My employer is very aggressive about grounding. This year, they dropped the corporate standards for grounding down to less than 1 ohm from the equipment to the transformer neutral, and 5 ohms from the transformer to Earth ground.
We're a sand mine, and the ground itself is absolutely lousy. BUT...we've met those requirements.
On the transformer ground rods, we got the threaded ones and simply drove one rod after another down until the readings dropped to below 25 ohms. As the dirt settled back in within a week, the readings dropped down to less than 5 ohms. Only had to come back and add an extra ground rod once.
As for the equipment, mostly we ended up adding lots of external grounds. Usually, the problem wasn't with the wiring itself. It was with the structure which after 25 years or so of rust simply wasn't all that well bonded anymore. So adding external ground supplemental ground wiring was simply easier and cheaper than trying to actually find the problems in the existing wiring.
It sort of makes sense to meet these requirements as well. Think about it. Most of the equipment is 460V. The Code requirement is less than 25 ohms. Even at 20 ohms, that means that you could have a fault current of I=V/R=23 amps. That's fine for your house but in an industrial setting, most motors are substantially larger than that. A 60 amp breaker or fuse wouldn't ever detect the fault, leaving you with a continuous current draw across the ground.
We're a sand mine, and the ground itself is absolutely lousy. BUT...we've met those requirements.
On the transformer ground rods, we got the threaded ones and simply drove one rod after another down until the readings dropped to below 25 ohms. As the dirt settled back in within a week, the readings dropped down to less than 5 ohms. Only had to come back and add an extra ground rod once.
As for the equipment, mostly we ended up adding lots of external grounds. Usually, the problem wasn't with the wiring itself. It was with the structure which after 25 years or so of rust simply wasn't all that well bonded anymore. So adding external ground supplemental ground wiring was simply easier and cheaper than trying to actually find the problems in the existing wiring.
It sort of makes sense to meet these requirements as well. Think about it. Most of the equipment is 460V. The Code requirement is less than 25 ohms. Even at 20 ohms, that means that you could have a fault current of I=V/R=23 amps. That's fine for your house but in an industrial setting, most motors are substantially larger than that. A 60 amp breaker or fuse wouldn't ever detect the fault, leaving you with a continuous current draw across the ground.
