Non Contact Voltage Detector Lighting Up on USB Cable

I am using one of those 6 port USB chargers, which plugs into the mains. When I place my non contact AC voltage detector at the tip, or anywhere along the USB wire, the detector is lighting up. The detector is no where near the charger, so it shouldn't be detecting any AC voltage.

I have tried to use my multi-meter to measure the output voltage of the charger and it only detects 5v DC, 0v AC. I have tried all the ports on the charger with the same results, AND, the detector is also lighting up on all the devices that the charger is powering. I also tested the detector on a USB cable that is connected to my PC's USB port, which it didn't light up.

Does anyone knows why this is happening?

Probably, the AC/DC converter in the device is generating a little bit of switching noise, and the probe is picking up some of that.

It's also not uncommon to AC connect the secondary GND to one side of the primary with a capacitor. And with a standard non polarized plug (like here in Europe) that can be the hot/line/life and that can trigger the pen.

To check if the secondary is free from the primary just do a continuity test with your DMM when not plugged in. Try all 4 possibilities.

Congratulations, you now have proven to yourself that such devices are not only a waste of time but potentially dangerous.

If you connect an AC voltmeter between the ground on the USB connector (generally by having it plugged into something such as an Arduino module to which you can actually access the ground) and ground on another appliance, you will measure a voltage.

This is not necessarily dangerous - it is because there is always some capacitance between primary and secondary of the transformer inside the power supply and the primary has an average AC potential. Such small coupling capacitances (the "detector" after all, is sensing the voltage by an even smaller capacitance) always couple an AC potential unless the secondary side is grounded (so you will see this with every non-grounded supply), but the capacitive impedance limits the current to a harmless value.

Paul__B:
If you connect an AC voltmeter between the ground on the USB connector (generally by having it plugged into something such as an Arduino module to which you can actually access the ground) and ground on another appliance, you will measure a voltage.

This is not necessarily dangerous - it is because there is always some capacitance between primary and secondary of the transformer inside the power supply and the primary has an average AC potential. Such small coupling capacitances (the "detector" after all, is sensing the voltage by an even smaller capacitance) always couple an AC potential unless the secondary side is grounded (so you will see this with every non-grounded supply), but the capacitive impedance limits the current to a harmless value.

Paul, your answer is right on the money! I did measure 94V AC by using the procedure you mentioned. How does one effectively ground the secondary side in this case if we want to(just for discussion sake), do we simply connect it to the earth connector if a 3 prong plug is used, and what if it were a 2 prong plug?

Byork:
do we simply connect it to the earth connector if a 3 prong plug is used

If you mean with "it" the secondairy GND, yes.

Byork:
and what if it were a 2 prong plug?

You can't...