CT Clamp Readings wildly different


I have a Flukso with a CT Clamp that comparing with my actual meter is 150w out over a 20kWh test

i have a Current cost device with CT clamp on the same wire and its reading 925w vs the Flukso 620w

replicated twice with 935w vs 658w so about 150% higher reading.

can they be this far out!?

i have set my calibration somewhere between .67 and .7 for now and will test again to replicate consistent results but i cant believe its that far out.

i know when i used it years ago against my meter then it was .95


The position of the wire relative to the CT can cause an error as the flux fields will not be even across the CT if the wire is not dead center in the opening through the CT. That said, unless you have a very large burden on the CT, an error of 150% is not really reasonable. I would guess that the two clamps you’re comparing have different ratios. Do both CTs have the same part #? If they do, you probably have a defective CT. If two or more of the CT turns are shorted, this will change the ratio.

Another possible failure point is the CTs resistor. I’m not familiar with the internals of the Flukso Clamp on CTs, but since they are designed for consumer use, I expect that there is also an internal resistor in the assembly so that the output of the assembly is a voltage rather than a current. (This would be due to the fact that an open circuited CT without some load across it’s terminals, can be very dangerous. If there is a current through the cable the CT is around, then the voltage at the output of the CT rises until the CT output current matches the current through the cable being measured (divided by the turns ratio). While there is a maximum voltage that a CT can produce, getting to this value usually damages the CT and is a shock hazard.)