Abstract:
Protective equipment for safely discharging electrical potential comprises an interface ( 10, 50, 60 ) adapted to be in contact with a wearer. A conductive path in the interface contacts the wearer, the conductive path reaching a ground. A first circuit device ( 30 ) is provided in the conductive path. The first circuit device ( 30 ) has at least one variable resistance so as to oppose a first level of variable resistance to decrease a conductivity in the conductive path as a function of an increase in potential difference between the wearer and the ground, to allow static discharge through the protective equipment.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present patent application claims priority on International Patent Application PCT/IB2006/052789, filed on Aug. 11, 2006, and on U.S. Provisional Patent Application 60/890,549, filed on Feb. 19, 2007. 
    
    
     FIELD OF THE APPLICATION 
     The present application relates to electric-shock prevention, and more particularly to protective equipment used as protection against electrical discharges. 
     BACKGROUND OF THE ART 
     Interactions of the human being with his/her environment (such as friction through movements) causes an accumulation of a static charge on the human being. Humans typically wear non-conductive clothing (e.g., rubber-sole shoes which are an efficient insulator), which enhances the accumulation of a static charge by the human. More specifically, clothing is often the interface between the human and the ground. When the interface has insulating properties (e.g., shoes with rubber soles), the human accumulates a static charge. As such, when an object representing a ground is contacted, the human feels an unpleasant static shock as the charge is discharged to the ground. 
     In some workplaces, the level of static charge on workers is controlled. As an example, in the field of electronics, a 100 V charge may cause damage to electronic components, such as the erasure of data on magnetic data medium, even though the worker does not feel the discharge of such a charge. In operating with hydrocarbons (petroleum and gas industries), a 50 V charge may spark flammable vapors. In the plastics industry, static charges may be of such magnitudes that severe injuries resulting from electrical shocks have occurred. The control of static charge is therefore important, whether it be for the security of workers or for maintaining levels of productivity. 
     Protective equipment, such as shoes, wrist bands, heel pads, ankle bracelets, has been developed to control the charge of workers. However, if such protective equipment is highly conductive, there is an increased risk of electrocution for the wearer. The wearer of highly conductive protective gear may become a ground for high-voltage machinery or wires. 
     A plurality of products have been developed and patents have been granted to address the need for conductive, insulated or antistatic gear. Examples of patents include U.S. Pat. No. 493,782, U.S. Pat. No. 2,261,072, U.S. Pat. No. 2,712,099, amongst others. 
     SUMMARY OF THE APPLICATION 
     It is therefore an aim of the present invention to provide electric-charge protective equipment that addresses issues associated with the prior art. 
     Therefore, in accordance with the preferred embodiment, there is provided protective equipment for safely discharging electrical potential comprising: an interface adapted to be in contact with a wearer; a conductive path in the interface contacting the wearer, the conductive path reaching a ground; and a first circuit device in the conductive path, the first circuit device having at least one variable resistance so as to oppose a first level of variable resistance to decrease a conductivity in the conductive path as a function of an increase in potential difference between the wearer and the ground, to allow static discharge through the protective equipment. 
     Further in accordance with the preferred embodiment, the first circuit device comprises a first resistance and a first pair of transistors, with the first pair of transistors arranged with respect to the first resistance so as to offer the first level of variable resistance according to a potential difference across the first resistance in a direction of a current flow. 
     Still further in accordance with the preferred embodiment, the protective equipment comprises a second circuit device in the conductive path in series with the first circuit device, the second circuit device having a second resistance and a second pair of transistors, with the second pair of transistors being arranged with respect to the second resistance so as to offer a second level of variable resistance according to a potential difference across the second resistance in a direction of a current flow. 
     Still further in accordance with the preferred embodiment, the transistors of the first pair are mosfets, with the transistors arranged on opposed sides of the first resistance so as to operate in depletion mode when current flows from the wearer to the ground. 
     Still further in accordance with the preferred embodiment, the transistors of the first pair and of the second pair are mosfets, with the transistors of the first pair and of the second pair respectively arranged on opposed sides of the first resistance and of the second resistance so as to operate in depletion mode when current flows from the wearer to the ground. 
     Still further in accordance with the preferred embodiment, the interface is a shoe, the shoe having an insole, an insulating midsole, and an outsole, with the circuit device being accommodated in the midsole and in contact with the insole and the outsole. 
     Still further in accordance with the preferred embodiment, the insole has a conductive ribbon in contact with a conductive layer and with the circuit device, the conductive layer contacting a foot of the wearer. 
     Still further in accordance with the preferred embodiment, the outsole comprises at least one of a conductive heel portion and a conductive tab portion in contact with the circuit device. 
     Still further in accordance with the preferred embodiment, the interface is any one of a wrist band and an ankle band, with the interface being grounded. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a top exploded view of an electric-charge protective shoe sole in accordance with an embodiment of the present invention; 
         FIG. 2  is a bottom exploded view of the electric-charge protective shoe sole of  FIG. 1 ; 
         FIG. 3  is a schematic view of an electric-charge protective wrist bracelet in accordance with another embodiment of the present invention; 
         FIG. 4  is a schematic view of an electric-charge protective ankle bracelet in accordance with another embodiment of the present invention; and 
         FIG. 5  is a circuit used by the electric-charge protective gear of  FIGS. 1 to 4 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings, and more particularly to  FIGS. 1 and 2 , an electric-charge protective shoe sole constructed in accordance with an embodiment is generally shown at  10 . The shoe sole  10  has from top to bottom a conductive layer  12 , an insole  14 , an assembly base  16  for upper, a middle sole  18  and an outsole  20 . Other layers can also be found in the shoe sole  10 , or some of the above described layers may be optional. 
     The conductive layer  12  is the interface between the foot of the wearer and the shoe sole  10 . 
     The insole  14  is the cleanliness layer, and is typically made of an absorbent material so as to absorb odors and humidity. The insole  14  is made of an electricity-insulating material. 
     The assembly base  16  is used to connect the upper to the shoe sole  10 . Although not shown for clarity purposes, the upper may be integrally part of the assembly base  16 . The assembly base features an opening  16 A. 
     The middle sole  18  interrelates the assembly base  16  to the outsole  20 . The middle sole  18  is made of an electricity-insulating material. 
     The outsole  20  is the interface of the shoe sole  10  with the ground. 
     Still referring to  FIGS. 1 and 2 , an electric-charge protective device as used with the shoe sole  10  is generally shown at  30 . The protective device  30  is an electronic component that is accommodated in the shoe sole  10 . The protective device  30  is associated with conductive elements so as to be part of the dissipation path for electric charges of the wearer. 
     More specifically, the protective device  30  has a top electrode  31 A and a bottom electrode  31 B. The top electrode  31 A contacts a top conductive ribbon  32 A. The top conductive ribbon  32 A contacts the conductive layer  12  as well by passing through a pair of openings in the insole  14 . As the insole  14  preferably consists of an insulating material, the conductive ribbon  32 A forms the conductive path between the wearer and the protective device  30 . 
     A bottom conductive ribbon  32 B contacts the bottom electrode  31 B of the protective device  30 , and contacts both conductive tab portion  33  in the front of the outsole  20 , and conductive heel portion  34  in the rear of the outsole  20 , by passing through the opening  16 A in the assembly base  16  and a pair of openings in the middle sole  18 . As the middle sole  18  preferably consists of an insulating material, the conductive ribbon  32 B forms the conductive path between the protective device  30  and the ground. The tab portion  33  and the heel portion  34  are typically made of conductive materials, such as selected conductive polymers or rubbers, amongst other materials. The conductive ribbons  32 A and  32 B are typically made of a metallic material, and may be as an alternative PCBs in the various layers of the shoe sole  10 . 
     In order to overcome the issues associated with prior-art devices, the electric-charge protective device provides variable resistance as a function of the electric charge accumulated by the wearer. The variable resistance of the protective device is offered in the form of a circuit devices exhibiting varying levels of resistance for discharging various levels of electric charge potentials. Referring to  FIG. 5 , a circuit of the protective device  30  is generally shown at  40 , and has the top electrode  31 A and the bottom electrode  31 B. 
     When discharge occurs from the wearer to the ground, thus from electrode  31 A to electrode  31 B, current must pass through transistors  41 AB/ 41 BA (first circuit device) and  42 AB/ 42 BA (second circuit device). The transistors are in an embodiment depletion-type mosfets (i.e., metal-oxide-semiconductor field-effect transistor), although other types of field-effect transistors can be used as well. The transistors  41 AB/ 41 BA and  42 AB/ 42 BA are selected so as to vary in resistance according to the discharge potential at one of the electrodes, their transistor threshold voltages and the voltages present at their gate (g), source (s) and drain terminals respectively, so as to operate in a depletion mode. In the present illustration, the depletion mode of operation is assured for transistors  41 AB and  42 AB when a positive voltage is present at electrode  31 A, that is, current flows from wearer to ground, since in such a case, Vgs is negative. 
     Similarly, the depletion mode of operation is assured for transistors  41 BA and  42 BA when a positive voltage is present at electrode  31 A, that is, current flows from wearer to ground, since in such a case, Vgs is positive. 
     With negative Vgs voltages present at the respective gate and source terminals of transistors  41 AB and  42 AB, and positive Vgs voltages present at the respective gate and source terminals of transistors  41 BA and  42 BA, all the transistors will decrease their conductivity as a function of the potential difference across electrodes  31 A and  31 B. That is, the overall resistance of the discharge path increases as a function of charge potential on the wearer. 
     Resistances  43  and  44  are respectively down-stream of the transistors  41 AB and  42 AB and provide more resistance to the discharge path, as well as means to detect whether or not each transistor shall operate in the depletion mode. 
     If the voltage present at the electrode  31 A, for example, is below a minimum threshold value, the transistors operate such that current does not flow from drain to source (or from source to drain) of transistors  41 AB/ 41 BA and  42 AB/ 42 BA equally, and thus offer a resistive path with a lower resistance than in the case where the voltage present at the electrode  31 A is greater than a threshold value. 
     If the voltage is above a maximum threshold value, the transistors  41 AB/BA and  42 AB/BA oppose resistances  43  and  44  to the current, along with their own internal resistances which vary according to the voltage differences established across each of the resistances  43  and  44  respectively. 
     The action of the transistors  41 AB/BA and  42 AB/BA combine to define intermediate values between the minimum and the maximum threshold values, with the effect of either one of the resistances  43  and  44  defining the respective operational resistivity or conductivity of each transistor, and thus the overall resistance offered by the resistive discharge path. 
     In one embodiment provided for illustrative purposes, the protective device  30  opposes an average resistance lower than 0.02 mega ohms for a voltage up to 5 V, an average resistance of about 0.4 mega ohms between 5 V and 100 V, an average resistance of about 1.0 mega ohms between 100 V and 300 V, and an average resistance about of 1.4 mega ohms above 300 V. Other threshold values could be set according to the contemplated use of the protective device. The above-referred threshold values are suitable to allow static charges of the wearer to be safely dissipated to ground, while protecting the wearer against static shock from ground. 
     It is noted that the protective device  30  offers protection in the case a greater potential is found at electrode  31 B (or ground) compared to electrode  31 A (wearer) since transistor  42 BA (or transistor  41 BA in case a single set of transistor is provided) blocks any discharge to the wearer. 
     The above-described protective device  30  can be adapted to offer similar variable resistance protection in the case a discharge path is required to flow from electrode  31 B to electrode  31 A (second direction) by inversing the n-type for p-type transistors, and the p-type for n-type in illustration of  FIG. 5 . Various other configurations are considered as well. 
     Although the circuit  40  is shown as having pairs of transistor/resistance sets in series in either direction, it is considered to limit the circuit  40  to a pair of transistor/resistance sets in a single direction. Also, a single transistor/resistance set or numerous transistor/resistance sets could be provided for the circuit  40  so as to customize the protective device  30  to its contemplated use. 
     It is contemplated to provide the variable-resistance protection in other formats. As an example, referring to  FIG. 3 , an electric-charge protective wrist bracelet is generally shown at  50 . The wrist bracelet  50  is conductively connected to wrist of the wearer, and features the protective device  30  as connected to the ground G by way of a wire  51 . In another example, referring to  FIG. 4 , an electric-charge protective ankle bracelet is generally shown at  60 . The ankle bracelet  60  is conductively connected to the ankle of the wearer, with the protective device being connected to the ground by way of a wire  61 .