Re: Alarm & Grounding
These supplemental ground terminals can be bonded to the GEC conductor if building steel, water pipe, or no other electrode is available.
I would be wary of using building steel or a water pipe however. Case in point: We regularly install and service small to medium sized telephone systems from a well known manufacturer. Some time ago we got a call from a law firm that we had installed at least two years prior. The complaint was that the entire system was down.
When I arrived I determined that the processor module was completely dead. This module has the line cord for the system as well as a supplemental ground screw. The manufacturer also suggests that this screw be connected to building steel or a cold water pipe. Since this was on the third floor of a large office building, when this was installed I put a clamp on an "I" beam above the dropped ceiling to provide the supplemental ground. There is usually nothing else (economically) available in these offices.
I replace the module with a new one ($750.00), reconnected the supplemental ground wire and plugged the line cord in. Everything worked fine. Two days later I again get a call that the system is down.
Again the processor module is dead.
I replace the module (and start to think about power surges) but this time I just happen to notice that after a few minutes the line cord is warm! It's an 18ga three wire cord but this only draws about 75 watts. For some reason (luck I guess)I disconnected the supplemental ground wire from the screw before I pulled the plug and got an arc! Got the meter out and there was 50 volts between the "I" beam and the ground on the receptacle. Why now after 2+ years? Customer said that there was some kind of electrical work being done for the building owner at night apparently by some "fly by nighters".
Now, the current flow between the grounds wouldn't be so bad if the morons at this "well known company" had hardwired with a piece of wire the supplemental ground screw within the module to the green ground wire of the line cord. At worse the wire would have burned up. Instead they used a circuit board trace to connect the two and it had a bunch of other connections to it along the way. Once the trace burned in half the voltage differential destroyed components on the board and that was the end.
Moral of the story is we no longer provide a supplemental ground unless we can connect it to the same point (and within close proximity) as the receptacle ground.