FAILURE MODES AND
PRACTICAL APPLICATION
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

Which is More Conductive ?
Bronze = 16.4% IACS
(typical cast 83600 cu alloy)
Aluminum = 37% IACS
(typical cast 356-T6)
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
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
Bronze Hardware
Copper & Bronze Connections

FAILURE MECHANISMS

OXIDATION
4Al
(s)
+ 3O
2(g)
2Al
2
O
3(s) o Occurs in the presence of oxygen o Forms a high-resistance coating on metallic surface o 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

 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

 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
INHIBITORS - TYPES
– 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

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.
Approximate Temperatures at Which
Creep Becomes Pronounced
Metal
Aluminum Alloys
Titanium Alloys
Low-alloy Steels
High-temperature Steels
Nickel and Cobalt superalloys
Refractory Metals (Tungsten, Molybdenum)
Temperature degrees C
200
325
375
550
650
1,000
Source: Askland, Donald, The Science and Engineering of Materials

THERMAL EXPANSION AND
THERMAL RATCHETING
Material
Aluminum
Copper
Stainless Steel
Thermal Expansion
Coefficient ( o
C
-1
)
2.5 x 10
-5
1.7 x 10
-5
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

THERMAL RATCHETING
FACT or MYTH?
 Are Set Screws In Current Path
Susceptible to Thermal Ratcheting?
 Can You Over Tighten a Belleville
Washer?

A LEADING FAILURE WE SEE ON
RETURNED GOODS
Is over torque worse than over torque?

BOLT ELONGATION
Excessive Torque will exceed yield strength and accelerate creep ultimately providing less clamping force.
Cross Section of ½-13 Bolt
2024-T4 Aluminum Bolts
43 KSI Yield @ 50C = 6098 lb!

2500
2000
1500
1000
500
0
4500
4000
4182
3500
3000
1
2945
2
2473
3
Clamp Force Reduction with Bolt Reuse
12mm (0.472 in) Steel Bolts
2203
2158
2113
4 5
Reuse #
6
2113
2023
2091
1978
7 8 9 10
Fastener Technology International / October 2005

AUTOMATICS – FAILURE MODES

SPLICE INSTALLATION FACT or MYTH?
How close together should splices be installed?
To each other? To a terminating device?
(auto & compression)
See anything wrong with these pictures?

WHAT DOES ANSI SAYS ON THE SUBJECT?
For Mechanical Testing of Deadends, Splices, Compression…
Basically All Mechanical Connectors

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.

AUTOMATIC SPLICES ON SLACK SPAN
Should automatic splices be installed on slack span?
See anything wrong with these pictures?

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