7.0 Potential Ignition Sources
A number of devices or activities could start a fire in Hall B. This section attempts to enumerate all possible ignition sources, and to identify all ignition modes of each ignition source. No systematic attempt is made to evaluate the credibility of a gi ven source; the relative likelihood of these sources actually creating a fire is estimated in See Also: Specific Fire Scenarios - Hazard Analysis and Risk Estimates .
7.1 Electrical Power
Broadly speaking, there are at least four modes in which electrical devices in Hall B can be ignition sources. These are: inadequate connections, incorrect connections, sparks or flame from electrical or electronic components, or loss of cooling.
Inadequate connections which have higher resistance than the design value can generate more heat than the connector/cable combination is designed to manage. This can be characterized in terms of the power dissipated in the connection, written for resistiv
e loads by the familiar equations 
. For a given resistance, the power dissipated increases rapidly for increasing voltages and currents. In the situation of an improper connection, the resistance often increases with time as well, due to oxidation associated with elevated temperatures or
humid conditions. The higher the voltages or currents involved in the connection, the greater the risk of overheating an inadequate connection. The problem is most severe when the junction resistance developed is small compared to the load resistance, for
high-current loads. In this case the current and voltage are maintained at approximately their nominal value, but substantial power is being deposited at the junction; safeguards based on overcurrent and overvoltage protection are not activated.
Incorrect connections involve the element of human error more directly than the other ignition modes. It is more difficult to predict what possible incorrect connections could be made without some assumptions about the level of training, competence, and g ood will on the part of the person making the connection. The most straightforward example of incorrect connections is replacing a power fuse with one of an incorrect value unintentionally. Power leads which are unlabeled or poorly labelled and which can extend to more than one connecting point could be incorrectly connected. Miscommunication (between a technician and a supervisor, for example) and poor labelling could lead to an incorrect connection. An individual's response to time pressures or sleep de privation can also be implicated in making incorrect connections.
Sparks or flame from electronic components are often short-lived and low-energy; sparks in particular are unlikely to be able to ignite massive fuels directly. The main vulnerability to sparks or flame from components is to low-mass materials with low ign ition temperatures such as paper or small cables. These materials, when ignited, can then propagate the flame to more massive fuels.
Loss of cooling of electrical or electronic components can result from breakage or blockage of cooling water lines, failure or blockage of HVAC systems, or clogged cooling fan air filters.
7.1.1 Drift Chamber On-board Electronics
M. Mestayer
The drift chambers have two kinds of boards mounted directly on the chambers: high voltage translation boards (HVTB's) which distribute the high voltage to the wires, and signal translation boards (STB's) which route signal pulses from the wires to single inline package (SIP) amplifiers. The STB components are resistors, high-voltage capacitors, regulators and SIPs. Both boards have small areas at high electrical potential (-1000 to +2000 VDC). Low voltage power to the STB's is transported via individuall y fused wires.
One ignition mode for this source is deposition of power in an inappropriate spot. The power for the Region III drift chamber is typically 1.25 A at 7.5 VDC. The fuses are set at 2 A. This leaves 7.5 V, .75 A (5.6 W) "extra" power which could be dissipated at inappropriate places on the board, such as bad connections or in shorted SIP's. (As a general rule, the fuses are set 20% over the measured maximum current and rounded up to the next commercially available fuse.)
A second ignition mode arises if a fuse is accidentally replaced with one of a larger value, and a failure occurs in a component which deposits power in a flammable material. The maximum power available from the power supply is 50 A at 8 VDC, or 400 watts . However, the internal trip circuit would have to fail (or be set incorrectly) in order for this much power to be available (see See Also: Internal and External Protection of DC Low Voltage Supplies ) in addition to both a component failure and incorrect fuses in both the supply and return lines.
All of this electronics is located in an inerted atmosphere, making it a relatively implausible ignition source.
7.1.2 High Current Fast Electronics
M. Mestayer
The largest concentration of high-current electronics is associated with the drift chamber system. The drift chamber electronics (rack-mounted) is comprised of 11 FASTBUS and 31 VXI crates which have high power dissipation. These (and other miscellaneous modules) are located in 5 rooms on the space frame. The total power dissipation in such fast electronics crates is anticipated to be about 200 kW. For these devices, the units which supply power to a given crate are located within the crate. Many of the r acks used to house these supplies are closed and interlocked.
