Fire Hazard Analysis for Hall B

Probability: marginally possible. Severity: severe impact. Risk class: 2

Normal currents are low (~1-2 A); increased resistance would only decrease the current, so the full current allowed by the fuse is not available. Attaining 400-600 degrees C temperature is unlikely because of heat conducted away by the electrical conducto r, but might be possible; if 1 watt were transferred to 1 g of plastic with no heat losses, a naive calculation using a typical heat capacity for plastics of 1 J/goC predicts 400 degrees is attained in 7 minutes. Heat losses, however, will occur through t he air, local wires, and other plastics, increasing this time significantly. The main safeguard here is the inerting of the drift chamber endplate region, which would prevent any flame from occurring. Self-extinguishing cables (CL2 rating) could probably not be ignited by a single small flame, and would not propagate the fire if ignited. Besides the cables, there are essentially no flammable materials on these boards with the exception of a Molex brand connector, which could not be made to burn in informa l ignition tests.

8.2 Drift Chamber Low-Voltage Power Ignites Cables at Disconnect

A connection develops resistance, depositing 5 watts of power into a small plastic junction connector located at the upstream end of the torus. The fuse rating is not exceeded, so it does not open. The plastic part attains its self-ignition temperature an d develops a small flame which spreads to the cables nearby. Because of the large cable density in the area, part of which is in a vertical orientation, the fire grows rapidly.

Specific mitigations for this hazard include:

Other mitigations which may be relevant to this hazard include:

Probability: marginally possible. Severity: moderate impact. Risk class: 1

Normal currents are low (~1-2 A); increased resistance would only decrease the current, so the full current allowed by the fuse is not available. Attaining 400-600 degrees C temperature is unlikely because of heat conducted away by the electrical conducto r and other materials, but might be possible (see comments in previous scenario). Self-extinguishing cables (CL2 rating) could probably not be ignited by a single small flame, and would not propagate the fire significantly if ignited; linear heat sensors on these fuels would sound an alarm. Vertical runs of cable in this area are limited in length. The Sniffer system would be likely to see gases if the heat deposit to the plastic were large, and the VESDA system would see smoke products after ignition (an d possibly before ignition also).

8.3 High Current Electronics Ignites Cables by Inadequate Connection

Scenario: In the case of an inadequate connection to the power cable or bus of the high-current fast electronics, dripping flaming cable insulation or other dislodged burning material may drop through the open floor grating or open area beneath the racks on all four carriages. (The current being produced by the supply may not be detected as excessive since these supplies normally draw large currents, and depending on where the heating occurs the over-temperature sensor may not activate.) This open floor a rea is dense in cables which are oriented both vertically and horizontally. These cables are configured both as thick cable bundles and individual small bundles spaced widely apart, so a wide range of potential propagation conditions are present. Once a s ufficient number of cables ignite under the floor (the floor is an open grating), the fire can propagate along the cables to the detectors (and to the bulk cable delay rooms on the mobile carriages), propagating upward and horizontally via burning, and do wnward by dripping burning material down any vertical openings to spaces below.

A similar scenario occurs for loss of power supply cooling if there is a failure of the overtemperature protection. This scenario can also result from miscellaneous other smaller power supplies which have similar failure modes.

Specific mitigations for this hazard include:

Other mitigations which may be relevant to this hazard include:

Probability: possible. Severity: moderate impact. Risk class: 2

Employee training on how to clean and attach the supply leads properly will reduce the likelihood of the condition arising. The prefire condition should be detected by VESDA and Sniffer. Fire rated cables and predominantly horizontal runs will slow fire g rowth; cables may self-extinguish at a small radius from the ignition point. The vertical cable runs within the rack or in front of the rack may, however, burn to completion before firefighters arrive if the fire is not detected in the incipient stage. Li near heat sensors may respond early depending on the diameter and location of the fire.

8.4 High Current Fast Electronics Ignites Cables by Sparking

Scenario: A component failure inside a crate power supply causes sparks or flames to be emitted from the chassis for a short time. Paper, cardboard, or small, easily ignited cables catch fire from the sparks or flames, and these materials burn longer and produce enough heat to ignite larger cables. The fire then propagates along all available cables.

A similar scenario can arise from one of the other miscellaneous smaller power supplies.

