Knomentous Customer Login Cable Tracking System
Menu

Neutral Corrosion – How Much Is Too Much?

July 7, 2010

Q:

I am looking for some guidance on what percent of bare concentric neutral corrosion can be tolerated on an underground primary cable before it needs to be replaced. I wondered if the technical staff at Novinium happens to have any ideas as to where I may get some information on this subject.

A:

Novinium has access to millions of feet of records of treated cable. They have analyzed over 70 million feet of rejuvenated cable that had been scanned with a time-domain reflectometer (TDR). The incidence of neutral corrosion is way less than many suppose. In an August 1996 article, “Neutral Corrosion Problem Overstated” in Transmission & Distribution World, Bob Gurniak of Pennsylvania Power & Light (PP&L) described this overstatement using data from the AEIC Cable Report and IEEE ICC Task Force on Cable Neutral Corrosion (6-21). There are two notable exceptions in North America … the Appalachian Mountain Region (in PP&L territory) and Wisconsin suffer more than the normal amount of corrosion because of the low soil electrical conductivity in those regions. The T&D article is available at …

http://tdworld.com/mag/power_neutral_corrosion_problem/index.html

… without the Table and Figures in the printed version. I have recreated that table below and provided similar Figure 1 and Figure 2 illustrations.

1986

1987

1988

1989

1990

1991

1992

1993

AEIC Cable Report
Failures

3,363

3,299

3,195

3,697

3,277

3,427

Reported Mileage

47,804

49,078

54,399

54,790

50,854

51,876

Failure Rate

7.0

6.7

5.9

6.9

6.4

6.6

IEEE ICC Task Force 6-21 Cable Neutral Corrosion
Neutral problems

612

468

552

539

154

209

94

Reported Mileage

89,949

78,494

80,225

78,346

62,475

58,960

32,100

Failure Rate

0.68

0.60

0.69

0.69

0.25

0.35

0.29

Ratio

9.9

9.9

10.0

9.4

26.8

 

Fig. 1. Technician Analyzes the TDR readout to pinpoint bad sections of cable.

Fig. 2. Waveform from the TDR.

As a rule of thumb, most circuit owners with 100% neutrals accept up to 50% local loss of neutrals. What I mean by local loss is that neutral corrosion is almost always limited to just a few feet as shown in the photograph above. The purposes of the neutral, enumerated in Section 4 of IEEE 1617-2007 (Guide for Detection, Mitigation, and Control of Concentric Neutral Corrosion in Medium-Voltage Underground Cables) are not compromised.

The reason for the locality of typical neutral corrosion is that the predominant cause of concentric neutral corrosion is differential aeration which is an inherently local phenomenon. See Section 6.4 of IEEE 1617-2007.