Neutral to Neutral = Dc Voltage

[First-time-buyer]

A bit of a strange one...

Recently I have installed a few extra sockets (7 doubles) in the stud walls upstairs. The sockets were run with a view to extend the ring main at the single socket on the landing. One leg was going to pulled out and Junction Boxed, and the other pushed up the stud and into the socket. Hey presto...

With the old socket apart (stranded uPVC) the ring was present i.e Neutral to Neutral, Live to Live etc. Also there was a 2.5mm T&E running off to a spur socket in the lounge.

Carrying out a voltage check prior to starting work, I noticed that the neutral from the ring to the neutral of the spur kicks out a DC Voltage on the Fluke. Unsure how much, as the fluke only has lights and not a gauge.

Afterwards, when its all put back together prior to turning power back on, I carried out a continuity check beftween Live-Neutral, Live-Earth, Neutral-Earth. Both Live-Earth, Live-Neutral and Neutral-Earth show continuity. Live-Earth was more of a chirp than a solid Live-Neutral due to appliances still plugged in etc. I think that's one for the morning...

Anyhows, DC Voltage in the picture below? A bit of a random one. I had thought it could be one of the many phone chargers, or the alarm panel battery etc discharings...

However.. when the lighting circuit is pulled the DC Voltage goes away. Looking at the Spur, the only thing that is close to it lighting purposes if the wall lights... Mrs FTB assured me that the wall lights were turned off..

Cant see how 2 lighting circuits borrowed neutral will matter, as the house only has 1 lighting circuit...

 

[First-time-buyer]

Not to be distracted by the Live-Earth waffle, that was probably caused by a light being on somewhere in the house, the board not being turned off. Therefore the churp is the PME connection...

The question is DC voltage from the spur..

 

[JohnW2]

The question is DC voltage from the spur..

What exactly are you detecting/measuring this with?

Kind Regards, John

 

[First-time-buyer]

http://m.supremetechnology.com.au/mobile/Product/ProductDetails.aspx?ProductId=613

Hi John,

It's a Fluke T3 tester.

I should of broken out the multimeter.

 

[bernardgreen]

Measuring voltage across the ends of a ring final will show some "strange" results as the loop of cable is acting as a aerial and will have voltages induced in it from a variety of sources.

A high impedance test device will react to these induced voltages and produce what appear to be strange readings. One strange result is to find a length of wire ( the ring ) has a negative resistance. This is due to the presence of an induced voltage and the way that meter measures resistance.

The most common source of induced voltage is from adjacent cables that are Live

To get sensible ( accurate ) readings of voltage the meter should be low impedance or shunted with a small 230 volt filament lamp that will absorb the induced energy, ( or glow brightly if there is 230 volt to be measure ).

To get sensible reading of resistance any induced voltages need to be swamped by using a test current higher than that which most digital multi meters use. ( always test for voltage before testing the resistance )

 

[JohnW2]

http://m.supremetechnology.com.au/mobile/Product/ProductDetails.aspx?ProductId=613
Hi John, It's a Fluke T3 tester. I should of broken out the multimeter.

As Bernard has said, one can get very strange results due to induced voltages. As Bernard implied, even with a (digital, high impedance) multimeter you may well have seen some sort of voltage unless you shunted the meter with some sort of load (like the filament lamp he suggested).

Why your gadget was indicating DC voltage (which is clearly wrong) is anyone's guess.

Kind Regards, John

 

[First-time-buyer]

Emf induced by the lighting circuit due to cables within the same capping/conduit?

The house currently has a 4 way board, which is built up of 1x Ring Main, 1x Lighting Circuit, 1x Immersion, 1x Cooker (which is the kitchen sockets = bodge). The plan in the future is to have a 10-way dual rcd board fitted.

If this was a neutral issue, the only way around it would be to put all 4 old ways onto one side of the board....

 

[JohnW2]

Emf induced by the lighting circuit due to cables within the same capping/conduit?

Sure, that's definitely a possibility - the emf can be induced by any 'live' cables in the proximity - obviously more likely as they get closer.

If this was a neutral issue, the only way around it would be to put all 4 old ways onto one side of the board....

I very much doubt whether you have any 'issue' - you're just seeing artefactual indications/measurements. Have you tried repeating your tests with the entire board switched off?

Kind Regards, John

 

[bernardgreen]

Why your gadget was indicating DC voltage (which is clearly wrong) is anyone's guess.

No guess work.

Many "economically designed" meters only measure the positive peak voltage when measuring AC voltage and scale it to display the equivalent RMS voltage. This reduces the complexity of the circuitry in the meter and gives reasonably accurate results provided the input is a symmetrical sine wave

 

[JohnW2]

Why your gadget was indicating DC voltage (which is clearly wrong) is anyone's guess.

No guess work. Many "economically designed" meters only measure the positive peak voltage when measuring AC voltage and scale it to display the equivalent RMS voltage. This reduces the complexity of the circuitry in the meter and gives reasonably accurate results provided the input is a symmetrical sine wave

If we were talking of a cheap multimeter, I would agree. However we appear to be talking about a device (far from cheap {before it was discontinued} and from a very reputable manufacturer) specifically designed to indicate (qualitatively) whether an AC or DC pd exists between its probes - and it is clearly getting it wrong.

Kind Regards, John

 

[ericmark]

Having voltage is fair enough already explained but to be DC requires a diode some where. However aluminium oxide will work as a diode so likely some where there is an aluminium box with some corrosion on it.

But I would not consider it is anything to worry about unless you are a radio ham. I know with Ham radio rusty bolts can cause all sorts of harmonics but for most people it's just nothing to worry about.

 

[JohnW2]

Having voltage is fair enough already explained but to be DC requires a diode some where. However aluminium oxide will work as a diode so ....

As I've implied, I very seriously doubt that there is any 'DC' present, at least in the usual sense of the term. I suspect that, although expensive and reputable (Fluke) this testing device was, for some reason, just getting it wrong.

In any event, it's extremely unlikely that any corroded aluminium (or corroded iron - iron oxide can also create a diode) would exist in, or connected to, the conductor path of a ring final, isn't it?

Kind Regards, John

 

[ban-all-sheds]

iron oxide can also create a diode

Yup.

You can (could?) stand close to rusty wire fences near the 1500m transmitter at Droitwich and listen to the broadcast..

 

[First-time-buyer]

I guess time will tell, eventually as we redecorate all old cabling will be replaced.

We just hope that we can pull the new cables up the existing conduit/capping. Just leaving minor filling where the back boxes have been removed.

I see 50mtrs of 1.0mm T&E is £18 odd. Is there anywhere else which is cheaper.?

 

[EFLImpudence]

£16.88 at Toolstation.