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You are an expert at summarizing long articles. Proceed to summarize the following text: 
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Application Ser. No. 62/108,768 filed on Jan. 28, 2015, the entire disclosure of which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to devices and methods for selective firing of perforating guns. 
     BACKGROUND 
     One of the activities associated with the completion of an oil or gas well is the perforation of a well casing. During this procedure, perforations, such as passages or holes, are formed in the casing of the well to enable fluid communication between the well bore and the hydrocarbon producing formation that is intersected by the well. These perforations are usually made with a perforating gun loaded with shaped charges. The gun is lowered into the wellbore on electric wireline, slickline or coiled tubing, or other means until it is adjacent the hydrocarbon producing formation. Thereafter, a surface signal actuates a firing head associated with the perforating gun, which then detonates the shaped charges. Projectiles or jets formed by the explosion of the shaped charges penetrate the casing to thereby allow formation fluids to flow from the formation through the perforations and into the production string for flowing to the surface. 
     In some situations, a gun train having a series of guns is successively fired. These configurations typically include devices for selectively arming such guns. The present disclosure relates to methods and devices for selective arming of guns in a gun train. 
     SUMMARY 
     In aspects, the present disclosure provides a switch for selectively firing a perforating gun train that includes at least a first perforating gun and a second perforating gun. The switch may include a casing having a bore, a piston assembly, a contact assembly disposed in the casing bore, a pin disposed in the bore, a predetermined quantity of lubricant deposited in the bore, and a spring. The piston assembly may have a first end and an exposed end, a plurality of flanges formed at the first end, a plurality of grooves formed at the exposed end, an insulating sleeve enclosing the plurality of flanges and electrically isolating the piston assembly from the casing. The insulating sleeve and the plurality of flanges may be at least partially disposed in the casing bore. The pin may be slidable between a first position and a second position. The pin electrically contacts the piston assembly and is electrically isolated from the contact assembly in the first position, and the pin is electrically isolated from the piston assembly and electrically engages the contact assembly in the second position. The spring may urge the pin into engagement with the piston assembly when the pin is in the first position. The predetermined quantity of lubricant may be approximately 0.3 grams. 
     In aspects, the present disclosure provides a method for selectively firing a perforating gun train. The method may include forming the perforating gun train to include includes at least a first perforating gun and a second perforating gun, forming an electrical connection between the perforating gun train and a surface location using at least one switch as described above; conveying the perforating gun train into a wellbore with the pin in the first position; firing the first perforating gun, the firing causing the pin to move to the second position; and firing the second gun. 
     It should be understood that certain features of the disclosure have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the disclosure that will be described hereinafter and which will in some cases form the subject of the claims appended thereto. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For detailed understanding of the present disclosure, references should be made to the following detailed description taken in conjunction with the accompanying drawings, in which like elements have been given like numerals and wherein: 
         FIG. 1  schematically illustrates a side sectional view of a pre-activated switch according to one embodiment of the present disclosure; 
         FIG. 2  schematically illustrates the  FIG. 1  embodiment after being activated; and 
         FIG. 3  schematically illustrates a perforating gun assembly that incorporates switches according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to devices and methods for preventing an unintended activation of one or more downhole tools. The present disclosure is susceptible to embodiments of different forms. There are shown in the drawings, and herein will be described in detail, specific embodiments of the present disclosure with the understanding that the present disclosure is to be considered an exemplification of the principles of the disclosure, and is not intended to limit the disclosure to that illustrated and described herein. 
     Referring to  FIG. 1 , there is schematically illustrated one embodiment of a switch  100  made in accordance with the present disclosure. The switch  100  includes a casing  110 , an electrically conductive piston assembly  130 , a contact assembly  150 , an electrically conductive pin  170 , a spring  190 , and a lubricant  210 . The switch  100  may be configured to actuate any desired device. One non-limiting device is an electrical device that may be used to change the polarity of current that passes through a circuit. Such devices include, but are not limited to, a diode assembly  220 . Sealing elements  220  form fluid barriers between the casing  110  and adjacent structures and sealing elements  222  may be used to provide fluid isolation for the interior of the casing  110 . 
     The casing  110  may be a tubular body having a bore  112  for receiving the piston assembly  130  and the contact assembly  150 . The lubricant  210  may be deposited in the bore  112  and proximately at the region wherein the piston assembly  130  and the contact assembly  150  face one another. 
     The piston assembly  130  includes a piston body  132  having a first end  134  that is enclosed by an electrically insulating sleeve  136 . In one arrangement, the first end  134  includes a plurality of flanges  138  (e.g., two flanges). The flanges  138  are circumferential projections such as a rib that has surfaces oriented transverse to a circumferential surface of the piston body  132 . These transverse surfaces ensure the detonation impact that is applied to the piston body  132  is distributed over a large amount of shear area when the insulating sleeve  136  applies pressure to the lubricant  210 . Such an arrangement reduces the risk that the piston body  132  does not shear through the insulating sleeve  136  when gun detonation pressure is applied and charge debris impacts an exposed end  140  of the piston body  132 . Such an arrangement may also increase the pressure rating to at least 20,000 psi (after gun detonation). In another arrangement (not shown), one flange is used and the flange is positioned on isolated bore side  139  of the sealing element  222 . In some embodiments, the piston body  132  may include two or more grooves  142  formed at the exposed end  140 . The most distal groove may be used to connect a wire (not shown). The interior groove reduces the cross section of the piston body  132  such that the piston body  132  can bend and break, which may protect the piston seal area at the insulating sleeve  136  from damage. 
     The contact assembly  150  selectively forms an electrical path when the circuit is completed by the pin  170 . That is, the contact assembly  150  may have to conductors, here concentrically arranged, that are electrically isolated. The pin  170  upon entering the contact assembly  150  forms an electrical connection between these two conductors. The contact assembly  150  may have suitable connection points at which electrical leads may be connected. The contact assembly  150  may also include suitable bores or cavities to receive the pin  170  and the spring  190 . 
     The lubricant  210  is a pressure transmitting fluid body that transfers pressure applied by the piston body  132  to the pin  170 . In one non-limiting arrangement, the lubricant  210  may be grease. The amount of lubricant may be 0.3 grams. If less lubricant is used, the force applied to the pin  170  may not be sufficient to fully seat the pin  170  into the contact assembly  150  and maintain electrical conductivity. If more lubricant is used, the impact force may be reduced, which may result in inadequate seating of the pin  170  into the contact assembly  150 . 
     The pin  170  slides axially away from the piston assembly  130  toward the contact assembly  150  when sufficient pressure is supplied by the lubricant  210 . In one embodiment, the pin  170  may be a rod-like member having a tapered seat  172  that is shaped to ensure an inner diameter of the spring  190  does not bind on the pin  170  when the pin  170  is seating into the contact assembly  150 . Further, the outer surfaces of the pin  170  are substantially free of sharp shoulders or projections that the inner diameter of the spring  190  can bind upon as the pin  170  enters the contact assembly  150 . In embodiments, the pin  170  may include one or more ridges  174  in an upper end  176  to provide a shoulder on which a wire and solder interface (not shown) may adhere. This configuration also provides increased shear area of the solder to reduce the chances of the wire/solder interface breaking loose from the pin  170  when the perforating gun (not shown) is detonated. 
     In the pre-activated position of  FIG. 1 , a wire (not shown) from a detonator (not shown) of a downhole perforating gun (not shown) is connected at the outermost groove  142  of the piston body  132 , the piston body  132  and the pin  170  are in physical contact with one another, and another wire (not shown) in electrical communication with a firing panel (not shown) at the surface is connected to the pin  170 . Thus, electrical signals travel via the pin  170  and the piston body  132  to the downhole detonator (not shown). The contact assembly  150  is not part of this circuit. 
     After detonation, the switch  100  has the configuration shown in  FIG. 2 . The pressure generated by the firing of the downhole perforating gun (not shown) displaces the piston assembly  130  toward the pin  170 . The piston assembly  130  reduces a volume of the bore  112 , which pressurizes the lubricant  210 . The pressurized lubricant  210  flows along the bore and toward the pin  170  and applies a pressure to the pin  170 , which displaces the pin  170  into the contact assembly  150 . Thus, the pin  170  is no longer in electrical communication with the piston assembly  130 . Instead, the pin  170  is driven into the contact assembly  150  and wedges into a fixed relationship with the contact assembly  150 . The engagement between the pin  170  and the contact assembly  150  forms an electrical path  221  ( FIG. 1 ) from the surface to the detonator or other equipment of the uphole perforating gun (not shown). This electrical path may include the electrical equipment such as a diode assembly that allows selective transmission of DC electrical power. The pressure of the lubricant  210  overcomes the spring force of the spring  190  and effectively locks the spring  170  with the contact assembly  150 . 
     Referring to  FIG. 3 , there is shown an illustrative use of a switch  100  according to the present disclosure. In  FIG. 3 , there is shown a section of a perforating gun assembly  200  in a wellbore  202  drilled in an earthen formation  204 . The wellbore  202  may include a wellbore tubular such as a casing  206 . The perforating gun assembly  200  may include a plurality of perforating guns  210   a,b,c . In one arrangement, the perforating gun assembly includes two switches  100   a,b , each of which have an associated diode assembly  220   a,b , respectively. A lower switch  100   a  is configured to pass only negative polarity DC current after activation. An upper switch  100   b  is configured to pass only positive polarity DC current activation. A detonator  214   a  is configured to detonate the perforating gun  210   a , a detonator  214   b  is configured to detonate the perforating gun  210   b , and a detonator  214   c  is configured to detonate the perforating gun  210   c.    
     During use, the perforating gun assembly  200  is placed at a desired depth and the operator applies a positive DC current at a surface shooting panel (not shown) to fire the lowermost perforating gun  210   a . The current flows through the detonator  214   a  and thereby fires the bottom perforating gun  210   a . The pressure pulse associated with the firing of the bottom perforating gun  210   a  actuates the lower switch  100   a . This actuation causes an associated diode assembly  220   a  to block positive DC current. Because the diode assembly  220   a  on the first switch  100   a  blocks positive DC current, current does not reach the detonator  214   b  and the second perforating gun  210   b  does not fire at this time. 
     When the operator is ready to fire the second perforating gun  210   b , a negative DC current is be applied at the shooting panel (not shown). The negative DC current is allowed to pass through the diode on the switch  100   a  and the detonator  214   b  detonates, which fires the second perforating gun  210   b . As a result, the pin on the upper switch  100   b  is pushed up, which actuates activates the upper switch  100   b . This actuation causes an associated diode assembly  220   b  to block negative DC current. Because the diode assembly  220   a  on the first switch  100   a  blocks negative DC current, the diode assembly  220   b  on the upper switch  100   b  blocks negative DC current, which prevents current reaching the detonator  214   c  and does not cause the third perforating gun  210   c  to fire. 
     When the operator is ready to fire the third gun  210   c , a positive DC current is applied at the shooting panel. The positive DC current is allowed to pass through the diode on the switch  100   b  and the detonator  214   c  detonates, which fires the third perforating gun  210   c.    
     The foregoing description is directed to particular embodiments of the present disclosure for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible without departing from the scope of the disclosure. It is intended that the following claims be interpreted to embrace all such modifications and changes.

Summary:
A switch includes a casing having a bore, a piston assembly having a first end and an exposed end, a plurality of flanges formed at the first end; an insulating sleeve enclosing the first end, a contact assembly disposed in the casing bore; a pin disposed in the bore; a predetermined quantity of lubricant deposited in the bore; and a spring urging the pin into engagement with the piston assembly. The insulating sleeve and the plurality of flanges are at least partially disposed in the casing bore. The pin has: (i) a first position wherein the pin electrically contacts the piston assembly and is electrically isolated from the contact assembly; and (ii) a second position wherein the pin is electrically isolated from the piston assembly and electrically engages the contact assembly. The predetermined quantity of lubricant may be approximately 0.3 grams.