Effective Approaches to Concentric
Neutral/Shield Corrosion
Assessment
Ben Lanz
Director, Applications Engineering
Pr o b le m So lvin g
Concentric Neutral/Shield Assessment
2
T h e Pu r p o s e o f t h e
N e u t r al/Sh ield
Concentric Neutral/Shield Assessment
3
Why Assess the
N e u t r al?
Concentric Neutral/Shield Assessment
4
R O O T C A U SES
Concentric Neutral/Shield Assessment
Foreign
Objects
Cutback
Issues
Contaminants
Stress
Enhancements
Water Trees
Electrical
Trees
Insulation
Voids
Cable
System
Failures
Semicon
Damage /
Chem Attack
Overheating
Voids
Insulation Cuts
Lack of Void
Filler
(e.g.:Grease)
Incorrect
Dimensions
How common is neutral
corrosion?
5
A s s e s smen t
Me t h o d s
Concentric Neutral/Shield Assessment
• AC or DC Resistance
• Time Domain Reflectometry (TDR)
6
A s s e s smen t
Me t h o d s
Concentric Neutral/Shield Assessment
7
A s s e s smen t
Me t h o d s
Concentric Neutral/Shield Assessment
Comparison Parameter
Test Method
HR TDR
AC
& BSIR
Resistance
Can be performed online
No
Yes
Location of corrosion/breakage
Yes
No
Connection issues detected
Yes
No
Verifies equipment function
Yes
No
Impervious to neutral currents
Yes
No
Detects thinned/altered geometry
Yes
No
Smallest neutral loss likely detected
25%
60%
8
A SSET MG MT.
A PPR O A C H ES
Concentric Neutral/Shield Assessment
40
Reactive
Failures/100miles/year
35
30
25
Proactive Insufficient
20
15
Proactive Costly
10
Optimal
5
0
1
2
3
4
5
6
7
8
9
10
Year
9
Ex a mp le s o f
C o r r o s io n D a ma g e
Concentric Neutral/Shield Assessment
Drain wires likely broken by corrosion and through fault current
10
Ex a mp le s o f
C o r r o s io n D a ma g e
Concentric Neutral/Shield Assessment
Broken neutral wires cause pitting
11
Ex a mp le s o f
C o r r o s io n D a ma g e
Concentric Neutral/Shield Assessment
Reduced neutral feeder cable
after multiple recloser events
and through faults
12
Ex a mp le s o f
C o r r o s io n D a ma g e
Concentric Neutral/Shield Assessment
Poor contact with semicon leads to pitting
13
Ex a mp le s o f
C o r r o s io n D a ma g e
Concentric Neutral/Shield Assessment
Tape shield corroded at overlap yields pitting
14
Su mma r y
Concentric Neutral/Shield Assessment
• Neutral corrosion is discrete and is likely < a 5% issue
– Select target population
– Consider ROI of assessing the insulation of the cable and
accessories in a capital program
– Perform effective assessments
– Act based on performance data
• Avoid reclosing and ‘thumping’ cable systems
• Protect system with surge arresters –especially open pts.
• Specify
• Sufficient concentric wire designs (no tin) e.g. ≤1/6th for 1000kcmil
• Crimped or shear bolt neutral connectors- high current apps.
• Jacketed cable with water blocking capability
15