Capacitive Reactance

[Choice_Gorilla]

I was reading through Soares Grounding and Bonding and it says:

“…any circuit with insulated conductors at different instantaneous potentials and a potential from ground will have capacitance and capacitive reactance.”

Are they stating that all AC circuits, regardless of the presence of a capacitor in the circuit, will have some level of capacitive reactance adding to the overall impedance? Would this be due to the insulation along the length of the wire “storing a charge”, treating the entire conductor like a capacitor that never discharges? Or am I missing something?

 

[electrofelon]

I was reading through Soares Grounding and Bonding and it says:

“…any circuit with insulated conductors at different instantaneous potentials and a potential from ground will have capacitance and capacitive reactance.”

Are they stating that all AC circuits, regardless of the presence of a capacitor in the circuit, will have some level of capacitive reactance adding to the overall impedance? Would this be due to the insulation along the length of the wire “storing a charge”, treating the entire conductor like a capacitor that never discharges? Or am I missing something?

Basically yes. A conductor with a potential forms a capacitor whether you intend it to or not.

 

[kwired]

I was reading through Soares Grounding and Bonding and it says:

“…any circuit with insulated conductors at different instantaneous potentials and a potential from ground will have capacitance and capacitive reactance.”

Are they stating that all AC circuits, regardless of the presence of a capacitor in the circuit, will have some level of capacitive reactance adding to the overall impedance? Would this be due to the insulation along the length of the wire “storing a charge”, treating the entire conductor like a capacitor that never discharges? Or am I missing something?

A capacitor is nothing more than conductive material sandwiched between layers of insulating material. Exactly same thing you have with conductors in a raceway or cable. But in a designed unit built to gain extra capacitance they put many layers inside to increase the density of the "capacitor".

The conductors in raceway or cable have capacitance but the amount is negligible for most of what we are interested in, but is enough that it can have effects that matter at times.

 

[Choice_Gorilla]

Awesome, thank you. That’s basically what I was thinking, I just wanted to clarify that I was on the right track.

 

[kwired]

This capacitance is why there can be "ghost voltage" on an other wise dead line that is run with other energized conductors. A low impedance meter will draw enough current to short out the weak capacitor where a high impedance meter will show a reading.

Capacitance is also behind the principle of how non contact voltage testers work.

 

[Choice_Gorilla]

Capacitance is also behind the principle of how non contact voltage testers work.

That’s interesting, I always thought it was electromagnetic induction. The NCVT would pick up the magnetic field from the alternating current. I never really looked into it though

 

[jim dungar]

That’s interesting, I always thought it was electromagnetic induction. The NCVT would pick up the magnetic field from the alternating current. I never really looked into it though

There are several types of contact less sensors.
The cheap, common, ones use capacitance and are better at detecting voltage than they are non-voltage, especially if you are wearing gloves.
There are 'field' sensing types for <600V but they are common for MV systems where meter lead contact is not often practical.

 

[ptonsparky]

Before I had a Megger, I had a tester that I would use on long runs of underground. You could hear it charge up and stop, just as when field testing a start capacitor. Some conductors would hold that charge. Those that started over when the tester was reapplied, were suspect.

A Megger does the same. Word of warning…discharge that good conductor!

 

[LarryFine]

Many years ago, a mechanic friend tried to hand me an automotive ignition points condenser.

I told him to touch the terminal to the can first. He was impressed that I knew what to suspect.

 

[ramsy]

There are several type of contact less sensors.
The cheap, common, ones use capacitance and are better at detecting voltage than they are non-voltage, especially if you are wearing gloves.
There are 'field' sending types for <600V but they are common for MV systems where meter lead contact is not often practical.

Can anyone explain why those contactless testers don't work on the neutral ?

 

[LarryFine]

Can anyone explain why those contactless testers don't work on the neutral ?

You mean a grounded conductor? Because it's at the same voltage as the earth around your body.

Now, one will find the now-energized side of a break in a neutral.

 

[ramsy]

You mean a grounded conductor? Because it's at the same voltage as the earth around your body.

Now, one will find the now-energized side of a break in a neutral.

If neutral has no voltage, how does it zap us off the ladder when we touch it ?

 

[LarryFine]

If neutral has no voltage, how does it zap us off the ladder when we touch it ?

I need more detail to answer you, unless you mean static discharge.

 

[electrofelon]

Can anyone explain why those contactless testers don't work on the neutral ?

You mean a grounded conductor? Because it's at the same voltage as the earth around your body.

So expanding on this, here is a thought experiment: say you have a typical single phase distribution line on poles. How does the capacitance of the line compare if it was ungrounded vs if one leg was grounded?

 

[ptonsparky]

If neutral has no voltage, how does it zap us off the ladder when we touch it ?

It's an open neutral .

 

[jim dungar]

Can anyone explain why those contactless testers don't work on the neutral ?

Because they are not good at detecting no-voltage conditions.

For the most part the neutral conductor is connected to ground/earth. You are also somewhat connected to earth, due to its physical size, therefore there is not much of a capacitive leakage so the testers are fooled. In other situations, such as when you are wearing rubber gloves the testers can also be confused.

These testers rarely have false positives so if they say there is voltage believe them. However they often have false negatives, so if they say no voltage, confirm it with a contact type meter.

 

[LarryFine]

So expanding on this, here is a thought experiment: say you have a typical single phase distribution line on poles. How does the capacitance of the line compare if it was ungrounded vs if one leg was grounded?

I'd say you'll still receive a positive indication. Floating systems form a virtual capacitive "neutral."

 

[Choice_Gorilla]

Because they are not good at detecting no-voltage conditions.

For the most part the neutral conductor is connected to ground/earth. You are also somewhat connected to earth, due to its physical size, therefore there is not much of a capacitive leakage so the testers are fooled. In other situations, such as when you are wearing rubber gloves the testers can also be confused.

these restore rarely have false positives so if they say there is voltage believe them. However they often have false negatives, so if they say no voltage, confirm it with a contact type meter.

So if I understand you correctly, a non contact voltage tester is basically picking up on capacitive leakage to ground through an imperfect dielectric (the insulation on the wire). It doesn’t pick up the neutral because it should be at the same potential as earth and would have far less, if any leakage to earth.

 

[tburdick]

The milwaukee 2203-20 dual volt tester reads neutral voltage.
It is my favorite non contact tester.

 

[Jpflex]

This capacitance is why there can be "ghost voltage" on an other wise dead line that is run with other energized conductors. A low impedance meter will draw enough current to short out the weak capacitor where a high impedance meter will show a reading.

Capacitance is also behind the principle of how non contact voltage testers work.

I witnessed a dead wire show voltage on it while ran within the same conduit of another line side hot switch wire. Or at least the trick tracer beeped on this wire.

Although the two conductors separated by insulation can create a capacitive effect, I thought the effect of voltage seen on the wire not connected to power to be caused more through inductance and changing of AC field from the hot wire rather than the capacitive portion