---- Doug Hutchison <specialq....@ntlworld.com> wrote: > Yup...thats just what I mean...experts.....bah humbug!! > > D > > On 01/08/2010 19:44:48, Eric Lemmon (wb6...@verizon.net) wrote: > > That is so true! When you add "corporate ego" to the mix, things get > > really > > murky. A case in point: Back in the mid-eighties, one very large and > > well-known computer equipment manufacturer was contracted to install > some > > facility monitoring equipment at Space Launch Complex Six at Vandenberg > > AFB, > > a site that was to be (until the Challenger disaster) the west-coast > > Space > > Shuttle launch pad. The monitoring equipment was divided into two major > > pieces, on separate floors of the Launch Control Center and about 200 > > feet > > apart. From the moment the system was energized, a number of data > > channels > > had 60 Hz common-mode noise corrupting the data on the RS-422 > > circuits. > > > > My crew was attempting to investigate the noise issue, when we > discovered > > that the > > supplier's technicians had deliberately "floated" the upstairs > > cabinets by using insulating washers and plastic sheets to avoid > contacting > > any grounded facility items such as embedded rebar. When advised by > our AHJ > > (Authority Having Jurisdiction) inspector that the installation violated > > Article 250 of the NEC, the manufacturer's > > engineer explained that this was > > the way his company did these installations, and- > > here's the corporate ego > > part- they had been doing%2
I must admit that I don't see the problem with the sensitivie equipment not finding ground thru the floor! In the 1980 I have some telephone central offices built by TRW. Each of the equipment racks were mounted on thick plastic sheets, and the mounting bolts were inserted thru insulating shoulder washers. Each equipment rack had a "dedicate home run" ground back to the main central office ground buss plate. All worked well, with no lightning problems. Some many years later I was still working for the same telephone company when I was asked to go back to that office to see why the new equipment addition would not work. It was needed badly as the C.O. was out of lines and needed this expansion to work. A quick inspection showed that the new equipment had been mounted directly to the concrete floor with out the insulation kit. I had the CO crew there help jack the rack up and slide a rubber floor mat under it for temporary insulation. We verified that the ground strap was correctly installed, and when we returned that equipment cabnet to service , it came up and worked fine. The equipment installer was forced to return at night to correct his problems. So not everthing should be chassis grounded to the floor. Jeff
