Presentation on theme: "FAILURE MODES AND PRACTICAL APPLICATION CONNECTOR THEORY Your Presenter: Arnie Cobb Product Manager Distribution Connector Products."— Presentation transcript:
FAILURE MODES AND PRACTICAL APPLICATION CONNECTOR THEORY Your Presenter: Arnie Cobb Product Manager Distribution Connector Products
THE ENTIRE ELECTRICAL SYSTEM
And all the expensive equipment…
….depend on the integrity of power connectors to deliver valuable electric service, … and this won’t cut it!
WHY DO YOU CHOOSE A PARTICULAR CONNECTOR? Based on Application For Mechanical Strength For Conductivity Combination of Strength & Conductivity Sometimes Best Compromise For Reliability Choice Will Affect Installed Cost
MECHANICAL STRENGTH Full Tension, ANSI Class 1 (mainly auto splices & deadends) Typically 95% of Conductor RBS Some Cases 90% (guying products) Partial Tension, ANSI Class 2 (bolted, compression) 40% of Conductor RBS Minimum Tension, ANSI Class 3 (jumpers, etc) 5 % of Conductor RBS NEW TENSION CLASS (Included in ANSI C119.4, 2011) NORMAL TENSION, ANSI Class 1A (mainly bolted deadends) 60% of Conductor RBS
Bronze = 16.4% IACS (typical cast cu alloy) Aluminum = 37% IACS (typical cast 356-T6) CONDUCTIVITY, Bronze vs Aluminum Which is More Conductive ? Then why use Bronze? Mechanical strength, Expansion/Contraction, Corrosion issues
PAD-TO-PAD CONNECTIONS ACTUAL CONTACT ZONE Actual contact zone is concentrated at high pressure, low resistance, area close to bolt holes. – Only about 5% of pad surface area. Effective contact zone can be increased (resistance reduced) by using conductive hardware. The number of bolted holes more significant than total pad area.
HARDWARE Bronze Hardware Copper & Bronze Connections Conductive Hardware Provides maximum conductivity in single-metal connections. Aluminum or Bronze Hardware will have 4 – 5 times the conductivity of steel 2024-T4 Al Bolts Aluminum-to-Aluminum connections
FAILURE MECHANISMS Oxidation and Corrosion Creep and Stress Relaxation Thermal Expansion & Ratcheting
OXIDATION oOccurs in the presence of oxygen oForms a high-resistance coating on metallic surface oOxides form immediately on copper and aluminum – Thickness depends on temperature on copper – Oxide growth rate on aluminum depends on humidity and temperature – Oxide growth is self-limiting as metal oxide coating covers surface. oOxide coating protects metal structure from further attack. – Good for structural metals – Bad for electrical connections. Acts as an insulator on conductor.
OXIDATION - FACT or MYTH? Do You Need To Scrub New Conductor Prior To Installation? Does Copper Oxidize? Are You Supposed To Scrub Conductor Dry Or Through Inhibitor?
CORROSION – A BIMETALLIC ISSUE! ♦ Refers to the deterioration of a metal in the presence of water ♦ Loss of electrons to the cathode metal – Generates the release of ions into the solution – Causes the release of material from the anode metal ♦ Material with the more negative galvanic potential corrodes – Aluminum corrodes in galvanic action with copper
CORROSION / OXIDATION PREVENTION Contact Sealants and Joint Compounds Seal out both water and air Water out => No galvanic corrosion Air out => No oxidation of contact surfaces including conductor strands. Seals out contaminates in polluted atmospheres. Must remain pliable over wide temperature range Synthetic types have highest temperature stability
CORROSION - FACT or MYTH? Does Corrosion Only Happen with Bi-Metallic Connections? Which Connector On Top (aluminum or bronze) & Why? Will Tin Plating Reduce Corrosion Rate? Do You Wire Brush Tin Plated Connectors? Why or Why Not?
Base Material – Petroleum Base Original base material Good product but will tend to break down at high temperatures & evaporates leaving a dry mass – Non Petroleum Base (Veg Oil, Synthetic) Usually more stable at higher temperatures Will not harm rubber gloves or cable polymeric materials Grit Type – Non Gritted For use on Grooves/Bolted & Pads – Non Conductive Grit For use on Compression only Grit usually made from Silicon Oxide which is an Insulator Grit helps improve gripping strength Can cause threaded fasteners to gall – Conductive Grit For use on Compression, Grooves (away from bolt threads), & Pads Has a micro grit that will not cause arc points like non-conductive grit varieties on grooves and pads Grit helps improve gripping strength as well as conductivity INHIBITORS - TYPES
CREEP AND STRESS RELAXATION Applied Mechanical Stress from Clamping force… – Material may creep (elongate) and eventually fail. – Applied stress should be less than the yield strength of material. Creep is Plastic Deformation, Intensified at High Temperatures – Reduction of residual contact pressure over time.
THERMAL EXPANSION AND THERMAL RATCHETING Metals expand when exposed to heat. Different metals have different rates thermal expansion. Thermal ratcheting – Repeated process of expansion and contraction – From changes in temperature due to current load cycling.
BELLEVILLE WASHERS A Properly Sized / Designed Belleville Washer balances the Clamping Force of the Bolt, Which in our case is approximately 4400 Lbf. Split Ring Lock Washers compress at only about 350 Lbf
Are Set Screws In Current Path Susceptible to Thermal Ratcheting? Can You Over Tighten a Belleville Washer? THERMAL RATCHETING FACT or MYTH?
OVER TORQUE A LEADING FAILURE WE SEE ON RETURNED GOODS Is over torque worse than over torque?
Cross Section of ½-13 Bolt 2024-T4 Aluminum Bolts 43 KSI 50C = 6098 lb! Excessive Torque will exceed yield strength and accelerate creep ultimately providing less clamping force. BOLT ELONGATION
Reused Bolts - Clamping Force Loss
AUTOMATICS – FAILURE MODES
How close together should splices be installed? To each other? To a terminating device? (auto & compression) See anything wrong with these pictures? SPLICE INSTALLATION - FACT or MYTH?
For Mechanical Testing of Deadends, Splices, Compression… Basically All Mechanical Connectors WHAT DOES ANSI SAYS ON THE SUBJECT?
OUR RECOMMENDATIONS No closer than ANSI testing procedure specifications for exposed conductor length. A good “Rule of Thumb” is: No closer than 3X the length of the splice for 4/0 and smaller conductor. Example: Splice for #2 conductor is 12” long. Therefore no closer than 3 feet. If possible, the longer the distance the better. When ANSI test procedure is exposed conductor length is longer than rule of thumb, ANSI rules apply. Minimum tension requirement of 15% or greater (Competitors and Lineman’s Handbook agree) Splices in slack span (less than 10%) are not recommended.
Should automatic splices be installed on slack span? See anything wrong with these pictures? AUTOMATIC SPLICES ON SLACK SPAN
REVIEW Aluminum conductor should be wire brushed. Formally recommended through inhibitor, now okay dry Inhibit, Inhibit, Inhibit Non Gritted - For Use On Grooves/Bolted & Pads Non Conductive Grit – For Use On Compression Only Conductive Grit - For Use On Compression, Grooves (away from bolt threads), & Pads Torque Is An Issue. Don’t Over-Torque! Bi-metallic Connections – Aluminum On Top Remember Copper Salts Do Not Wire Brush Tin Plated Connectors. Tin Plating Is Very Thin And Is Easily Scrubbed Off Do Not Reuse hardware. Do Not Use Automatics (Splices Or Deadends) On Slack Span. Minimum Distance Between Splices = 3X The Length Of The Splice or ANSI Spec