There are a number of other crates distributed throughout the hall which service the other detector systems. These include FASTBUS, VME, VXI, CAMAC, and NIM crates. In general, the heat from the high-current supplies is ejected into the electronics rooms by fans; the rooms themselves are air conditioned. All of these supplies are commercially manufactured, although a few include custom features.
A typical ignition scenario involves dissipation of power in unintended locations such as at cable junctions or in under-sized cables; with most of these devices the normal current draw is large compared to what is required to generate enough heat for a f ire, so over-current protection or fusing may not prevent a problem from arising. Another ignition mode for these is overheating due to cooling failure, such as blocked or inoperative cooling fans. A third ignition mode involves internal component failure resulting in sparks or flames coming from within the metal chassis of the supply, which subsequently ignite nearby combustibles. A fourth ignition mode arises if there are any power cables which are poorly labelled and which are long enough to connect to the wrong location. Typically these supplies are adjacent to many cables as well as temporary transient combustibles such as notebooks. Because there are so many of these supplies, all in close proximity to several types of fuels, this is one of the most probable ignition sources in Hall B for remote operation.
7.1.3 High Current Magnet Power Supplies
There are five large, high-current magnet power supplies in Hall B. These include the torus supply (3,861 A through a superconducting load), the minitorus/pair spectrometer supply (10,000 A at 40 VDC), the photon tagger magnet supply (2,400 A at 70 VDC), and a pair of dual-use supplies (each 8,000 A at 40 VDC) which power either the Møller polarimeter magnets or the photon beamline sweeping magnets. In addition to these, for polarized target experiments there will be a polarized target magnet suppl y (500A at 10 VDC).
All of these devices are commercially manufactured. Three of the larger supplies are located on Level 0 of the space frame, that is, on the hall floor and not adjacent to much fuel. The other two are on space frame Level 1, also relatively remote from com bustibles. For all of these supplies the housing is made of metal, and any penetrations through them are minimal. Therefore, it is essentially impossible for flames or sparks to be ejected directly from the chassis.
The primary ignition mode for these devices involves depositing power at unintended locations such as cable connections or in undersized conductors, or in a shorted magnet coil. An additional consideration is that the current from these supplies travels t hrough long conductors which are routed between the supply chassis and the load. While the voltage is fairly low, it is clearly possible to involve these leads in a mechanical accident which results in a short circuit, in which case sparks and splattered molten metal would be likely to result. A third ignition mode for the non-superconducting magnets could result from a loss of cooling water. A fourth ignition mode is for sparks or flames to be emitted from apertures in the power supply chassis, which is unlikely as noted above. A fifth ignition mode is for cables to be long enough and poorly labelled so that they could be connected to the wrong location. In a system such as the minitorus leads, if one pair of the flexible cables were interchanged by acci dent, tested briefly, and then remote operations were to proceed, the mis-cabling might not be detected, and remote operations would begin even though one lead is carrying several times its rated current. (In general these leads are so massive that it is only possible to connect them one way.)
There are only a few of these supplies, and they are expensive, highly engineered devices which have many safety features. Nonetheless, as ignition sources the overall system must be considered (supply + extended power leads + load). These supplies must b e considered as significant potential ignition sources both in remote running conditions and while the hall is occupied, if they are left on. This is particularly true in that some of the loads are reconnected frequently, and the power leads travel throug h locations where many activities take place.
7.1.4 Miscellaneous Power Supplies and Instrumentation
While the high-current supplies and their respective loads offer the largest current sources, even a small power supply can start a fire, and these supplies and their leads and loads may receive less safety engineering attention. An example is the rasteri ng magnet power supply. This device provides 10 A to the rastering magnets, and it is not a commercially produced supply. Other examples include power supplies for drift chambers and for photomultiplier tube high voltage, and the low voltage power supplie s for the drift chambers. For some of these devices the possibility of accidental mis-cabling is much greater. In an environment where outside users contribute significantly to the hall instrumentation, there is always the possibility that some small, `te mporary' device is configured in a way which is significantly less fire-safe than the highly engineered ones.
