FAILURE MODES AND PRACTICAL APPLICATION

CONNECTOR THEORY

Your Presenter:Arnie CobbProduct ManagerDistribution 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 83600 cu alloy)

Aluminum = 37% IACS (typical cast 356-T6)

CONDUCTIVITY, Bronze vs AluminumWhich 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 BoltsAluminum-to-Aluminum connections

FAILURE MECHANISMS

• Oxidation and Corrosion

• Creep and Stress Relaxation

• Thermal Expansion & Ratcheting

OXIDATION

4Al(s) + 3O2(g) 2Al2O3(s)

o Occurs in the presence of oxygeno Forms a high-resistance coating on metallic surfaceo Oxides 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.

o Oxide 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.Metal

Temperaturedegrees C

Aluminum Alloys 200Titanium Alloys 325Low-alloy Steels 375High-temperature Steels 550Nickel and Cobalt superalloys 650Refractory Metals (Tungsten, Molybdenum) 1,000

Source: Askland, Donald, The Science and Engineering of Materials

Approximate Temperatures at WhichCreep Becomes Pronounced

THERMAL EXPANSION AND THERMAL RATCHETING

MaterialThermal Expansion

Coefficient (oC-1)Aluminum 2.5 x 10-5

Copper 1.7 x 10-5

Stainless Steel 1.1 x 10-5

• 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 RATCHETINGFACT or MYTH?

OVER TORQUE

A LEADING FAILURE WE SEE ONRETURNED GOODS

Is over torque worse than over torque?

Cross Section of ½-13 Bolt

2024-T4 Aluminum Bolts43 KSI Yield @ 50C = 6098 lb!

Excessive Torque will exceed yield strength and accelerate creep ultimately providing less clamping force.

BOLT ELONGATION

Reused Bolts - Clamping Force Loss

Clamp Force Reduction with Bolt Reuse 12mm (0.472 in) Steel Bolts

4182

2945

2473

2203 2158 2113 2113 2023 20911978

0

500

1000

1500

2000

2500

3000

3500

4000

4500

1 2 3 4 5 6 7 8 9 10

Reuse #

Forc

e (lb

)

Fastener Technology International / October 2005

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

Questions?