Questions about identifying wires with a continuity tester

[M4gery]

Today I did some work in a really old house, half of the walls are open and half are closed so I have to mixed and match old and new wiring and box things where necessary.

In the bathroom I had a feed coming into a switchbox, a 3-wire coming out of the switchbox into a light, then a 2 wire coming out of the light and going off to another receptacle down the line. So the 3-wire coming out of the switch has a neutral, hot, and switched hot. This is really old BX with that cloth type covering on the wire that breaks away easily, unfortunately I had to re-use that 3-wire coming up from the switch, this is where I ran into trouble.

I wanted to see if I could identify those 3 wires using a continuity tester, I didn't want to open the switch box and possibly damage those wires (just by moving them). The first thing I did was try to read continuity (with my Fluke 322) from a known grounded source to 1 of the 3 wires- None of them rang out. I figured the neutral would show continuity since it's bonded to ground in the panel. I measured 120V from hot to this same grounding point so I should have been able to read continuity from a neutral wire to this point, right?

Second, I tried to read continuity thru 2 out of the 3 wires while the switch was in the on position, this should go off when the switch is off. Unfortunately I couldn't read continuity thru any sequence of the 3 wires. IMO it should have rang out between the hot wire and the switched hot wire when the switch was turned on.

Neither of these methods worked and I am wondering why...?

I ended up opening the switch box and identifying the wires that way, everything worked when I was done, but I am still stumped on why I wasn't able to get it to work with a continuity tester...

 

[hurk27]

When I get into this type of wiring I set up a good 3-wire extension cord to a known properly wired receptacle, I use the extension cord to test to find the real hot and neutrals, and to make sure there is a ground if the wiring method supports one, I have no idea why you couldn't read the contacts of the switch other then if this is the old tin plated copper wiring that maybe there was enough corrosion to not allow your test leads to make a connection with the conductor but when you twisted the wire together it worked.
The reason for the extension cord is many times I have found where the white wire is hard to distinguish from the black and an earlier person did not take the time to make sure they connected the correct wire together, and this can lead to having reverse polarity of the down stream circuit.

 

[SG-1]

M4gery I agree with Hurk27 that copper oxide probably foiled the continuity test. I would re-create the situation & experiment with the meter. The continuity setting is supposed to beep or tone on 35 ohms or less.

 

[hurk27]

M4gery I agree with Hurk27 that copper oxide probably foiled the continuity test. I would re-create the situation & experiment with the meter. The continuity setting is supposed to beep or tone on 35 ohms or less.

Unless you are using a Ideal Volt-Con which will start to tone at 600k ohms.:happyyes:

 

[M4gery]

Thanks for the responses gentlemen.

After I tried the tests that I mentioned above (and they did not work) I opened up the switchbox and read continuity from each lead in the switchbox to the end of the whip. When doing this I touched the test leads to the ends of the wire in the same way as when I did the initial test, so with that in mind I'm not sure if oxidation was the culprit

 

[stevenje]

Thanks for the responses gentlemen.

When doing this I touched the test leads to the ends of the wire in the same way as when I did the initial test, so with that in mind I'm not sure if oxidation was the culprit

Take your knife and scrape off the oxidation off. Then you know you have a good contact point. I have been fooled more than once by not doing this.