Fluke meters measuring 110 on 220 outlet (US) with no neutral - Page 1
Forum section: electronics / testgear

author: tmorris9
author_rank: Newbie
timestamp: October 24, 2022, 07:35:06 pm
content: I am in the U.S. and was having an issue with a 220 outlet, my Fluke meters (77 & 101) both said I had 110 from neutral to either hot which is what there is suppose to be but the equipment would not work properly. I had an electrician come out and his meter (a plug in one with just lights for each voltage) showed no voltage across the neutral to hots, he found in another box that was daisy chained to this one the neutral had come loose.My question (and one that concerns me very much) is why did my Fluke meters show 110v when they should not have? BTW I plugged mine in with electrician there and showed him it said it basically had the neutral and was OK, he just said I don't know but there was definitely no neutral hooked up to that outlet. He hooked up the loose neutral and everything worked.Now I have used Fluke's for 30+ years and I just lost confidence in them.Can anyone explain why it showed proper voltage with no neutral?Thanks!

author: bdunham7
author_rank: Super Contributor
timestamp: October 24, 2022, 07:42:17 pm
content: Quote from: tmorris9 on October 24, 2022, 07:35:06 pmYour meters are fine, but they are high impedance (~10M) and put no load on the circuit. Depending on how loose the neutral was and what else might be on the circuit, you'll get results like yours that are known as 'ghost voltages'. The voltage is actually there, but goes away as soon as you put any load on it. Meters and testers with a Lo-Z function have a lower input impedance in that range and thus put at least a very small load on the circuit. Your old 77 and entry-level 101 don't have this Lo-Z feature.https://www.fluke.com/en-us/learn/blog/digital-multimeters/dual-impedance-digital-multimeters

author: electr_peter
author_rank: Unknown
timestamp: October 24, 2022, 07:49:03 pm
content: DMM has high input impedance (around 10 MOhm) and pick ups stray voltage (also called ghost voltage) on the line (from capacitive or inductive coupling). It is "real" voltage, but it has very high impedance (i.e. cannot provide real power). Small load (in the order of few kOhm) on the line is needed to eliminate ghost voltage. This is typically done with LoZ mode on DMM.Some material from Fluke on ghost voltages and mitigationdual-impedance-digital-multimetersHow To Determine If Voltage Is Real Or a Ghost Voltage Using your Fluke Digital Multimeter

author: electr_peter
author_rank: Unknown
timestamp: October 24, 2022, 08:08:57 pm
content: There are special adapters for regular DMMs to eliminate ghost voltage. DIY is also an option if you know what you're doing.Fluke SV225 stray voltage eliminatordiy-fluke-sv225-stray(ghost)-voltage-adapter

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 24, 2022, 08:18:26 pm
content: In another word your Fluke meter is correct that is if you touch between the hot and the neutral you would get a shock because there is 110V potential between them. Only when you connect a load does the voltage went away.

author: alm
author_rank: Super Contributor
timestamp: October 24, 2022, 08:27:22 pm
content: Unless the wires are very long, you probably won't get a shock, although obviously touching mains wiring without proper precautions (lock out / tag out) is never recommended. Most likely the tiny current won't do much. Kind of similar to touching a metal case of a device that's designed to be grounded but connected to an ungrounded outlet. 110 V across 10 MOhm is only 11 uA, which is I believe is below what feel under normal circumstances.

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 24, 2022, 09:42:48 pm
content: But if I shut down the circuit. Locki it out. Check it with my Fluke and it says 110V I would not touch it. Because when my Fluke says 110V it's 110V there. How much current it can supply is unknown. I wouldn't trust the low Z meter for that because they have 3KOhms impedance and my body resistance is a lot more than that.

author: bdunham7
author_rank: Super Contributor
timestamp: October 24, 2022, 11:05:52 pm
content: Quote from: BeBuLamar on October 24, 2022, 09:42:48 pmThe resistance goes up quickly with voltage. At 110V indicated, its resistance would be about 15K or so. But if it indicated 110V in Lo-Z, then you'd know not to touch it. The real question is what do you do if it reads 20 volts? The answer is that it still might give you a buzz.