05-18-2006, 09:45 PM #1
- Join Date
- Feb 2005
- Haymarket, VA
Isolated Ground Receptacles for Sensitive Electronics
Within the past two weeks, I have received a two telephone calls and four emails asking for my assistance in providing a solution for high noise and other electrical problems associated with the ubiquitous orange receptacles with the triangle on the lower left hand side of the devices. Because of two systems failures within a month and other disruptions, quality power surveys were requested at two of the locations. The results indicated there was more normal and common mode noise in addition to normal problems associated with a modern electrical grid or building wiring.
I provided what I considered sound advice. For those facilities who did not conduct power surveys, I recommended they be conducted and provided names of companies providing such services in the respective geographical locations. In addition, I provided the following additional information which I want to share with you. Incidentally, all the recommended companies recommended the same solutions.
*Isolated Ground (IG) Wiring and the Dedicated Line
IG receptacles should not be used for the reduction or elimination of electronic noise. It should be understood that the specific technique or techniques for installing an isolated ground circuit is not described in Articles 250-146(D), Isolated Receptacles, 408-20, Grounding of Panelboards or 647-7(B), Isolated Ground Receptacles, NFPA 70, National Electrical Code ® NEC ®. It is for this reason and this reason alone the following was written.
Manufacturers have traditionally required their customers to install a dedicated line and isolated ground to provide clean power for their equipment. As this equipment was replaced by more sophisticated networking systems, the practice continued because many felt that dedicated lines and isolated ground would provide a level of protection from the disturbances created on AC power lines. Dedicated line and isolated grounds are usually expensive to install and the installation process can cause serious disruption.
A true dedicated line with IG consists of three individually isolated wires (line, neutral and ground) that are run from the point where the utility wires enter the building (the service entrance) to the receptacle to which the system’s equipment will be connected. Typically, and as an accepted practice, the dedicated line originates at any convenient subpanel. So then, the term “IG” refers to a separate installed ground conductor as we previously learned. No other equipment or receptacles are to be connected to this circuit.
While the line and neutral conductors tend to originate at a convenient electrical subpanel the insulated ground conductor must carry through any subpanels to the service entrance ground bond. Where more than one insulated ground enters the subpanel and isolated, insulated ground bus bar can be used to consolidate the isolated grounds. A single larger ground connector can conduct this isolated ground bus to the ground bond at the service entrance.
Another concern is what to do about noise impulses that are generated externally. Utility grid switching, power factor correction, heavy industrial loads and lightning can produce very high levels of noise impulses. These higher energy disturbances can frequently destroy or degrade system components. The fact is that the dedicated line/isolated ground does very little to protect the system from these external noise impulses. It is true that long wire runs usually associated with dedicated lines add inductive impedance that will attenuate the noise somewhat. Usually, the impedance is inadequate to attenuate the noise to a safe level on your system and may actually increase noise coupling between the server and other loads in the same raceway. Hence, the dedicated line/isolated ground does not necessarily provide a safe or protected isolated environment for sensitive electronic equipment. Because of the inductance and capacitive coupling phenomena other measures are required to decrease fast rise time impulses that exist in all modern facilities.
Hence, the purpose of dedicated lines and IG is to isolate sensitive electronic systems from noisy electrical loads such as refrigeration compressors, HVAC and other bulk electrical loads. Whenever these devices are turned on they draw a very large in-rush or startup current for a short period of time. Coupling this with the laws of physics, particularly Faraday’s Law, which says that voltage equals the inductance of the wiring times the change in current over the change in time (E=L di/dt), dictates that noise impulses are a natural phenomena which occurs in all modern facilities. This noise can range from a few volts to thousands of volts! The theory is that this internally noise is dissipated throughout the facility’s electrical systems and utility system rather than being reflected into the dedicated lines.
Power Conditioning Developments
The modern low impedance transformer based power conditioners developed approximately 15 years ago were designed to remove harmful noise levels including lightning from the power line. The typical low impedance power conditioner consists of three major assemblies — an input protective network, a unique isolation transformer and an output filter assembly. The main purpose of the transformer is to allow the bonding of neutral and ground conductors on the output side of the transformer. This effectively eliminates common mode noise allowing the ground reference to be reestablished. The transformer also provides excellent noise rejection between line and neutral. The output filter assembly ties line and neutral together at high frequencies. This assures that all remaining noise is eliminated or attenuated to a safe level, yet passes the harmless lower frequencies necessary for efficient power transfer.
In contrast to the dedicated line/isolated ground; these low impedance power conditioners were designed to attenuate all harmful electrical noise impulses to a level that is safe for networked systems. Because the conditioner is normally connected very close to the server or workstation, noise generated both within the building as well as from external sources is attenuated to a safe level at a significant savings over the cost of installed dedicated lines and isolated grounds.
Typically, a “plug and play” power conditioner at a workstation will cost much less than special IG wiring. Whereas IG/Dedicated lines cost $500.00 to $1,000.00 per location, the power conditioner at the workstation or server can be purchased for $150.00 to $300.00 depending upon size required. Or, for conditioning and battery back-up power, a conditioned UPS can be utilized, typically for less than $500.00. The bottom line is that these devices can protect your networked systems from “internally” generated noise impulses as well as or better than dedicated lines with isolated grounds. Plus, they have the added benefit of protecting your system against externally generated noise. (*Source: “©Isolated Grounding, Is It Worth The Expense For Your Network?” Herb Goldstein, 1997, Advanced Power Solutions, Inc. ®)
Hopefully this will be of assistance for those of you facing the same problem.
Enjoy the day,