Summing CT Measurements

Posted on Feb 4, 2014

This could be achieved by monitoring separate points either through the same recorder, or worse still, two separate recorders. In both cases the results need to be artificially summed to get the total current draw. Two problems exist with this, the first being the actual VA, VAR, kW, etc cannot be measured as `inter distribution point` currents ar

Summing CT Measurements
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e not taken into account. The ideal solution would be to have the selected distribution points all supplied from one common supply, or at least all the required phases available for common clamping in one CT in order to affect measurement. This is not always practical. Wiring CTs in series, each CT with it`s own termination resistor, will only apply if the input to the recorder is greater than 1000 times the sum of all the termination resistors, this is usually an impractical figure. Wiring in parallel requires that the recorder has a near zero resistance input, this not being the norm as most fix-on or clamp-on CTs output a voltage relative to the measured current. Wiring either in series or parallel also creates a further complication if certain CTs need to be taken out of circuit. The circuit is a simple current to voltage converter. In this configuration the op-amp will always try to ensure the negative input (pin 2) remains equal in voltage to the positive input (pin 3). Ignoring CT inputs 2 through 4, if a current of +1mA was introduced on CT input 1 the op-amp would swing it`s output negative by 1 volt to introduce a current of -1mA (1 volt through 1k ©) to counteract the input current and bring pin 2 back to the voltage level of pin 3. As pin 3 is at ground potential the impedance at pin 2 is therefore 0 ©. If each input were to have +1mA current presented on them the output would now, in order to compensate, be -4...

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