**Attachment 8**

**Guidance for Fire Non-Suppression**

**Probability Analysis**

[Task 2.7.1 - Fire Detection Analysis]{.underline}

The dominant means of fire detection may vary somewhat depending on the
type of fire postulated (e.g. hot work fires with a fire watch present),
fire location within the room (e.g., proximity to fire detectors).
Hence, the fire detection analysis is nominally conducted on a scenario
specific basis. However, in practice, the same analysis result will
apply to multiple fire scenarios:

> Example: The fire area under analysis has fixed detection installed.
> There is no finding against the fire detection system, and the system
> provides full room coverage. In this case, a fire detection time is
> calculated using the fire characteristics of a typical fire ignition
> source in the room, and using a typical fire location. The result is
> applied to all fire scenarios in the room.

The following general rules will be applied in the fire detection
analysis:

-   Prompt fire detection is assumed for two cases, and in these cases
    the time to detection is set to zero. The two cases are:

    -   Postulated hot work fires so long as a hot work fire watch is
        posted and there is no finding of degradation against the
        licensee's hot work permitting and fire watch programs, or

    -   Fires postulated in a fire area with a continuous fire watch
        posted so long as there are no findings against the fire watch
        and the point of fire origin (the fire ignition source) can be
        directly observed by the fire watch.

-   For other general fire scenarios, it will be assumed that automatic
    fire detection systems will be the first line of defense in fire
    detection. That is, if a fixed fire detection system is in place,
    the response time of that system will determine the fire detection
    time for all scenarios. This time is estimated using fire modeling
    correlations such as those included in the USNRC Fire Dynamics
    Tools, NUREG-1805.

-   For areas covered by fixed fire suppression system, but not an
    independent fixed detection system, the actuation of the suppression
    system should universally result in a fire alarm signal. Hence, for
    these cases the time to fire suppression system actuation will be
    taken as the fire detection time. This time will be adjusted to
    reflect findings against the suppression system as appropriate.

-   Barring any other means of detection, manual detection (detection by
    plant personnel) will be considered. Manual detection will apply in
    fire areas that lack fixed detection, in cases where a detection
    system is found to be highly degraded, or in cases where the
    detection signal is tied to actuation of a fixed suppression system
    and that system is found to be highly degraded. The manual detection
    time is estimated based on a qualitative evaluation of factors such
    as room occupancy, frequency of routine entry into a room, fire
    watch rotation periods when applicable, general room accessibility,
    and fire severity.

[Detection by a Continuous Fire Watch]{.underline}

In fire areas with a continuous fire watch posted, prompt detection of
fires will be generally assumed:

-   For a continuous fire watch (fire watch is maintained constantly
    within the physical room) the time to detection by the fire watch
    would be close to or at the time of ignition. No delay in detection
    is assumed:

t~detection~ = t~ignition~ = 0

Discretion is available to assess the effectiveness of a fire watch in
the context of specific fire hazards. For example, in some fire areas a
continuous fire watch posted in a particular location cannot observe the
entire fire area. In this case, the fire area may be treated as
continuously occupied for "hidden" fire ignition source scenarios and a
nominal delay time may be assigned in fire detection for these
scenarios.

[Detection by a Fixed Detection System]{.underline}

If a fire area is covered by a fixed fire detection system, but is not
covered by a continuous fire watch, then the response time of the fixed
system will be assumed to dominate the fire detection time. Fire
detection response time is estimated using the following NRR fire
analysis worksheets.

[Cross-zone Detection]{.underline}

In some circumstances, the analysis of a cross-zone fire detection
strategy is needed. In a cross-zone strategy, a minimum of two
detectors, one on each of two separate detection circuits, must actuate
to generate the desired signal. This is most common when the actuation
of an automatic fire suppression system is tied to a fire detection
signal. Common applications include: pre-action or dry-pipe fire
sprinklers, water deluge systems, water curtains, and gaseous
suppression systems.

If a cross-zone detection strategy is encountered, the total detection
time will be dominated by the detector located farthest from the fire
ignition source. Identify which detectors are assigned to each of the
fire detection circuits and locate the nearest detector in each of the
circuits. Of these two, the detection time is generally dominated by the
detector located further from the fire ignition source (radial
horizontal distance from fire center to detector location). Exceptions
include:

-   Cases where one of the detectors is located in a different beam
    pocket from the fire ignition source,

-   Cases where one detector has a slower time response that another
    (e.g., a heat detector will generally respond more slowly than a
    smoke detector).

Identify the detector that is the limiting factor in the time response
and base the actuation analysis on the time response of that detector.

[Detection by a Roving Fire Watch]{.underline}

Fire watches may be implemented by licensees either as a compensatory
measure, or as a part of routine plant operation. All fire watches, at a
minimum, provide a fire detection function. Hence, if a fire area is
covered by a roving fire watch, and is not covered by an operational
fixed fire detection system, then the fire watch recurrence frequency is
used to estimate the time to fire detection. When crediting a fire watch
with detection, the detection time is assumed to be one-half the
recurrence time. The following examples illustrate this approach:

-   For a roving fire watch on a 15 minute recurrence schedule (roving
    patrol) the time to detection by the fire watch is assumed to be ½
    the duration of the roving patrol

t~detection~ = 7.5 minutes

-   For an hourly fire watch:

t~detection~ = 30 minutes

The detection time by general plant personnel should also be checked
consistent with the discussion immediately below. The lowest detection
time dominates the process and is taken as the final estimate.

[Detection by General Plant Personnel]{.underline}

In the absence of a fixed fire detection system (or a fire detection
signal tied to actuation of a fixed fire suppression system), or given a
highly degraded fixed detection system, detection of the fire will be
assumed to occur by plant personnel. One of two factors will be
utilized:

-   If the fire area is continuously manned by plant personnel (but not
    by a fire watch) the fire detection time will be assumed to be 5
    minutes.

-   In the absence of any other means of detection, a maximum fire
    detection time of 15 minutes will be assumed.

Again, the dominant manual detection time will be the least of these
values and is taken as the final detection time. Hence, in no case
should the manual detection time be assumed to be greater than 15
minutes

[Task 2.7.2 - Fixed Fire Suppression Systems]{.underline}

General rules to be applied in the fixed suppression system analysis
include the following:

-   The actuation of a non-degraded, fully functional fixed fire
    suppression system that is deemed by the inspector to be effective
    for the fire ignition source scenario (e.g., properly positioned to
    apply suppressant to the ignition source) will be assumed to disrupt
    the development of the fire scenario. That is, if such a system
    actuates, it will be assumed the fire growth will be arrested and no
    further fire damage will occur.

-   Judgments are expected to be made as to whether or not the
    suppression system, degraded or not, will be effective against the
    specific fire threat being postulated (i.e., is the system installed
    and configured such that a fire involving each specific fire
    ignition source will be controlled given actuation of the
    suppression system?). If it is judged that the system will not be
    effective (e.g., the system provides partial coverage, and a
    specific fire ignition source is outside the coverage zone, or the
    fire source is such that the fire suppression system would likely be
    overwhelmed), then the system will not be credited on a
    scenario-specific basis (e.g., it might be credited in some
    scenarios and not in others).

-   The assessment of any fixed fire suppression system includes
    application of nominal system reliability factors (generally a
    random failure probability of 0.02 is applied). Those cases where
    the fire suppression system fails on demand, fire suppression is
    totally dependent on manual fire fighting provisions (see Task
    2.7.3). Recovery of the failed fire suppression system will not be
    considered in the SDP Phase 2 analysis.

-   If the inspection finding is against a fixed fire suppression
    system, the finding may result in allowing only partial or no credit
    to the system. The degradation may be reflected as a reduction in
    overall reliability or a delay in actuation time.

-   If the fixed fire suppression system is automatically actuated, the
    time to actuation will be calculated based on engineering
    correlations.

-   If the fixed fire suppression system is manually actuated, the time
    to actuation will be based on the estimated fire brigade response
    time, plus a nominal period of two minutes to assess the fire
    situation and actuate the system.

-   Fixed gaseous suppression systems have discharge delay timers. The
    actuation time for such systems will be the estimated time to a
    valid actuation demand signal (based on engineering correlations)
    plus the discharge delay time.

