Background: The current passing throughout two or more ground electrodes with a ground resistance between them, generate circulating current that could emit levels of electromagnetic energy that cause noises, interference (EMI & RFI) and affect the equipment performance and/or expose personnel to safety hazards.
Purpose of IG: Isolated Ground is intended to provide a straight and low impedance path to the source under the following conditions:
1. Steady State: Reduce electrical noise (RFI & EMI- electromagnetic interference) on the grounding circuit caused by power apparatus, electronic equipment and or harmonics generated in the system.
2. Transient: Limit the overvoltage and current surges to safe values to avoid hazardous electrical shocks and momentary noises.
Ground Reference:
In practical application, the maximum recommended total resistance should be less than 1 Ohms. Cable should be sized to contribute with less than 0.1 Ohm to the overall resistance. Cu stranded soft-drawn # 4/0 AWG isolated conductor exothermically connected to the ground grid are sufficient for many applications on the power industry such as DCS.
Supplementary ground electrode:
To reduce the IG to low values typically engineers and equipment manufacturers recommend installing three rods arranged in a triad configuration 10 ft apart and interconnected via an insulated ground conductor. The ground grid is connected to one of the ground rods while the IG conductor run to the to equipment Isolated ground bus via the test well used for maintenance and testing
CAUTION:
1.In electronic equipment using DC I/O common bus shall not be connected to the isolated grounding electrode because can created hazardous electrical potentials between the AC and DC systems.
2.IG is not recommended to be bonded
3.Enclosure of equipment is permitted to connected to the building steel frame structure in individual ground (daisy change is not recommended).
4.Use of an isolated equipment-grounding conductor does not relieve the requirement for grounding the raceway system.
CODE & STANDARS(Partial List):
NEC:
Section 250-2(d) "The earth shall not be used as the sole equipment gnd. conductor or fault current path."
Section 250-54 Do not apply that supplementary electrodes be bonded together. (See 250-50 & 250-58).
Section 250-74. Connecting receptacle (Withdrawn on NEC 99).
Section 250-96(b) permit to have an isolated ground and this should also serve as your equipment ground.
Section 250-146(d). Isolated Receptacles. Where required for the reduction of electrical noise (EMI…
(FPN: Use of an IG equipment conductor does not relieve the requirement for gnd. the raceway system).
Section 384-20: Exception IG is permitted to pass through the pnlbd w/o connected to equipment gnd. bar.
Section 517-19(g). Health caring facilities.
COMMENTS: Ground loops is not a concern for NEC since is not unsafe if grounded or not to the building. Designer can specify the reference ground to be bonded to the building electrode. If the designer elects not to bond the reference ground to the building electrode this is okay by the NEC.
IEEE:
Std 142 (GREEN BOOK) Recommended Practice for Grounding of Industrial and Commercial Power Systems.
Std 487 “Recommended Practice for the Prot.Wire-Line Communication Facilities Serving Electric Power Stations “
Std 518, “Guide for the Inst. of Elect. Eq. to Minimize Elect. Noise Inputs to Controllers from External Sources”
Std 1100 (EMERALD Book) "Recommended Practice for Powering and Grounding of Sensitive Electronic Equipment". (Section 9.10.12.6 discusses recommendation for IG circuits).
COMMENTS: IEEE specifications are more stringent than those contained in the NEC.
ANSI/TIA/EIA Std 607 “Commercial Building Gnd and Bonding Requirements for Telecommunications”
UL Std 1950. Standard for Safety for Information Technology Equipment…
FM data sheet
Suggestion: In addition to what is in the suggested references, you may encounter some specific terms for the ground you are working on, e.g. computer ground, instrument ground, quiet ground, noise-free ground, etc. as indicated on older drawings and in older project documents before some of those posted standards were created.
There are also Electromagnetic Interference (EMI) documents that you may be interested in to make sure that you do not obtain some unwelcome noises on your type of ground. EMI documents tend to be of a military or commercial origin.
