GE Mark VIe System Manual page 136

6.5.3 Signal Reference Structure
On modern equipment communicating at high bandwidths, signals are typically differential and/or isolated electrically or
optically. The modern Signal Reference Structure (SRS) system replaces the older single-point grounding system with a much
more robust system. The SRS system is also easier to install and maintain.
Note The provisions covered in this document may not apply to all installations.
The goal of the SRS is to hold the electronics at or near case potential to prevent unwanted signals from disturbing operation.
The following conditions must all be met by an SRS:
• Bonding connections to the SRS must be less than 1/20 wavelength of the highest frequency to which the equipment is
susceptible. This prevents standing waves. In modern equipment using high-frequency digital electronics, frequencies as
high as 500 MHz should be considered. This translates to about 30 mm (1 in).
• SRS must be a good high-frequency conductor. (Impedance at high frequencies consists primarily of distributed
inductance and capacitance.) Surface area is more important than cross-sectional area because of skin effect.
Conductivity is less important (steel with large surface area is better than copper with less surface area).
• SRS must consist of multiple paths. This lowers the impedance and the probability of wave reflections and resonance
In general, a good signal referencing system can be obtained with readily available components in an industrial site. All of the
items listed below can be included in an SRS:
• Metal building structural members
• Galvanized steel floor decking under concrete floors
• Woven wire steel reinforcing mesh in concrete floors
• Steel floors in pulpits and power control rooms
• Bolted grid stringers for cellular raised floors
• Steel floor decking or grating on line-mounted equipment
• Galvanized steel culvert stock
• Ferrous metallic cable tray systems
• Raceway (cableway) and raceway support systems
• Embedded steel floor channels
Connection of the Protective Earth (PE) terminal to the installation ground system must first comply with code requirements
and second provide a low-impedance path for high-frequency currents, including lightning surge currents. This grounding
conductor must not provide, either intentionally or inadvertently, a path for load current. The system should be designed so
that there is no way possible for the control system to be an attractive path for induced currents from any source. This is best
accomplished by providing a ground plane that is large and low impedance, so that the entire system remains at the same
potential. A metallic system (grid) will accomplish this much better than a system that relies upon earth for connection. At the
same time all metallic structures in the system should be effectively bonded both to the grid and to each other, so that bonding
conductors rather than control equipment become the path of choice for noise currents of all types.
In the control cabinet, the base is insulated from the chassis and bonded at one point. The grounding recommendations call for
the equipment grounding conductor to be 120 mm
point of building ground system. PE plated copper bus to Earth pit/connection point resistance should be 1 Ohm or less. The
Functional Earth (FE) is bonded at one point to the PE ground using two 25 mm
jumpers. These grounding recommendations are illustrated in the following figure.
136 GEH-6721_Vol_I_BP
2 (4/0 AWG) gauge wire of shortest possible length connected to the nearest
GEH-6721_Vol_I Mark VIe and Mark VIeS Control Systems Volume I
Public Information
2
(6 AWG or larger) green/yellow bonding