-   Gaseous fire suppression systems that are degraded due to an
    inability to maintain adequate concentration will be credited with
    providing some time delay in the progress of the fire (based on the
    demonstrated suppressant soak time that is available). However,
    manual fire response will be needed to complete fire suppression.
    See discussion on page 9-8.

-   Credit will be given to gaseous suppression systems that provide a
    multiple discharge capacity (this typically requires manual actions
    to initiate a repeated discharge).

-   There are a number of time delays that may apply to gaseous systems,
    deluge, pre-action sprinklers, or dry-pipe water systems. These
    delays must be accounted for. In general, the correlation for
    actuation time is applied. This time reflects the time that a demand
    signal is generated. The time to actual discharge is the sum of the
    time to actuation of the demand signal plus any applicable discharge
    timing delays. Delays to be considered are:

    -   For gaseous suppression systems there will be a built-in timer
        that delays discharge to allow for personnel evacuation. The
        inspector should determine this time (typically on the order of
        30 seconds to 2 minutes).

    -   There may be a delay for cross zoned detection system, i.e., the
        automatic suppression system will not begin actuation sequence
        until after the second detector is actuated. If cross-zoning is
        used, the detection time analysis should be reviewed to ensure
        that the cross-zone detection criteria are met. The time to
        generation of the actuation signal will be dominated by the
        slower detector (typically the detector farther from the fire
        ignition source).

    -   Also, there may be a time lag for suppressant to get to the
        hazard (for example, low pressure CO2 may have to travel an
        extended distance) or for pipes to fill with water prior to
        discharge (dry or preaction systems). If the delay is not known,
        use 1 minute. If it is known, the delay should typically be
        between 30 and 60 seconds.

[Task 2.7.3 - Plant Personnel and the Manual Fire Brigade 5.2.7.3Task
2.7.3: Plant Personnel and the Manual Fire Brigade]{.underline}

Fire suppression by manual fire fighting is assessed based on historical
evidence provided by fire event data. In Task 2.7.3, one of the
pre-calculated fire duration curves must be selected to be applied to
each scenario. The same curve may be used for multiple scenarios, if
appropriate, or different curves may be chosen for each scenario.

Additional analyses of the raw fire event data is neither expected nor
required as a part of the Phase 2 SDP analysis; rather, one of the
pre‑calculated curves should be applied to each fire scenarios being
analyzed based on the fire type and/or location.

In no case should an attempt be made to generate a new fire duration
curve to suit a particular analysis. The various pre‑calculated curves
for specific conditions should cover the vast majority of fire
scenarios. If none of these specific condition curves provide a
reasonable match to the conditions of the fire scenario, the \"all
events\" curve should be applied. The \"all events\" curve represents a
composite analysis of all of the events that went into each of the other
individual fire duration curves.

Note that some of the curves apply to fires in a particular location
(e.g., the main control room or containment). However, most are
applicable to particular fire ignition source scenario. The cases that
are covered by these pre‑calculated curves are:

1\. All events

2\. Hot work (welding) fires

3\. Transient fires

4\. Electrical fires

5\. Cable fires

6\. Transformer/switchyard

7\. Main Control Room

8\. Turbine Generator

9\. Energetic Arcing Faults

10.Containment fires (non-inerted containments)

The mean non-suppression probability curves for each of these fire types
are presented at the end of this attachment. The following table
presents the same information in a tabulated format. The tabulated
values of PNS~manual~ should address most situations. As an alternative,
the PNS~manual~ value can be calculated using the following formula:

![](media/image1.wmf)

Where 'λ' is the mean rate constant (1/min) for the given fire type and
't' is the fire duration time (time to damage after detection) in
minutes. The values for 'λ' for each of the ten fire type/location
categories are provided in the last row of the PNS~manual~ table.

