What's your definition of 'ghost voltage' ?

[Lucien Nunes]

I don't like the term 'ghost voltage' a.k.a. 'phantom voltage*' much, not least because it's rather loosely defined and probably means different things to different people. I also suspect that quite a few people use the term without really understanding what it is they are referring to.

My definition of what constitutes 'ghost voltage' is quite specific, but first, what is yours?

*OTOH 'phantom power' (as used to power condenser mics and other audio sources via the signal cable) is a different kettle of spooks and very clearly defined.

 

[R-fur]

The National Grid Co have very specific terms for voltages on conductors and how they got there. I think they have four terms from memory, Trapped charge, capacitive coupled, induced voltage, cant remember the other one. They are talking about EHV circuits obviously the theory is similar.

 

[buzzlightyear]

funny you mention the post ,I had one to day on my megger no voltage present
and started do show OL ghost voltage , the last time it came up I rang megger the person on the other phone said how spooky , I started to singing goat busters bang me head against a wall.lol

 

[Darkwood]

It's obvious that none of you have ever seen a supernatural film, a ghost voltage is the thing that makes the lights flicker and the tv come on when it's not plugged in... seriously, where you been!

 

[dmxtothemax]

My definetion of ghost voltage is a measurable voltage that is of such a low current it can do no real work.

 

[telectrix]

it can do no real work.

a bit like an Aussie then.

 

[Lucien Nunes]

My definition. All three crtitera need to be satisfied to call it a ghost voltage:

A voltage present with respect to earth, or to some other identified point
a) on a conductor or component presumed to be or intended to be unconnected (but not necessarily safely isolated) i.e. floating.
b) derived from and having the characteristics of the supply (or one of the supplies) within the installation.
c) with a source impedance in the range 1MΩ - 100MΩ at freequencies of interest, or one decade outside this range under special circumstances

Point (a) excludes abnormal voltages on energised circuits due to a high resistance connections etc. and normal functional voltages that are not a recognisable supply voltage. In day-to-day electrical work many things are either connected to the supply or not connected, whereas in electronics most points have some voltage on them but are not directly connected to a supply. It does not make sense to me, to use the term 'ghost voltage' of a voltage expected to be present, just because it has a high source impedance, i.e. that it is not 'stiff'. This point also excludes low voltages of low source impedance such as differences between two connected earthing or bonding conductors due to circulating currents in a CNE.

Point (b) excludes kinds of voltages that exist on random conductors that have nothing to do with electrical wiring or are transient, e.g. triboelectricity (static), residual charge and dielectric absorption recharge, low thermal or electrochemical voltages etc. These usually have a signature that allows them to be immediately identified.

The lower resistance bound of point (c) puts the maximum current to earth on a 230/400V supply at 0.23mA, i.e. below the lowest threshold for permissible earth leakage on any appliance, while the upper bound ensures that a ghost of voltage 50V O/C gives a reading of at least 5V on a meter with 10MΩ input resistance. Working in electronics, especially where high-impedance circuits are involved, one gets a feel for source impedance without having to measure it, especially if one can reasonably estimate the O/C voltage and compare that with the actual reading.

Every isolated conductor on the planet has stray admittances (inverse of impedances). If it's large and well insulated, its susceptance even at 50Hz from its stray capacitance to earth will dominate. If it is part of a wiring system, the next most significant admittance is also likely due to capacitance, to other conductors. A floating conductor in a cable will often take up a potential between those of nearby cores and earth, and one can with a bit of logic and maths make all kinds of inferences from the readings that it gives: length to a break, position within a multicore, likely devices connected at the far end.

I've heard all sorts of nonsense about 'ghost voltage', such as 'it's not real voltage' and 'you can't measure it', whereas from an electrical point of view, it's just as legitimate a thing to measure and analyse as any other voltage.

 

[Ian Mills]

Here in France it is a big thing, because they use a lot of conduit with many cables in it from different circuits you get a lot of inductance in cables that should be 0 volts I have seen LED lights glowing all night from this

 

[MPTESE]

This may or may not be related or useful context for your question. In the film industry, there's a common term called "Ghost load". It's often used when dimming lighting fixtures with traditional dimmer packs. Sometimes the dimmer pack requires a minimum load to operate effectively and smoothly. If the load is minimal, often with LED lights, the solution is to attach a "ghost load". Sometimes we'll burn a 1k incandescent in line and point it away from set, and it solves our dimming problems.

Perhaps this is similar to what @dmxtothemax replied with.

 

[Lucien Nunes]

Same word, different meaning and context. Ghost load is a deliberate load that is hidden from view, ghost voltage is a potential from such high source impedance that it 'disappears' when tested with anything other than a high-resistance meter.

 

[HappyHippyDad]

Same word, different meaning and context. Ghost load is a deliberate load that is hidden from view, ghost voltage is a potential from such high source impedance that it 'disappears' when tested with anything other than a high-resistance meter.

