Prioritization of Cables for Assessment and Testing

The size and nature of the cable population and the nature of the adverse environments and service conditions must be considered. Cables with the greatest safety and operational

importance that are subjected to adverse conditions should be given the highest priority. Early-generation cables (those manufactured before 1975 to 1977 having XLPE, butyl, or black EPR insulation) should be given higher priority. Later-generation cable having silane-treated clay or

Cable Aging Management Process

discharge-resistant insulation (cables produced by Kerite) could have lower priority. Normally energized cables having limiting conditions for operations, such as those connecting startup and auxiliary transformers to busses, should have high priority, as should safety-related cables, especially those that are continuously energized.

The prioritization method should include consideration for criticality of the connected

components, as defined by INPO AP-913, Equipment Reliability Process Description, [99] and risk significance, as defined by 10CFR50.65 (Maintenance Rule) [97]. Cables that are not Maintenance Rule risk-significant can be given low priority. However, care must be taken that protective relaying and circuit breakers associated with non-risk-significant cables and loads are included in plant maintenance and test programs to ensure that they function when called on should a non-risk-significant cable fail. Prioritization could be further associated with

environmental stressors (wetted environment, elevated temperature) and their severity, the cable and accessory design (water impervious, insulation type, and so on), site-specific and industry operating experience, and duty cycle (normally energized and loaded, normally energized and unloaded, periodically energized, or normally de-energized).

Weighting factors can be assigned for each of the factors that plant personnel consider important to longevity. The summation of the weighting factors would indicate the relative priority of cables with respect to aging. This prioritization could be used as a method to prioritize cable testing, refurbishment, and replacements.

9.2.1 Risk Ranking Methodology

This section provides a method that could be used for prioritization. It is an example, not a requirement.

9.2.1.1 Maintenance Rule and Criticality Screening

Using screening criteria from Maintenance Rule scoping, before applying significance factors and weighting factors, will reduce the number of cables that must be analyzed. The screening criteria identify cable criticality based on the criticality of the source or the load. This is important because a cable failure can result in loss of load and sometimes the source. If a non-safeguard load, especially one that is noncritical or run to failure, could trip a non-safeguard feeder circuit that is critical, the cable should be considered critical. Criticality screening should include Maintenance Rule scope and risk-significance classification if criticality scoping does not.

Cables that feed or are fed by components with a risk-significant designation should be of equal risk rank as critical components. The highest risk factor would be those that are critical and risk significant, followed by those either critical or risk significant, then noncritical, and then run to failure.

Cable Aging Management Process

9.2.1.2 Insulation Type

Insulation type significance is based to some degree on operating history and known

manufacturing process improvements that have occurred over time. This combination is evident in that XLPE insulation manufactured before 1975 and black EPR manufactured around the same time should have a high significance factor with respect to aging [5]. Butyl rubber and cables having a compact design such as UniShield should be ranked high. Pink, brown, and gray EPRs would be lower-significance insulation types. TR-XLPE cables have not accumulated sufficient operating time but are likely to have significance factors similar to those of pink EPR.

9.2.1.3 Significance Factor for Jacket Types or Water-Impervious Designs

Cables with sealed metal jackets (lead or aluminum) or having a water-impervious design used in newer cables with sealed, corrugated metallic shields are less susceptible to water degradation compared to other types of cables. Most jacket materials provide some barrier to water

absorption, but they do not stop water migration. Lead-sheathed cable was used in few plants.

Sealed, corrugated metallic shields have just started to be used in some applications starting in about 2005.

9.2.1.4 Significance Factor for Operating Experience

A higher significance factor should be applied to cables of the same type in similar conditions as those that have had a failure or several cable failures in other plants.

9.2.1.5 Significance Factor for Diagnostic Test Results

Cables that have diagnostic test results indicating that they are in an aged but not severely degraded state should be given higher priority (reduction of the period between tests). Cables that have not been tested and are in an adverse environment so that their condition is unknown should be given higher priority than cables that have already been tested and found to be

acceptable. Cables that have or have not been tested but are in acceptable dry locations should be valued at the lowest significance but should be assessed to determine whether they are exposed to adverse thermal, radiation, chemical, or oil environments.

9.2.1.6 Significance Factor for Voltage and Insulating Level

Cables with insulation thicknesses greater than that required for the operating voltages will have lower dielectric stresses within their insulation and should be significantly less at risk than cables with the minimum required insulation thickness for the applied voltage.

9.2.1.7 Significance Factor for Operating Conditions

Cables that are wet and normally energized are of higher significance than normally de-energized cables because the conditions could lead to water-related degradation. In dry

environments, cables with high currents with respect to their ampacity could have adverse ohmic heating. For wet cables, the most rapid aging is expected to come from being energized with no load or the opposite extreme, energized with high load.

Cable Aging Management Process

9.2.1.8 Weighting Factor for Adverse Environment

Wetting has been identified as a factor in the highest percentage of operating experience failures for medium-voltage cables and should have the highest weighting factor for adverse

environments. Thermal, radiation, ohmic heating, and high-resistance connections should be afforded weight values equally, unless a specific situation is known to be excessive or in combinations (such as a known high-temperature area (>122°F [>50°C]) and high radiation (5 Mrd [50 kGy]).

In most cases, adverse environments affect specific portions of a cable run, but in the case of risk ranking, the worst-case condition that a cable circuit experiences should be used to determine the risk factor for the whole circuit. If more than one adverse environment exists, both weighting factors should be applied to the significance factor and the two results should be added together to account for the higher risk. Both electrical testing of the cable to determine the condition of the wet section and inspection of the dry section with the adverse environment or service condition will be necessary.

9.2.1.9 Weighting Factor for Current and Amperage Level

Sizing cables in nuclear power plants generally used derating factors for cables (safety-related and non-safety-related) so that ampacities were reduced by 20% or more. Reducing the ampacity to 80% results in low to moderate conductor temperatures and limited thermal aging. A

weighting factor for operating current should be applied if it is significantly greater than 80% of the conductor’s rated ampacity.