7.1.5 Photomultiplier Tube Bases
Of the several thousand photomultiplier tube bases in Hall B, there are only a few distinct electrical and mechanical designs. The list of different designs includes: forward calorimeter, forward time-of-flight, Cerenkov counter, large angle calorimeter, large angle time-of-flight, tagger energy, tagger timing. In any photomultiplier base the typical current draw ranges from a fraction of a milliampere to two milliamperes. The maximum power deposited is approximately 5 watts, and the typical power deposit is approximately 1 watt.
Because the bases contains high voltage components, they are fitted with protective housings. Generally these housings are completely enclosed and are made of metal or thick plastic to prevent accidental shocks. In order to burn, the power available has t o be deposited in a small flammable component which ignites, and the flame has to spread to other flammable components; the power supply must also not trip off. If the fuel of the first ignited component runs out, or the available oxygen runs out, the fla me is extinguished. The ignited internal components must then ignite or burn through the massive or metallic housing of the base in order for the fire to spread. If the housing is made of plastic, there must be enough heat available to raise its surface t emperature to the ignition point. In the case of the time-of-flight bases, for example the housing is made of thick plastic (unplasticised PVC, schedule 80) and a substantial amount of heat would be required to attain ignition temperatures on its inner su rface.
While full ignition of a photomultiplier tube base may not be impossible, it is very unlikely because of the small available power, the limited amount of available fuel within the housing, and the massive or nonflammable housing of the base.
7.1.6 Drift Chamber Wires
M. Mestayer
Under normal circumstances the individual drift chamber wires carry minute amounts of current, therefore ohmic heating is negligible. In the case of a fault these wires could heat up to some extent, however, because of their mechanical fragility and elect rical resistance, the wires cannot carry much current without breaking. (The most robust wires are the guard wires, which are also closest to the nylon gas bag.) Sparks due to high voltage, on the other hand, are likely to occur, since the chamber will op erate at potentials of up to 2000 V relative to ground. Sense wire to field wire potential differences will be as high as 3000 V. Intense, prolonged arcing is unlikely due to the trip capability of the power supply and to the mechanical fragility of the w ires. Since the wires are in an inerted environment, the sparks are unlikely to be a significant ignition source. In addition, the amount of energy contained in a given spark is limited by the high voltage power supply and by the capacitance in the system . The closest combustibles to the drift chamber wires are the gas bags and the plastic portion of the wire feedthrough (regions I and III only). Adjacent combustibles include the gas bag, which is made of aluminized nylon, and the feedthroughs, which are injection-molded plastic.
The power available from the high voltage supplies and delivered to these wires is current limited by a trip circuit (in distinction to the low voltage supplies). Under normal circumstances the power available cannot exceed 0.12 watts. Each output channel has its own separate high voltage `generator' circuit, so it is extremely unlikely for multiple internal channels to contribute to a single external channel by a failure internal to the supply.
7.1.7 Static Electricity
M. Mestayer
Static electricity can produce low-energy sparks which can ignite flammable gas mixtures. In order to accumulate static electricity, an insulating surface in a dry environment is required. In the hall, the only flammable gas is inside the target, usually in liquid form. The beam pipe is a conducting material which completely surrounds the target. In the vicinity of the beamline area the only insulating surfaces are the drift chamber gas bags, which are metallized on one side. The insulating side is in con tact with the ambient air in the hall which is maintained at 30-50% relative humidity, making it unlikely that any charge could accumulate. In the event of a simultaneous catastrophic failure of both the target cell and the beam pipe, the gas could come i nto contact with the bag. Although unlikely, there is a remote possibility of ignition from this source.
7.1.8 Other Electrical Appliances
There are many other electrical devices which may be operating in the hall at a given time. These may include soldering irons, computer terminals, oscilloscopes, extension cords, incandescent and fluorescent lights (both fixed and portable), and an assort ment of other devices. These are all possible ignition sources; in highly protected facilities, it is often true that devices of this type initiate fires, since less safety engineering attention has been applied to their use compared to the fixed, high-va lue devices such as the high-current power supplies.
