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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for excavating a hole by sucking debris and spoil out of the 
     2. Discussion of Background 
     Such an apparatus is shown for example in German utility model DE 29902562 U1 which discloses a vehicle with a suction tube to be inserted into a hole or excavation to suck debris and spoil out of the hole, an air pump to cause the suction and a filter to remove the spoil from the airflow. 
     Excavating holes using a suction excavator as in the above German utility model is advantageous because holes with a much smaller horizontal cross-section can be dug using this device than previous excavations made using drills, spades etc. Excavating using a suction excavator is much quicker than conventional digging techniques, reduces the amount of spoil produced from the hole and the amount of tarmac required to re-fill the hole, causes less damage to tree roots and to other utility pipes and cables and causes much less disruption to pedestrians and drivers if used in a street. However, because of the large volume of air that is sucked through the vehicle, any fuel gas from a leaking gas main for example is likely to be sucked into the vehicle. Any sparks produced within the vehicle, for example from the air pump or static build-up within the vehicle due to the fast movement of air through pipes etc., is likely to cause a spark, igniting the gas and causing an explosion. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided a suction excavator with a gas sensor arranged to detect for the presence of gas in the passage of air through the suction excavator. 
     The provision of a gas sensor within the suction excavator enables the presence of potentially dangerous explosive gas within the suction excavator to be determined so that appropriate action can be taken. 
     There is preferably provided a control means which when gas is detected flushes air through the suction excavator. 
     All of the components of the suction excavator through which suction air is passed, such as the air pump, suction tube etc., are preferably electrically bonded to each other and a connection between the connected components and earth provided to discharge any electrostatic charge built-up. The bonded components may be connected to earth via electrostatically conducting tyres when the suction excavator is mounted on a vehicle or via an electrically conducting strap, for example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     An example of a vacuum excavator according to the present invention is shown in the accompanying drawings in which: 
     FIG. 1 shows an operator excavating a hole by directing the nozzle of a suction tube into the hole; 
     FIG. 2 diagrammatically shows some elements of the suction excavator; 
     FIG. 3 shows a hopper arranged to receive spoil from the excavation; 
     FIG. 4 shows a control system connected to a gas sensor of the suction excavator; and 
     FIG. 5 is a flow diagram showing operation of the control system shown in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows an excavation  10  which may for example be made to reach a subterranean pipe or cable. When in urban areas and the excavation is made into a road or a pavement, a pneumatic drill may initially be used to break into the hard tarmac surface of the ground. A nozzle  11  is connected to an air pump and a filtering unit, which in this case are mounted on a vehicle  12 . The nozzle  11  is used to suck up spoil from the excavation  10 . If necessary whilst sucking up spoil through nozzle  11  the ground in the excavation  10  may be broken up using, for example, a pole, a spade, a fork, or more preferably an air knife as is well known in the art for delivering a high velocity jet of air. This suction nozzle  11  has a circular cross section of about 25 cm diameter and in this case the periphery of the tip of the nozzle  13  follows an undulating path which is less likely to damage subterranean pipes which the free end of the nozzle  11  may encounter. The nozzle  11  is provided with couplings or brackets  14  into which any number of extension pipes  15  may be inserted to extend the length of the nozzle  11 . In this example the nozzle is made from aluminium which is strong and light. The nozzle is provided with an on/off switch, in this case on a handle  16  used by the operator to direct the nozzle. The on/off switch immediately starts/continues or stops suction through the nozzle  11 . The switch is preferably arranged such that an operator must constantly apply pressure to it to continue the sucking operation. When the operator stops applying pressure to the switch suction is then immediately stopped. The ability to immediately disengage suction is particularly useful to enable blockages to be cleared from the end of the nozzle and to prevent injury in case the operator or his clothes are accidentally caught in the nozzle. The nozzle is provided with a flexible hose  17  which may be made from heavy duty rubber to connect the nozzle to a boom  18  which may be hydraulically supported for easy operation and which is mounted on the vehicle  12  containing the air pump and filtering equipment. 
     FIG. 2 diagramatically shows an example of the suction and filtering equipment. Suction air and entrained spoil is passed from the nozzle  11  shown in FIG.  1  through boom  18  to a hopper  20 , in this case a drop box hopper, to remove the vast majority of the spoil entrained in the suction air. The suction air then passes to a cyclone  30  where it is accelerated and then to a filter  40  where dust and smaller particles are removed from the air. The air then passes through an air pump  50  which in the present example is arranged to pump between 1100 and 1900 cubic metres of air per minute, and suction air is then discharged through exhaust system  60  which includes one or more silencers. 
     FIG. 3 shows the drop box hopper  20  in more detail. Suction air is passed from boom  18  into the hopper  20  past a gas sensor  21  as is well known in the art. Spoil entrained in the incoming air falls under the influence of gravity to the bottom of the hopper  22  where it is collected. The base  23  of the hopper is hinged along one edge  24  and is arranged such that when a particular weight of spoil  22  has accumulated at the bottom of the hopper  20  the base  23  rotates about a hinge along the edge  24  to pass the spoil  22  down a chute  25  for collection or disposal. The base plate  23  is urged upwardly when in use by the passage of the suction air and is only lowered when the weight of spoil exceeds the upward force provided by the suction air. If desired a counter-balance  26  may be provided on the hinged base  23  to adjust the weight of spoil that causes its ejection down chute  25 . 
