If all appears to be ok, or results are inconclusive, then
move the whole monitoring to the lamp circuit
© 13.04.04 |
MEASUREMENT TECHNIQUES: