We have Cargo containers that are being used as offices, powered by generators with the generator frame grounded to a driven ground rod. Now engineering wants another ground rod with #6 solid wire installed to ground the shell of the cargo for lightning protection. The panel mounted on the shell, fed from the generator is already grounded by the EGC. Wouldn't another rod create a differance in potential. Electrical storms are an everyday occurance and they are concerned about personell safety.
Lightning protection
- Thread starter high90s
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ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: Lightning protection
Bonding the container is a good idea, especially if it is 5/16" thick and can act as an air terminal itself. Additional rods are a good idea, as long as they are interconnected with the other rod to make a grounding electrode system.
Bonding the container is a good idea, especially if it is 5/16" thick and can act as an air terminal itself. Additional rods are a good idea, as long as they are interconnected with the other rod to make a grounding electrode system.
rick hart
Senior Member
- Location
- Dallas Texas
Re: Lightning protection
A single #6 and ground rod is not going to do much for lightning. Multiple rods and jumpers bonded together may do it. I wouldn't worry to much about a ground loop- the cargo frame is already in contact with the Earth.
A single #6 and ground rod is not going to do much for lightning. Multiple rods and jumpers bonded together may do it. I wouldn't worry to much about a ground loop- the cargo frame is already in contact with the Earth.
- Location
- Massachusetts
Re: Lightning protection
Unless it is sitting on some substantial wood leveling blocks it strikes me that a metal cargo container sitting on the ground is about as 'grounded' as it is ever going to be.
As far as personal safety if your in the box when lightning strikes it you will be fine. (Faraday Cage)
If your outside touching the box when lightning strikes it I doubt any extra grounding electrodes will save you.
Unless it is sitting on some substantial wood leveling blocks it strikes me that a metal cargo container sitting on the ground is about as 'grounded' as it is ever going to be.
As far as personal safety if your in the box when lightning strikes it you will be fine. (Faraday Cage)
If your outside touching the box when lightning strikes it I doubt any extra grounding electrodes will save you.
Re: Lightning protection
Under U.S. rules the containers do need to have a local grounding electrode connected to the EGC at the panelboard unless the container is wired with a single circuit. The philosophy is that if somebody is dependent on ground rods then putting in more ground rods on the system helps lower ground resistance. There are some places where if you are using ground rods you MUST use 2.
I do not know how big your generator is. Installing 4 or more ground rods for a large generator would be reasonable.
The 25 Ohm rule is from the days of telegraph systems that used the earth as one of the circuit wires. A 25 Ohm ground led to good efficiency because on longer lines a 25 ohm ground was small compared to the resistance of the line. A 25 Ohm ground is almost useless for a power system if an open neutral occurs. A 5 Ohm ground actually has a chance at stabilizing an open neutral condition.
Under U.S. rules the containers do need to have a local grounding electrode connected to the EGC at the panelboard unless the container is wired with a single circuit. The philosophy is that if somebody is dependent on ground rods then putting in more ground rods on the system helps lower ground resistance. There are some places where if you are using ground rods you MUST use 2.
I do not know how big your generator is. Installing 4 or more ground rods for a large generator would be reasonable.
The 25 Ohm rule is from the days of telegraph systems that used the earth as one of the circuit wires. A 25 Ohm ground led to good efficiency because on longer lines a 25 ohm ground was small compared to the resistance of the line. A 25 Ohm ground is almost useless for a power system if an open neutral occurs. A 5 Ohm ground actually has a chance at stabilizing an open neutral condition.
Re: Lightning protection
Dereck:
Please fill us in. I am guessing this actually has some meaning for telecomm or similar applications.
From my perspective (areas like building power), an open neutral is just an open circuit. There is no "instability" involved to stabilize.
Steve
Dereck:
Please fill us in. I am guessing this actually has some meaning for telecomm or similar applications.
From my perspective (areas like building power), an open neutral is just an open circuit. There is no "instability" involved to stabilize.
Steve
bphgravity
Senior Member
- Location
- Florida
Re: Lightning protection
Since the utility systems are grounded, and since building and structure systems are grounded, and since the GEC is bonded to the system grounded conductor, a sufficently low impedance through Earth would allow proper operation of the system.
So hence the problem. The NEC requires a fairly low Earth resistance, yet wants to limit objectionable current on non-current carrying metallic parts. Both can't be done at the same time.
Since the utility systems are grounded, and since building and structure systems are grounded, and since the GEC is bonded to the system grounded conductor, a sufficently low impedance through Earth would allow proper operation of the system.
So hence the problem. The NEC requires a fairly low Earth resistance, yet wants to limit objectionable current on non-current carrying metallic parts. Both can't be done at the same time.
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