Current Transformer giving a reading when not clamped around a wire

A split core current transformers is currently clamped around a live wire. One end of the transformer's output wires are connected to the analog pin of an Arduino and the other end has a 2.5V bias created by tapping between two 470k ohm resistors connected across GND and +5V of the Arduino.

If the current transformer is removed and A0 is connected to the 2.5V point, Arduino gives a steady reading of 512 which is expected..

However when the current transformer is connected as described and A0 is connected to the other end of the current transformer (as described), Arduino readings of pin A0 will fluctuate around between 503 and 519 even when the clamped wire is unplugged or even when removed.

What is causing the current transformer to still give a reading?

Additionally, if we have 2-3 split core current transformers about 1-3 inches apart from each other, will the readings of each current transformers be affected?

• Does the "spurious" reading reduce when the clamp is closed? What range of readings do you get when operated properly with a decent load current? – Andy aka Mar 21 '14 at 13:15