Download Suggestions for Grounding a Computer Room

Tech Tips
Suggestions for Grounding
a Computer Room
by Jeff Jowett
Megger
I
n a previous article, we described the concept of the single point ground as
essential to the proper grounding of a computer room in order to eliminate
voltage gradients and stray currents that can damage equipment and disrupt
operations. Here we will expand with some suggestions for effectively tying
equipment into such a system.1
The goal is to establish and maintain an equipotential plane for all equipment in
order to avoid voltage differences. This begins with a good neutral-to-ground bond
at the power treatment device or in the secondary of the transformer feeding the
computer room. The isolated transformer establishes a separately derived system
that keeps the computer room free of noise existing in large building systems.
However, the grounded system protecting the computer room should be interconnected with the building ground in order to maintain an equipotential plane. It
is often felt that an isolated computer ground will keep the system separate from
general building noise. But, in fact, all this does is establish a potential gradient
that attracts noise (ground currents) into the computer room system.
The isolated transformer should be as near the computer as possible. This is
because the impedance of long conductors can establish potential differences
that produce electrical noise and interference sufficient to disrupt the smooth
operation of the computer. Conductor sizing is important in further reducing or
eliminating this problem. Logic circuits of computers, which began operating at
comparatively enormous levels around 50 V, have by development been progressively reduced to more and more efficient and tighter voltage “windows”: 12, 5,
1.8, and 1.2 volts. Speed of operation increased apace. With these increasingly
demanding parameters, the leading edge of a noise-generated voltage rise can
readily appear like a logic signal. Enough noise- generated voltage and the computer becomes a random number generator. The larger the gauge of the isolated
grounding conductor, the lower the impedance of the return, in turn reducing
noise. The recommendation is the grounding conductor be sized equivalent to the
phase conductors, or at a minimum #8 AWG. If the color green is used for the
safety ground conductor, then green with yellow stripes is used for the isolated
grounding conductor. The latter must be extended with the circuit conductors,
neutral and safety grounding conductor and go through the panel or reference
control panels. It is not connected to conduit or secondary distribution panels
through which it passes, and terminates on an isolated bus bar, main isolated
ground cable or receptacle, and at the single point ground of the power source.
www.netaworld.org Balancing a multiphase system will
reduce neutral current, but this is not
possible for single-phase circuits. In
such instances, neutral sizing is critical.
A convenient source of information
is Table 9 in the National Electrical
Code® (NEC®). For a typical acceptance value of no more than 2 Vpp, a
convenient formula is:
R = 1000 / (Icb x Lm)
Where:
R = resistance in ohms per 1000 ft
Lm = max length in ft
Icb = current capacity
From this calculation and the data
in Table 9, the appropriate AWG of
copper or aluminum can be selected.
It is important to note the difference with the goals of the National
Electrical Code. The NEC is concerned with safety…protection from
personnel hazards like fire and electrocution…more than with performance.
A grounded system utilizing metallic
conduit and panels is fine from the
standpoint of safety and may well
meet Code but at the same time be
unsatisfactory in promoting system
performance.
A good way to monitor the equipotential balance of the system is to
check the voltage between neutral
Spring 2009 NETA WORLD
and ground. There should be no current flowing in the
grounding conductor. Ideally, the grounding conductor
terminates at some insulation barrier, say, at a motor housing that is isolated from circuitry by insulation. If there
is some breakdown of this protection, then current flows.
If the system is carrying ground current during normal
operation, a voltage is generated between neutral and the
grounding conductor which represents a voltage drop in the
neutral. To check correct bonding, with no current flow in
the grounding conductor, a voltage measurement between
the neutral and grounding conductors at, say, a subpanel or
receptacle, will indicate generated voltage from that point
through the neutral-ground bond at the main distribution
panel. A well maintained system with all connections offering minimal resistance will hold this to a minimum. Voltage
should be the product of current times the resistance of the
neutral conductor. Any other resistive connection between
the neutral-ground bond and the point of measurement
will add to this.
As an example, 100 feet of #12 copper, from a table in
the NEC, would be expected to have an impedance of 0.17
ohm. If the neutral carried 5 amperes, the voltage would be
expected to be 0.85 Vrms. If an actual measurement proved
to be more than 2 V peak-to-peak, there could be a loose or
high resistance connection in the neutral. If unusually low,
neutral and ground must be touching at some point short
of the neutral-ground bond. Manufacturers of electronic
equipment will specify the maximum neutral-ground voltage allowable, and the system must be so designed, with the
neutral sized accordingly, to assure a voltage drop within
this allowance. This can be accomplished by increasing
the size of the neutral in order to reduce impedance or by
decreasing the length of the circuit to footage of acceptable
impedance.
The safety ground protecting the building at large can be
a source of ground loops and the flow of noise currents, but
separating these connections at the computer room creates
voltage gradients, as has been discussed. This conflict can
be resolved by the installation of a floor that serves as a
ground plane. A thin metal plate has low surface impedance
but lacks strength and is difficult to connect. A successful
alternative is an interconnected ground grid used as support for the floor and as ground plane. The ceramic floor
tiles can fit within the squares of the grid while permitting
access underneath.
The structural supports that constitute the grid are arranged in the form of two-foot squares. Such a structure
proves adequate to noise levels up to 30 megahertz.1 Above
this frequency, noise signals can be filtered or ignored.
The various members of the structure are bonded at less
than one milliohm, in order to serve as would a very thin
plate at low frequencies. Such connections must be sealed
against oxidation, maintained under pressure, and able to
withstand the weight of the load on the floor and vibrations.
The floor is a lightly conductive material (typical value: 109
ohms/square1) which is connected to the grid. It serves to
drain static charges and impede charges of personnel on
the floor. Signal cables under the floor run near the surface
NETA WORLD Spring 2009
of the ground plane in order to reduce noise. Cables that
leave the floor area should follow a properly bonded extension of the ground plane. Lightning protection in the form
of a ground ring was described in the previous article on
single-point grounding.
The installation is energized with its own dedicated
transformer, typically 208Y/120 V. This can be an isolation
transformer providing only common mode attenuation, or
can have a Faraday shield with constant voltage or a line
conditioner to provide regulation and attenuation. This constitutes a separately derived system that must be connected
to ground in accordance with Section 250-30 of the NEC.
The bonding jumper from neutral to ground must not be
sized smaller than the grounding electrode conductor, as
specified in Table 250-66 of the NEC.
If an uninterruptible power source (UPS) or standby
power supply (SPS) is installed, the transformer must be
on the load side. Power distribution units (PDUs) can also
be used. Multiple PDUs can be connected at the output
of a UPS in order to service multiple computer rooms or
other equipment.
As can now be easily recognized, computer room grounding must studiously be applied while avoiding haphazard or
merely generic wiring practices. Levels of noise that would
ride through much electrical equipment will adversely affect
computer room operation. Only close adherence to a specific
set of proven practices will assure smooth operation.
1 Source of recommended practices: Grounding for Electrical
Distribution Systems, ALLTEC® Corp., Cohasset, MA
Jeffrey R. Jowett is Senior Applications Engineer for Megger in Valley
Forge, Pennsylvania, serving the manufacturing lines of Biddle®, Megger®, and Multi-Amp® for electrical test and measurement instrumentation. He holds a BS in Biology and Chemistry from Ursinus College.
He was employed for 22 years with James G. Biddle Co. which became
Biddle Instruments and is now Megger.
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