2 way switch induced voltage?

[Lucien Nunes]

Got a 2 way switch that gives 43 volts down one strapper when off, could this be an induced voltage, and if so why isn't it on the other strapper when it is off?

Draw the circuit - think about the permutations. However it is wired, one strapper will always be at line potential. What is on the other(s) depends on which ends the PL & SL are connected.

Scheme 1) PL & SL to L1 & L2, all three terminals of both switches strapped
Light off: Strapper 1 at line volts, strapper 2 at or near neutral via SL & load, common wire could be either.
Light on: Both strappers and common at line voltage.

Scheme 2) PL to one com, SL from the other, L1 & L2 strapped
Light off: One strapper at line volts , other at or near neutral via SL & load
Light on: One strapper at line volts, other floating.

I.e. only in the last situation will one of the strappers be floating and likely to show a ghost voltage, unless the load is O/C in which case one strapper will be floating whenever the switches are in an off state.

A switch that physically sticks after being operated under load, has probably got badly pitted contacts. The tiny weld that forms at each operation, especially if the load has high inrush, can interlock the rough surfaces together. I had this on an MEM grid module, that would jam almost solid after switching about 500W of switch-start fluo, but worked normally off-load.

Rated voltages are nominal based on lifespan and safety and in practice the maximum usable voltage depends on the kind of load. A switch specced for 250V will probably switch 500, but with seriously reduced life and poor safety margin and a tendency to flash over with inductive loads. At very light load it might even handle 750.

 

[Midwest]

It's far more likely to be a capacitively coupled voltage rather than induced.

A phantom voltage such as this will not interfere with your dead testing due to the fact the circuit will be dead! Theoretically it could be an issue where multiple circuits are in close proximity and some remain energised, but I've yet to come across this being a problem for an MFT.

I'm gonna have to disagree with you there davesparks, but I know you'll put me right.

On a couple of occasions, my Megger mft (1730) would not conduct continuity tests, because it detected 'voltage'. Complete isolation of the installation, removed 'phantom, ghost, induced' etc voltage, and the MFT functioned correctly.

 

[davesparks]

I'm gonna have to disagree with you there davesparks, but I know you'll put me right.

On a couple of occasions, my Megger mft (1730) would not conduct continuity tests, because it detected 'voltage'. Complete isolation of the installation, removed 'phantom, ghost, induced' etc voltage, and the MFT functioned correctly.

As I said I've never yet had the problem, or at least can't remember it being a problem. That's not to say it can't happen, and from your experience obviously does happen.

Did you try discharging the dead conductors by briefly connecting to earth? I doubt it would make a difference but am curious?

 

[Midwest]

Did you try discharging the dead conductors by briefly connecting to earth? I doubt it would make a difference but am curious?

Have to say I didn't at the time due to my lack of expertise, but we all gain from our experiences. Now know what to think about in those circumstances.