I used non-contact voltage tester to test burned out Christmas lights. It says it can only detect in the range of 50-600V. One of the strings has 40 bulbs in series and none of the lights are burned out, so plugging it into a 120V outlet means there should be a 3V drop across each bulb, so my 50-600V tester shouldn't be able to detect that, but it does pick it up. Does anyone know why this would be?

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    \$\begingroup\$ Don't look a gift horse in the mouth is my advice. \$\endgroup\$
    – Andy aka
    Dec 13, 2020 at 17:25
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    \$\begingroup\$ What do you mean by saying it detects a 3 V drop? \$\endgroup\$ Dec 13, 2020 at 17:31
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    \$\begingroup\$ It's hard to make this kind of tester accurate to a few volts. If they have 1V accuracy, they can sell their tester as "50-600V, +/-2%", which will sound reasonable to most buyers. "3-600V, +/-30%" probably won't. \$\endgroup\$ Dec 14, 2020 at 9:15
  • \$\begingroup\$ How do you test the voltage across a single bulb? How are the terminals exposed? \$\endgroup\$
    – Hot Licks
    Dec 14, 2020 at 14:09
  • \$\begingroup\$ Questions seeking fundamental understanding of test equipment are not "usage" questions \$\endgroup\$ Dec 15, 2020 at 6:53

1 Answer 1


plugging it into a 120V outlet means there should be a 3V drop across each bulb, so my 50-600V tester shouldn't be able to detect that

Your tester is not detecting the voltage drop across a bulb, but rather that of the AC electric field produced by the "hot" lead of the system.

If the geometry of the system were other than it is, such that there could be a place where the tester were near only the "cold" end of the string just a few bulbs up from the neutral, then it's possible that at that point there might be insufficient field strength to activate the detector. But the usual design of these folds the loop, so there is no such place.

Additionally, just because the tester is hoped to detect 50 volts or more does not mean it will not detect less, doing so is simply not within the stated design goals.

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    \$\begingroup\$ That 50-600V label is there to cover the butt of the company. If someone died on a <50V line and tried to sue the company, the company will simply state that the detector was never designed for <50V and can't be liable. Can it possibly be detected? Sure. Are they legally responsible if it doesn't? No. \$\endgroup\$
    – Nelson
    Dec 14, 2020 at 4:41
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    \$\begingroup\$ The other thing with holiday lights is that they frequently use non-polarized plugs. Depending which way you plug them in the folded cable may be the neutral or it may be live. In the latter case you would detect voltage with the non-contact meter all the way to the end. In the former you would probably lose detection down the line as the voltage fades. \$\endgroup\$
    – J...
    Dec 14, 2020 at 11:41
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    \$\begingroup\$ Typically holiday lights have bulbs on both side of the fold alternating rather than just one side and a return. \$\endgroup\$ Dec 14, 2020 at 13:17
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    \$\begingroup\$ @Nelson And that 50V threshold conveniently coincides with the definition of "low-voltage wiring" and its accompanying legal allowances. \$\endgroup\$ Dec 14, 2020 at 17:15
  • \$\begingroup\$ Apparently there may actually be some sorts of holiday lights that have a "cold" end if they pass through to a socket on the far end and have two strings of bulbs which can independently work or fail. \$\endgroup\$ Dec 15, 2020 at 6:54

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