     The substantially spoil-free air passes out of the hopper  20  through a conduit  27  to cyclone  40 . Conduit  27  is provided with a valve  28  and valve actuator  29  arranged, when actuated, to block the passage of air from hopper  20  to conduit  27  and instead admit air from outside into conduit  27 , in this case via conduit  27   a.  When not actuated, the valve  28  admits air from hopper  20  into conduit  27  and blocks the passage of air from conduit  27   a  into conduit  27 . 
     The air from conduit  27  is then sucked through a cyclone  30 , as is well known in the art, to accelerate the air and then to a filter  40  as is also well known in the art to remove any dust from the suction air. The filter  40  may be regularly cleaned to prevent dust from causing clogging and preventing the passage of air therethrough. 
     Air from the filter  40  is sucked to the air pump  50  which is preferably powered by the gearbox of the vehicle  12  upon which the air pump and filtering equipment is mounted. 
     Air from the pump  50  is then passed to silencers  60  as are well known in the art to vent the air and reduce noise. 
     FIG. 4 shows a control system including a control means  70  such as a microprocessor for receiving a signal from gas detector  21 . When a signal is received by control means  70  indicating that gas detector  21  has detected explosive gas or a predetermined amount of explosive gas in the boom  18 , control means  70  instructs valve actuator  29  to open valve  28  thereby preventing the further suction of air from excavation  10 . Instead air is drawn from conduit  27   a  which is in communication with fresh air, for example from above the vehicle  12  to pass the fresh air through the cyclone  30 , filter  40 , air pump  50  and exhaust system  60  thereby flushing out any fuel gas. A further gas sensor  51  is preferably provided at the suction air inlet of the air pump  50 , the actuation of which also opens valve  28  to prevent the further suction of air from the nozzle  11  and instead flushes clean air through the suction excavation system. An audible or visual alarm is preferably activated when a gas detector  21 ,  51  is activated to advise an operator of the reason for the interruption in suction from the nozzle. In order to reactivate the suction excavation system, a manual re-set  71  must be activated to ensure that the operator is aware of the situation. However, the manual re-set  71  will not close valve  28  until the fuel gas concentration detected by sensor  21 , and if used also sensor  51 , has fallen below the predetermined level which caused its actuation. 
     FIG. 5 shows the operation of the control system. The control means  70  continually monitors gas sensors  21  and  51  to see whether a predetermined concentration of explosive gas has been detected. As soon as a predetermined concentration of explosive gas is detected from either sensor, valve  28  is opened and is not closed to permit further suction excavation until the concentration of explosive gas has fallen below the predetermined level and the manual re-set  71  has been activated. The control means  70  preferably continually monitors the concentration of gas detected by the gas sensors  21 ,  51  and may store the received concentrations, for example on a data logger such as a RAM of a computer for subsequent analysis. The control means  70  may be set to open valve  28  when any predetermined gas concentration is detected, for example 1% fuel gas in air. The gas sensors  21 ,  51  and control means  70  are preferably calibrated so that a particular signal from a gas sensor  21 ,  51  corresponds to a known concentration of gas. 
     The on/off switch to be engaged by the operator and which in this case is mounted on the operator&#39;s handle  16 , shuts off suction by opening valve  27  which provides a much faster shut off than turning off the air pump  50  for example which would take time to slow down through inertia. However, the operator&#39;s on/off switch mounted in this case on handle  16  cannot override the opening of valve  28  as a result of a signal from a gas sensor  21 ,  51 . 
     Since the movement of air through the components of the suction excavation system may generate static charge, this raises the possibility of sparks being generated which could possibly cause an explosion, especially if explosive gas is present. To prevent this, each component through which air is passed by the suction system is electrically bonded to each other to enable electrostatic charges to pass therebetween and the system is connected to earth, for example, via electrostatically conducting tyres or via an electrostatically conducting strap connected from the system to earth. 
     If desired, the control means  70  may be arranged to open valve  28  when any number of potentially explosive situations arise such as an overheating engine or drive belt or dangerously low oil levels. Again the manual reset will not be able to close the valve until the cause of the opening of the valve  27  has been rectified.

Summary:
A suction excavator including an air pump for generating a flow of air and a nozzle through which air is drawn under the influence of the air pump, the nozzle being arranged when in use to suck up spoil from an excavation, and a mechanism for separating spoil from the air drawn through the nozzle. The excavator is provided with a fuel gas detector to detect for the presence of fuel gas in the flow of air drawn up through the nozzle. When fuel gas is detected a valve may be actuated to interrupt the flow of air up through the nozzle and instead to admit air from a substantially fuel gas free source to flush any fuel gas out of the system reducing the risk of an explosion.