Power Systems

Isolated Grounding by Liebert Corporation

(Traducción en Español)

SUMMARY

Isolated grounding (IG) is a technique often used with sensitive electronic equipment to reduce common mode noise. IG insulates the grounding of the sensitive load equipment from the conduit and raceway grounding system and controls the connection to the power grounding system. Thus, ground potential shifts due to stray ground currents flowing in the conduit system are eliminated, and raceways and conduits provide EMI/RFI shielding. IG is sometimes misinterpreted to mean a separate "isolated" ground for the sensitive load, and configurations based on this interpretation are usually unsafe and in conflict with National Electrical Code requirements. If currents induced in the isolated load equipment ground are allowed to flow over data, communication, and control cabling, IG wiring may actually contribute to inductively coupled common mode noise when applied to circuits that have interconnected load equipment.

Common mode noise is any unwanted signal that is common to all circuit conductors simultaneously with respect to ground. The difference in potential between neutral and ground is one form of common mode noise. Another more troublesome form is the differences in ground potentials throughout an electrical system. Further, the surge suppression, wiring, shielding, and grounding of the building electrical system (including the control, data, and communication cabling) can have a pronounced effect on the levels of common mode signals to which sensitive electronics are exposed.

Because equipment ground potentials (or changes in them) have been observed to affect the operation of certain electronic devices, there are often specific and special grounding instructions. Most instruction are based on empirical testing rather than rigorous analysis, and the basic principles of electricity sometimes are ignored. Keep in mind that the primary purpose of grounding is personnel safety, not the reduction of noise. These two goals can be mutually exclusive. If that’s the case, safety must prevail .

One grounding technique used in low voltage AC power systems to reduce common mode noise is isolated grounding (IG). IG is allowed in the U.S. by the National Electrical Code (NEC)³ and in Canada by the Canadian Electrical Code (CEC).4 In both cases, IG is an exception to the standard grounding requirements. NEC 250-74 and 250-75 allow IG only "where required for the reduction of electrical noise."

What is Isolation?

Isolation ground refers to an isolated (really an insulated) ground path from the computer back to the power grounding point. It is not a separate "clean" grounding system for the computer, isolated from the "dirty" utility ground. There can only be a single ground. Establishing a second, separate ground is not only dangerous and a code violation, it can cause more noise problems than it solves.

The IG concept can be seen by comparing a standard receptacle to an IG receptacle as shown in Figure 1 . In the IG receptacle, the receptacle grounding terminals are electrically insulated from the metal outlet box and the metal conduits and raceways. Thus, there are two isolated grounding paths back to the single power system ground. IG receptacles are often colored orange or marked with an orange triangle.
 
 



As a code minimum, the conduit or raceway is relied upon to ground the outlet box. When non-metallic conduits and most flexible conduits (which do not provide an effective ground path) are used with IG receptacle, the NEC requires a separate grounding conductor to ground the outlet box.

The basic reasons for grounding AC power systems are to limit the circuit voltages, stabilize the circuit voltages to ground, and facilitate the operation of the overcurrent protection device (OPD) in the event of a ground fault. For solidly grounded, low voltage AC power systems, the NEC-250-51 requires that all of the metallic enclosures of the electrical systems be effectively grounded to minimize the electrical shock potential and to facilitate the operation of OPD to clear a ground fault. The NEC defines effectively grounded as having a ground path that (1) is permanent and continuous, (2) has ample current-carrying capacity to handle potential ground fault current, and (3) has sufficiently low impedance to allow the operation of the OPD to clear a fault quickly. These requirements necessitate that an equipment grounding conductor permanently connect all of the metal enclosures of the electrical system and any other conductive parts that could become energized. In order to facilitate the operation of the OPD to clear ground fault, the equipment grounding conductors must be connected to the power system grounding point.

Ground Path

With conventional receptacles, the equipment grounding conductor is in parallel with the conduit ground path. Although ground path impedance is improved, there may be noise on the conduit ground. With isolated ground receptacles, the equipment ground path is separate from the conduit to avoid coupling noise to the computer ground.

