Voltage Question

[DBoone]

We are starting a new house today and this morning I had to install the bonding jumper on our temporary. The neutral bar is insulated from the panel and the ground lug is mounted to the panel. They provide a solid piece of copper wire to bond with.

Before I installed the bonding jumper I checked voltages and out of curiosity I checked voltage from a hot leg to the 4x4 post that the set up is mounted on. It's a treated post and probably holds some moisture and I was reading anywhere from 60-80 volts.

That made me wonder. If the bonding jumper wasn't installed and a fault occurred, obviously there wouldn't be a low impedance path and trip the breaker quickly but lets assume that the 4x4 post provide a path through the ground and back to the source.

What would that do to the voltage on the metal parts affected by the ground fault?

I know If there is no path then metal parts would be at line voltage. What happens when there is a high impedance path back to the source.

This scenario could also happen in a home where the neutral and ground bars are isolated from each other and someone only lands the GEC on the ground bar thinking that will "ground" everything.

Thanks

PS- very busy today.... Will check back in as soon as I can.

 

[don_resqcapt19]

...
What would that do to the voltage on the metal parts affected by the ground fault?

I know If there is no path then metal parts would be at line voltage. What happens when there is a high impedance path back to the source.
...

The parts would remain energized at the line voltage. The connection via the 4x4 in the ground or even a ground rod will not reduce this voltage, except that for a very small area around the 4x4 or the ground rod the voltage of the earth would be raised some reducing the potential in this small area.

Think of it this way. The earth is a load. What happens to the voltage on the line side of a load when the load is drawing power?

 

[fmtjfw]

Low Impedance Meters -- or chaos

Low Impedance Meters -- or chaos

Never trust a high-impedance volt meter unless you are repairing vacuum-tube-based equipment or computer internal circuits. Most DVMs are (were) high impedance and will read all sorts of flaky voltages on essentially open circuits.

You need to be sure your voltmeter is low impedance (often identified as low-Z). If you have a high-impedance voltmeter, Fluke sells a resistor in a box that plugs into most COM--VOLTS lead receptacles in a meter and allows you to plug the leads into it. It is rated CAT IV 600V - part number SV225 Stray Voltage Eliminator.

 

[DBoone]

The parts would remain energized at the line voltage. The connection via the 4x4 in the ground or even a ground rod will not reduce this voltage, except that for a very small area around the 4x4 or the ground rod the voltage of the earth would be raised some reducing the potential in this small area. Think of it this way. The earth is a load. What happens to the voltage on the line side of a load when the load is drawing power?

The way I see it, if the earth is the load then everything on the line side would be at line voltage. The metal parts and EGC affected by the fault would just become extensions of the hot wire?

 

[GoldDigger]

You could certainly look at it that way.
Or you could look at it as series circuit with a high impedance st each end of the earth path (the fault electrode and the POCO electrode(s)), and lower wire resistances in many other pieces along the wired part of the path.
When the "load" is not intended, can we really call it a load or just another part of the fault current path?

Do we go by whether the voltage at a point is more or less than half the line voltage?

 

[don_resqcapt19]

The way I see it, if the earth is the load then everything on the line side would be at line voltage. The metal parts and EGC affected by the fault would just become extensions of the hot wire?

Yes, the connection to earth will flow very little current so there will be almost no voltage drop and every thing will be at line voltage.

 

[edlee]

Phantom voltage from high-impedance digital meter.