Fluke 179 reads 12vAC with com disconnected? - Page 1
Forum section: electronics / testgear

author: Steve Zarg
author_rank: Contributor
timestamp: December 13, 2021, 05:37:10 pm
content: Hi,I have become confused by my multimeter. It is a Fluke 179 and yesterday and this morning I was measuring USA 120vAC 60Hz house line voltage while trying to trace out a wiring scheme, and became confused by the readout on my DVOM.When the hot and com testing leads were connected to an outlet I saw the expected 120vAC, but when I disconnected the com test lead I noticed that the meter still read 13vAC. In other words with just a single lead connected to the meter it reported 13 volts. I wouldn't have been surprised by some small auto ranged millivolt figure but would have anticipated zero volts. I tested this repeatedly and got the same reading. I changed batteries and got the same reading.I am wondering if I have damaged the Fluke and caused it to report voltage erroneously, or if there is some aspect of the internal filtering that makes it display the 13 volts.It is possible I have never paid attention to the meter display while it was not properly connected, and did not notice that it does this.I am unnerved by the idea that the meter may not be functioning properly.Any thoughts or suggestions?Thank you!

author: tooki
author_rank: Super Contributor
timestamp: December 13, 2021, 05:58:01 pm
content: It’s perfectly normal, it’s capacitive coupling through the air. The high input impedance of digital multimeters makes them capable of measuring voltages with practically no current.They even make adapters to suppress these “phantom” voltages.Also, for fun, watch what happens if you touch the common test lead while the red V lead is connected to live.(IN VOLTAGE MODE ONLY!! DO NOT ATTEMPT IN CURRENT MODE!)This is the same effect that lets you take a modern ultra-efficient LED, like a white or emerald green one, connect one lead to earth ground, and touch the other lead, and in a dimly lit room you’ll see it light up!

author: AVGresponding
author_rank: Super Contributor
timestamp: December 13, 2021, 05:58:38 pm
content: Was the red lead still connected to the hot side of the mains socket?

author: Steve Zarg
author_rank: Contributor
timestamp: December 13, 2021, 06:11:32 pm
content: Quote from: AVGresponding on December 13, 2021, 05:58:38 pmYes.

author: Steve Zarg
author_rank: Contributor
timestamp: December 13, 2021, 06:13:56 pm
content: Quote from: tooki on December 13, 2021, 05:58:01 pmThank you. This is not the first time I have been confused by not fully understanding what to expect from a meter during unintended use scenarios.

author: AVGresponding
author_rank: Super Contributor
timestamp: December 13, 2021, 06:18:24 pm
content: Then yes, that's what you'd expect to see. As Tooki said, the COM is effectively capacitively coupled to ground, allowing a (very) tiny current to flow and causing the reading you got. You can alter the reading by holding the meter in your hand or putting it down.I wouldn't advise holding the COM probe while the red is connected to hot. If there happened to be some contamination inside the meter, you'd get an unpleasant surprise.

author: Steve Zarg
author_rank: Contributor
timestamp: December 13, 2021, 06:31:40 pm
content: Thank you.It was a rare circumstance where I was testing several possible common/ground paths to confirm they were disconnected so it seemed convenient to leave the hot (red) attached while connecting and disconnecting the Com (black) with the possible pathways.If I had followed Flukes instructions and connected the hot last and disconnected it first I would not have become confused.Thank you!

author: BILLPOD
author_rank: Frequent Contributor
timestamp: December 14, 2021, 02:19:18 pm
content: Good Morning Steve Zarg,Some DMMs have what is referred to as a 'Low-Z' function.My new EEVBlog 121GW has it. It puts a small load on the circuit to counteract these 'Ghost/Phantom' voltages.

author: electr_peter
author_rank: Unknown
timestamp: December 15, 2021, 07:12:35 pm
content: Just an example on low-Z importance. Today I measured 83V AC from grounded DIN PSU case to isolated outputs (+/-) in normal high impedance mode on Agilent U1272A. Same measurement in low-Z mode showed no AC voltage. Mains here is 230V, same measurement would be ~2x smaller in 110V AC mains location.