I am working on installation of lightning protection system. I am facing a problem that in NFPA 780 (4.13.1.3) (NEC 250.60) it is mentioned that earthing pits for lightning protection system, pits for telecommunication system, pits for electrical system and pits for instruments should be connected to each other; my problem is that when high currnet is grounded through lighhtning pit it may flow towards the instruments or electrical equipments through interconnected pits so it can cause a damage to the instruments. Please suggest some protection in this regard.
Interconnection of Earthing Pits
- Thread starter eeali
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bphgravity
Senior Member
- Location
- Florida
Intution will tell that is what will happen, but that is not the case. By bonding each of the grounding electrodes of each of the systems you describe above, you reduce the potential difference across these systems when the ground potential rise occurs during the surging event. With all the grounded parts rising in potential and falling in potential at the same duration and intensity, the flow of hazardous currents cannot develop.
In any case, you should be employing a full spectrum of protection to fully establish the most effective protective system possible, such as the addition of Article 280 and 285 surge protection devices.
Since you already have your NFPA 780 out, take a look at the Annexes in the back of the Standard, Annex C in particular.
In any case, you should be employing a full spectrum of protection to fully establish the most effective protective system possible, such as the addition of Article 280 and 285 surge protection devices.
Since you already have your NFPA 780 out, take a look at the Annexes in the back of the Standard, Annex C in particular.
brian john
Senior Member
- Location
- Leesburg, VA
Assuming you keep all bonding on the outside that is not running conductors through the building to other pits.
Think of it this way if they are not bonded on the outside, they are all connected together inside one way or the other by it accidentally or intentionally.
Think of it this way if they are not bonded on the outside, they are all connected together inside one way or the other by it accidentally or intentionally.
Even if there is no intentonal interconnecting conductor between pits, all the pits below ground are inherently interconnected through the ground/ earth. Remember that we are trying to make the ground resistance as smal as possible. And the addition of interconnecting conductor is to ensure that the resistance between pits underground is small thus the potential differences between pits are small during current sinking to the ground (due to lightning, groundfault or static discharge.
Imagine if we did not interconnect the pits and there is a ground resistance of 5 ohm on one of the pits (say pit no.1). A ground fault (or lighting discharge) of 10000A sinking into pit 1 would induced a potential difference of 10000V (V=IR=10000x1) between pit 1 and a remote pit. But a connecting conductor of 0.01 ohm resistance would limit this potenial diference to 100V (V=IR=10000x.01).
Imagine if we did not interconnect the pits and there is a ground resistance of 5 ohm on one of the pits (say pit no.1). A ground fault (or lighting discharge) of 10000A sinking into pit 1 would induced a potential difference of 10000V (V=IR=10000x1) between pit 1 and a remote pit. But a connecting conductor of 0.01 ohm resistance would limit this potenial diference to 100V (V=IR=10000x.01).
bphgravity
Senior Member
- Location
- Florida
Two comments Bryan...
1. Eliminate the term "ground fault" from your thought process when discussing system grounding and especially lightning protection. Ground faults are really not faults to the earth, thus do not play a role in the grounding design. Ground fault protection is provided by effective fualt paths and overcurrent devices, not the earth.
2. Current of any kind doesn't "sink" to the ground (earth), regardless of its source. Lightning isn't simply dumping current into the earth where it magically is soaked up like in a giant sponge. Earth based objects have charges, passing storm clouds have charges. Sometimes these imbalance of charges "equilize" in the form of lightning. And don't think of lightning as a current source, at least when considering the NEC / NFPA 780 requirements.
1. Eliminate the term "ground fault" from your thought process when discussing system grounding and especially lightning protection. Ground faults are really not faults to the earth, thus do not play a role in the grounding design. Ground fault protection is provided by effective fualt paths and overcurrent devices, not the earth.
2. Current of any kind doesn't "sink" to the ground (earth), regardless of its source. Lightning isn't simply dumping current into the earth where it magically is soaked up like in a giant sponge. Earth based objects have charges, passing storm clouds have charges. Sometimes these imbalance of charges "equilize" in the form of lightning. And don't think of lightning as a current source, at least when considering the NEC / NFPA 780 requirements.
bphgravity
Senior Member
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- Florida
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