Energized coax cables

[satman]

FYI,
I just installed some electronics in a State building with services from Comcast. The product went up in smoke!. I found 55VAC on the center conductor of the coax. Seems that Comcast field techician neglected to remove the power shunt from the amplifier feeding the building.

Also I was trouble shooting signal related probems with a home run cable installation at a High School. I removed the coax from on classroom and instantly receive a healthy shock. My volt meter measured 75VAC to the coax shield and rack.

I found that the electronics connected to the coax in the classroom went through 3 daisy chained outlet strips. The voltage between neutral and ground was 75VAC. Upon further investigation the outlet strip plugged into the wall had the ground broken off and the plug was rotated around since the plug wasn't polarized.

Think safety
Be safe,
satman

 

[unimo]

What a nightmare!

Of course the power strips were the problem in your case, but what a lot of people don't know is that it is common for the neutral to not be at ground potential. This is especially true in large commercial and industrial buildings, but I've seen it in houses too.
The reason is that the neutral wire has resistance, the longer it is, the more resisitance it has. So at the point of use, the load is returning probably 120 or 277 volts to ground, and if the resistance of the neutral is high, it acts as a load in series with the load it is hooked to, and gradually gets closer to ground potential as it goes back to the panel. Then add in harmonics from non linear loads, and you can get shocked worse from a neutral than a hot wire sometimes !
And if it is returning to a subpanel, the neutral busbar there may not be at ground potential for the same reason.

I'll be careful of coaxes after your story though.

 

[don_resqcapt19]

unimo,

So at the point of use, the load is returning probably 120 or 277 volts to ground, and if the resistance of the neutral is high, it acts as a load in series with the load it is hooked to, and gradually gets closer to ground potential as it goes back to the panel.

How does the voltage go down as you get closer to the panel? The voltage to the grounding condcutor or earth from a grounded conductor is equal to the voltage drop on the grounded conductor. Anything more than a volt or two is excessive and is the result of conductor overload or a poor connection in the grounded conductor between the load and the power source.
Also current does not return to ground....it only returns to its source.
Don

 

[unimo]

How does the voltage go down as you get closer to the panel? The voltage to the grounding condcutor or earth from a grounded conductor is equal to the voltage drop on the grounded conductor. Anything more than a volt or two is excessive and is the result of conductor overload or a poor connection in the grounded conductor between the load and the power source.

Don[/quote]

In theory this is true, but I troubleshoot all day long, and i've measured 80 volts to ground on a neutral from a 277 volt fluorescent lighting load a few hundred feet from the panel. Of course some of this is harmonic. Everything was working fine though. I thought it was a problem, and opened every box back to the panel and checked all connections, and made sure the neutral busbar was tight. The neutral voltage gradually got lower as it got closer to the panel. It's from the voltage drop caused by the resistance of the long neutral itself. The neutral wasn't overloaded. True, you could save power by upsizing the neutral, but there were maybe thirty lights on the circuit, and my job was just to make sure it was working properly, not redo stuff. The store had about 600 lights in it.

 

[ryan_618]

Then add in harmonics from non linear loads, and you can get shocked worse from a neutral than a hot wire sometimes !

How can this be? The current flow through the body during an electrical shock is a function of two things:
1) The voltage of the circuit, and
2) The resistance of the body.

(I=E/R)

How can harmonic currents change either of these two items?

 

[dereckbc]

unimo,
IMO if you were to have more than 7-volt drop from N-G along any point on a 277 circuit, then you have a major problem on your hands period. 80-volts is catastrophic and totally unaccetable under any circumstances. Do some basic Ohm's Law equations and you will quickly understand a significant malfunction , design error, or a fire.

 

[roger]

Unimo, what type of meter were you using when you took these measurements? Was it a DVM by any chance?

Roger

 

[unimo]

Then add in harmonics from non linear loads, and you can get shocked worse from a neutral than a hot wire sometimes !

How can this be? The current flow through the body during an electrical shock is a function of two things:
1) The voltage of the circuit, and
2) The resistance of the body.

(I=E/R)

How can harmonic currents change either of these two items?

It changes the E. It stores voltage from a previous cycle, then returns it later to add to the present cycle. that's why neutrals have to be larger than phase wires in a highly harmonic circuit.

 

[unimo]

unimo,
IMO if you were to have more than 7-volt drop from N-G along any point on a 277 circuit, then you have a major problem on your hands period. 80-volts is catastrophic and totally unaccetable under any circumstances. Do some basic Ohm's Law equations and you will quickly understand a significant malfunction , design error, or a fire.

but that's normal operation. i know it doesn't look good on paper, but does anyone else do troubleshooting out there? Go to the last fluorescent light on a circuit in a large building, a couple hundred feet from the panel, magnetic ballasts, and measure neutral to ground and tell me what you get. This was back when I was using an analog voltmeter. Would digital be different?

 

[unimo]

I forgot to mention that 80 volts is the highest I have seen after doing this for 25 years. it's often over the 7 you mention, i would say it averages closer to 30 volts. I think it is from harmonics.

 

[don_resqcapt19]

unimo,

but that's normal operation. i know it doesn't look good on paper, but does anyone else do troubleshooting out there?

