Your UPS is only as reliable as your grounding system.
Without a properly installed grounding system, your UPS won't function correctly. A grounding system allows circuit protection to clear a ground fault, and provides paths for diverting surge current away from the UPS and for removing undesirable currents from the critical load. So what can you do to ensure your grounding system allows your UPS to do its job?
One of the first steps to getting the grounding system right is knowing whether to design for a separately derived system, which depends on the detailed arrangement of the bypass neutral. The NEC defines a separately derived system as “a premises wiring system whose power is derived from a battery, from a solar photovoltaic system, or from a generator, transformer, or converter windings, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.” This discussion will focus on the requirements for such a system.
According to 250.30, you must bond the grounded circuit conductor — usually the neutral — at its source to the equipment safety grounding conductor. The grounding circuit conductor must also be bonded to a local grounding electrode conductor that is connected to the nearest grounded building steelwork, metal water pipe, or other effectively grounded man-made grounding electrode. Consult 250.66 when sizing the grounding electrode conductor for a separately derived system for the derived phase conductors. You must also connect the grounded conductor of the derived system to the grounding electrode.
For multi-module systems, connect the separately derived source to a common grounding electrode conductor sized per 250.66. Make connections at an accessible location, using exothermic welding or irreversible compression connectors listed for the purpose.
A multi-module UPS system fed from a 3-phase, 3-wire, grounded wye supply is a separately derived system because the neutral doesn't connect to the output of the UPS modules. In this case, you should isolate the ground and neutral buses, which are located inside the modules, from each other. But you must bond the neutral bus and ground bus in the system cabinet together and connect them to the local grounding point. Typically, the grounding point is a copper bar mounted on insulators in the electrical room and bonded to the local building steelwork.
Whenever you use a delta-connected supply for a UPS system, you must create an artificial neutral. In such cases, a three-resistor network typically provides a logic reference point for the bypass input. Bond the neutral of the UPS output to the local UPS room ground bus. Some UPS modules come equipped with an input isolation transformer, but these don't influence whether you deem the system to be a separately derived source.
For multiple battery cabinets incorporating battery disconnects, the cabinets are bolted together, forming a single lineup with the UPS. The cabinet grounds are inherently connected to the UPS ground bus via the metal chassis. You must connect the supporting racks of a wet-cell installation to the battery disconnect and to the UPS module ground bus. You should connect the racks directly to the local UPS room ground bus only if required by local electrical codes.
Grounding at data centers.
Thanks to the dot.com explosion, data centers have popped up in large numbers throughout the last few years. And despite the subsequent implosion, the continued demand for data centers at banks, financial institutions, and telecommunications facilities means it's still important to know how to ground them. At such facilities, you must ground the complete UPS system, so you'll need to understand a few terms and concepts first.
The exterior ground ring (EGR) is used for lightning protection — particularly for prefabricated concrete buildings — and consists of a 4/0 AWG bare copper ground wire buried in a trench at least 30 in. below grade, with ½-in. copper ground rods driven at least 10 ft below the trench. Ground wells must be located at the four corners of the building with ground rods spaced in between at 10-, 20-, or 30-ft intervals. This ring should be about 2 ft beyond the building drip line; all underground connections must be exothermically welded. Many designers will choose to use the steel structure of a building instead of the EGR. In that case, the main ground bus (MGB) should connect directly to the building steelwork.
The interior perimeter ground loop consists of a 2 AWG bare copper ground wire connected to the signal reference ground (SRG) at every interior ground bar. Metallic parts entering the protected area must bond to this perimeter ground loop. However, this loop isn't always installed around the perimeter of the computer room when the building has a steel structure.
Special considerations apply to raised-floor installations with pedestal-bolted stringer connections. Copper conductors bolted to alternate supporting pedestals constitute the SRG, and you must bond equipment grounds located on the raised floor to the perimeter loop. Be sure to keep grounding straps as short as possible and eliminate loops. You must bond to the SRG junction boxes beneath the raised floor, and PDUs, RPCs, and A/C units. PDUs, ASTS units, and other computer equipment should also be bonded to the nearest interior ground bar using green, insulated 8 AWG grounding wire. Finally, bond UPS cabinets to the nearest interior ground bus, using a green, insulated 2 AWG grounding wire. Installers typically mount the main ground bar (MGB) of a UPS room 24 in. above the finished floor and use zones for best grounding results.
Your UPS will do its job only when it has the right infrastructure to support it. Deciding whether or not your system is separately derived and figuring out how to configure and install the grounding system are determining factors in ensuring the UPS will perform as designed.
Russell is an associate with EYP Mission Critical Facilities, Inc., in Los Angeles.
Sidebar: Getting More From Your UPS
Consider the following guidelines to improve performance of your UPS:
The service entrance ground should be a quality, stable connection to earth with impedance low enough to meet equipment requirements specified by the vendors.
Use a lightning protection system.
Use a two-stage surge arrester scheme (high energy at the service entrance, lower energy in downstream distribution).
Use dedicated feeders and branch circuits, separating noise generators, such as electronic ballast lighting, from noise-sensitive loads.
Use correctly sized equipment-grounding conductors, per 250.122, to reduce ground system impedance.
Locate a separately derived system, such as a PDU isolation transformer, as close to the sensitive load as possible. This minimizes common-mode noise.
Select a low-impedance local grounding electrode, per 250.30, when grounding a separately derived system.
For large data centers with raised floors, install a signal reference grid system (SRG) to create an equipotential ground plane. Connect the SRG to the single-point power distribution unit PDU ground. In the case of multiple PDUs, connect the raised-floor stringer system to each PDU ground. The SRG could be a prefabricated ground grid installed beneath the raised floor, a grid consisting of 2 AWG copper conductors bonded to every other pedestal with special clamp connectors, or bolted stringers bonded to the pedestals.