Capacitance between parallel conductors in a cable

[Porque223]

can cause Phantom Voltages which confound troubleshooting efforts.
Do you in the UK have this problem?

 

[Spark123]

Yes, we certainly do. It becomes apparent too in 2 way lighting circuits where energy saving lamps are in use causing them to flash when they are turned off.

 

[RF Lighting]

It is more pronounced if there isn't a CPC running between the conductors.

A simple capacitor or neon indicator connected between line and neutral at the problem fitting cures the problem 99 out of 100 times.

 

[333rocky333]

Not in the same cable, but recently called to a socket which hasnt worked since refit.
Fed in twin and earth a fluke tester measured 35 volt between live and earth terminals,even lighting the red indicator on the tester, followed the cable and it was entirely cut off at the other end, the cable was bunched and tied with aprox 10 other twin and earths on a straight run of cable tray aprox 10 metre long

 

[Porque223]

Affirmative on the two way switches and the ground conductor.
We use Romex and it measures about 100 pF/ft so 50' of cable gives you 120vac in series with a 5000 pF capacitor. I doubt it supplies enough current to feel a shock, though.

IIRC, Fluke and others make Lo-Z meters for this, plus there are solenoid voltmeters. I use an incand. bulb to load down residential wiring.

Thanks, folks. . .

 

[JohnW2]

We use Romex and it measures about 100 pF/ft so 50' of cable gives you 120vac in series with a 5000 pF capacitor. I doubt it supplies enough current to feel a shock, though.

Indeed. If my mental arithmetic is right, that's about 0.5MΩ at 60Hz, so about 240μA at 120V - as you say, quite probably too low to be felt. Of course, if there's also a CPC ('earth') in the cable, then the capacitance from that to the other two conductors changes things quite a lot.

IIRC, Fluke and others make Lo-Z meters for this, plus there are solenoid voltmeters.

I've certainly seen meters which have a switchable shunt resistor across the input to lower the input resistance/impedance, but it's obviously very easy to improvise that with any voltmeter. What do you mean by a 'solenoid' voltmeter - an analogue moving coil one?

Kind Regards, John

 

[333rocky333]

During the 80's before RCd's become common, i had a Drummond tester, not really a meter but a hand held two prong tester, this had a solenoid in and moved to indicate the voltage as well as vibrating.
guarenteed it would trip the RCD though, which in them days was supplying the whole dist board.
These and the "Leb" used, bulb type tester were the most reliable testers IMO

 

[Porque223]

What do you mean by a 'solenoid' voltmeter - an analogue moving coil one?

This thing might pull 30 mA at 120vac

http://en.wikipedia.org/wiki/Solenoid_voltmeter

 

[Lectrician]

I have a Drummond volt meter with a bulb, a customer gave it to me, brand new in box, when clearing their fathers house when he died.

Much prefer using that than my modern meters, and Mr NICEIC Inspector liked it to

 

[ericmark]

I seem to remember doing balanced feeders and impedance matching in University and being told how will extremely long distances how even with low frequencies as low as 50Hz one can get a null point and in South Africa they had to use DC.

From memory the calculations for a feeder include current so any cable must be a compromise and likely you will need some correction. I have used aerial tuning units and fitted balums many times with higher frequencies there is no reason why one should not do the same with lower frequencies.

The distance between the conductors will vary two things. The inductance and the capacitance and if the distance is correct they will balance out. I have used 300 ohm ribbon and 50, 75, and 90 ohm coax with radio frequencies and if you look at 300 ohm ribbon cable it is very like twin and earth cable.

I also remember the calculations we did in University and how they filled pages and I would not really want to calculate what inductance or capacitance would be required to correct the un-balanced feeder (twin and earth) used. With the extra low voltages used in radio we use a VSWR meter which measures the un-balance and we trim to try and get a 1 to 1 relationship. However never seen a meter of this type which is designed for low voltage.

The question is will it required extra inductance or capacitance to match. If capacitance then quite easy to add one but with inductance not so easy. From what I have read on this forum I am lead to believe capacitors are sold to correct this out of balance but I have not really gone into how to work out which way or how much needs adding.

 

[JohnW2]

What do you mean by a 'solenoid' voltmeter - an analogue moving coil one?

