3V from capacitive coupling?

[xAlex]

Just taking an aspect which did not get answered in a previous thread .... can the galv capping on a lighting cable buried in a wall, measure 3V between it and the Neutral of the circuit, as a result of capacitive coupling?

Thanks

 

[ericmark]

Although capacitance and induction can induce currents and voltages into other items there are also many other causes. I had an AVO multi-meter designed for Auto Electricians that measured 300 amp going through a length of 2.5mm cable which did not blow the 10A fuse in the circuit. The RF from a two way radio was completely messing up the meter. If however you are measuring to some other point then again there are many reasons why you may get 3 volt. Zinc the coating on capping when some electrolyte is used will give about 1.5 volt when compared with carbon (Copper will give similar result). Although zinc chloride and ammonium chloride dissolved in water is normally used as an electrolyte damp plaster will also work. During certain atmospheric conditions electrostatic induction can cause 1000’s of volts differential from to tip of a house roof to ground level and with tall building lighting conductors are used to prevent building damage. Holding a florescence tube under electric pylons can cause the lamp to light under certain conditions and Nikola Tesla did some odd experiments in sending power from point to point without wires. Neutral and Earth are often connected together as power comes into the house but as power is sent through neutral wires and not (we hope) through earth wires then as we move away from where they are bonded together the voltage of the neutral wire will rise slightly to the value of the line wire and on a power circuit 5.75 volt would be acceptable. That is half of a 5% volt drop on a 230 volt circuit.
So although capacitance and induction is not ruled out it is far more likely to be something other than that.
Eric

 

[xAlex]

Thanks Eric. I'm guessing then that the only other thing it could be is low insulation resistance resistance on the lighting cable.

 

[JohnD]

are you assuming the capping to be earthed? I imagine it is just in contact with the wall.

What voltage do you get on the cable between N and E at that point?

What is the earthing method in the house?

Is it a 1-way or a 2-way switch? And is it run in T&E , and is the CPC properly earthed?

 

[xAlex]

Hi John,

As you say, I really doubt that the capping is earthed (I'm surprised there is capping at all!). I will check the N-E voltage when I get home.

The switch is a 2-way 3-gang, but only the 1-way are being used. There is one supply into the switch and the other 2 gangs are linked at the switch supplying power to 2 cables for wall lights and 2 ceiling lights on separate switches. In all, 5 cables into the switch, with neutrals and Earth's twisted together. The earthing arrangement is TN-C-S. As mentioned in my other thread (which I am afraid to mention again as someone will tell me off for repeating), the circuit goes back to a MCB on the non-RCD protected side, yet when I touched the dead L and N together at the lighting point, the RCD tripped. It's all crazy. I will do a continuity on the CPC now you mention it. And yes - all T&E

 

[tim west]

Considerable voltages albeit with very low currents can be induced in surrounding unearthed metalwork from HF sources, we have trouble with sets coming in with HF Control gear for Florries and also for Neons which have been found to induce as much as 100volts in surrounding unearthed metalwork as far away as 6-8 inches, earthing this metalwork is usually enough to dispel this induced emf

 

[ericmark]

A friend of mine who worked at Orford Ness told me how they had to earth brick ties!