Grounding systems for I&C and Computer/Data Processing systems and equipment shall be provided to minimize damage to equipment, interference with equipment operation or signal processing, and shock or other electrical hazards to personnel.
Guidance: Grounding systems should be designed to meet the following major goals:
Provide for personnel and equipment protection and life-safety required by various regulatory agencies.
Maintain all equipment and circuits at the same reference ground potential.
Provide a safe, high ampacity fault return path for those power distribution systems that have the source or generating system referenced to ground.
Maintain a low inductive loop area between the power distribution system and the fault return path for equipment that has a potential for high fault currents.
Provide a low impedance leakage path for any static charge that may accumulate on equipment.
Provide a low impedance discharge path for energy storage devices such as capacitors and inductors that are installed for the suppression of high voltage transients or electrical noise.
Minimize noise interference in instrumentation systems by providing common reference planes of low relative impedance between devices, circuits, and systems.
Assure that all ground system conductors that must carry high frequency signals (greater than 10 kHz) are selected for low inductance characteristics. At 1 Megahertz, the impedance of an average length ground conductor is around 4,000 ohms.
Conductive enclosures that contain I&C and computer/data processing system components shall be appropriately connected to ground to ensure that shock hazard risks are minimized for personnel.
Guidance: The connection should provide a low resistance path to ground for any fault currents that may be produced by mechanical failures, insulation failures, component failures, accidents, etc. Low resistance paths to ground maintain low potential differences between metal components and reduce the chances of a fault-induced current flowing through personnel in contact with system components. Grounding is especially important in an environment where conductive elements may be present in the flooring, piping, ductwork, or other equipment.
The grounding of I&C and computer/data processing systems shall provide protection against self or adjacent equipment generated or induced electrical noise.
Guidance: The following information provides insight on potential sources of electrical noise, its effects on I&C and/or computer/data processing systems, and the application of proper corrective grounding techniques:
Computer/control and data processing systems utilize high speed, low level switched signals for operation. At the high frequencies at which these systems operate, electrical noise will propagate, traveling between two conductors or between an insulated ground conductor and other grounds or metallic components in the area. It is important that the system ground be connected in such a way that it does not act as part of a transmission line to couple noise into the computer system. This can be avoided by keeping this ground very short, tying directly to the reference ground plane or ground node, or by insuring that only one conductor is connected to the system and all other signals enter on fiber optics.
Noise can be avoided by segregating equipment that generates electrical noise from computer circuitry. Relatively small amounts of high frequency electrical noise can disrupt computer operation and cause downtime, loss of function, or spurious equipment operations.
When using LAN’s, such as Ethernet, and low frequency noise is encountered, the loop may be broken by installing ground isolation devices in the communication network at each node. The ground isolation device will appear as a high pass filter inserted in the communication link. Ensure ground isolation of the communications network at each node.
All connections in signal cable should consider possible noise coupling points and should be made carefully with special consideration given to the shield connection. Anytime the shield of a coax cable is broken a coupling path is created for high frequency noise from the outside environment to enter the inside environment of the coax cable shield.
The biggest contributor to signal inaccuracy is noise injected into input/output signals. The best way to minimize this noise is through proper grounding and wiring methods of the I/O signal hook-up. IEEE Standard 1050 should be used as a reference on shielding and grounding for instrumentation cables.
For control and computer/data processing communications protocols that utilize nonisolated systems to transfer data (RS232, RS422, RS423, etc.), the Data Terminal and Communication equipment shall be powered and grounded by the same source as the device providing the signal to prevent ground loops. Peripherals connected to optically isolated communications can be grounded to any grounding system of adequate integrity.
E. Facility grounding systems shall be evaluated to ensure the system is adequate for the applicable I&C and/or computer/data processing system and equipment.
Guidance: Large inductive electrical loads cause electrical noise on all conductors in the vicinity and a typical facility ground may have loops that will pick up very large noise voltages. The inadvertent connection of a computer system across such a loop may couple large noise signals into the computer system. To avoid the inadvertent second connection to facility ground, it may be preferable to run a separate ground node for the computer system. This ground node should still tie to the facility ground at a single point for safety reasons. The facility ground system should be evaluated to determine if the network impedance is suitable for a proper ground system. If it is not, then it will be necessary to install a new ground system network that is connected to earth at the same point as the facility ground. Grounding methods should be in accordance with IEEE Standard 142, which complements the NEC.
For I&C and computer/data processing distributed systems, grounding conductor runs over 250 feet shall be avoided. If conductor runs over 250 feet are necessary, a new single point ground node shall be created for all equipment that is located within the 250 foot run limit and connected to the single point earth ground for the facility/system.
Guidance: It is possible to treat different system nodes as essentially separate systems as far as grounding is concerned. This adheres to the distributed ground concept in IEEE 1050. Every effort should be made to ground equipment that may communicate in any way to the same earth ground. If more than one piece of equipment is tied to separate earth grounds, the earth currents will create a potential difference between the equipment. A lightning strike or power fault in the vicinity can create hazardous potentials between earth grounds. When distances from a system or equipment to the nearest node become excessive, a new node should be created.
Note: As the frequency increases, the impedance of the ground conductor increases. At 10 Megahertz, the impedance of a typical ground conductor may be in the order of 40,000 ohms and will no longer serve the purpose of providing a common reference point. Where high frequency grounds or connections are required, conductor shape and length must be selected for low inductance (impedance).
Guidance: The codes, standards and guidelines identified in this section provide grounding practices that should be consistent with most equipment manufacturer requirements. However, these codes, standards and guidelines should be used in conjunction with the manufacturer’s computer control and data processing systems grounding recommendations. The manufacturer’s grounding specifications should be reviewed for consistency with relevant standards and industry practices. Grounding schemes requiring a dedicated ground conductor routed separately to special earth points would not be acceptable. The I&C and/or computer/data processing system design and installation should be in compliance with the applicable portions of the National Electric Code. Safety takes precedence over potentially conflicting considerations.