What NEC says about design constraints for grounding systemsPostedOCT 22 2014byEDVARDinENERGY AND POWERwith0 COMMENTS

What NEC saysabout designconstraints for grounding systems (on photo: Encased Grounding Electrode by CTR via Flickr)Design constraintsSeveral key designconstraintsfor grounding systems from the NEC are as follows. These are paraphrasedfrom the code text:Electrical systemsthat are grounded must be grounded in such a manner asto limit thevoltageimposed bylightning, line surges, or unintentionalcontactwith higher voltage lines and that will stabilize the voltage to earthduring normal operation /Article 250.4(A)(1)/.In other words, if a system is considered solidly groundedtheground impedancemust be low.

Figure 1 Solidly grounded system

If the system can be solidly groundedat 150 V to ground or less, it must be solidly grounded /Article 250.20(B)/.There is therefore no such system as a 120 V UngroundedDelta in use, even though such a system isphysically possible.If the system neutral carries current it must be solidly grounded /Article 250.20(B)/. This is indicative ofsingle-phase loading and is typical for a4-wire wye(figure 2 left) orcenter-tapped 4-wire delta(figure 2 right) system.

Top: 208Y/120-V, 3-phase, 4-wire wye system; Bottom: 120/240-V, 3-phase, 4-wire delta system

Certain systems are permitted, but not required, to be solidly grounded. They are listed as electric systems usedexclusivelyto supplyindustrial electricfurnacesfor melting, refining, tempering, and the like, separately derivedsystems used exclusively for rectifiers that supply only adjustable-speed industrial drives, and separatelyderived systems supplied by transformers that have a primary voltage rating less than 1000 volts provided thatcertain conditions are met /Article 250.21/.If a system 50-1000 VAC is not solidly-grounded,grounddetectorsmust be installedon the system unless thevoltage to ground is less than 120 V /Article 250.21/.Ground detectors are used to provide a visual indication, anaudiblesignal, or both, toALERT SYSTEMoperators and maintainers of a ground-fault condition in the electrical system.With notification of the ground-fault condition, rather than automatic interruption of thecircuit, the operators of the process supplied by the ungrounded system can then take the necessary steps to effect an orderly shutdown, determine where the ground fault is located in the system, and can safely perform the necessary repair.Certain systems cannot be grounded.They are listed as circuits for electric cranes operating over combustiblefibers in Class III locations as provided in Article 503.155, circuits within hazardous (classified) anesthetizinglocations and otherisolated powersystems in healthcarefacilities as provided in 517.61 and 517.160, circuitsfor equipment within electrolytic cell working zone as provided in Article 668, and secondary circuits of lightingsystems as provided in 411.5(A) /Article 250.22/. Some of the requirements for hazardous locations and healthcare facilities are covered in section XVI.For solidly-grounded systems,an unspliced main bonding jumpermust be used to connect the equipmentgrounding conductor(s) and the service disconnect enclosure to the grounded conductor within the enclosurefor each utility service disconnect /Article 250.24(B)/.

Main Bonding Jumper

For solidly-grounded systems, anunspliced system bonding jumpermust be used to connect the equipmentgrounding conductor of a separately derived system to the grounded conductor. This connection must be madeat any single point on the separately derived system from the source to the first system disconnecting means orovercurrent device /250.30(A)(1)/.A grounding connection on the load side of the main bonding or system bonding jumper on a solidly-groundedsystemis not permitted/Articles 240.24(A)(5), 250.30(A)/.Ground fault protection of equipment must be provided for solidly grounded wye electrical services, feederdisconnects on solidly-grounded wye systems, and building or structure disconnects on solidly-grounded wyesystems under the following conditions: The voltage is greater than 150 V to ground, but does not exceed 600 V phase-to-phase. The utility service, feeder, or building or structure disconnect is rated 1000 A or more. The disconnect in question does not supply a fire pump or continuous industrial process.

Articles 215.10, 230.95, 240.13Where ground fault protection is required per Article 215.10 or 230.95 for ahealth carefacility, an additional stepof ground fault protection is required in the next downstream device toward the load, with the exception ofcircuits on the load side of an essential electrical system transfer switch and between on-site generating units forthe essential electrical system and the essential electrical system transfer switches /Article 517.17/.Health care facility definition -Buildings or portions of buildings in which medical, dental, psychiatric, nursing, obstetrical, or surgical care are provided. Health care facilities include, but are not limited to, hospitals, nursing homes, limited care facilities, clinics, medical and dental offices, and ambulatory care centers, whether permanent or movable.The alternate source for an emergency or legally-required standby system is not required to have ground faultprotection. For an emergency system, ground-fault indication is required /Articles 700.26, 701.17/.

Ground fault protection

All electrical equipment, wiring, and other electrically conductive material must be installed in amannerthatcreates a permanent, low-impedance path facilitating the operation of the overcurrent device. This circuit mustbe able to safely carry the ground fault current imposed upon it. /Article 250.4(A)(5)/.The intent of thisrequirement is to allow ground fault current magnitudes to be sufficient for the ground fault protection / detectionto detect (and for ground fault protection to clear) the fault, and for a ground fault not to cause damage to thegrounding system.High-impedance grounded systems may utilized on AC systems of 480-1000 V where: Conditions of maintenance and supervision ensure that only qualified persons access the installation. Continuity of power is required. Ground detectors are installed on the system. Line-to-neutral loads are not served.

Article 250.36For systems over 1000 V: The system neutral for solidly-grounded systems may bea single point groundedormultigrounded neutral.Additional requirements for each of these arrangements apply /Article 250.184/. The system neutral derived from a grounding transformer may be used for grounding /Article 250.182/. The minimum insulation level for the neutral of a solidly-grounded system is 600 V. A bare neutral ispermissible under certain conditions /Article 250.184 (A) (1)/. Impedance grounded neutral systems may be used where conditions 1, 3, and 4 for the use of highimpedance grounding on systems 480-1000 V above are met /Article 250.186/. The neutral conductor must be identified and fully insulated with the same phase insulation as the phaseconductors /Article 250.186 (B)/. Zig-zag grounding transformersmust not be installed on the load side of any system grounding connection/Article 450.5/.When agrounding transformeris used to provide the grounding for a 3 phase 4 wire system, the groundingtransformer must not be provided with overcurrent protection independent of the main switch and common-tripovercurrent protection for the 3 phase, 4 wire system /Article 450.5 (A) (1)/. An overcurrent sensing device mustbe provided that will cause the main switch or common-trip overcurrent protection to open if the load on thegrounding transformer exceeds125% of its continuous current rating/Article 450.5 (A) (2)/.IMPORTANT NOTE! These points are not intended to be an all-inclusive reference for NEC grounding requirements. They do,however, summarize many of the major requirements.When in doubt, consult the NEC.Reference:System Grounding Bill Brown, P.E., Square D Engineering ServicesRecommended EE articles How to get rid off the harmonics in electrical system?JULY 16, 2014 6 Factors Influencing Requirements for a Good Grounding SystemJUNE 6, 2014 Instrument transformers (CTs, VTs) in the systemMAY 19, 2014 Case study Improper grounding practice of computers in buildingsMAY 14, 2014

Article Tags //electrical system, ground fault protection, ground-fault, grounded systems, grounding, grounding systems, solidly grounded,Filed Under Category //Energy and PowerAbout Author //

EdvardCsanyiEdvard-Electrical engineer, programmer and founder ofEEP. Highly specialized for design of LV high power busbar trunking (