Originally posted by electure:
A neighbor wants to put a 60A subpanel in his detached garage, about 30' from the house. He's doing it himself, has already run a UG 1" conduit with 3 #4, & a #8 EGC.
AHJ said he's required to install another grounding electrode @ the garage, but [b]why?? I don't have a good answer for him.
His reasoning is that if the sub was on the fence, 30' further, it wouldn't be required. I can't argue with that. [/B]
I am going to take a stab at this even if I get my head bit off. The difference lies in the panel being in a building which is a structure that is meant to be occupied at least part of the time by people. The reason for the additional electrode at the second structure is to limit the voltage difference between the exposed conductive surfaces in the building and the grounded surfaces in the building during lightning strikes and fault conditions. If the earth and the conductive surfaces that will conduct stray currents to it are held to nearly the same potential during these events the hazard to personnel is reduced. The entire purpose of installing a Grounding Electrode System is to avoid a human being becoming the Grounding Electrode Conductor between the exposed conductive surfaces and ground.
One example of the hazard would be the existence of a fault between a ungrounded conductor and a grounded box. The sweaty home owner is kneeling on his garage floor, using a three wire tool on his kid's soap box derby racer. Without the grounding electrode at the garage the voltage drop over the Equipment Grounding Conductor may raise the potential of the drill in the owners hand several volts above the grounded concrete garage floor. If a tenth of an amp flows between the occupants hand and his knee his heart may go into fibrillation. Unless effective CPR is begun at once and a defibrillator reaches him within six minutes he is a dead man. Not likely you say, I was the first due Emergency Medical Technician on the call. The patient was not discovered immediately and died of his injuries. The current was not high enough to burn either his hand or his knee. We only discovered the source of the shock by trying to turn the lights on in the garage to work on the patient. The breaker had tripped. After the medic unit left with the patient I tried to reclose the breaker. It tripped immediately and in the darkened interior of the garage I could see a visible spark between the drill surface and the garage floor. I asked the engine company to secure the scene and came back with my work van. When the cops were finished with the scene I moved the drill to a wooden work bench. I attached a fluke DMM set to hold the highest voltage reading between the drill housing and a scrap of braided copper lightning protection conductor laying in a puddle of normal saline I poured onto the garage floor. The fluke DMM showed a voltage of 39 volts after I reclosed the breaker and it tripped again. I do not know if a fluke DMM is fast enough to actually measure the full voltage rise on the circuit but the voltage rise I did measure was an indication of the presence of a substantial voltage rise during the fault condition. The voltage rise during a lightning strike would have been far greater. There was a water spigot supplied via galvanized iron piping a few feet from the garage. I took a piece of stranded #10 AWG and ran it between the ground terminal of a cord cap and a ground clamp I installed on the spigot riser. I plugged the cord cap in to a receptacle near the garage door close to the spigot. When I then reclosed the breaker the fluke DMM did not measure any significant rise although it did beep as if the voltage had changed. The breaker that tripped was protecting the lighting circuit. The breaker for the circuit supplying the outlets never tripped. The garage did not have GFI receptacles so I plugged the drill into the portable GFI I use to protect myself and repeated the experiment with and without my improvised Grounding Electrode Conductor. Neither one tripped the plug-in GFI. I then took the heat gun I use for heat shrink tubing on UF splices and wired it between Ungrounded Conductor and Equipment Grounding Conductor on the receptacle circuit. With the switch thrown to the high setting of 2000 watts the voltage across the ungrounded conductor and the Equipment Grounding Conductor dropped to 102 volts verses an unloaded voltage of 123 with the same meter. There was no such voltage drop when the same measurement was taken between the Grounded Circuit Conductor and the Ungrounded Circuit Conductor. The voltage dropped only two volts on the normal circuit. So with nineteen volts of voltage drop attributable to the impedance of the Equipment Grounding Conductor it was not hard to imagine how the current necessary to stop the victims heart might have flowed. I cant be sure that a properly installed Grounding Electrode System would have saved that man and I realize that my post incident investigation was crude at best. The police called in people from the state to do a more formal investigation after I told them what I found. I never heard if they got the same kind of results or not. I was told that the fault was traced to an internal failure in a photo voltaic sensor controlling the garage's outdoor lights the accident having occurred at dusk.
