Signal Reference Grids: Why the literal ground your equipment stands on, matters
Ensuring your entire electrical system is grounded properly is one of the most significant benefits you can offer your system in regard to power quality. Regardless of whether your facility is a school, data center, manufacturing plant, hospital, etc., proper grounding techniques can ensure your equipment functions as it is intended. Grounding helps stabilize voltage under normal operating conditions, which maintains the voltage at one potential relative to ground. Having a set voltage is very important for electronic equipment that uses the ground connection as a 0-reference point. However, a facility can face a problem when electronic equipment utilizes a ground that may not be 0. Your facility may already minimize harmonics in the system and sensitive process equipment may receive power conditioning, but electronics can create their own grounding issues based on operating frequency.
Effects of Distance
Distance from the actual ground connection to earth can contribute to impedance issues on the grounding system. Copper inherently has a resistance, which is why the National Electric Code (NEC) recommends accounting for voltage drop during design. NEC articles: Branch Circuits 210.19(A), Feeders 215.2(A)(1), Underground Service Conductors 230.31(C), and Ampacities for Conductors 310.15(A)(1) size conductors to limit feeder and branch circuit drop to a total of 5%, as discussed in a previous blog post. However, many people do not realize that frequency effects the maximum grounding length. The Institute for Electrical and Electronics Engineers (IEEE) conducted a study (outlined in IEEE Std 1100-1999 [Emerald Book]) that determined the maximum grounding conductor lengths based on equipment frequency. Equipment grounding conductors and bonding is easy to take for granted since the maximum distance allowable at 60 Hz is 150 miles. However, at 1 MHz the maximum effective distance is only 49.5 feet, and only 1.19 inches at 500 MHZ. Any piece of equipment with microprocessor controller or on-board CPU can easily exceed 1 MHz.
When equipment frequencies reach 1 MHz and beyond, a condition called “skin effect” begins. Skin effect occurs when the conduction of electric current concentrates only on the outer surface of the conductors and the core sees little to no current. This results in only the wire’s “skin” (surface of the conductive material) being used by electrons and gives the property its name. The problem with only using part of a conductor is the fact that despite proper sizing during design, the connection appears to be undersized and potential may find its way onto the grounding system. The same IEEE study 1100-1999 determined that at higher frequencies, circular conductors can actually stop conducting. A system earth ground with a resistance of 1 ohm does very little when the wire used to reach it has a resistance of thousands of ohms.
Signal Reference Grid
A signal reference grid (SRG) is a tool intended to help with power quality of electronic equipment, by providing a solution to distance constraints and skin effect. SRGs allow an entire area to have the same conductive potential. Multiple flat, wide conductors are bonded to the building’s grounding electrode system (i.e. earth) instead of a single, circular equipment grounding conductor. Since the equipment sits directly on the grid, ground connections can be limited to inches.
SRG Installation Methods
When raised floors were in fashion for data centers, the flooring grid often conveniently filled the role of an SRG. The ease of using raised floors as an SRG led to SRGs primarily being used in data centers, however any type of facility can benefit. Heating and power delivery advances have seen the industry move away from raised floors, but the benefit of an SRG shouldn’t be forgotten. Options other than a raised floor also exist. A snap together floating floor allows for identical installation as if a raised floor is used and can work in almost any type of space. An office setting with a ceiling grid can use the ceiling to function as an upside down raised floor. Grounding and bonding connections for equipment simply go to the ceiling instead of the floor. Copper straps can be embedded in the concrete flooring for manufacturing facilities that prefer their floors remain concrete. A conductive coating can be painted onto a floor to serve the same function, if little vehicle traffic, such as fork or scissor lifts, is expected.
Advantages to SRG Installations
Low-impedance return path for RF noise currents.
Containment of EM (noise) fields between their source and the grounding plan.
Increased filtering effectiveness of contained EM fields.
Shielding of adjacent sensitive circuits or equipment.
Functionality as a static dissipative floor.
Benefits Extend Beyond Data Centers
As you can see, while data centers and concentrations of computers benefit from SRGs, these advantages can extend to any type of facility. Fitzemeyer & Tocci Associates, Inc. recently helped design SRG flooring within an electronics manufacturing facility. Circuit boards for electronics with very strict tolerances were manufactured utilizing multiple pieces of equipment, each relying on a microprocessor. A common ground was important since any variation could build off previous stages in the assembly process. All the equipment sat on and was bonded to an SRG in the form of a conductive coating on the concrete floor to minimize variations in the final product. If your facility depends on electronics in the form of computers or microprocessor-reliant equipment, Fitzemeyer & Tocci Associates, Inc. can help you determine the best way to design a space to maximize its potential.