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Data Quality and Grounding Considerations in a Mixed-Use Facility Marcus O Durham, PhD, PE THEWAY Corp / U Tulsa Tulsa, OK Robert A Durham, PE D2 Tech Solutions, Inc. Tulsa, OK Abstract Facilities: Problems: Issues: Systems: Attention: depend heavily on networks signal quality & noise electrical safety & code eleven different grounds bonding, isolate earth & power Gordian Knot All for one Power co, Telco, Computer group Install for parochial interest Interferes w/ sensitive network Traditional power & personnel safety Computer & Communication Lower levels & higher freq Grounding – Do Not connect all together Types of Ground Terms Grounding metal path, call ground metal & earth Grounded current carrying neutral, common Shielding metal around signal intercept noise Bonding possible energized between metal Transient equal potential grid lightning, etc Application Requirements of systems quite different Systems power, comm, data, EMI & RFI Ground convenient common pt Bonding eliminate potential Arrangement limit circulating currents Susceptible & Cause Most susceptible systems are often cause • Switched mode power supply (SMPS) • On sensitive electronics Non-linear load causes harmonics Neutral noise & poor power quality Fix • 12 pulse • Isolation transformer National Electric Code “ The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity.” Article 90 National Electric Code Article 250 >25 pages Other pages grounding neutral, transients, specifics National Electric Code Concern Allows 25 Ω power ground On 120 volt circuit I = 120 V / 25 Ω = 4.8 A Will not trip 20A Shock at 6 mA Target 1 Ω July August Single point Accessible Insulated from other metal Bond back to here Reference point for all systems Reference Eleven Grounding Systems 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. Building structural steel Electrical enclosure grounding conductor Transformer wye common Neutral at main Transient protection gnd Telephone reference gnd Data (dc) power common Back-up battery common Shielding Isolated (ac) power gnd Network equi-potential reference gnd Ground Design 11 Unique points Shows complexity of EE – bond & isolate All insulated to prevent shorting except enclosure Conductor size fault current inductance w/ transient freq Conduit bonds & isolation Bond non-electrical metal w/in 6’ EE Neutral White, current carrying Ground at main disconnect ONLY Isolate at sub-panels • if not, circulating current in ground Harmonic analysis • affects size of neutral • may need filter or isolation xformer Transformer main distribution Hot H Neutral N Ground G wye bond single point Communications Includes instrumentation & networks More sensitive to noise, transients, shifts Both analog & digital Isolated from power ground Special considerations • cable tray & racks • isolated earth • isolated power Ground Noise Much of noise introduced from ground System imbalance contributes Harmonics travel through grounding conductors Isolated ground reduces noise Uncontrolled Ground Paths 3 sources of electrical connection • ac power • cable tray • building metal – not grounding conductor Separate grounds All wire insulated Equipment Room Grounding Star design radial For small close One-point Bus bar, visible, insulated Rat race ring For larger area Equi-potential Ground 2+ locations More Robust Isolated insulated conductors around room Connect to isolated triad @ 2 pts One rack – single ground conductor RACK 1 RACK 2 POWER SUPPLY 24 VDC + 24 - COM CHASSIS CHASSIS RADIO 1 TELE 1 RADIO 2 TELE 2 RADIO TERMINATION CONNECTION GROUND ROD SURGE PROTECTOR TELEPHONE TERMINATION Racks and Raceways 1. Insulated conductor 2. Conductor along each tray 3. Connect each rack – 2 locations Remove paint Fix connector & bolt 4. Each chassis bond to rack Jumper or strip 5. Crimp connectors & reinsulate 6. NEVER allow wire to touch metal Isolated Earth Ground Triad earth w/ 3 rods Space 2.2 * length 6’ separation from metal Connect w/ uninsulated copper Exothermic weld connections ACME CONCRETE Ring Bus Isolated Earth Ground Run #1 corner to equipment room Run another different route Run #3 corner to reference ground remove if interference per NEC Green insulated wire Continuous - no underground splice Bends – minimize Z > 8” radius Minimize # to reduce L Buffer dissimilar metals to reduce R Isolated Power Not always req’d All electrical power & ground isolate from external metal 1:1 transformer Connect w/ pvc – provides isolation No building ground here New distribution panel on insulated sheet Transient protection Use isolated ground Receptacles w/in racks PVC Main Power H Power N Ground PVC Isolated Distribution Power H Power N Ground Cathodic Protection CP applied to some metal in earth Provides low Z Path to earth Cannot bond to other metal – would short CP – If necessary to bond, use resistor • prevents use as fault path • better than open circuit Has inherent personnel protection Conclusions Effective data quality & grounding considers neutral power bonding shielding transient Ground path < 1Ω Conclusions Special consideration grounding enclosure – rack equi-potential earth ground consider isolated electrical Analog & digital separate Conclusions Reference ground for interconnection of structure enclosure wye neutral transient telephone data battery shielding isolated equi-potential QUESTIONS ?