<table style="width:100%;">
<colgroup>
<col style="width: 12%" />
<col style="width: 8%" />
<col style="width: 8%" />
<col style="width: 8%" />
<col style="width: 8%" />
<col style="width: 8%" />
<col style="width: 9%" />
<col style="width: 8%" />
<col style="width: 8%" />
<col style="width: 8%" />
<col style="width: 8%" />
</colgroup>
<tbody>
<tr class="odd">
<td colspan="11"><strong>Table A8.1 - Non-suppression Probability Values
for Manual Fire Fighting Based on Fire Duration (Time to Damage after
Detection) and Fire Type Category</strong></td>
</tr>
<tr class="even">
<td rowspan="2"><blockquote>
<p>T<sub>Damage</sub> - T<sub>Detection</sub></p>
<p>(min)</p>
</blockquote></td>
<td colspan="10"><strong>Mean manual non-suppression probability curve
values - PNS <sub>manual</sub></strong></td>
</tr>
<tr class="odd">
<td><blockquote>
<p>All Events</p>
</blockquote></td>
<td><blockquote>
<p>Hot Work - Welding</p>
</blockquote></td>
<td><blockquote>
<p>Transients</p>
</blockquote></td>
<td><blockquote>
<p>Electrical Fires</p>
</blockquote></td>
<td><blockquote>
<p>Cable Fires</p>
</blockquote></td>
<td><blockquote>
<p>Switchyard</p>
</blockquote></td>
<td><blockquote>
<p>Main Control Room</p>
</blockquote></td>
<td><blockquote>
<p>Turbine Generator</p>
</blockquote></td>
<td><blockquote>
<p>Energetic Arcing Faults</p>
</blockquote></td>
<td><blockquote>
<p>Containment</p>
</blockquote></td>
</tr>
<tr class="even">
<td>0</td>
<td>1.00</td>
<td>1.00</td>
<td>1.00</td>
<td>1.00</td>
<td>1.00</td>
<td>1.00</td>
<td>1.00</td>
<td>1.00</td>
<td>1.00</td>
<td>1.00</td>
</tr>
<tr class="odd">
<td>1</td>
<td>0.93</td>
<td>0.93</td>
<td>0.87</td>
<td>0.89</td>
<td>0.84</td>
<td>0.97</td>
<td>0.78</td>
<td>0.98</td>
<td>0.95</td>
<td>0.94</td>
</tr>
<tr class="even">
<td>2</td>
<td>0.87</td>
<td>0.86</td>
<td>0.76</td>
<td>0.79</td>
<td>0.70</td>
<td>0.95</td>
<td>0.62</td>
<td>0.96</td>
<td>0.90</td>
<td>0.89</td>
</tr>
<tr class="odd">
<td>3</td>
<td>0.81</td>
<td>0.80</td>
<td>0.66</td>
<td>0.70</td>
<td>0.59</td>
<td>0.92</td>
<td>0.48</td>
<td>0.94</td>
<td>0.86</td>
<td>0.84</td>
</tr>
<tr class="even">
<td>4</td>
<td>0.76</td>
<td>0.74</td>
<td>0.58</td>
<td>0.63</td>
<td>0.49</td>
<td>0.90</td>
<td>0.38</td>
<td>0.92</td>
<td>0.81</td>
<td>0.80</td>
</tr>
<tr class="odd">
<td>5</td>
<td>0.71</td>
<td>0.69</td>
<td>0.51</td>
<td>0.56</td>
<td>0.41</td>
<td>0.88</td>
<td>0.30</td>
<td>0.90</td>
<td>0.77</td>
<td>0.75</td>
</tr>
<tr class="even">
<td>6</td>
<td>0.66</td>
<td>0.64</td>
<td>0.44</td>
<td>0.50</td>
<td>0.35</td>
<td>0.85</td>
<td>0.23</td>
<td>0.88</td>
<td>0.74</td>
<td>0.71</td>
</tr>
<tr class="odd">
<td>7</td>
<td>0.62</td>
<td>0.59</td>
<td>0.38</td>
<td>0.44</td>
<td>0.29</td>
<td>0.83</td>
<td>0.18</td>
<td>0.86</td>
<td>0.70</td>
<td>0.67</td>
</tr>
<tr class="even">
<td>8</td>
<td>0.58</td>
<td>0.55</td>
<td>0.34</td>
<td>0.39</td>
<td>0.24</td>
<td>0.81</td>
<td>0.14</td>
<td>0.84</td>
<td>0.66</td>
<td>0.63</td>
</tr>