Figure 2 is an example of a typical low voltage AC power system using standard receptacles. Compare that with Figure 3, a typical system using IG receptacles as allowed by the NEC. The ground terminal of the receptacle in Figure 3 is not connected to the conduit grounding system at the receptacle. Instead, an IG wire is connected to the receptacle ground terminal and is routed with the power conductors, passing through one or more panelboards, remaining insulated from the metal conduit and enclosure grounding system until its termination at the power system grounding point at the service entrance.

The isolated grounding conductor from the load equipment must be routed through the conduit. Experimental evidence5 indicates a substantially lower impedance for a grounding conductor in a conduit as opposed to one routed outside a conduit.
 
 


 
 

If a ground fault were to occur at the load equipment of either the conventionally grounded or the IG grounded system, Figures 4 and 5, both schemes provide an effective ground path.



Hard-Wired Load Equipment

One other form of IG wiring is allowed in NEC 250-75 (also by exception). It is for hard-wired load equipment as shown in Figure 6. Because there is no IG receptacle, a non-conductive bushing or conduit fitting is inserted where the conduit or raceway terminates at the load equipment enclosure.
 
 



The NEC just recently added the exception for hard-wired equipment, but its effectiveness and safety are still in question. To isolate the grounding of the load equipment, the metal frame of the load equipment must be insulated from its grounded surroundings, perhaps the building itself. There is concern that this could permit shock potential or side flashes to occur between the grounded surroundings and the load equipment enclosure when large ground currents flow, e.g., during lightning strikes.

IG for a Separately Derived Source

Packaged power distribution centers with separately derived sources (defined in NEC/NFPA 70-1993) generally provide the best grounding for computer systems. They’re usually in the computer room, minimizing the length of the wiring to the load equipment. Long runs of IG wiring between the load equipment and the grounding point can cause common mode noise problems because of high frequency impedance and resonance.

Figure 7 shows the wiring of an IG receptacle with a separately derived source. When isolated ground receptacles are used with separately derived sources, the isolated ground system is terminated at the separately derived source and not at the service entrance.
 
 

Incorrect and Unsafe IG Wiring

Figure 8 shows an incorrect and unsafe interpretation of IG wiring. This approach is an apparent attempt to isolate the load equipment ground from the "dirty" power ground. Sometimes extraordinary efforts are made to ensure a good connection to earth in the hopes of providing a "quiet ground" for sensitive electronics.


 
 

This approach does not provide an effective ground path as required by the NEC. Consider the possibility of a ground fault at the load equipment as depicted in Figure 9.
 
 


 
 

There is no effective ground path between the isolated ground and the power source (service entrance) grounding electrode. The ground path between the two earth electrodes may or may not be permanent, continuous, or of ample current-carrying capacity. Moreover, it’s unlikely the ground path has a low enough impedance to allow the OPD to clear the ground fault quickly and safely. The impedance of ground electrode connections to earth is measured in ohms, while the required ground fault path impedance must be in the milli-ohm range.

Because the isolated ground is considered quiet and the power system ground is considered dirty, there is an assumed difference in potential between the isolated ground and the power source ground. Any such differences will manifest themselves as common mode (N-G) voltage at the load equipment.

So, while the original intent of the isolated ground was to preclude electrical noise, the result of this incorrect IG wiring is actually an increase in the common mode noise potentials. Significant ground potential differences can occur whenever there are large ground currents flowing, such as during ground faults, lightning, or even when electrically charged storm clouds move over the earth. A common result of incorrect isolated IG wiring during these events is damage to the connected load equipment.

Load equipment served by incorrect IG wiring may operate normally except under specific conditions such as during a ground fault or a lightning storm.

The Benefits of IG Wiring

Clearly the conduits and raceways provide EMI/RFI shielding of the power and IG conductors contained within them. A more practical benefit, though xxxxx IG wiring minimizes stray ground currents (see Figure 10).




 
 

Stray ground currents flowing on the grounding system cause changes in the ground potentials throughout the grounding system. Stray ground currents are a reality with virtually every power system and exist under a variety of conditions, most of which are dynamic. They can be the result of electrostatic discharge to the enclosures, ground fault currents, or even the capacitively coupled ground current surge when a load is energized.

As Figure 10 shows, any stray ground current will cause the ground potential of the panelboard enclosure to rise relative to the power ground reference at the service entrance. With the standard grounding configuration, the computer system’s equipment ground reference relative to the power ground will also rise because the ground terminal at the panelboard is connected to the enclosure and changes as the enclosure’s ground potential changes.

