Most electronic hardware demands power in short bursts. These bursts create voltage spikes that can be seen between the power conductors. These spikes are the demand for power taken from the fields that exist between the power conductors. Power line filters in the hardware can supply power on a momentary basis. This limits the extent of the spikes that propagate as fields towards the source of power.

Isolated grounds (a form of equipment grounding permitted by code) add unwanted loop area to every power filter circuit. This reduces the effectivity of every power line filter and in turn increases the interference field in the facility. For this reason isolated grounding is not recommended for facilities.

The Ground Plane

A ground plane is any conducting surface. Commercial ground planes are often made from a lattice of stringers supported by stanchions. This mechanical arrangement allows the plenum chamber that is formed to be used for air flow and cable routing. This floor arrangement is often used for supporting racks of electronic hardware.

A ground plane functions by limiting the nature of the electromagnetic field at its surface. Because the electric field can only be vertical at the surface of a ground plane, cables routed on a conducting surface do not couple well to any interfering field. Unfortunately cables are usually routed on the cement floor several feet below the ground plane and field coupling is not controlled. This negates the effectivity of the ground plane.

To be a part of the ground plane, the racks housing electronic hardware should be correctly bonded to the ground plane. This bonding must use a wide conducting path that follows the cables that exit or enter each rack. Because this approach is difficult to implement, a ground plane is often constructed on the floor where the cables rest. This plane can be formed from sheets of steel or by using a bonded wire mesh. This approach is a lot less expensive then a commercial ground plane.

For several reasons the rebars used in the concrete are not acceptable as a ground plane: the rebars may not be bonded at every crossing; the rebars may not be grounded and bonded to the grounding electrode system at many points; and there is no way to connect and extend the ground plane to the racks above the rebars.

Just because rebars are present does not imply that a useable ground plane is provided. The rebars that are in nearby concrete pose no difficulty. An effective ground plane is visible, can accept multiple electrical connections, is an integral part of an installation, and is a part of the grounding electrode system.

Ground planes should never be single-point grounded. They should be multiply connected to building steel and to every grounded conductor that enters the area. The idea is to distribute interference currents over a wide surface area to limit field intensity. For example the surface resistance of a metal surface is typically microhms per square inch. For a single-point ground, a lightning pulse would see ohms. A 10,000 ampere pulse of lightning would melt such a single point connection. The resulting voltages and arcing could do damage to hardware and the facility.

The equipment grounding conductors in a facility form a pseudo ground plane. This crisscross of conductors exists near all hardware. Even when an intentional ground plane is not provided this structure provides some benefit. The use of isolated grounds removes this pseudo ground plane usually with negative results.

Isolation

The term isolation has no single meaning. The term electrical isolation is no better. The term applied to a transformer implies shielding but the use of the shields is undefined. There are isolation transformers on the market but the user is left with the problem of how to use the shields. In most cases adding an isolation transformer after the fact is usually ineffective in reducing facility interference.

The transformers used in hardware provide isolation by avoiding direct connections to the power conductors. Capacitances between windings still allow power currents and interference currents to flow in the hardware connections. Shielded transformers are rarely used in hardware design as circuit techniques are available that limit this current flow.

When many pieces of hardware are used together, the parallel connection to the power line can introduce problems. The interference that results can often be traced to a neutral voltage drop. This problem can be limited by using a separately derived source of power. The National Electric Code allows the use of a separate distribution transformer that is regrounded at its secondary. This regrounding allows the new neutral conductor to be short. Further, the current in the new neutral is limited to the local load. The neutral ground must be a part of the grounding electrode system for the facility. This separately derived power source should be a part of the initial electrical installation.

Power centers are available on the market that provide separately derived power using an integral transformer. These power centers provide shielding, filters, breakers, and surge protection. The Code permits these centers to be mounted directly on a locally prepared ground plane.

Conclusion

There are many ways to design the way power is distributed in a facility. There are methods available that reduce the interference that results from operating hardware. Retrofitting a poorly designed facility can be very expensive. This series of articles discussed such topics as power routing, isolated grounds, the use of separately derived power, and ground planes. The reason these approaches are effective rests directly on simple physical principles.

By Ralph Morrison

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