When does GES impedance matter?

[EBQ]

Some theoretical questions for understanding GES impedance... How high is too high for GES impedance? Would a 100-ohm GES impedance work fine in practical real world scenarios? Is the 25 ohms requirement more or less an arbitrary number, or is it supported by physics and data? How would a 100 ohm GES and 25 ohm GES differ (in realistic terms) in the following scenarios?

1. Static discharge / potential equalization: no real world difference? hard to imagine static build up would be faster than a 100 ohm bleeder resistor could dissipate for most buildings?
2. Ground fault: higher GES impedance helps direct more current through EGC back to the breaker to clear the fault?
3. High voltage fault (high voltage transmission line falling on to low voltage distribution line): higher GES impedance lowers potential surge current and heat/fire risk?
4. Open neutral: does higher GES impedance lower step voltage? since a low impedance GES does not safely lower touch voltage anyway
5. Indirect lightning surge: both service neutral and hots provide a higher gauge lower resistance path for surge dissipation than the #6 or #8 GECs.
6. Direct lightning strike: lightning's going to ground and will destroy everything it travels through anyway, does 25 or 100 ohms make a difference?

Any other applicable scenarios?

 

[kwired]

Some theoretical questions for understanding GES impedance... How high is too high for GES impedance? Would a 100-ohm GES impedance work fine in practical real world scenarios? Is the 25 ohms requirement more or less an arbitrary number, or is it supported by physics and data? How would a 100 ohm GES and 25 ohm GES differ (in realistic terms) in the following scenarios?

1. Static discharge / potential equalization: no real world difference? hard to imagine static build up would be faster than a 100 ohm bleeder resistor could dissipate for most buildings?

think I mostly agree here

2. Ground fault: higher GES impedance helps direct more current through EGC back to the breaker to clear the fault?

GES is not for facilitating overcurrent protection even if you get down to 5 ohms, @ 120 volts that will limit fault current to 24 amps if the GES is the only return path. That is not going to trip even a 15 amp breaker very quickly, and shouldn't trip 25 amp or higher breaker at all. Bonding is what allows for low impedance return paths for ground faults and results in high enough current to get rapid response from OCPD's.

3. High voltage fault (high voltage transmission line falling on to low voltage distribution line): higher GES impedance lowers potential surge current and heat/fire risk?

IMO it simply gives such faults more places to go, good and bad paths can be involved.

4. Open neutral: does higher GES impedance lower step voltage? since a low impedance GES does not safely lower touch voltage anyway

No. Earth is very consistant base, only rise is near an electrode, and the voltage gradient zones are typically fairly small, meaning it don't take much distance to get across a high potential, unless you can achieve a very low impedance to earth to carry your neutral current, you will have significant voltage rise because of resistance of the electrode.


5. Indirect lightning surge: both service neutral and hots provide a higher gauge lower resistance path for surge dissipation than the #6 or #8 GECs.
6. Direct lightning strike: lightning's going to ground and will destroy everything it travels on anyway, does 25 or 100 ohms make a difference?

Lightning is very high voltage and high frequency, what seems conventional for low voltages and frequency doesn't always happen with lightining because one doesn't realize what may may affect impedance and divert current to where you may not expect it. The indirect strike, your larger service conductors potentially can allow more of the lightning current to make it to your service, But your GEC is sized per your ungrounded conductors until you get to 3/0 copper then it isn't required to get any larger. But at same time only 6 AWG is required for a rod or pipe electrode because they figure the rod or pipe can't dissipate any more energy than a #6 can deliver to it - similar concept for CEE's and ground rings.

Any other applicable scenarios?

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