Grounding

American Iron and Steel Institute’s Technical Session

Steel Distribution Poles - The Material of the Future Electrical Considerations - presented by -

Brian Lacoursiere, P.Eng. Product & Technical Support Manager IUSI (International Utility Structures Inc.) American Iron and Steel Institute

Electrical Considerations

Topics to be discussed: • Grounding • Lightning & BIL • Touch & Step Potential American Iron and Steel Institute

Grounding

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Grounding Objective: The NESC Handbook states that “the object of protective grounds on electric circuits or equipment is to keep some point in the electric circuit or equipment at, or as near as practical to, the potential of the earth in the vicinity.” American Iron and Steel Institute

Grounding

Designed to Achieve Two Main Criteria: • Safeguard utility employees and the public from injury • Minimize property damage caused by accidental contact with live conductors, electrical equipment, or lightning strikes American Iron and Steel Institute

Grounding Components of Typical Grounding System: • Grounding Conductors (Down-Ground) • Grounding Electrodes • Soil Resistance

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Grounding Grounding Conductors (Down-Ground): • Steel pole acts as main down-ground (saves cost of installing copper wire)

• Equivalent current-carrying capacity: CL.5=350 mcm; CL.3=400 mcm; CL.2=500 mcm

• Threaded insert provided about 5’ down from pole top to bond system neutrals, arrester leads, equipment tank grounds • Threaded insert provided just above groundline to allow connection to ground rod or other acceptable ground electrode American Iron and Steel Institute

Grounding Grounding Electrodes: Existing Electrodes (NESC Rule 94A)

• Installed for purposes other than grounding, such as metallic water pipes, and steel re-bar in concrete foundations

Made Electrodes (NESC Rule 94B) • Driven rods, metal strips and plates, wire wraps, and concrete-encased metal installed specifically for grounding American Iron and Steel Institute

Grounding Soil Resistance: • Major component of resistance in any grounding system • 25 Ω or less for single electrode • 4 or more grounds per mile of line • Directly related to soil resistivity è è

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Soil Type

Resistivity (Ω - m )

Loam Topsoil Inorganic C lay Silty C lay C lean G ravel Sand

26 33 190 800 2,585

Lightning & BIL

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Lightning & BIL

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Lightning & BIL Lightning: • A series of stroke pulses that become a constant current source once a path to ground is established • Takes the least resistance path(s) to ground (e.g., a steel pole) • During lightning conditions, steel poles typically provide a low impedance, virtually instantaneous path to ground, allowing protective devices to sense a fault quickly American Iron and Steel Institute

Lightning & BIL

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Lightning & BIL BIL: • “Basic Impulse Insulation Level” • A relative measure of a structure’s insulation withstand strength against overvoltage impulses caused primarily by lightning strikes • 300 kV BIL level is typical for distribution voltages, and is recommended by RUS • May not be the same for steel and wood American Iron and Steel Institute

Lightning & BIL Ways to Increase BIL Level (If Req’d): • Fiberglass pole top pin on center phase • Increase spacing between center phase and pole (e.g., longer crossarm) • Insulators rated above operating voltage • Add bells to suspension insulator string (e.g., at deadends, corners, angles) • Vertical (armless) construction on horizontal post insulators • MOV arresters at strategic line locations American Iron and Steel Institute

Touch & Step Potential

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Touch & Step Potential

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Touch & Step Potential Touch: • single point of contact (hand or arm) Step: • potential drop between feet • exponential decrease in potential from point where current enters ground

Mesh: • multiple contact points Transferred: • direct contact with fault current American Iron and Steel Institute

Touch & Step Potential • A steel pole line typically provides more efficient, multiple paths to ground compared to a wood pole line, which may have only a few grounded structures per mile • The touch and step potential at each individual steel pole should be reduced, as the fault current is spread over numerous paths to ground along the steel pole line American Iron and Steel Institute

Conclusions

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Conclusions • The conductivity of steel poles offers numerous benefits: • enhanced grounding capability • pole acts as main down-ground • virtually instantaneous path to ground for lightning or fault currents • reduced touch and step potentials at each structure in a steel pole line

• Steel poles will not shatter or catch fire when struck by lightning American Iron and Steel Institute

Any Questions?

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