Specific mitigations for this hazard include:

Other mitigations which may be relevant to this hazard include:

Probability: possible. Severity: moderate impact. Risk class: 2

Housekeeping with respect to transient trash will reduce the likelihood of the condition arising. The fire condition should be detected by VESDA and Sniffer. Fire rated cables will slow fire growth; horizontal cables may self-extinguish at a small radius from the ignition point. Any vertical cable sections may, however, burn to completion before firefighters arrive if the fire is not detected in the incipient stage. Linear heat sensors on ignited vertical cables may respond early after initial flame occur s. Although most of these areas are remote from the time-of-flight detectors, the fire in an advanced stage could propagate via vertical cables to these detectors, which may not be self-extinguishing. While this is possible with any late-stage fire, there is a slightly greater vulnerability in this case in that sparks can ignite a fire several feet away from the crate, and the appearance of the flame occurs almost immediately. There may not be an incipient stage to such a fire; therefore, rapid response b y firefighters is critical (and killing electrical power may not help). Fortunately, a key component to this scenario is controllable; if there is no transient trash under the fuel, the sparks are extremely unlikely to have enough energy to light any exis ting fuel directly. Momentary flames from the supply chassis are a more serious threat, but can also be avoided by routing small cables away from the chassis or by adding buffer space between the cables and the chassis.

8.5 Minitorus Magnet Supply Ignites Cables by Inadequate Connection

Scenario: After multiple reconnections and after the conducting surfaces have been handled frequently, the minitorus power leads acquire enough surface oxidation to produce an average resistance of 150 mW. This is only a 3% change in the total load seen b y the supply, so the software controls don't identify it as a problem. However, at 6000 A (the nominal operating current) the power deposited in the junction reaches 5,400 watts in a small spot, and the water cooling flow (and to the heat conduction of th e system) is no longer able to cool the junction. The cable jacket heats and burns, and ignites some of the many other cables nearby, providing some radiant and convective heating as well. Boiling in the cooling water lines exacerbates the heating problem . A runaway heating condition might result, where the resistance grows rapidly at the junction, resulting in escalating heat production producing sparks or an electrical explosion.

Specific mitigations for this hazard include:

Other mitigations which may be relevant to this hazard include:

Probability: possible. Severity: moderate impact. Risk class: 2

Employee training on how to clean and attach the power supply leads properly will reduce the likelihood of the condition arising. The prefire condition should be detected by VESDA and Sniffer. Linear heat detectors on the magnet leads would be likely to p rovide an early warning of the problem, before any flame breaks out. Fire rated cables will slow fire growth; horizontal cables may self-extinguish at a small radius from the ignition point. The vertical cable sections may, however, burn to completion bef ore firefighters arrive if the fire is not detected in the incipient stage. Linear heat sensors on ignited vertical cables may respond early after initial flame occurs.

8.6 Pair Spectrometer Magnet Supply Ignites Cables by Sparks

Scenario: One of the aluminum welds in the pair spectrometer magnet high-current leads fails. These leads are approximately 150 feet long and they are made in sections joined together by approximately four dozen welds. The failure could be due to undetect ed mechanical damage (forklift backing into the unprotected leads), corrosion at the weld due to contaminants in the aluminum, or other problems. The weld area heats up, causing mechanical stresses, which cause further damage. The PVC tubing around the le ad melts and sags away, exposing the conductor, or the electrical tape holding the tubing in place loses its strength and allows the tubing to fall away from the conductor. The damaged weld reaches a critical temperature and shorts out in a shower of alum inum sparks. If the short occurs behind the forward carriage when it is in the `maintenance' position, the shower of sparks could easily reach any of the three levels of the forward carriage. Paper left in these areas ignites from the sparks, and the pape r lights the cables. The cables accessible to this spot include the signal delay cables on Level 0, a huge supply of fuel.

Similar considerations would apply when considering the (upstream) Møller polarimeter magnets, which have extended welded aluminum leads passing by cable fuels.

