Grounding is the creation of a
to an electrically conductive body, such as the earth, which maintains a zero potential (not positively or negatively charged) for connecting to an electrical circuit. This is usually done by connecting the data center equipment at the power source to an earth-grounding electrode subsystem which is a network of
rods, plates, mats, or grids installed to establish a low-resistance contact with the earth. The purpose of the earth connection is to provide safety from
to personnel and to protect the data center equipment from voltage gradients which could cause failures or fires. All metallic objects at the site that enclose electrical conductors or that are likely to be energized by electrical currents (for example, circuit faults, electrostatic
, or lightning) should be grounded for human safety, reducing fire hazards, protecting equipment, and to maintain optimal system performance.
A final reason for proper grounding is noise control, an important aspect of power quality.
Bonding is the means by which two or more grounding rods are connected. Proper bonding techniques are critical to proper grounding. You don't want to connect a grounding electrode to the central ground using a material that would act as an insulator, as this would add resistance to the path the electricity would take. The means by which you bond different grounding materials is specified by code. NFPA 70 1999, Article 250, sections 90 through 106, gives specific information on bonding. NFPA 70, section 250-90, defines bonding in general as "Bonding shall be provided where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed."
A solid and well-bonded grounding system will allow circuit breakers to perform correctly, and ensure that devices like surge protectors and power sequencers connected to grounded outlets have a safe path to ground if an overcurrent situation occurs. In areas where
situations are likely, you can ground the metal chassis of a rack to the grounding system.
The common point of grounding can be connected to any number of sources at the service
(main power feed), for example:
Driven earth rod
Whatever the sources, the ground should be carried through the entire system from these sources. Ideally, the central point of grounding at the service entrance will be connected to redundant ground sources such as building steel, buried grid, and cold water piping. A single source sets up the potential for a broken ground. A water pipe might be
. Building steel could accumulate resistance over several floors. By tying into multiple grounds, ground
are avoided, disruptions are minimized, and redundancy is achieved.
A university on the
seaboard lost all power from a problem with poorly grounded generators on the main power line. In the postmortem, it was found that there really was a postmortem. A raccoon seeking warmth had climbed into the generator housing and shorted out the circuit, creating a grounding loop, and knocking out the power. When everything was finally back online, another raccoon climbed into the generator and self-immolated, taking the power with it. After that, chicken wire was installed around the generator.
Compliance With the NEC
All grounding design should
with the National Electrical Code (NFPA 70 or NEC) unless superseded by other codes. Article 250 of NFPA 70 1999 " covers general requirements for grounding and bonding of electrical installations, and specific requirements in (1) through (6).
Systems, circuits, and equipment required, permitted, or not permitted to be grounded.
Circuit conductor to be grounded on grounded systems.
Location of grounding connections.
Types and sizes of grounding and bonding conductors and electrodes.
Methods of grounding and bonding.
Conditions under which
, isolation, or insulation may be substituted for grounding."
NFPA 70 1999 in section 250-2 (d) "Performance of Fault Current Path" states:
" shall be permanent and electrically continuous."
The ground should be continuous from the central grounding point at the origin of the building system. If the path is installed in such a way that damage, corrosion, loosening, etc. could impair the continuity, then shock and fire hazards can develop. The ground should be dedicated and continuous for the whole system to avoid a ground differential that can occur from using various grounds.
" shall be capable of safely carrying the maximum fault likely to be imposed on it."
Fault currents can be many times normal currents, and such high currents can melt or burn metal at points of poor conductivity. These high
can be a
, and they can destroy the continuity of the ground-fault path.
" shall have sufficiently low impedance to facilitate the operation of overcurrent devices under fault conditions."
A properly designed system will have as low an impedance as possible. If the ground-fault path has a high impedance, there will be hazardous voltages whenever fault currents attempt to flow. Also, if the impedance is high, the fault current will be limited to some value so low that the fuse or circuit breaker will not
promptly, if at all.
"The earth shall not be used as the sole equipment grounding conductor or fault current path."
You have to use circuit breakers and valid grounding systems. You can't just rely on the fact that the building is connected to earth as the sole means of grounding.
NFPA 70 1999 Section 250-50 state that each of the items below " shall be bonded together to form the grounding electrode system."
Metal frame of the building, where effectively grounded
Furthermore, if metal underground water pipe is used, "continuity of the grounding path or the bonding connection to interior piping shall not rely on water
or filtering devices and similar equipment." Additionally, a supplemental electrode is required.
NFPA 70 1999 section 250-52 states that if none of the previous grounding items are available, then, and only then, should you use the following:
The material in this section is reprinted with permission from NFPA 70, The National Electrical Code
Copyright 1999, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the complete and official position of the National Fire Protection Association on the referenced subject, which is represented only by the standard in its entirety.
Equipment Grounding Conductor Impedance
The data center must have its own grounding plan which will tie into the earth ground for the rest of the building. The system should have sufficiently low resistance to allow circuit breakers, surge protectors, and power sequencers to respond to this overcurrent state very quickly. This resistance should be in the 1 to 5 ohm range. In the U.S., a 25-ohm maximum resistance value is the minimum standard for most "normal" grounding systems, according to the NEC. While this level of resistance is acceptable in a normal office environment, data centers should use the 5 ohms of resistance as the acceptable maximum resistance level for their grounding system.
The NEC and local codes require electronic equipment to be grounded through the equipment grounding conductor and bonded to the grounding electrode system at the power source. The impedance of the equipment grounding conductor from the electronic equipment back to the source neutral-ground bonding point is a measure of the quality of the fault return path. Poor quality connections in the grounding system will give a high impedance measurement. Properly installed equipment grounding conductors will give very low impedance levels. Equipment grounding conductors should have levels meeting code requirements with a value of less than 0.25 ohm.