Unsafe Electronic Equipment Racking Systems the Norm, not the Exception
Mike,
I want to take this opportunity to thank you for your many well written articles. They provide us with critical knowledge and warnings on many relevant issues. I value your opinion on electrical issues and request a bit of your time addressing what appears to be a ground safety issue with commercially purchased Electronic Equipment Racking Systems. I believe that a rack safety ground should be mandatory on all equipment racks and these systems do not provide or recommend the grounding of their equipment racks.
I was an Electrician in the US Navy and for the past 33 years a Hardware Engineer for an aerospace company. While on the job I have received substantial electrical shocks when coming in contact with a number of ungrounded equipment racks containing defective hardware. Even though I haven’t experienced an electrical shock from the current racking systems, when considering the power required for data processing equipment there is a greater potential for an electrical shock. As a point of reference, one (1) of our IBM Blade Server Racks can require eight (8) PDUs, each supplied by a 208 volt 30 amp power cord. If an electrical ground fault should occur in either a PDU, IBM server, third party chassis or a power cord is chafed via one of the server slides, the ungrounded rack would become a dangerous shock hazard.
A typical example of an Electronic Equipment Racking System is the IBM T42 Equipment Rack with Power Distribution Units (PDUs). Please note IBM is only an example and definitely not the only company that provides similar racking systems. All of the equipment rack’s metal surfaces are painted, including the mounting rails. The surfaces of the rack mounted PDUs are also painted. These PDUs (32P1766, 32P1736) do not provide ground studs/ground straps, nor is there any bare metal to metal contact made during mounting. The rack is ungrounded except for a possible intermittent ground path through the slide rail of a server chassis, which does not provide the permanent, low-impedance circuit as required by the NEC [250.4(A)(3) and (5)].
The IBM Rack Installation Manuals (32P0096, 32P0099) and the Rack PDU manuals (02R2739, 02R2738) do not mention a rack safety ground. I consider this design non-compliant with the National Electrical Code (NEC), ANSI/IEEE STD 142, Mil-HDBK-454 and good engineering practices. In correspondence with IBM, they believe their rack designs are completely compliant and that the racks do not have to be grounded.
Since the NEC requires an electrical ground on equipment racks for Information Technology Systems [645.15] and Audio Equipment Racks [640.22], I would think the grounding requirement should also apply to Data Processing Equipment Racks.
The following codes justify my position:
645.15: All exposed non-current-carrying metal parts of an Information Technology System shall be grounded in accordance with Article 250 or shall be double insulated.
640.22: Metal equipment racks and enclosures shall be grounded.
250.4(A)(5): Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source.
ANSI/IEEE STD 142, “IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems,” Section 2.1.2. states: ”… many personal injuries are caused by electric shock as a result of making contact with metallic members that are normally not energized and expected to remain non-energized. … to avoid exposed shock hazards an equipment ground conductor must be present.”
Mil-HDBK-454: “The design and construction of equipment shall insure that all external parts, surfaces, … are at ground potential at all times…,” which implies the need for a rack safety ground.
My Position: Each power distribution unit within a rack should be grounded to the rack since they can originate from different power distribution panels. Connecting the green safety ground wire from each power source to the rack would provide a direct connection to the source through the power cord. This would ensure the lowest possible ground fault path.
As a work around an external ground wire could be added between the rack and the raised floor metal grid, but this would not provide a direct path to the source nor would it be compliant with NEC 250.102 or 250.134.
I am currently using the following work around when a vendor cannot provide a PDU with an appropriate ground connection, which is often the case. I purchase at least one (1) PDU with an appropriate ground stud from a third party vendor providing at least one safety ground for each rack. While this is not the optimum design, it does provide a measure of safety that can prevent injury or death from electrical shock.
Please provide guidance relative to this issue and if you find the basis of my concerns valid, consider the establishment of a rack grounding requirement for ALL Equipment Racks as part of the revised NEC.
Thank you for your time,
Bob Cheke
Mike’s Comment: Bob, I agree with you 100% that the metal racks should be grounded (bonded) to an effective ground-fault current path in accordance with 250.2 and 250.4(A)(3) and (5). Maybe this newsletter will get the attention of the industry and it will result in changes in the design of electronic equipment racks as well as enforcement by the AHJ.