No its not normal operation, and yes I troubleshoot circuits.
When you measure voltage between the grounded (neutral) conductor and the grounding conductor, you are really measuring the voltage drop on the grounded conductor between the main bonding jumper and the point where you are measuring the voltage. If you have a 30+ volt drop, the lights are not likely to be working as they should.
Don

 

[dereckbc]

Don I would add whatever drop is incurred on the neutral is also incurred on the phase conductor, so you can double whatever voltage drop we are talking about 80 +80 = 160 277-160 only leaves you with 117 volts. The circuit cannot function. Of course I have assumed single phase with N & P being equal sized.

 

[unimo]

Don't forget this was a 277/480 multiwire circuit. Three hots and a neutral,
all magnetic ballasts before they understood harmonics. So the neutral current could have been 1.73 times the phase current.

 

[don_resqcapt19]

unimo,

Three hots and a neutral,
all magnetic ballasts before they understood harmonics. So the neutral current could have been 1.73 times the phase current.

That doesn't change anything. The circuit can't work properly with that much voltage drop.
Don

 

[unimo]

unimo,

Three hots and a neutral,
all magnetic ballasts before they understood harmonics. So the neutral current could have been 1.73 times the phase current.

That doesn't change anything. The circuit can't work properly with that much voltage drop.
Don

I understand it doesn't work on paper, but I'm simply reporting field observations on circuits that work perfectly well. I don't have the engineering knowledge to tell you why they work, but they do. i'll concede that when I measured 80 volts on the neutral it was before I was troubleshooting, and I was just replacing ballasts and making measurements out of curiousity because other electricians had warned me of getting shocked off the neutral and I didn't believe them. Also it was a non-RMS analog meter.
But now that I've been troubleshooting for most of my career, I have measured 50 volts maybe 15 times, and 30 or under very often. Half the time you'll measure close to nothing. That's why I was asking if there's anyone out there doing troubleshooting who can make actual measurements in the field.
the systems where you see this are three phase, entirely magnetic fluorescent loads, completely balanced loads. Maybe thirty ballasts on each leg to neutral. So if it wasn't for harmonics and current lag there should be nothing on the neutral.
I think you're over simplifying what's happening. In theory, the ballast doesn't need the neutral, and operates in series with another ballast. The circuit is primarily inductive impedance, and if you measure amperage on the neutral at the panel, it is low.

 

[don_resqcapt19]

Half the time you'll measure close to nothing.

And in the other half there is something wrong with the electrical system. If there wasn't some problem with the system, you would not see that voltage between the grounded and grounding conductors.
Don

 

[ryan_618]

Then add in harmonics from non linear loads, and you can get shocked worse from a neutral than a hot wire sometimes !

How can this be? The current flow through the body during an electrical shock is a function of two things:
1) The voltage of the circuit, and
2) The resistance of the body.

(I=E/R)

How can harmonic currents change either of these two items?

It changes the E. It stores voltage from a previous cycle, then returns it later to add to the present cycle. that's why neutrals have to be larger than phase wires in a highly harmonic circuit.

How does voltage have anything to do with wire size? Perhaps you mean current...such as you say later in your sentance "highly harminic current. Also, the NEC defines a nonliner load as one whose current waveform doesn't match the voltage waveform. I don't think the voltage changes, but the current does. This is why it is impossible to receive a higher shock value from a nuetral than a phase conductor, all other things being equal.

 

[unimo]

It changes the E. It stores voltage from a previous cycle, then returns it later to add to the present cycle. that's why neutrals have to be larger than phase wires in a highly harmonic circuit.

How does voltage have anything to do with wire size? Perhaps you mean current...such as you say later in your sentance "highly harminic current. Also, the NEC defines a nonliner load as one whose current waveform doesn't match the voltage waveform. I don't think the voltage changes, but the current does. This is why it is impossible to receive a higher shock value from a nuetral than a phase conductor, all other things being equal.[/quote]

Yes I think you're right about this. A number of electricians had told me you could get shocked worse off the neutral and that's what got me started measuring neutral to ground voltage. I've never measured higher than the phase wires on the neutral.
I got the impression that it could be higher because I had read that triplen harmonics (multiples of three) can add to the voltage waveform, but now I think that causes only voltage waveform distortion, and only adds to the current magnitude.
I wrote too quickly about wire size. That's only current, of course.

 

[unimo]

Half the time you'll measure close to nothing.

And in the other half there is something wrong with the electrical system. If there wasn't some problem with the system, you would not see that voltage between the grounded and grounding conductors.
Don

We'll have to agree to disagree on this one. After years of experience measuring this phenomenon in systems that appear to be working correctly, i've decided it's OK. If you think you could convince the customer that something's wrong when hundreds of lights are operating normally, power bill is normal, current readings at the panel are normal, nothing overheating, then you could make a living correcting this "problem". It exists in at least a third of all large stores that are older systems, using fixtures with F96T12 tubes and magnetic ballasts, where three hots and a neutral are sent down each row of lights, the lights being 277 volt, and three circuits on each boat.
Hasn't anyone else reading this observed the same thing?

 

[dereckbc]

Uniomo,
I think you are taking this wrong. We are not trying to gang up on you, tell you that it cannot or will not happen. I have seen it, but always have found the cause. The cause can come from any one of multple factors or a combination of factors.

First couple of things I would look at are current on neutral, load balance, distance, and connections. More than likely the answer lies within. For example your customer as you stated my not complain from high bills, becuase he/she has always been paying high bills and think it is normal. At 80-volts drop, lots of power is being wasted.

So don't go away thinking we are beating up on you, we are not. I know some of the responders, and between us we are sharing around 75 years of experience with you. If you coutn Ryan 75-1/2 years :lol: