FirePRA:Prior HEAF Guidance
Prior HEAF Fire Ignition Sources, Counting Guidance, and Ignition Frequencies
| Bin | Plant Location | Ignition Source | Description | Count (how) | Counting Reference | Fire Ignition Frequency (Mean) | Fire Ignition Frequency Reference | |
|---|---|---|---|---|---|---|---|---|
| 16.a | Plant-Wide Components | High Energy Arcing Faults - Low Voltage Electrical Cabinets (480-1000 V) | High-energy arcing faults are associated with switchgear and load centers operating between 480 and 1000 Volts. For this bin, similar to electrical cabinets, the vertical segments of the switchgear and load centers should be counted. | Each vertical segment of the switchgear and load center for low voltage (480-1000 V) electrical cabinets is counted separately. MCCs are not included, unless the MCC is associated with switchgear that is used directly to operate equipment such as load centers. | EPRI 1011989 / NUREG/CR-6850 | 1.52E-04 | EPRI 3002002936 (NUREG-2169) | |
| 16.b | Plant-Wide Components | High Energy Arcing Faults - Medium Voltage Electrical Cabinets (>1000 V) | High-energy arcing faults are associated with switchgear and load centers. Switchyard transformers and isolation phase buses are not part of this bin. For this bin, similar to electrical cabinets, the vertical segments of the switchgear and load centers should be counted. Additionally, to cover potential explosive failure of oil filled transformers (those transformers that are associated with 4.16 or 6.9kV switchgear and load centers) may be included in vertical segment counts of the switchgear. | Each vertical segment of the switchgear and load center for medium voltage (above 1000 V) electrical cabinets is counted separately. | EPRI 1011989 / NUREG/CR-6850 | 2.13E-03 | EPRI 3002002936 (NUREG-2169) | |
| 16.1 | Plant-Wide Components | HEAF for segmented bus ducts | A bus duct where the bus bars are made up of multiple sections bolted together at regular intervals (transition points). Here, the bus bars are contained within open-ended sections of metal covers that are bolted together to form a continuous grounded enclosure running the full distance between termination points.
Segmented bus ducts are able to accommodate tap connections to supply multiple equipment termination points. – Segmented bus ducts tend to be longer in comparison to the nonsegmented bus ducts. Segmented bus ducts are used in cases where the required lengths and/or geometries make the use of nonsegmented bus ducts impractical. – The length of each segment may vary depending on supplier and installation details. – Segmented bus ducts tend to connect end devices that are remote from each other. Example: A segmented bus duct might be used to connect an oil-filled transformer located in an outdoor area to equipment (e.g., switchgear) located inside the plant buildings. Note: This bin does not cover nonsegmented or continuous bus ducts or cable ducts. The arc faults for these two categories are inherently included in the treatment of the end device, and no further treatment is needed. |
The analyst will need to choose between one of two recommended practices for counting segmented bus ducts as a fire ignition source. The choice will be dependent on whether or not the transition points can be identified based on an external visual inspection of the bus duct.
Counting approach 1: If the transition points along the length of the segmented bus duct can be identified by external visual inspection, or based on plant electrical construction drawings, then count the total number of transition points. Note that transition point counting excludes the bus end termination points, which are considered a part of the end device for fire frequency purposes. Transition points may be identifiable based on visual observation or review of design drawings. Transition points for the bus bars may, or may not, correspond to junctions in the outer ducting that surrounds the bus bars. It is not intended that the protective duct be removed to identify transition points. However, industry feedback indicates that the joints or junctions in the outer ducting surrounding a bus duct cannot be assumed to correspond to junctions in the bus bars themselves without confirmation. A representative sample of plant applications should be inspected to ensure that the internal bus bar transition points and external duct junctions do in fact align with each other. Once the total count of transition points has been obtained, the plant-wide fire frequency is then partitioned to a specific location based on the number of transition points in the location of interest divided by the total number of transition points for the entire plant. Counting approach 2: If the transition points cannot be identified based on external visual inspection, or by plant electrical construction drawings, then the partitioning of fire frequency to a specific fire scenario is based on apportioning of the fire frequency equally along the length of the bus duct. Hence, the analysis must estimate the total length of segmented bus duct present in the plant under analysis. A “per linear foot” fire frequency can then be estimated by dividing the plant-wide fire frequency by the total length of segmented bus duct in the plant. That is, the fire frequency for a given fire scenario would be based on the ratio of the length of duct for which identified targets fall within the bus duct arc fault zone of influence to the total length of bus duct in the plant. A lower limit to the assumed fire frequency for any given fire scenario is also applied. That is, if the length of bus duct for which the identified target(s) fall within the zone of influence is less than 12 linear feet, then a minimum length of 12 feet should be assumed. This lower bound is based on the assumption that, lacking specific information on segment lengths, a nominal segment length of 12 feet should be assumed. Any single scenario is then assigned a fire frequency equivalent to that associated with one bus bar segment 12 feet in length (i.e., equivalent to one nominal transition point). |
FAQ 07-0035, Section 7 of Supplement 1 | 1.10E-03 | EPRI 3002002936 (NUREG-2169) | |
| 16.2 | Plant-Wide Components | HEAF for iso-phase bus ducts | A bus duct where the bus bars for each phase are separately enclosed in their own protective housing. The use of iso-phase buses is generally limited to the bus work connecting the main generator to the main transformer. | There should generally be one iso-phase bus per unit (an iso-phase bus includes all three phases). If there is more than one iso-phase bus, simply count the total number of iso-phases buses per unit. For individual fire scenarios, the plant-wide frequency is applied (i.e. partitioned) equally to each end of each iso-phase bus duct counted. | FAQ 07-0035, Section 7 of Supplement 1 | 5.91E-04 | EPRI 3002002936 (NUREG-2169) |