I remember a while back I mistakenly said ghost voltage is 'not real' and you instructed me otherwise. I still can't quite get my head around why we can't get a shock from it if it is 'real'?

I tested a 19 core cable and got around 200V between one of the conductors (which was completely disconnected) and earth when tested with my MFT. I then tested with the drummond and got no light at all (so <50V). Is this useable voltage and if so why don't I get a shock? Is it 200V as the MFT says or is it <50V as the drummond says?

 

[DPG]

Technically speaking you could get a shock from a 'ghost' voltage. In fact consider the ghost voltages and currents available on transmission lines.

 

[Lucien Nunes]

Voltage is potential, the ability to do work by driving a current through a resistance. But most of the effects that electricity can produce, like heating an element or giving you a shock, depend on the value of the current. If the current is drastically limited in some way, then even if the source voltage is high, the effect is also drastically limited. In the case of ghost voltage, the current is limited by a very high impedance between the source (a truly live conductor) and the conductor οn which the ghost voltage is detected.

If that current-limiting factor (e.g. a stray capacitance from a truly live core of a cable to another that you are testing) has an impedance of one megohm, on a 230V circuit no matter what happens the current available to the touch is limited to 230/1,000,000 = 0.23mA which will not give you a shock, or light a lamp, or give an indication on a traditional Drummond tester. However, 0.23mA is plenty to operate a digital multimeter's voltage range, and it will give you a reading in the order of 200V. It's really 200V while it's there, but connect a lamp (which has a much lower resistance than the meter) or bridge it to earth with your hand, and the voltage collapses because of that very limited available current.

When I have more time later I'll set up some examples and examine them as voltage dividers.

 

[HappyHippyDad]

Voltage is potential, the ability to do work by driving a current through a resistance. But most of the effects that electricity can produce, like heating an element or giving you a shock, depend on the value of the current. If the current is drastically limited in some way, then even if the source voltage is high, the effect is also drastically limited. In the case of ghost voltage, the current is limited by a very high impedance between the source (a truly live conductor) and the conductor οn which the ghost voltage is detected.

If that current-limiting factor (e.g. a stray capacitance from a truly live core of a cable to another that you are testing) has an impedance of one megohm, on a 230V circuit no matter what happens the current available to the touch is limited to 230/1,000,000 = 0.23mA which will not give you a shock, or light a lamp, or give an indication on a traditional Drummond tester. However, 0.23mA is plenty to operate a digital multimeter's voltage range, and it will give you a reading in the order of 200V. It's really 200V while it's there, but connect a lamp (which has a much lower resistance than the meter) or bridge it to earth with your hand, and the voltage collapses because of that very limited available current.

When I have more time later I'll set up some examples and examine them as voltage dividers.

That's a great explanation Lucien, thankyou, I understand it alot more now. I guess it's the same with the 1000V IR test on the MFT. I remember the tutor giving me a shock with it, but it's not really that bad which I assume is due to the 'current limiting factor'.

 

[Vortigern]

Speaking from an installer/tester (be I ever so 'umble sir) the more technical aspects you bring up do not usually press to the front of considerations in such work. In common parlance ghost voltage is a voltage that is not derived directly from the DB. It is generally understood to be an induced voltage from other cables close to the one being tested and running along with it. It is important on a pragmatic level to distinguish between the voltage which is inducted and the voltage arising from a fault in the wiring/installation. So we introduce a higher load in the tester to see if the voltage is sustained under load which give a pretty sure indication of which of the two we are dealing with, fault or inducted voltage. It is of note that in the instruction manual for the MTL20 drummond tester that this voltage is referred to as interference voltage and phantom voltage and perhaps where the term ghost voltage came from.
The finer point you have brought to scrutiny while giving food for thought as well as education would rarely feature in on the ground (so to speak) day to day work. Not denigrating what you contribute but finding practical applications for it would be a challenge. Notwithstanding the foregoing I would encourage your suggestion of some worked voltage divider simulations of such situations as a picture says a thousand words and do sincerely look forward to the same!

 

[Lucien Nunes]

However, faultfinding and analysis are an art/craft/science that can be taken to whatever level one likes. Even when confronted with simple, everyday problems, you can still benefit from the use of advanced techniques and insights because they can deliver devastating speed and efficiency.

Example: Some here would say that voltsticks are the work of the devil and have no place in a proper electrical toolkit. But, on one occasion, using just a voltstick, I was able to locate and identify which strapper cable was not making a good connection, in which switch, in a 2-way + intermediate setup in about 30 seconds without taking any faceplates off and hence without even visiting the CU. I was then able to isolate the lighting circuit, remove the requisite switch, reterminate the loose conductor and put it back together with a total downtime of maybe 2 minutes. Using only the so-called dangerous voltstick, aided by my knowledge of its method of operation and parameters, and a knowledge of the shapes of the metal components inside an intermediate switch and the shapes of the electric fields they will produce in the plane of the front plate. Simple problem + advanced method = happy customer + £300/hour. QED.