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In Reply to: Help with dedicated line question posted by Sean H on June 18, 2000 at 16:45:28:
Back in June of 1991, one of my clients suddenly started to have significant problems with a minicomputer that had been 100% stable up until that time. First, an 8 way comm board fried. This was somewhat typical; static electricity from a user's keyboard could toast the controller chip on that particular serial controller.Then a memory card went. At first, we thought that it was a thermal problem: the offices were new, it was the first summer, and the system was in a warmer part of the building. So, we installed air, fans, etc.
Then the CPU card went.
We thought it was the age of the system; it was now 5 years old. Maybe the power supplies were going south and taking components with them. So, we replaced the power supplies.
Then the disk controller card fried.
What the heck: even though the system was plugged in an Orange Hubbel outlet, which in accordance with code means a dedicated circuit, we put a voltage recorder on the circuit for a week.
Low and behold, every day at 10:30A we recorded a series of 15 volt spikes on the ground. We traced the ground a couple hundred feet, and found out that the electrician had "cut corners" at the request of the building owner and simply tied dedicated ground to common ground back at one sub-panel.
We then realized that a brand new, big, industrial microwave oven had been purchased at the end of May, and that the office staff made popcorn every day at about 10:30A. Turns out that microwave was sending that voltage spike over ground...
The moral of the story: dedicated circuits really do need a dedicated ground to be effectual. We forced the landlord/electrician to properly isolate the ground, and the system regained stability.
Oh, by the way, that same minicomputer is still running just fine this very night.
The little voices told me to stay at home and clean the guns today.
Nice story and informative. One thing though, you said "We forced the landlord electrician to properly [isolate] the ground." Also that "the electrician had 'cut corners' ......and simply tied dedicated ground to common ground back at one subpanel."
You need to be careful about the use of the word "isolate" as there are those who continue to believe this means you stick a separate rod in the earth and attach your ground to it. What you should have said is that the electrician was forced to "re-terminate" the dedicated ground at the correct point (which I am assuming was at the main panel, not the subpanel) as you did not give details.
I had tons of noise coming into my test bench equipment. Just like Bob's situation, it was all coming in through the ground. Since all power line filters shunt the interference to ground, they were completely in-effective. Luckily, we were able to find the source of the problem and it's been much better ever since.The moral of both stories: Don't under-estimate the importance of a good ground system. Sean
>
He he he, that's a very interesting story. Wow. I was suspicious of this, that I someone would mention dedicated grounds for a dedicated circuit. And I have thought of this too. But after reading other posts in the past and doing some searches of the archives I'm finding that in itself can be a bitch to do. Some ideas call for extremely long piping be insterted in the ground, some say to use a "mesh" ground well into the ground. There is even some sort of arguement that because the the dedicated line coming off the box will have its own ground it may conflict with the ground everything else in the box is grounded to- dual grounds.(??) So, yes this is another issue and I need to gain a lot more information I guess. Any further suggestions or comments?Thanks,
Sean Heisler
D e s i g n _P o r t f o l i o
There seems to be some confusion about what a dedicated ground is. A dedicated ground is part of a dedicated circuit. No other device is connected to that ground, but it is grounded at the service panel. Also, a ground may be isolated. This means it has no bonding to conduits or boxes and is insulated all the way back to the point of connection at the service panel. Those funny orange outlets are isolated ground outlets. The ground connection is not common with the mounting tabs. I would not suggest using another ground other than the service panel ground. You are risking your equipment and will most likely have higher noise levels as you have seperated the ground and neutral (by many megohms).The lowest noise will be from a isloated, dedicated ground, connected at the service panel, which is the zero voltage reference point for your wiring.It is exactly the same concept as star grounding inside your equipment.
There is a misstatement of fact in your post. Just being orange in color does not make a receptacle isolated ground. Hospital grade and isolated ground hospital grade receptacles come in colors other than orange, although this color is popular because of its high visibility. A hospital grade standard ground receptacle is identified by a green dot regardless of face color. A hospital grade isolated ground receptacle is further identified by a green triangle. The green triangle is the only mark on an IG receptacle that confirms the presence of an isolated ground. There are also cover plates marked "isolated ground" which are used to help identify the presence of an IG.
I stand corrected. I do now remember seeing white outlets with green triangles in use. I have yet to see an orange outlet that was not isolated ground, maybe not hospital grade, but still isolated ground. I would believe they may be out there somewhere, however, my point was to try and clear up what an isolated ground cicuit is. Thank you!
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