<tr class="odd">
<td>9</td>
<td>0.54</td>
<td>0.51</td>
<td>0.29</td>
<td>0.35</td>
<td>0.20</td>
<td>0.79</td>
<td>0.11</td>
<td>0.83</td>
<td>0.63</td>
<td>0.60</td>
</tr>
<tr class="even">
<td>10</td>
<td>0.50</td>
<td>0.47</td>
<td>0.26</td>
<td>0.31</td>
<td>0.17</td>
<td>0.77</td>
<td>0.09</td>
<td>0.81</td>
<td>0.60</td>
<td>0.57</td>
</tr>
<tr class="odd">
<td>12</td>
<td>0.44</td>
<td>0.41</td>
<td>0.19</td>
<td>0.25</td>
<td>0.12</td>
<td>0.73</td>
<td>0.05</td>
<td>0.78</td>
<td>0.54</td>
<td>0.51</td>
</tr>
<tr class="even">
<td>14</td>
<td>0.38</td>
<td>0.35</td>
<td>0.15</td>
<td>0.20</td>
<td>0.08</td>
<td>0.69</td>
<td>0.03</td>
<td>0.74</td>
<td>0.49</td>
<td>0.45</td>
</tr>
<tr class="odd">
<td>16</td>
<td>0.33</td>
<td>0.30</td>
<td>0.11</td>
<td>0.15</td>
<td>0.06</td>
<td>0.66</td>
<td>0.02</td>
<td>0.71</td>
<td>0.44</td>
<td>0.40</td>
</tr>
<tr class="even">
<td>18</td>
<td>0.29</td>
<td>0.26</td>
<td>0.09</td>
<td>0.12</td>
<td>0.042</td>
<td>0.62</td>
<td>0.013</td>
<td>0.68</td>
<td>0.40</td>
<td>0.36</td>
</tr>
<tr class="odd">
<td>20</td>
<td>0.25</td>
<td>0.22</td>
<td>0.07</td>
<td>0.10</td>
<td>0.029</td>
<td>0.59</td>
<td>0.008</td>
<td>0.65</td>
<td>0.36</td>
<td>0.32</td>
</tr>
<tr class="even">
<td>25</td>
<td>0.18</td>
<td>0.15</td>
<td>0.03</td>
<td>0.05</td>
<td>0.012</td>
<td>0.52</td>
<td>0.002</td>
<td>0.59</td>
<td>0.28</td>
<td>0.24</td>
</tr>
<tr class="odd">
<td>30</td>
<td>0.13</td>
<td>0.11</td>
<td>0.017</td>
<td>0.03</td>
<td>0.005</td>
<td>0.46</td>
<td>0.001</td>
<td>0.53</td>
<td>0.21</td>
<td>0.18</td>
</tr>
<tr class="even">
<td>35</td>
<td>0.09</td>
<td>0.07</td>
<td>0.008</td>
<td>0.017</td>
<td>0.002</td>
<td>0.40</td>
<td>*</td>
<td>0.48</td>
<td>0.17</td>
<td>0.14</td>
</tr>
<tr class="odd">
<td>40</td>
<td>0.06</td>
<td>0.05</td>
<td>0.004</td>
<td>0.009</td>
<td>0.001</td>
<td>0.35</td>
<td>*</td>
<td>0.43</td>
<td>0.13</td>
<td>0.10</td>
</tr>
<tr class="even">
<td>45</td>
<td>0.05</td>
<td>0.03</td>
<td>0.002</td>
<td>0.005</td>
<td>*</td>
<td>0.31</td>
<td>*</td>
<td>0.39</td>
<td>0.10</td>
<td>0.08</td>
</tr>
<tr class="odd">
<td>50</td>
<td>0.03</td>
<td>0.02</td>
<td>0.001</td>
<td>0.003</td>
<td>*</td>
<td>0.27</td>
<td>*</td>
<td>0.35</td>
<td>0.08</td>
<td>0.06</td>
</tr>
<tr class="even">
<td>55</td>
<td>0.02</td>
<td>0.02</td>
<td>*</td>
<td>0.002</td>
<td>*</td>
<td>0.24</td>
<td>*</td>
<td>0.31</td>
<td>0.06</td>
<td>0.04</td>
</tr>
<tr class="odd">
<td>60</td>
<td>0.02</td>
<td>0.01</td>
<td>*</td>
<td>0.001</td>
<td>*</td>
<td>0.21</td>
<td>*</td>
<td>0.28</td>
<td>0.05</td>
<td>0.03</td>
</tr>
<tr class="even">
<td colspan="11">* Value is less than 0.001. Screen using
PNS<sub>manual</sub>=0.001 or use formula to calculate actual
value.</td>
</tr>
<tr class="odd">
<td>Mean Rate Constant (1/min)</td>
<td>0.069</td>
<td>0.075</td>
<td>0.137</td>
<td>0.117</td>
<td>0.177</td>
<td>0.026</td>
<td>0.242</td>
<td>0.021</td>
<td>0.051</td>
<td>0.057</td>
</tr>
</tbody>
</table>

[Task 2.7.4 - Probability of Non-Suppression]{.underline}

The purpose of Task 2.7.4 is to estimate the overall probability of fire
suppression failure. Failure in this context means that suppression was
not achieved before the FDS of interest is reached. Fire suppression
will eventually be achieved for all fires, but if the FDS is reached
before suppression, then in the SDP context, fire suppression has failed
to prevent fire-induced damage consistent with the postulated FDS
scenario.

[Fixed fire suppression systems]{.underline}

Both the estimates of fire damage time and the time to fixed suppression
system suppressant discharge contain considerable uncertainty. Hence,
the probability that the fire suppression system suppresses the fire
prior to critical damage is not based on a simple comparison of the time
to damage versus time to suppressant discharge. Rather, a probability of
non-suppression is assigned based on the "margin" between time to damage
and time to suppressant discharge.

The time margin/likelihood relationship is described in the table below.
The first column presents the difference in minutes between the time to
damage and the time to suppressant discharge. If the two times are
close, or damage occurs before suppressant discharge, a high likelihood
of damage will be assumed (PNS approaches 1.0). If the time to
suppression is shorter than the time to damage, the PNS value decreases
reflecting a higher likelihood of suppression success. As the time
difference reaches 10 minutes, PNS approaches zero. Note that in
quantification, the likelihood that the fire suppression system fails on
demand is explicitly treated.

  ---------------------------------------------- ------------------------
  **Table A8.2 - Probability of Non-suppression  
  for Fixed Fire Suppression Systems Based on    
  the Absolute Difference Between Damage Time    
  and Suppression Time**                         

  **Time Delta: (t~Damage~ - t~Suppress~ )**     **PNS~Fixed~**

  Negative Time up to 1 Minute                   1.0

  \> 1 Minute to 2 Minutes                       .95

  \> 2 Minutes to 4 Minutes                      .80

  \> 4 Minutes to 6 Minutes                      .5

  \> 6 Minutes to 8 Minutes                      .25

  \> 8 Minutes to 10 Minutes                     .1

  \> 10 Minutes                                  0.0
  ---------------------------------------------- ------------------------

[PNS treatment for degraded gaseous fire extinguishment systems -
inadequate soak time]{.underline}

One specific type of degradation that may be identified for a gaseous
fire extinguishment systems involves the inability of the system to
maintain the design concentration of fire suppressant for a sufficient
time to assure the complete extinguishment of a deep-seated fire. The
required suppressant concentration and maintenance time are established
by the system design criteria. This degradation is commonly referred to
as an "inadequate soak time." This can be an issue for Halon and Carbon
Dioxide fire extinguishment systems, as well as for other gaseous
suppression systems (e.g., Halon replacements).

For the inadequate soak time degradation case, special consideration is
required to estimate the probability on non-suppression (PNS). So long
as the system can deliver an initial concentration that meets the design
criteria, then some credit is given to the system for disrupting the
fire growth and spread process. For this case, the following assumptions
are made:

-   Fires involving cables or other electrical and electronic components
    will be deep-seated.

-   If a gaseous suppression system cannot maintain adequate
    concentration for a sufficient time to assure fire extinguishment
    (per design specifications), manual fire fighting must respond and
    must achieve final fire suppression.

-   The fire will be held in check during the time that the fire
    suppressant concentration is maintained at design level.

-   Assuming that the system actuation is timely (i.e., adequate margin
    between discharge time and the estimated fire damage time) the
    systems effectiveness will be reflected as a corresponding delay in
    the estimated fire damage time. As a result, the manual fire brigade
    is given an additional time to effectively respond to the fire.

