Thanks Mike, great expose on “false positives” from non-contact voltage measurements. I wish the author of the original question had described the situation that prompted their question. What were they trying to measure and why? There is a lot of subtlety in these kinds of measurements and your example of a cell phone charger “false positive” made me wonder how you could tell the difference between a good cell phone charger and a faulty one if even a good one has enough line voltage leakage to trigger a non-contact tester? I assume there are varying levels of quality (e.g., sensitivity) non-contact testers out there; do you know if some of them could distinguish between a good and a leaky charger and some can not? Or, should you need to follow up a positive with an ac leakage current measurement as a definitive test? It would be great if you could do a follow-up article that tells your audience what to do if they do get positive so they can rule out an actual safety issue. Again, thank you for sharing a great topic and learning opportunity.
As you’re aware, a fault current leakage test is the gold standard but can a little tricky to do. I’m thinking that a reversed pair of LEDs in parallel with an appropriate series current limiting resistor would be safe tester. If the LED bulbs glow when connected between the metal phone body and ground, there must be at least a few mA of available fault current. Thoughts?
Just got out my NCVT and it lit up quite brightly near my iPhone 14 Plus when it is connected to either of the two different chargers I tried. The flashing and beeping was comparable to what it does when inserted into the hot side of a 120 VAC receptacle. My tester says it is rated to detect >70 V. I get the same result with the plastic case on or off but no response if I hold the phone in one hand while holding the tester in the other hand with the case on or if I remove the case and touch the metal edge of the phone with one hand and hold the tester in the other hand (as expected). I have no experience of an electrical shock when touching it. An LED test sounds interesting but how to connect it is not so obvious. I wonder how UL does the leakage test and if their method would provide some guidance?
Thanks Mike, great expose on “false positives” from non-contact voltage measurements. I wish the author of the original question had described the situation that prompted their question. What were they trying to measure and why? There is a lot of subtlety in these kinds of measurements and your example of a cell phone charger “false positive” made me wonder how you could tell the difference between a good cell phone charger and a faulty one if even a good one has enough line voltage leakage to trigger a non-contact tester? I assume there are varying levels of quality (e.g., sensitivity) non-contact testers out there; do you know if some of them could distinguish between a good and a leaky charger and some can not? Or, should you need to follow up a positive with an ac leakage current measurement as a definitive test? It would be great if you could do a follow-up article that tells your audience what to do if they do get positive so they can rule out an actual safety issue. Again, thank you for sharing a great topic and learning opportunity.
As you’re aware, a fault current leakage test is the gold standard but can a little tricky to do. I’m thinking that a reversed pair of LEDs in parallel with an appropriate series current limiting resistor would be safe tester. If the LED bulbs glow when connected between the metal phone body and ground, there must be at least a few mA of available fault current. Thoughts?
Just got out my NCVT and it lit up quite brightly near my iPhone 14 Plus when it is connected to either of the two different chargers I tried. The flashing and beeping was comparable to what it does when inserted into the hot side of a 120 VAC receptacle. My tester says it is rated to detect >70 V. I get the same result with the plastic case on or off but no response if I hold the phone in one hand while holding the tester in the other hand with the case on or if I remove the case and touch the metal edge of the phone with one hand and hold the tester in the other hand (as expected). I have no experience of an electrical shock when touching it. An LED test sounds interesting but how to connect it is not so obvious. I wonder how UL does the leakage test and if their method would provide some guidance?