Specific mitigations for this hazard include:

Other mitigations which may be relevant to this hazard include:

Probability: marginally possible. Severity: moderate impact. Risk class: 1

Significant corrosion is unlikely since the corrosion properties of this water supply (the Low Conductivity Water, `LCW') are monitored. Mechanical damage of the conductors is unlikely to be overlooked. The insulating cover on the conductors is mechanical ly robust and unlikely to come off. The aluminum conductors are sized for much larger current than is practically used. Prefire condition should be detected by linear heat detectors on magnet leads. Fire rated cables will slow fire growth; horizontal cabl es may self-extinguish at a small radius from the ignition point. The vertical cable sections, such as those on the back of Level 0 of the forward carriage, may burn to completion before firefighters arrive if the fire is not detected in the incipient sta ge. Linear heat sensors on ignited vertical cables may respond early after initial flame.

8.7 Photomultiplier Tube Base Ignites Time-of-Flight Detectors

Scenario: A fire due to electrical component failure occurs in a photomultiplier tube base attached to a time-of-flight counter near the bottom of one of the side carriages. The flames propagate to the plastic cover of the detector itself and ignite the s cintillator material. The flames propagate across the surface of the detector plane, baking the outer support surface of the Region III drift chamber which then collapses and burns, exposing the inner two drift chambers to radiant heat and smoke damage.

Specific mitigations for this hazard include:

Other mitigations which may be relevant to this hazard include:

Probability: marginally possible. Severity: severe impact. Risk class: 2

The photomultiplier tubes are a very unlikely ignition source ( See Also: Photomultiplier Tube Bases ). When heated, both the PVC of the tube base and the Lucite light guide emit gases to which the Sniffer system is sensitive; the Sniffer tube at the apex of the spectrometer would be likely to detect a pre-fire condition. The linear heat sensors on the T OF light guides would be likely to give a signal before the flame got to the scintillator; both of these systems would be reported by the PLC system and the linear heat sensors would provide specific position information. The return air duct VESDA would g ive an elevated output at some stage of the fire as well. The degree of access to the fire depends strongly on the location of the problem and whether the carriage is in the operating or maintenance position; the access may be very simple if the fire is i n an early stage on the bottom of the spectrometer, or totally inaccessible if the fire is, for instance, in the region between the large angle calorimeters with the carriage in the maintenance position.

8.8 Transient Electrical Device Ignites Cable

Scenario: A transient electrical device such as a soldering iron, portable halogen lighting, or damaged extension cord produces heat. The most likely location is in or near an electronics area. Transient combustibles such as paper come into contact with t he heat. This could happen through direct carelessness of a person putting the two into close proximity, or the paper could be blown by a draft onto the hot object; strong drafts are produced, for instance, by the local cooling blowers. The transient comb ustible ignites and proceeds to ignite other fuels.

Specific mitigations for this hazard include:

Other mitigations which may be relevant to this hazard include:

Probability: possible. Severity: moderate impact. Risk class: 2

8.9 Target Failure Causes Hydrogen Explosion by Sparks

Scenario: A sudden catastrophic failure of the vacuum pipe also ruptures the target cell (or vice versa), and hydrogen gas and (briefly) liquid fill the volume of the Region I drift chamber and its vicinity. The failure ruptures the drift chamber gas bag from flying pieces of the vacuum pipe, or from sudden overpressure, and a spark from the drift chamber wires ignites the hydrogen-air mixture, which explodes. The explosion destroys all three regions of drift chambers and starts fires in several locations including the TOF detectors.

Specific mitigations for this hazard include:

Other mitigations which may be relevant to this hazard include:

Probability: marginally possible. Severity: severe impact. Risk class: 2

The pressures in the system at any given time are low, which makes it unlikely that both the gas cell and the beam pipe would rupture simultaneously. Engineering tests of components are performed before they are used; no new stresses are put on the system when the hydrogen is introduced. Beam pipe failure is likely to be an implosion (or a slow leak) and may not produce any flying parts which could damage the chamber. An explosion would not be directly detected by fire safety related systems, but any ensu ing fires would. If the explosion were not audible in the counting house, it would be detected by various alarms from the hydrogen target monitoring system.

8.10 Welding or Grinding Ignites Cable or Scintillator by Sparks

Scenario: A welding or grinding activity produces sparks which ignite transient trash. The transient combustible ignites and proceeds to ignite other fuels. This can happen anywhere in the hall, including locations with no other ignition sources; examples include the cable storage rooms (Level 0 of forward carriage and south carriage) or the TOF scintillators.

Specific mitigations for this hazard include:

Other mitigations which may be relevant to this hazard include: