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Is Grounding the Culprit Marcus O. Durham, PhD, PE Robert A. Durham, PhD, PE ThewayCorp.com What is the purpose of grounding? Can grounding issues cause ignition or shock? Where is a ground or return path used? What is the difference between ground, bond, & neutral? 11 1 2 12 3 4 10 8 9 Why Ground ? 7 M.O. Durham Theway Corp. 6 5 Grounding Complex Topic Critical for electrical safety Biologic Property NEC - > 28 pages NESC – Specific Requirements IEEE – Multiple Standards NFPA – Myriad Publications Significance Grounding is major element of every electrical system, but rarely understood “Hot” cause shock or ignition only with return path – complete circuit Either neutral (controlled) or ground (uncontrolled) provides return Electrical activity on neutral or ground – fault involving ground system Ground is code and legal issue Grounding System Three components Grounding Electrode Contact point with earth ○ NEC <25Ω or additional electrode required ○ NESC - < 25Ω. ○ IEEE 142 - 25Ω not satisfactory – 1-5Ω Grounding Electrode Conductor Connects grounding electrode to system Must handle fault currents – NEC size Z V 120V 6 I 20 A Grounding system Bonding Connects metal surfaces to grounding system Required between grounding electrodes NEC FPN –bond all metal even if not specified Without bond – Potential difference exists between metals Potential difference causes current flow I Vd Neutral Current carrying conductor Controlled return path for 1Φ & DC Single point connection to ground If multiple points – current flows through grounding system If poor connection – circuit current can flow Transformer through ground Entrance Load Power H Power H Neutral Neutral Power H Ground Ground Utility Ground Service Ground Water & Other Metal Stray Current Unintentional current flow through earth Three causes Neutral grounded at multiple points Fault of “hot” wire to ground Difference in potential of ground connections Multi grounded neutral allows ~60% return current in earth Allows V between metals and between metal and earth Multi Point Neutral Portion of In flows through earth Disturbances cause I flow in alternate routes Risk of shock or equipment damage In Transients Short duration disturbances Should be shunted to ground Causes Lightning Utility Operations Every On/Off of electrical circuit Transients - Lightning Lightning is natural phenomena Uncontrolled – damage Can be controlled Industry standards and practices Controlled – no or limited damage Energization Issues Three issues Energized Ground Floating Neutral Energized Neutral Energized Ground I from another circuit Caused by poor cnxns and poor ground Z Melted insulation on ground wire or jacket Energization Issues Floating Neutral Poor N connection Allows return current on alternate path Uncontrolled V levels Energized Neutral I returns on gnd (water pipe, etc.) Melted neutral or ground insulation Uncontrolled I flow Faults Three forms Loss of Insulation High Impedance Connection Breaking faults Loss of Insulation V breakdown of dielectric Heat, ejecta, loss of material Easily recognized Faults High Z connection Most common fault Misaligned contacts Poor connections Partially damaged insulation Not recognized by standard breakers Heat exceeds ignition Faults Breaking Faults Switching Pulling apart cable High frequency component Sudden increase in voltage Causes damage to insulation particularly at high inductance points (bends) Why Fail Now? A failure may not result in immediate catastrophe Catastrophic failures are the result of multiple improper conditions. Seldom do systems have serious consequences when only one component is improper. Grounding Purpose To ensure that all systems (facility and service) are operating at the same POTENTIAL reference To prevent circulating CURRENTS from developing in ground system To allow building and service protection systems to operate effectively and as designed in TIME Recap Improper grounding and bonding is frequent problem Electrical ignition and shock that is “undetermined” is often grounding Codes and standards are minimum requirements. Not following creates hazards Grounding issues are code, then legal issues. Questions?