When adding the resistance between ground systems, that's a minimum of 40 Ohms. Lightning is going to damage anything that is connected between both grounds.
By Douglas Krantz
Often times, for lightning protection at the signal wire entrance to a building, someone will drive a copper stake into the earth, but there's a problem. For the building's ground system, the main electrical wire entry already has a copper stake driven into the earth; a second copper stake driven into the earth for the signal carrying wires is a separate earth ground system.
Using two different earth ground systems for lightning protection is as bad as having no lightning protection at all.
Lightning and Ground
With two ground rods that aren't connected together, lightning puts most of its power into one earth ground connection or another. One ground stake is going to be affected with a voltage spike much more than any other.
Earth Ground Resistance and Voltage
Any earth ground connection has some resistance. As a best case scenario, the lowest resistance to the earth that most earth ground connections have is about 20 ohms.
This may not seem like much, but lightning has a lot of current (often it's over 10,000 peak amps). Using Ohm's Law of E (voltage) = I (current) times R (resistance), one can see that 10,000 amps peak current times a resistance of 20 ohms equals a total of 200,000 peak volts.
This peak voltage has a very short duration, but it's there.
When lightning strikes a building, the inside building ground system can have a spike voltage of 200,000 volts or greater. The whole building leaps to 200,000 volts at once; everything attached to the building ground system gets that same 200,000 volt hit. Because everything is connected to the building ground, the voltage between each device in the building doesn't change much.
Second Ground Rod - No Voltage Spike
As the building gets the 200,000 volt spike, the second ground rod holds the voltage on the other incoming wires closer to earth ground voltage potential. This voltage difference, between the incoming signal wires and the building electrical ground, could be 200,000 volts or more.
There is equipment standing between those incoming signal wires and building ground. Be it a fire alarm panel, a security panel, a cable TV amplifier, a CCTV camera, a telephone, etc., the only route for that 200,000 volts is from the building electrical ground, through the equipment, and into the wires being held to earth ground potential at the signal wire entrance. Something in the equipment is going to give.
That's not lightning protection.
Lightning Coming on the Wire Entry
Of course, if lightning strikes a power pole outside, or the PIV (Post Indicator Valve), or another building connected with signal wires, the same story can be told in reverse; but the result is the same: lightning damages the equipment.
This is why, according to the National Electrical Code (NEC), if an extra copper stake is driven into the earth, by using heavy wire, that extra copper stake needs to be connected directly (bonded) to all other copper stakes driven into the earth. That way, when the building gets its voltage spike, all earth ground connections will get approximately the same voltage spike, and there will be better lightning protection for the whole electrical system.
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