Effective ground fault path

[mbrooke]

Just to display my ignorance, it seems to me that bonding protects against grabbing the water line and gas line while standing on a rubber mat and getting zapped, because if properly bonded there is no potential difference between them. However, if I'm in my bare feet standing on a damp concrete basement floor, I can still get shocked (or worse). If those bonded pipes are now also grounded, as in connected to to the ground bar in the panel, then I can touch those pipes in my bare feet because the potential on the pipes should have tripped the circuit breaker, assuming something like a bolted fault. I'm not so very protected if there is a high impedance fault (say, only a few hundred milliamps) and the source is not connected to a GFCI device; I'll just get equally shocked whichever pipe I grab.

Bonding pipes assures that a hot touching them will trip a breaker, removing dangerous voltage.


Well how many milliamps is your high Z fault? Complimenting Mike's post above-

In my simplification, typically, the voltage a user will experience during a high Z fault can be calculated via Vs x VR2/(R1+R2) = V touch

Where R1 is the total impedance of the hot wire going back to the POCO transformer plus the high Z fault and R1 is the EGC back to the utility transformer.

Assuming 100 feet 12 guage NM and 50 feet of 1/0 AL back to the POCO transformer, R1 comes out to about 0.2 ohms and R2 to 0.2 ohms.

Adding a high impedance fault of 500 milliamps at 120 volts, V/I=R gives us 240 ohms.

240+0.2=240.2 ohms

So, plugging these value into the original equation gives us 0.1 volts touch voltage to remote earth.

Assuming 400 ohms ( very low end body resistance) 0.25 milliamps of current, below the no let go threshold.

So the user is safe under high Z fault conditions because of the EGC.

 

[mbrooke]

With no EGC, the body simply becomes a series resistor with the high Z fault current- so 240 ohms + 400 ohms = 640 ohms. 120v/640ohms= 187.5 milliamps. This is above the no let go value, well into the V-fib value.

Now, a bit of bonus- if we drive a 25 ohms ground rod on the device with its 500 ohms high Z fault, using the same resistive divider equation VsxR2/(R1+R2)= touch voltage, the voltage on the metal shell becomes 5.7 volts to earth.

Someone with dry hands or dry shoes typically will not precept 5.7 volts, creating the idea that a ground rod is removing dangerous voltage- and in some ways it is- until that 500 ohm fault becomes a 0.03 ohm fault.

It is possible the in the old days with questionable insulation that many fault started out as high Z faults, where driving a ground rod stopped the shocking under average body resistance (>5000 ohms) creating the myth that earth brought things to a safe potential.

 

[Carl Johnson]

NEC 250.4 does a pretty good job of explaining the purposes of grounding and bonding and why each is necessary.