This thing might pull 30 mA at 120vac [http://en.wikipedia.org/wiki/Solenoid_voltmeter

Thanks. As they say at the start of the article, it's really just a type of moving iron voltmeter.

Kind Regards, John

 

[ericmark]

I have used these meters in the past but the neon indicator with 4 neons was much cheaper and replaced them in the main.

There was a special AVO meter made for auto electricians which presented a reasonable load on the voltage range I think around 200 ohms per volt to stop reading voltage through a bit of water and salt and causing one to jump to wrong conclusions. However although good idea in practice they had a huge problem in that they were affected by RF and if there was a two way radio in the car would give wrong readings.

The old tin box separate volt and ammeter was far better. Can't remember who made them but were standard equipment with most auto electricians.

I suppose the modern meter is more like the old valve volt meter which did not draw any current. I have not seem a ohms per volt written on a meter in ages. I remember when I got my AVO Mk8 being really impressed by the ohms per volt and reading the voltage on two scales and working out the no load voltage.

Most Auto Electricians liked the Mk7 as the scale was better for them. To day I read a battery cell voltage on a 200 volt range would have never dreamed of doing that in old days.

Back to original subject though I do wonder if in USA with 60Hz there will be more of a problem then we get with 50Hz? Hard to tell as also they use a different voltage but in theory the space between conductors would need to be greater in USA to UK to produce a balanced feeder.

 

[JohnW2]

I seem to remember doing balanced feeders and impedance matching in University and being told how will extremely long distances how even with low frequencies as low as 50Hz one can get a null point and in South Africa they had to use DC.

It's an interesting subject, presumably of some interest/concern to those designing and operating distribution systems, but I suspect that the distances have to be very long before significant issues arise. A half-wave at 50Hz is about 3,000 km (about 2,500 km/1,500 miles at 60Hz). As for the transmission line characteristics of HV and EHV distribution, the overhead conductors are, of course, widely spaced from both each other and from the ground.

To what extent such considerations have to be taken into account, or 'dealt with', in very long distribution networks, I haven't a clue. Maybe someone here knows?

Kind Regards, John

 

[JohnW2]

I suppose the modern meter is more like the old valve volt meter which did not draw any current. I have not seem a ohms per volt written on a meter in ages.

The concept of 'ohms per volt' (strictly 'ohms per volt range') is really only applicable to moving coil meters, where it is simply a way of expressing the full-scale-deflection current of the meter. Hence, a simple moving coil multimeter with a 100 μA FSD meter will have an input resistance of 10,000 ohms per volt (i.e. an input restistance of 1MΩ on a 100V range, 5MΩ on a 500V range etc.).

With a valve voltmeter or modern DVM equivalent, the input resistance/impedence is not only very high, but is also 'fixed' (i.e. the same, regardless of any issues of 'range') - so the concept of 'ohms per volt' does not really arise.

Kind Regards, John

 

[FDJ]

I seem to remember doing balanced feeders and impedance matching in University and being told how will extremely long distances how even with low frequencies as low as 50Hz one can get a null point and in South Africa they had to use DC.

From memory the calculations for a feeder include current so any cable must be a compromise and likely you will need some correction. I have used aerial tuning units and fitted balums many times with higher frequencies there is no reason why one should not do the same with lower frequencies.

The distance between the conductors will vary two things. The inductance and the capacitance and if the distance is correct they will balance out. I have used 300 ohm ribbon and 50, 75, and 90 ohm coax with radio frequencies and if you look at 300 ohm ribbon cable it is very like twin and earth cable.

I also remember the calculations we did in University and how they filled pages and I would not really want to calculate what inductance or capacitance would be required to correct the un-balanced feeder (twin and earth) used. With the extra low voltages used in radio we use a VSWR meter which measures the un-balance and we trim to try and get a 1 to 1 relationship. However never seen a meter of this type which is designed for low voltage.

The question is will it required extra inductance or capacitance to match. If capacitance then quite easy to add one but with inductance not so easy. From what I have read on this forum I am lead to believe capacitors are sold to correct this out of balance but I have not really gone into how to work out which way or how much needs adding.

Don't try to apply RF transmission line theory to internal 50Hz mains wiring.
Your memories are a bit vague I'm afraid