Low resistance path to earth

[kjroller]

Is the reason your grounding grid generally needs to be under 25ohms because if you have a ground fault the OCPD will not trip because it picks up a very low fault current when resistance is higher?

 

[don_resqcapt19]

Is the reason your grounding grid generally needs to be under 25ohms because if you have a ground fault the OCPD will not trip because it picks up a very low fault current when resistance is higher?

The connection to earth does not play much of part in fault clearing. The fault clearing path is via the EGC to the main bonding jumper and then to the neutral of the transformer that supplies the system.

 

[kjroller]

The connection to earth does not play much of part in fault clearing. The fault clearing path is via the EGC to the main bonding jumper and then to the neutral of the transformer that supplies the system.

What is the purpose of a High resistance system then? Reason I ask is because if I have a true ground fault like case to ground and my resistance is to high I don't believe the OCPD will clear.

 

[don_resqcapt19]

What is the purpose of a High resistance system then?

That replaces the main bonding jumper with an impedance or resistance to limit the fault current when there is a ground fault. Typically the ground fault current is limited to less than 10 amps. The fault can continue without doing excessive damage, There will be a ground fault alarm, and steps will be taken to find and eliminate the ground fault, but an OCPD will not open when there is a ground fault on a resistive grounded system.
The expanded content in NFPA Link says:

The grounding impedance is selected to limit fault current to a value that is slightly greater than or equal to the capacitive charging current. This system is used where continuity of power is required. Therefore, a ground fault results in an alarm condition rather than in the tripping of a circuit breaker. The alarm allows for the safe and orderly shutdown of a process for which a non-orderly shutdown could introduce additional or increased hazards.

Even though the commentary suggests this is only used where a non-orderly shutdown could result in additional hazards, that is not a requirement to use such a system. Many large systems use this to simply limit damage from a ground fault.

 

[kjroller]

For example, lets say you have 480 volt going to the case of a motor, let's say your ground system is 100 ohm ideally your amps to ground would be 4.8 amps and lets say its on a 20 amp OCPD then your ground fault would not trip the breaker correct? (This is just an Example for ease of understanding)

 

[kjroller]

What I am really trying to understand is what the purpose is for a low impedance ground system? If what I stated above is not true.

 

[don_resqcapt19]

For example, lets say you have 480 volt going to the case of a motor, let's say your ground system is 100 ohm ideally your amps to ground would be 4.8 amps and lets say its on a 20 amp OCPD then your ground fault would not trip the breaker correct? (This is just an Example for ease of understanding)

As I said in my first post the grounding system has nothing to do with the fault clearing of a ground-fault condition. The real fault path will be in the tenths of an ohm. The ground fault clearing path is an intentionally constructed low impedance path back to the source, and the path via the earth, while in parallel with the intentional fault clearing path, has an impedance thousands of times greater than that of intentionally constructed fault clearing path.

But yes with a 100 amp fault return path the OCPD will not clear the fault.

 

[kjroller]

As I said in my first post the grounding system has nothing to do with the fault clearing of a ground-fault condition. The real fault path will be in the tenths of an ohm. The ground fault clearing path is an intentionally constructed low impedance path back to the source, and the path via the earth, while in parallel with the intentional fault clearing path, has an impedance thousands of times greater than that of intentionally constructed fault clearing path.

But yes with a 100 amp fault return path the OCPD will not clear the fault.

Do you mean 100 ohm? And what your saying is the MBJ is my fault clearing path? Also if you have a way to clear the fault why do you need a grounding system then?

 

[kjroller]

As I said in my first post the grounding system has nothing to do with the fault clearing of a ground-fault condition. The real fault path will be in the tenths of an ohm. The ground fault clearing path is an intentionally constructed low impedance path back to the source, and the path via the earth, while in parallel with the intentional fault clearing path, has an impedance thousands of times greater than that of intentionally constructed fault clearing path.

But yes with a 100 amp fault return path the OCPD will not clear the fault.

I figured it out the Grounding grids purpose period is to provide equal potential throughout the system and metal parts. So you want low impedance because (I x R=E) so the lower your Resistance the less potential there is for shock in case of a ground fault or a lightning strike so ideally if you can get it close to 0 that's the best case scenario. Whereas the bonding jumper is your return path to X0 to complete your electrical circuit that trips during a ground fault condition. When a ground fault condition occurs if the breaker doesn't trip the ground system should still be at equal potential throughout limiting the electrical hazard that could take place however if main bonding jumper and OCPD have been hooked correctly it should clear. Let me know if any of this doesn't make sense or you think that is a good understanding of it?

 

[kjroller]

In other words it is important to ground metal parts at an industrial plant because if metal becomes energized then its all at the same potential so there is no way to get shocked.

 

[don_resqcapt19]

In other words it is important to ground metal parts at an industrial plant because if metal becomes energized then its all at the same potential so there is no way to get shocked.

Not important to connect them to earth(ground), but is important to bond them together.