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I live in an old house which was build in 1920s.

On the hallway, there is an outlet and a ceiling light which are both control by one switch.

Recently I use contact pen style tester to check the outlet.

I found out both prongs of the outlet carries live electricity at the moment the switch is at it's OFF position. When the switch at the ON position, one side of the outlet is hot, the other side is neutral, or it carries no electricity.

I decide use a multimeter to measure it's voltage reading of the switched outlet.

when the switch is at its ON position:

I put red probe of the multimeter into Hot, the black probe into Neutral, the voltage reading: 120 volts

when the switch is at its OFF position:

I put the red probe into Hot, the black probe into Neutral, the voltage reads: 2.8 volt

I put red probe into Hot, black probe into D-shape prong(Ground) the voltage reads: 0 volt

I put the red probe into Neutral, black probe into Ground voltage reads: 0 volt

I believe the Ground is not grounded, and there is no neutral wire when the switch is at OFF position

I use a long extension cord, plug it into an outlet of another room, to bring the neutral wire next to the outlet for testing.

I put red probe into the Neutral prong of the extension power cord that pluged into an outlet of another room

when the switch at its ON position: Hot prong of the outlet: reads 120 Volts (hot to Neutral of a different outlet) Neutral prong of the outlet: reads 0 Volt (Neutral to Neutral of a different outlet)

when switch at OFF position: Hot prong of the outlet: reads 120 Volts (hot to Neutral of a different outlet) Neutral prong of the outlet: reads 100 Volt (Neutral to Neutral of a different outlet)

The conclusion: the switch, the outlet, and the ceiling light, work normally. The switch controls ON and OFF of the light and the plug.

BUT, when the switch is at OFF position, Both prongs of the outlet carries live electricity, one prong with 120 Volts, an other prong with 100 Volts'

Is it normal? or is it dangerous?

  • edited as Harper suggest, thank you Harper.

Thank you, Retired Master Electrician Thank you, Kris Thank you, Ken

My problem is solved now.

Kris is right. In old connection, the hot wire was not switch, instead, it switched the neutral. I rewire the circuit, connect hot wire to the switch, twisted the neutral wires together and cap it.

After rewired the Outcome: When switch OFF: Hot to Neutral reads: 0. with pen tester, both prongs unlit When switch ON: Hot to Neutral reads: 120 volts, with pen tester, Hot prongs lit, Neutral prongs unlit

Thank you. All.

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    Instead of "prong 1 and prong 2", please edit your question to use the proper names for those prongs, "hot" and "neutral". This diagram shows where they are located on a receptacle. schneider-electric.us/en/faqs/FA156527. Also we don't need to know which color probe is going in which hole since you are measuring AC. "I measure 100v between hot and neutral" for instance will suffice. – Harper May 5 '17 at 16:46
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Houses of that age will not have a grounding system. Also houses that age have switched neutrals. Meaning the neutral was switched instead of the hot. Back in that time no one really saw that as a problem until it lit up enough electricians to become something that needed to be changed.

By the way I don't think the first NEC didn't come out until somewhere around 1927. So you might want to get with someone and make plans and a budget to rework the electrical in that house because I am sure it is full of surprises.

  • yes I forgot about the switched neutrals - that change was in 1920 I think can't remember - the reason for it was that changing a bulb left the socket hot and if I made sure I was good and grounded when I the consumer changed my bulb - I might experience a new hair style or alert others I was Electric. – Ken May 5 '17 at 19:51
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No it is not normal. Your "neutral" (larger prong of receptacle) on the outlet should always be a neutral at 0V (sometimes 1.7VAC because of inductance).

Your hot is supposed to be what is switched.

Is the outlet wired through the light fixture or direct?

Normally (when properly wired that is) the hot goes through the switch and out to the outlet. The neutrals are wired directly back to the circuit panel. The ground is normally wired back as well - however some people try to get around that by connecting the neutral to it So when a tester is plugged in it looks right to the tester [Completely wrong - against code and Unethical].

You stated:

one prong with 120 Volts, an other prong with 100 Volts

Measuring form where to where ?

What it looks like is you have two hots - by the line with 100V is what is bothering me - it must be passing through something - because there is a voltage drop of 20V (perhaps it goes through a light bulb.)

The line with 100V is the line that I suspect is incorrectly wired.

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In that era, it was not uncommon to switch the neutral instead of the modern, better and safer practice of switching the hot. I would suspect the 100 volts to be produced from a switched neutral.

I don't think it is a fire hazard (insurance companies may disagree) but it does create headaches for electricians who do not turn the breaker off while working on a light they assume is off because the switch is off

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