In the IG wiring shown in Figure 11, the ground reference for the load equipment is isolated from the metal conduit and enclosure ground system. Stray ground currents flow on the conduit and enclosure system, and ground potential changes are confined to the conduit and enclosure ground system. There are no stray currents on the IG wiring, so the ground reference for the load equipment is unaffected.
 
 


 
 


Disadvantages of IG Wiring Techniques

There is the possibility of induced current along the IG conductor and across cabling in interconnected systems.

In most electrical conduits or raceways, multiple individual conductors are used instead of manufactured cable (see Figure 12). Thus, the position of the IG conductor relative to the power conductors is random. Whenever the ground conductor is not equally spaced between the power conductors, the magnetic fields associated with the currents flowing in the power conductors will not be balanced in the ground conductor. The net magnetic AC field will induce current into the ground conductor if it is part of a complete path along which the current can flow (ground loop).
 
 

IG circuits would appear to preclude the problem of induced ground currents because the IG conductor, terminated to ground at only one end, does not form a complete loop through which current can flow-unless there are interconnected systems, linked by data, communication, or control cables between individual units of the load as shown in Figure 13.




 
 

Cables linking load equipment can complete the loop for induced currents in the IG conductor. And because the induced currents are forced to flow over the connecting cables, there is increased likelihood of upsetting or damaging the sensitive load. Induced currents on cables can be particularly troublesome if the signals traveling over the cables can be upset by the power system frequencies, i.e., 60 Hz and harmonics of 60 Hz. Audio and video equipment and analog signal processors are particularly sensitive to power system frequencies.

Induced currents on connecting cables have led to the widespread practice of grounding the shield of a cable at only one end. While this practice may break up the loop, it admits the possibility of damaging or unsafe voltages developing in the system, especially during ground fault, lightning, or other surge events.

Normally, standard grounding techniques yield fewer problems with induced ground currents. This is because induced ground currents tend to flow without practical consequence in loops formed by the ground conductor and the conduit system, bypassing higher impedance loops that include the interconnecting cabling.

Sometimes IG wiring techniques are inadvertently implemented when the conduit or raceway ground path is interrupted. A frequent cause is the use of non-metallic enclosures in corrosive environments. Another is the direct burial of non-metallic conduit in the earth or concrete. The result can be induced currents over interconnected systems. There may also be problems with EMI/RFI interference if metallic shielding is eliminated.

Rules of Thumb

An easy rule to remember when installing IG wiring is as follows:

From the isolated ground receptacle, the isolated ground conductor should follow the wiring back to the first neutral-to-ground bonding point, and be connected to earth only at that point. The isolated ground should not continue beyond that point, nor connect to a separate grounding electrode (building steel, metal water pipe, or driven rod). And the most basic of all rules when grounding for system performance-follow the NEC for safety first.

References:

H.W. Denny, Grounding for the Control of EMI, Don White Consultants, Inc., Gainesville VA, 1983. Good information on the fundamentals of noise control.

E.C. Soares, Grounding Electrical Distribution Systems for Safety, Marsh Publishing Company, Inc., Wayne NJ, 1966.

ANSI/NFPA 70-1993, National Electrical Code, National Fire Protection Association, Batterymarch Park, Quincy MA, 1992.

4 CSA Std. C22.1, Canadian Electrical Code, Canadian Standards Association, Ontario, Canada, 1990.

5 R.H. Kaufmann, "Some Fundamentals of Equipment Grounding Circuit Design," AIEE Transactions, November 1954, pp. 227-232.

Additional references for information on isolated grounding:

T.M. Gruzs, "Computer Systems Need Isolated Ground that is Safe and Noise Free," Computer Technology Review, Spring 1988, pp. 103-108

W.H. Lewis, "The Use and Abuse of Insulated/Isolated Grounding," IEEE Transactions on Industry Application, Vol. 25, No. 6, November/December 1989, pp. 1093-1101.

IEEE Recommended Practice for Powering and Grounding Sensitive Electronic Equipment, The Emerald Book, IEEE Std., 1100-1992.

T.M. Gruzs, "The How’s and Why’s of Isolated Grounding," Seventh International Power Quality Conference, Intertec International, October 1993, pp. 685-698.
 

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