-   Upon dissipation of suppressant, the fire will re‑flash and the fire
    growth and damage process will pick up where it left off.

The quantification process for this case is as follows:

-   Select the appropriate manual suppression fire duration curve
    corresponding to the fire ignition source.

-   Estimate the time to fire detection in the usual manner. In addition
    to any other fire detection capability, assume that a valid
    actuation signal for the gaseous suppression system will trigger a
    fire alarm.

-   Using the fire damage time calculated in Step 2.6 (t~damage~), and
    the estimated time to fire detection (t~detection~), calculate the
    value of PNS~manual~ from the selected fire duration curve in the
    usual manner (i.e., using t~damage~-t~detection~ as the time
    available for manual suppression).

-   Estimate discharge/actuation time (t~suppress~) for the gaseous fire
    extinguishment system in usual manner. Recall that actuation will
    either be automatic or manual, and that the pre-discharge
    alarm/warning time must be included.

-   Calculate the time margin ("time delta") between the actuation time
    and fire damage time in the normal manner:

> Time Delta = ( t~damage~ - t~suppress~ )

-   Use the general PNS~fixed~ probability table based on "Time Delta"
    to assess the likelihood that the suppression system actuation is
    timely in comparison to the estimated fire damage time.

-   If the PNS~fixed~ value assigned is 1.0, then the gaseous system
    will not be credited. In this case use the value of PNS~manual~ as
    calculated previously as PNS~scenario~ and the analysis is complete.

-   If the PNS~fixed~ value is less than 1.0, then the gaseous system
    will be credited. Continue this analysis to estimate PNS~scenario~.

-   Calculate a modified fire damage time as follows:

t~damage_new~ = t~damage~ + t~maintain_gas~

> where t~maintain_gas~ is the length of time that the desired gaseous
> suppressant design concentration [can]{.underline} be maintained.

-   Using t~damage_new~ (i.e., in place of t~damage~) and t~detection~,
    estimate PNS~gas_manual~ based on the selected manual fire
    suppression fire duration curve. That is, calculate
    \[t~damage_new~ - t~detection~\] as the modified time available for
    manual suppression, and estimate PNS~gas_manual~ in the manner
    normally applied to PNS~manual~.

-   Estimate PNS~scenario~ by combining PNS~fixed~ , PNS~manual~ , and
    PNS~gas_manual~ using the following equation:

> PNS~scenario~ = \[0.95 × (1 - PNS~fixed~) × PNS~gas_manua\ l~\] +
> \[(0.95 × PNS~fixed~) x PNS~manual~\] +
>
> \[0.05 × PNS~manual~\]

The calculation of PNS~scenario~ combines three cases. The first case is
that the suppression system works (no random failure - 95% general
reliability/availability factor), the actuation is timely (1-
PNS~fixed~) and the fire brigade responds following dissipation of the
fire suppressant concentration (PNS~gas_manual~). This is reflected in
the first term on the right hand side of the equation. In the second
case, the fire suppression system does [not]{.underline} fail randomly
(the 95% reliability/availability factor), but discharge of the fire
suppression system is not timely (PNS~fixed~). In the third case, the
gaseous suppression system suffers a random failure on demand (the 5%
unreliability/unavailability factor). For the last two cases, the fire
brigade must successfully suppress the fire within the originally
estimated fire suppression time (PNS~manual~).

This equation reduces to:

> PNS~scenario~ = \[0.95 × (1 - PNS~fixed~) × PNS~gas_manua\ l~\] +
> \[(0.95 × PNS~fixed~) + 0.05\] × PNS~manual~

-   Verify that (PNS~scenario~ ≤ PNS~manual~). As in other cases, the
    manual brigade response given the original fire damage time the
    minimum credit given to fire suppression for any scenario. If
    (PNS~scenario~ \> PNS~manual~), then reset (PNS~scenario~ =
    PNS~manual~).

[Manual fire suppression:]{.underline}

The following process is repeated for each fire scenario:

-   Subtract the fire detection time from the fire damage time.

-   Using the appropriate fire duration curve, read across the x-axis to
    the time difference from the above step.

-   Transfer up to the corresponding point of the fire duration curve,
    and read across to the left to estimate the PNS~Manual~.