3
\$\begingroup\$

I have searched Google, but get bombarded with "RCD" type answers. I read a few answers here that may have been relevant but turned out not to be, so I post my question. Being my first, I may unintentionally break some rules - hopefully not.

I was about to make a change to a domestic light circuit so tested the light socket to ensure the circuit was dead and I had pulled the correct fuse. (Yes this circuit is 50-60 years old, hence no CB!) What I discovered with the digital multimeter (set to 700V AC, as nearest higher setting) was that 33 AC volts existed whichever direction I connected the leads. And this is with the correct fuse removed!

Wanting to be more certain I used a light bulb to verify more digitally - i.e. 0 or 1.

I proceeded with the task which used the existing light wires as pull leads to get another cable down the wall. All finished I was set to reconnect the circuit exactly as before.

My understanding was that the Live wire went to the Common connection on the switch, so I reconnected the fuse and tested for the live wire with a neon single-connecton screwdriver and connected the one that 'glowed' to the Common terminal of the switch and the other to terminal 1.

At some point; either reconnection of the fuse or flipping the switch, the fuse blew. I have not replaced it, until I get to the bottom of this problem. So my questions are;

1 - first, does the live wire go to the common terminal on a two-way switch? (being used as 1-way currently, but hopefully soon to be two way).

2 - Has the neon screwdriver led me astray for which was the live wire?

3 - Is it possible that my house has neutral switching and if so, how do I find out? It has been illegal here in NZ for at least 60 years I would have thought. If neutral switching should I put the other wire to Common and the live one to terminal 1?

4 - Is 33 volts AC a concern when the circuit is off? I did not get any form of tingle when man-handling the wires. We have 240 V AC circuits in NZ for domestic wiring.

Thank you for any answers that you care to provide.

\$\endgroup\$
  • \$\begingroup\$ Thank you responders 3. I have been happy with wiring to date and various electrical technicians have worked on it over the years, so if there was a serious problem I would expect them to notice something. \$\endgroup\$ – Erg Mar 27 '15 at 7:32
  • \$\begingroup\$ Sorry, I ran into 5 min edit rule, thus the duplication. \$\endgroup\$ – Erg Mar 27 '15 at 7:55
4
\$\begingroup\$

It is normal for a typical multimeter to show some low voltage on disconnected wires. This comes from the capacitance and inductance of the wires themselves.

Since there is essentially no load (the meter puts a tiny insiginificant load on it) that voltage doesnt go away. But the actual current it could supply is tiny. It poses no safety hazard and can't shock you.

Some meters have a mode that puts a load on the circuit to eliminate such false readings. If yours doesn't the easiest thing to do is measure while a load like a lightbulb is still connected.

\$\endgroup\$
  • 1
    \$\begingroup\$ @Erg - Grant is correct. It doesn't take much coupling to create 33V on a floating (disconnected) wire, since there's no place for the electrons to flow. There's always some power coupling in residential wires, but the actual power transfer is insignificant. If you were to hook up a light bulb, the (relatively few) electrons would flow through the bulb filament, but it wouldn't be enough power to cause the bulb to light. A multimeter (across the bulb) would show something much closer to 0V. In fact, this is likely happening in most lights in most houses worldwide. \$\endgroup\$ – bitsmack Mar 27 '15 at 0:07
  • \$\begingroup\$ Thank you responders 3. @grant and bitsmack I wonder if I try my old analogue multimeter, if I get the expected zero? I am pleased to have a reason, as I doubt a fault really. The funny thing is, that today I replaced the fuse, as the burglar alarm was complaining of running on battery only, and there was no problem, all the lights and the alarm were working as normal and the fuse did not blow. So I can't explain why it went, but perhaps it was not related! - I therefore assume live to common is correct. Next task is to enable two way switching to a new light. Hope Term 1->2 and 2->1 & Live->C \$\endgroup\$ – Erg Mar 27 '15 at 7:54
0
\$\begingroup\$

There is something strange going on but, with a 240V supply, 33V sounds suspiciously like a Neutral-to-Earth kind of voltage accompanied by a not particularly inspired phase balancing.

Assuming your neutral is not switched, and along with the fuse blowing on reconnection, that would suggest that you've now got a live-to-earth short somewhere in your lighting circuit.

The simplest explanation, were this some kind of mystery puzzle, would be that in your initial work post-isolation but pre-measuring you introduced a short from a live wire to a metal case or trunking.

However, this being your house and you presumably being reluctant to burn it down, I'd suggest at the very least you take some systematic voltage measurements from a known good earth (eg metal piping gas at an earthing point) to each point in your circuit and a good few resistance measurements on disconnected circuits (eg casing to supply, live-to-neutral, etc).

But the real thing to recommend, us not knowing your skill as an electrician is to call in a professional or knowledgeable friend.

\$\endgroup\$
  • \$\begingroup\$ I have been happy with wiring to date and various electrical technicians have worked on it over the years, so if there was a serious problem I would expect them to notice something. Your postulation in paragraph three may well be the cause. Your suggestion to measure a few known good circuits sounds "sound", so I plan to do that. The fuse wire is a measly 5A, so very thin. Max 1200 W and it has perhaps 700W max, so OK there. About to add three 58W/33 fluorescents, but still within max. Are they 58W or 33W I know not. My skill is research, planning, asking and thinking, rather than recall. \$\endgroup\$ – Erg Mar 27 '15 at 7:44
  • \$\begingroup\$ Do take into account the issue that the other poster mentioned, though, that a DVM can show spurious readings on disconnected circuits. I actually still use an analogue meter when I'm just going around checking liveness and continuity as it's not likely to give spurious readings of this kind (takes real energy to move that arm), so tend to forget that possibility. \$\endgroup\$ – Dan Sheppard Mar 27 '15 at 12:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.