A Close Look at the Signal Reference Grid for Grounding

By Vincent Saturno, P.E., and Rajan Battish, P.E., RTKL Assocs., Baltimore March 28, 2006

Editor’s Note: In the Spring 2006 issue of Pure Power, authors Vincent Saturno and Rajan Battish present the second installment in their two part series on single- and mult-point grounding systems. In Part 1 (Spring 2005) they explained how hybrids of these systems work. In Part 2 they continued their discussion with a closer look at hybrid strategies. However, due to space constraints, we were forced to omit their discussion of signal reference grid. It appears below.

The main purpose of an SRG (Signal Reference Grid) is to provide an equipotential plane to which all electronic and electrical equipment can be referenced. In addition, all conduit, ductwork and metallic enclosures should be bonded to the SRG. This allows the voltage potential of connected equipment and objects to rise and fall together, keeping voltage from developing on equipment enclosures.

SRGs are intended for high-frequency grounding, normally on the order of tens of megahertz. Voltage surges at high frequency actually increase the impedance of the grounding conductor by increasing inductance and causing resonance. For this reason a standard single-point grounding system is ineffective for signal grounding. The length of the conductors would allow for such high inductance values that the grounding system would be rendered ineffective. An SRG provides numerous low-inductance (at a high frequency range) grounding paths and much less resistance than a single grounding conductor.

The construction of the SRG has evolved over the years. Its earliest application was solid sheet metal. This actually produces the most effective results, since it is one conductive mass, able to accept voltage surges and dissipate them easily.However, this method is very costly and difficult to deal with, should any maintenance need to be performed. Surge protection plates are used in many of today’s data center grounding designs. They rely on the same concept, but on a smaller scale. Less robust metal plates are provided below the raised floor underneath electronic equipment. Then grounding connections are made from the equipment terminals to the surge plate and from the surge plate to the SRG.This allows for more reliable surge dissipation from the equipment to the SRG and from the SRG to the equipment.

Currently, SRG designs are constructed by utilizing the metal stringer support structure of raised floors. This method of creating an SRG has many advantages.It is cost-effective, maintenance-friendly and very successful at protecting equipment from voltage surges. The recommended practice is to crisscross bare copper grounding conductors under the raised floor, attaching to every fourth support pedestal via bolt/clamp connection. Connecting at every fourth pedestal under a standard 2-ft. x 2-ft. raised floor design ensures SRG conductor lengths suitable to handle voltage surges ranging from DC to approximately 30 MHz.The vast majority of data center equipment will fall within that operating frequency range.

Connections from the equipment to the grounding grid are normally done with flat conductive metal straps. Bonding lengths are made to be as short as possible to accept the high-frequency surges associated with the equipment. These high-frequency surges cause inductance along the bonding strap. Therefore, longer lengths (exceeding 24 in.) would cause such a large inductance that the impedance of the strap would offset any grounding effects it would create.Equipment is also bonded at multiple points and with different bonding lengths.Employing multiple bonds ensures less impedance than a single bond, while using different bond lengths ensures that at least one bonding straps will not be subject to its resonant frequency at a high-frequency surge.

Every conductive object passing into critical equipment area should be bonded to the SRG to minimize potential differences within the room. These include electrical conduit, ductwork, sprinkler piping and other conduits. In some cases, the SRG can be bonded to the building steel. However, this practice should be abandoned if building steel is used as lightning down conductor. A high-frequency lightning surge could travel onto the SRG and disrupt or damage the equipment bonded to it.

In many data center applications, a separate ground bar is located under the raised floor. This can act a central hub for the bonding of the SRG conductors and a connection point for the grounding conductor to the main ground bus.