Re: Grounding for electronics
Karl, I contacted several equipment manufactures, telephone operating companies, peers, and consultants about the 5 ohm ground electrode system specification. Rather than list each verbal and written response I will try to summarize the consensus along with my personal assessment.
1. Every one contacted agrees it has nothing to do with normal equipment operation, it is for safety. Performance grounding is another issue all together.
2. The requirement comes from three related fields of military communication, radio communications, and the telephone industry.
3. Every one agrees the NEC requirement is too relaxed for environments where national security, public safety, or where unnecessary outages cannot be tolerated from outside events. Or IMO the GES should be engineered for sensitive sites and not be left up to the installation contractor discretion or interest
4. The GEC impedance of 5 ohms or less provides a planned path that is capable of withstanding any outside fault current that is likely to be imposed on it. It also insures that the voltages developed from a lightning strike will be minimized to the furthest extent possible. Provide a planned path with impedance far lower than any incidental or unplanned path for fault current to flow thereby attenuating any unplanned path voltage differences. Or in short, minimize voltage stress on sensitive components and conductor insulations.
I did hear one other thing that sparked my interest that is related to the military, EMP attenuation. The engineer that referenced it is retired from a military defense contractor and will not elaborate other than it is similar to lightning and requires extra care in reference to GES impedance.
I could go on further about specifics about radio sites, sub-staions, generator plants, and common mode noise, but it is outside the scope of this NEC forum.
Karl, I contacted several equipment manufactures, telephone operating companies, peers, and consultants about the 5 ohm ground electrode system specification. Rather than list each verbal and written response I will try to summarize the consensus along with my personal assessment.
1. Every one contacted agrees it has nothing to do with normal equipment operation, it is for safety. Performance grounding is another issue all together.
2. The requirement comes from three related fields of military communication, radio communications, and the telephone industry.
3. Every one agrees the NEC requirement is too relaxed for environments where national security, public safety, or where unnecessary outages cannot be tolerated from outside events. Or IMO the GES should be engineered for sensitive sites and not be left up to the installation contractor discretion or interest
4. The GEC impedance of 5 ohms or less provides a planned path that is capable of withstanding any outside fault current that is likely to be imposed on it. It also insures that the voltages developed from a lightning strike will be minimized to the furthest extent possible. Provide a planned path with impedance far lower than any incidental or unplanned path for fault current to flow thereby attenuating any unplanned path voltage differences. Or in short, minimize voltage stress on sensitive components and conductor insulations.
I did hear one other thing that sparked my interest that is related to the military, EMP attenuation. The engineer that referenced it is retired from a military defense contractor and will not elaborate other than it is similar to lightning and requires extra care in reference to GES impedance.
I could go on further about specifics about radio sites, sub-staions, generator plants, and common mode noise, but it is outside the scope of this NEC forum.