Using ohm meter to calculate current in the event of a ground fault

[Dsg319]

Can one use an ohm meter between say feeder neutral and EGC bonded to case to be able to calculate and do simple math (ohms law) in the event of a fault to the case. Example feeder neutral resistance to bonded EGC to case on disconnect, protected with 400amp fuse at service hundreds of feet away.

120v/1ohm=120amps (not capable of clearing the fault with a 400amp fuse)

I know fault current calculations are much more detailed and take a lot more into account. Was just curious if one could use simple dc resistance from an ohm meter reading do this.

 

[jim dungar]

Can one use an ohm meter between say feeder neutral and EGC bonded to case to be able to calculate and do simple math (ohms law) in the event of a fault to the case. Example feeder neutral resistance to bonded EGC to case on disconnect, protected with 400amp fuse at service hundreds of feet away.

120v/1ohm=120amps (not capable of clearing the fault with a 400amp fuse)

I know fault current calculations are much more detailed and take a lot more into account. Was just curious if one could use simple dc resistance from an ohm meter reading do this.

Most VOMs are not capable of measuring fractions of ohms. You would need to be using a micro-ohmeter

In your, example how many thousands of feet of 500kcmil conductor is required to reach 1ohm of resistance? There is also the problem of parallel ground paths.

 

[Dsg319]

Most VOMs are not capable of measuring fractions of ohms. You would need to be using a micro-ohmeter

In your, example how many thousands of feet of 500kcmil conductor is required to reach 1ohm of resistance? There is also the problem of parallel ground paths.

I assume higher resistance parallel ground paths would screw up the numbers and increase them?

I just was testing connections today after finishing a 400amp service with approx 400’ feet of open air aluminum that splices into copper that comes down a mast to a transfer switch and was checking that neutral and EGC was bonded and that nothing was reversed and the thought came to mind, when I seen a resistance of around 3ohms

 

[Speedskater]

a] need to be completely disconnected from AC power or the VOM~DMM will be very unhappy.
b] member 'gar' has instructions for DIY low Ohm measurements.
scroll far down the page:

BETATRONICS&reg, Electrical photos, communication, Long Cable, High Baud Rate, RS-232, fast, I232, vendor, supplier, manufacturer, industrial

Electrical photos.

www.beta-a2.com

note member 'gar' has not posted since last August.

 

[LarryFine]

And AC impedance will be different from DC resistance.

 

[jim dungar]

I assume higher resistance parallel ground paths would screw up the numbers and increase them?

I just was testing connections today after finishing a 400amp service with approx 400’ feet of open air aluminum that splices into copper that comes down a mast to a transfer switch and was checking that neutral and EGC was bonded and that nothing was reversed and the thought came to mind, when I seen a resistance of around 3ohms

Resistances in parallel produce less resistance not more.

Look at the NEC tables for conductor resistance and impedances. Note they are in ohms per thousand feet.
If your meter is showing multiple ohms, it probably is not an accurate measurement. VOMs usually do not have sufficient battery power to make accurate resistance measurements on long conductor lengths. Check your meter manual.

 

[Dsg319]

Thanks all.

 

[kwired]

And AC impedance will be different from DC resistance.

exactly. This impedance may change as current increases as well.

You can have both capacitive as well as inductive effects involved.

 

[wwhitney]

Suppose instead of an ohmeter, we use a 10 ohm test load and measure the voltage across the load, as well as the current. Plus we we measure the open circuit voltage.

E.g. we get 128.0 V open circuit, and 125.0 V and 12.5A with the 10 ohm test load.

Can we now say that the source, including the branch circuit wiring, has an impedance of 3.0V / 12.5A = 0.240 ohms, and if we had a dead short at the load, the fault current would be 120V / 0.24 ohms = 500A ?

Thanks,
Wayne

 

[LarryFine]

E.g. we get 128.0 V open circuit, and 125.0 V and 12.5A with the 10 ohm test load.

What about measuring the voltage across the resistor, calculating the current, and using that?

 

[wwhitney]

What about measuring the voltage across the resistor, calculating the current, and using that?

Sure, 125V would be the voltage across the resistor in my example, so if we know the resistance accurately enough, we don't need an ammeter and can just use a voltmeter for the two measurements.

Cheers, Wayne