Connector Theory - Failure Modes and Practical Applications

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29 Νοε 2013 (πριν από 3 χρόνια και 10 μήνες)

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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 83600 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

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

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

Material
Thermal Expansion
Coefficient (
o
C
-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 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 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
2091
1978
0
500
1000
1500
2000
2500
3000
3500
4000
4500
1
2
3
4
5
6
7
8
9
10
Reuse #
Force (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?