7.1.9 Lightning
B. Manzlak
Lightning strikes to the outside of the hall can in principle produce fire hazards in Hall B. In the unlikely event that something like this happened, a very substantial amount of power is available to be deposited in unplanned locations.
The ubiquitous feature of lightning is its unpredictability. While features such as the standard grounded lightning rods attached to the hall, the gas sheds, and the counting house may decrease the probability of lightning strike (by `defusing' the local electric field), there is always a certain degree of non-deterministic behavior as to whether a strike will occur, and to which point it will choose to touch down.
The current may travel to the hall through the earth covering the building, through electrical conductors entering the building, through piping entering the building, and typically some current may go through several such paths, since the potential differ ences are very large. The obvious candidates for Hall B include the electrical power distribution and its associated ground network, and the gas system piping, which travels from elevated locations outside the hall directly into the hall through stainless steel tubing. While there is an intentional section of insulator pipe in this line to isolate it electrically, it is unlikely to guarantee breakover isolation from the potentials available from lightning. The current-carrying capacity of the electrical p ower distribution system is higher than that of the gas piping, and circuit breakers may provide some protection, depending on which conductor carries the current and on how large the currents are.
There is a ground grid within the floor of the hall to which the carriages are grounded by a high-capacity conductor. The metal frames of the carriages present an immense area of ground potential. This may help to distribute the charge from a lightning st rike, depending on the route the lightning takes.
In any case, reasonable measures have been taken to avoid this intrinsically unlikely event, however, it cannot be ruled out as an impossible ignition source. The areas most subject to damage are the locations where electrical conductors come into the hal l and points connected to them (hall perimeter and all carriages), and the drift chambers near space frame Level 1 (from the gas piping).
7.2 Hot Work
B. Manzlak
Hot work is an important potential ignition source. It was historically the source of one small fire in Hall B during the construction phase, and while welding activity has decreased there will always be a continuing need for occasional welding or grindin g in the area. The primary mode of ignition is through hot sparks which, unnoticed, ignite low-mass combustibles such as paper or thin plastic, and these proceed to ignite more massive combustibles. While direct ignition of cables or scintillator is possi ble in principle, the typical situation is that the spark does not provide enough heat to raise the surface temperature of these combustibles to the temperature needed for self-sustained burning. This temperature is, for example, in excess of 600 degrees C for sheet polyvinyl chloride (minimum hot plate ignition temperature) 7 , the jacket material for almost all CLAS cables.
Other possible ignition modes include direct or radiant heating from the flame, or ignition of flammable gas mixtures, for which a single spark is sufficient. Under normal circumstances, however, there is are no welding, cutting, or burning activities in the hall while there is any hydrogen in the target; it is always pumped out before beginning any maintenance activities.
7.3 Electro-Mechanical Device Failure
7.3.1 Pumps
All the vacuum pumps used in the CLAS are closed metal housing commercial pumps. There are no belt-driven pumps. Therefore, it is unlikely that these would be involved in starting a fire, although sparks could be thrown from the vent holes in a housing, o r a circuit breaker could fail to trip when the motor seizes, causing the windings to overheat.
7.3.2 Motors
The HVAC fans are powered by commercial motors; there is at least one layer of metal housing surrounding the windings and brushes. Emission of sparks from the metal housing is possible but unlikely; there are sparks available during the normal operation o f the motor inside its housing, such that flammable gas could be ignited.
7.4 Smoking
B. Manzlak
A no-smoking policy is in effect within Hall B and its truck ramp, as well as all other parts of the counting house building. If this policy is violated, it is possible that a lighted cigarette, or the open flame which lights it, could ignite a combustibl e or flammable gas. This is an unlikely situation, since the heat available from a cigarette is limited. However, in combination with transient, low-mass combustibles such as dry scrap paper or cardboard, this becomes a credible ignition source.
7.5 Sparks
Mechanical accidents are capable of producing sparks. This potential ignition source produces sparks which may have a smaller energy than those produced by welding or grinding, but which have the accompanying characteristic of the element of surprise. Thi s may lead to situations where there are distractions from any consequences of the production of the sparks, and there are no engineering or administrative controls. As in welding and grinding, the sparks are not likely to carry sufficient heat to ignite cables or scintillator directly, but can ignite paper or thin plastics present as transient trash.
