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This can be a tricky situation, especially if the clamp is only capable of lower than the current being measured, or the clamp is physically too small to encompass all the cables that make up a phase connection. Although the usual advice is to employ Rogowski type current sensors, this does not help when stranded with a jaw type clamp and needing to measure a building's current draw. There is a typical situation and we'll discuss it first. Owing to skin effect, buildings that require a large supply will usually be fed with parallel cables but with each not exceeding 25mm in diameter. The first requirement is to measure the current in each cable that makes up the phase ensuring that the draw is equally shared between each cable. If not, then this may be an area that requires attention! Once established that things are all ok, leave the clamp on one of the cables and adjust the multipliers/scaling in the instrument to now show the full-scale as the current range of the clamp multiplied by the number of cables per phase. Just a reminder; should the current not be shared equally between all the cables that make up a phase tthen attention should be given to this as this may be the reason behind the cause of the power quality issues. The next method of reducing current into a clamp takes a small amount of ingenuity. As in the previous example, current is shared between two conductors that are wired in parallel. There is only one restriction to this method of operating, and that is you have access two open points along the length of the conductor being measured. What was not stated was that this is proportion to the ratio of cross sectional area and inversely proportional to the ratio of the lengths of each conductor ("Whaaaah! More maths!" I hear you cry. Fear not, we try to keep it simple). How this works is to 'short out' a known length of main conductor with a known length of thinner conductor. The ratio of the current flowing in each is the ratio of the two cross sectional areas. However, it is rather difficult to use a piece of cable exactly as long as the main cable, especially if it to go through a current clamp. Here we apply the second formula to correct for a longer piece of "sensor" cable to main cable. An easier way is to have a piece twice as long as the main cable and to then, following the guidelines of the previous section, wrap it twice round the clamp in question. This way we get away with not having to worry about the second part of the whole equation. Told you we'd keep it simple. The best idea is to have a few cables of differing lengths in the power quality bag for this function. Each cable is neatly ended off with the termination of your choice, mine is 4mm plugs for fitting into 4mm croc-clips. There is one thing that must be pointed out with this method. The iron of the clamp will create an inductor on the "bypass" wire and could reduce the reading on the clamp significantly. The way to get around his is to adjust the ratio accordingly when measured with a trusted clamp.
© 06.04.02 |
MEASUREMENT TECHNIQUES: