Abstract:
A linear light-emitting diode (LED)-based solid-state lamp using an AC current control scheme and shock protection switches operates normally with an electronic ballast. Due to the use of shock protection switches in the two lamp bases at two opposite ends, the ballast-compatible LED lamp fully protects a person from possible electric shock during initial installation, maintenance, and re-lamping no matter what the rated power and the brand of the LLT lamp are and no matter whether the electronic ballast is existing used ones that may be incompatible with the lamp or faulty, leading to an unacceptable leakage current.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 14/135,116, filed on Dec. 19, 2013, which is a continuation-in-part and claims the priority benefit of U.S. patent application Ser. No. 13/525,249, filed Jun. 15, 2012 and issued as U.S. Pat. No. 8,749,167 on Jun. 10, 2014, which are incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     This invention relates to linear light-emitting diode (LED) lamps that work with linear tube lamp fixtures configured to electrically connect to an electronic ballast, and more particularly to an electric shock hazard-free linear LED tube lamp with a shock-protection mechanism. 
     BACKGROUND 
     Solid-state lighting from semiconductor light-emitting diodes (LEDs) has received much attention in general lighting applications today. Because of its potential for more energy savings, better environmental protection (with no hazardous materials used), higher efficiency, smaller size, and longer lifetime than conventional incandescent bulbs and fluorescent tubes, the LED-based solid-state lighting will be a mainstream for general lighting in the near future. Meanwhile, as LED technologies develop with the drive for energy efficiency and clean technologies worldwide, more families and organizations will adopt LED lighting for their illumination applications. In this trend, the potential safety concerns such as risk of electric shock become especially important and need to be well addressed. 
     In today&#39;s retrofit application of a linear LED tube (LLT) lamp to replace an existing fluorescent tube, consumers may choose either to adopt a ballast-compatible LLT lamp with an existing ballast used to operate the fluorescent tube or to employ an AC mains-operated LED lamp by removing/bypassing the ballast. Either retrofit application has its advantages and disadvantages. In the former case, although the ballast consumes extra power, it is straightforward to replace the fluorescent tube without rewiring, which consumers have a first impression that it is the best alternative to fluorescent tube lamps. But the fact is that total cost of ownership for this approach is high regardless of very low initial cost. For example, the ballast-compatible LLT lamps work only with particular types of ballasts. If the existing ballast is not compatible with the ballast-compatible LLT lamp, the consumer will have to replace the ballast. Some facilities built long time ago incorporate different types of fixtures, which requires extensive labor for both identifying ballasts and replacing incompatible ones. Moreover, a ballast-compatible LLT lamp can operate longer than the ballast. When an old ballast fails, a new ballast will be needed to replace in order to keep the ballast-compatible LLT lamps working. Maintenance will be complicated, sometimes for lamps and sometimes for ballasts. The incurred cost will preponderate over the initial cost savings by changeover to the ballast-compatible LLT lamps for hundreds of fixtures throughout a facility. When the ballast in a fixture dies, all the ballast-compatible tube lamps in the fixture go out until the ballast is replaced. In addition, replacing a failed ballast requires a certified electrician. The labor costs and messy maintenance costs will be unacceptable to the consumers. From energy saving point of view, a ballast constantly draws power, even when the ballast-compatible LLT lamps are dead or not installed. In this sense, any energy saved while using the ballast-compatible LLT becomes meaningless with the constant energy use by the ballast. Eventually, ballast-compatible LLT lamps are more expensive and less efficient than self-sustaining AC mains-operated LLT lamps. 
     Even with the above mentioned disadvantages of the ballast-compatible LLT lamps, consumers may still choose to use such lamps, considering only very low initial cost associated with a saving for expensive fixture rewiring. When power is applied to an electronic ballast designed to operate fluorescent tube, a high AC voltage starts to be created to ionize the gases inside the gas-filled fluorescent tube before sustaining continuous lighting. When a ballast-compatible LLT lamp used with such a ballast, the high AC voltage originally generated for starting up a fluorescent tube may reach 600 V or even a higher voltage of 950 VAC across a longer lamp. Voltages at these levels represent a strong shock hazard. Person who improperly handles the ballast-compatible LLT lamp by directly touching an exposed bi-pin or electrical connectors can result in severe injury or death. Therefore, a ballast-compatible LLT lamp, as its AC mains-operable counterparts working at 110, 220, or 277 VAC, has a construction issue related to product safety and needed to be resolved prior to wide field deployment. This kind of LLT lamps may fail a safety test, which measures through lamp leakage current. Because the high AC voltage from the ballast applies to both opposite ends of the tube when connected, the measurement of current leakage from one end to the other consistently results in a substantial current flow, which may present a risk of electric shock during re-lamping. Due to this potential shock risk to the person who replaces ballast-compatible LLT lamps in an existing fluorescent tube fixture, Underwriters Laboratories (UL) uses its standard, UL 935, Risk of Shock During Relamping (Through Lamp), to do the current leakage test and to determine if the ballast-compatible LLT lamps under test meet the consumer safety requirement. However, this safety issue related to the ballast-compatible LLT lamps has been ignored because such ballast-compatible LLT lamps can pass an initial test in laboratories with a particular electronic ballast. In fields, when used in fixtures that do not have the same brand of the electronic ballast as used in the lab test or have an existing ballast which has been used for years and may be faulty, such lamps may exist an electric shock hazard when used with the ballast. Ironically, a ballast-compatible LLT lamp has compatibility issues with existing ballasts in the fixtures not only to operate the lamp but to show an electric shock hazard. The safety issue needs to be resolved to protect consumers from being injured during relamping. 
     To verify that there exists such a ballast-dependent electric shock hazard, the inventors have measured leakage current from an exposed bi-pin of a ballast-compatible LLT lamp with the other bi-pin connected to the ballast for various combinations of different brands of electronic ballasts and ballast-compatible LLT lamps with a little different power ratings. In the experiments, three types of brand new and one type of used electronic ballasts and three types of brand new ballast-compatible LLT lamps are used. The results show that all the three lamps used with the used ballast have the largest leakage current among the tests, which could severely burn a person&#39;s finger skin although the ballast can normally operate all the three ballast-compatible LLT lamps under test. Other combinations also show unacceptable electric shock levels, burning the tester&#39;s finger skin to a certain degree. UL 935 suggests that a measurement instrument with body impedance model and frequency sensitive network be used to measure electric shock current. However, passing maximum meter indicating unit (M.I.U.) of 7.07 required in UL 935 does not mean that there is no shock hazard. As defined in UL 8750, a risk of electric shock exists between any two conductive parts or between a conductive part and earth ground if the open circuit potential is higher than 42.4 V peak AC, and the available current flow between them exceeds 0.5 mA as determined by the leakage current measurement test. In one experiment using a brand new electronic ballast and a new ballast-compatible LLT lamp, an open circuit potential of 75 V rms AC appears at the exposed bi-pin and earth ground, and a current that flow between them reaches 131 mA, well above 0.5 mA limit. The voltage and current at this level represents an unacceptable electric shock hazard to users or installers. 
     Although electrical power to the entire fixture needs to be disconnected when servicing an existing fluorescent fixture and three or four ballast-compatible LLT lamps in the fixture, it is not always practical in situations where a large number of fixtures are controlled from the same power switch such as in open office areas. In this case, risk of electric shock is unavoidably high to the person who does servicing. Fluorescent lamp ballasts can fail in many failure modes such as leaving and operating burned-out lamps in the fixture, using the wrong size lamps, improper wiring, incorrect line voltage, operating at temperatures below or above the rated limits, power surges, and even the age. However, not all the ballasts fail and stop functioning-many just overheat. So a severe problem occurs when a ballast is still functioning but has significant amount of leakage current that may introduce a shock hazard, and when a user tries to replace a ballast-compatible LLT lamp in the fixture that has such ballast, without knowing the risk of such an electric shock. Many even mistakenly believe that through the electronic ballast as an electrical buffer, there is no risk of electric shock for an exposed bi-pin when the other bi-pin is installed and energized. 
     When there are various kinds of LLT lamps on the LED lighting market, there are various kinds of configurations of linear fixtures, and misapplications of power supply may occur. For example, installing a ballast-compatible LLT lamp in the fixture that is intended for an application of AC mains of 277 V may burn some of the electronic components not rated at 400 V peak in the lamp, which create a fire hazard. Above all, a power source of AC mains at 277 VAC is different from that of an electronic ballast, which has an internal protection circuitry to shut down the operation of the ballast once short circuit is detected. So the design of a ballast-compatible LLT lamp must take this into account by removing such a risk. 
     SUMMARY 
     In one aspect, a ballast-compatible linear LED tube (LLT) lamp comprises: an elongated housing comprising two ends, each end comprising a bi-pin used to receive power from an electronic ballast; an LED printed circuit board (PCB) with a plurality of LEDs in LED arrays; an LED driver; and a shock protection mechanism implemented with two shock protection switches, is used to replace a fluorescent tube in an existing tube lamp fixture that has an existing electronic ballast. A general myth is that through the electronic ballast, there is no risk of electric shock for an exposed bi-pin when the other bi-pin is installed and energized. In fact, the electric shock does happen if no shock protection mechanism is adopted in the ballast-compatible LLT lamp. When such a ballast-compatible LLT lamp with the shock protection mechanism employed is installed in the fixture, the shock protection switches on both ends of the LLT lamp can effectively block an electric current flowing from the installed bi-pin that is energized, through the LED driver and the LED arrays to an exposed bi-pin not yet installed in fixture sockets, no matter what ballast brand, and rated power are and no matter whether the electronic ballast is existing used one that may have unacceptable leakage current. Therefore, no high AC voltage or leakage current will possibly appear at the exposed bi-pin during initial installation, maintenance, or re-lamping, thus completely eliminating risks of electric shocks. 
     The LED driver of the ballast-compatible LLT lamp comprises an interface module that has two termination devices and a frequency sensitive circuit, a voltage sensing and control module, and an LED driving module. The frequency sensitive circuit possesses frequency-dependent impedance employed to control input current with frequency in a range of 35˜50 KHz from the electronic ballast in the fixture used, which substantially controls power of LED arrays. When such a ballast-compatible LLT lamp is accidently installed in a fixture intended for AC mains applications, the frequency sensitive circuit can effectively oppose 60 Hz current flowing into the voltage sensing and control module, which in turn suppresses a dangerously high voltage to be generated, thus eliminating possible damages to the electronic components in the LED driving module if there is no voltage limiting circuit therein. In one embodiment, each of the two termination devices in the interface module is connected between the two pins of the bi-pins through each of the shock protection switches to help the electronic ballast complete its preheat and arc discharge process and function to turn on the ballast-compatible LLT lamp normally. With this scheme, consumers can safely install such a ballast-compatible LLT lamp in an existing lamp fixture used to operate a conventional fluorescent tube no matter what the electronic ballast type is instant-start type or rapid-start type, without operational uncertainty and electric shock hazard. That is, if an instant-start electronic ballast exists in the fixture the lamp will operate normally no matter whether the lamp sockets in the fixture are shunted or not, and no matter how the lamp sockets are wired. In one embodiment, AC power from an electronic ballast coupling to any two pins of four pins in the LLT lamp can operate the lamp without operational uncertainty. If a rapid-start electronic ballast is in the fixture, the ballast-compatible LLT lamp will still work with the two termination devices that provide preheat and arc discharge paths to turn on the ballast-compatible LLT lamp. When the electronic ballast dies, consumers may choose to replace it with a new one and relamp such ballast-compatible LLT lamps in the fixture without worrying about possible electric shock hazard that may occur when they accidentally touch an exposed bi-pin on the other end of the lamp. 
     The claims and advantages will be more readily appreciated as the inventive concept becomes better understood by reference to the following detailed description and the accompanying drawings showing exemplary embodiments, in which like reference symbols designate like parts. For clarity, various parts of the embodiments in the drawings are not drawn to scale. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to aid further understanding of the present disclosure, and are incorporated in and constitute a part of the present disclosure. The drawings illustrate a select number of embodiments of the present disclosure and, together with the detailed description below, serve to explain the principles of the present disclosure. It is appreciable that the drawings are not necessarily in scale as some components may be shown to be out of proportion than the size in actual implementation in order to clearly illustrate the concept of the present disclosure. 
         FIG. 1  is a block diagram of a ballast-compatible LLT lamp employing double shock protection switches operable with an instant-start electronic ballast according to the present invention, in which the lamp sockets are shunted. 
         FIG. 2  is a block diagram of a ballast-compatible LLT lamp employing double shock protection switches operable with a rapid-start electronic ballast according to the present invention, in which the lamp sockets are non-shunted. 
         FIG. 3  is a preferred embodiment of a ballast-compatible LLT lamp employing double shock protection switches operable with a rapid-start electronic ballast according to the present invention. 
         FIG. 4  is another embodiment of a ballast-compatible LLT lamp employing double shock protection switches operable with an instant-start electronic ballast according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In  FIGS. 1-4 , a ballast-compatible LLT lamp employs double shock protection switches to operate with an electronic ballast. The ballast-compatible LLT lamp  100  ( FIG. 1  and  FIG. 4 ) or  101  ( FIG. 2  and  FIG. 3 ) comprises a housing having two ends; two lamp bases  660  and  760  having respective bi-pins  250  and  350  at each end of the housing; two actuation mechanisms  640  and  740  of double shock protection switches  610  and  710  respectively in the two lamp bases  660  and  760 ; an LED driver  801 ; and LED arrays  807 . The LED driver  801  comprises an interface module  802 , a voltage sensing and control module  805 , and an LED driving module  806 . The interface module  802  has four interface ports  402 ,  404 ,  406 , and  408 . 
     The protection switch  610  at the lamp base  660  is of double-pole single-throw type, which consists of one actuation mechanism  640  and two sets of electrical contacts. In the first set, the first electrical contact connects to the upper pin of the bi-pin  250  at an electrical contact  401  whereas the second electrical contact connects to the interface port  402  of the interface module  802 . In the second set, the first electrical contact connects to the lower pin of the bi-pin  250  at an electrical contact  403  whereas the second electrical contact connects to the interface port  404  of the interface module  802 . Similarly, the shock protection switch  710  at the other lamp base  760  comprises one actuation mechanism  740  and two sets of electrical contacts with electrical contacts  405  and  407  respectively connecting to the two pins of bi-pin  350  and the interface ports  406  and  408  of the interface module  802 . 
       FIG. 1  is a block diagram of a ballast-compatible LLT lamp employing double shock protection switches operable with an instant-start electronic ballast according to the present invention, in which the lamp sockets are shunted. The lamp bases  660  and  760  respectively use the bi-pins  250  and  350  to connect to the instant-start electronic ballast  700  through the shock protection switches  610  and  710  to the interface module  802 . The shock protection switches  610  and  710  are normally “off” when the lamp is not installed in fixture lamp holders  810  and  820 . When actuated (pressed in, twisted on, etc.), the actuation mechanisms  640  and  740  respectively actuate the protection switches  610  and  710  and turn on the connection between the instant-start electronic ballast  700  and the interface module  802 . The interface module  802  manages to provide an input impedance similar to a fluorescent tube to activate the ballast operation, receives power from the instant-start electronic ballast  700 , and adjusts the ballast output current to flow into the voltage sensing and control module  805  where the AC current is guided to flow into an LED driving module  806  which powers the LED arrays  807 . The interface module  802  also checks the frequency of the received voltage. If a high frequency such as 35˜50 KHz generated from the instant-start electronic ballast  700  is detected, the interface module  802  allows the current to flow into the voltage sensing and control module. When the ballast-compatible LLT lamp is accidently installed in an AC mains operable fixture, the interface module  802  can detect the frequency of 50˜60 Hz and oppose the current flowing into the voltage sensing and control module  805 , thus deactivating operation of the LED driving module  806 . 
     When someone tries to install the ballast-compatible LLT lamp  100  in the fixture with the instant-start electronic ballast  700  wired with the lamp sockets shunted as in  FIG. 1 , she or he needs to first insert, for example, the lamp base  660  to the fixture lamp holder  810 . The actuation mechanism  640  is actuated to turn on both sets of electrical contacts on the shock protection switch  610  such that a high start-up voltage generated from the instant-start electronic ballast  700  appear at the interface ports  402  and  404  of the interface module  802 . Although no current flowing into the voltage sensing and control module  805  because the lamp base  760  has not yet been inserted into the lamp holder  820  to form a current return path, the voltage sensing and control module  805  is energized, so as the LED driving module  806  and the LED arrays  807 . If no shock protection switch  710  is in the lamp base  760 , the high start-up voltage generated from the instant-start electronic ballast  700  through the interface module  802 , the voltage sensing and control module  805 , the LED driving module  806 , and the LED arrays  807  will appear at the bi-pin  350 . When the person touches the bi-pin  350 , a leakage current flowing through her or his body to the earth ground may burn her or his finger skin to a degree depending on how large the leakage current is. The leakage current varies from ballast to ballast and from ballast-compatible LLT lamp to lamp because their internal circuit configurations and rated power are different. If the shock protection switch  710  is in the lamp base  760  as shown, and as long as it is not inserted into the lamp holder  820 , the actuation mechanism  740  will not be actuated. So the shock protection switch  710  remains “off”, disconnecting any electric current flowing from the interface module  802 , to the exposed bi-pin  350 , and thus no leakage current can possibly flow out—no electric shock hazard. When the person who does the installation further inserts the lamp base  760  into the lamp holder  820 , the actuation mechanism  740  is actuated, which turns on the protection switch  710 , thus connecting the bi-pin  350  to the interface ports  406  and  408  of the interface module  802 . When the protection switch  710  is “on”, an electric potential difference exists between the interface ports  402  and  406 , and an electric current can flow from the input/output port  503  of the voltage sensing and control module  805  to the LED driving module  806  via input/output port  808  of the LED driving module  806 , further to the LED arrays  807  followed by a current return path from the LED driving module  806 , through the current return port  809  of the LED driving module  806 , the input/output port  503  of the voltage sensing and control module  805 , and the interface ports  406  and  408  to the bi-pin  350 , thus delivering power to the LED driving module  806 , which then powers and lights up the LED arrays  807 . 
       FIG. 2  is a block diagram of a ballast-compatible LLT lamp employing double shock protection switches operable with a rapid-start electronic ballast according to the present invention, in which the lamp sockets are non-shunted. The lamp bases  660  and  760  respectively use the bi-pins  250  and  350  to connect to the rapid-start electronic ballast  701  through the shock protection switches  610  and  710  to the interface module  802 . The shock protection switches  610  and  710  are normally “off” when the lamp  101  is not installed in the fixture lamp holders  810  and  820 . When actuated (pressed in, twisted on, etc.), the actuation mechanisms  640  and  740  respectively actuate the protection switches  610  and  710  and turn on the connection between the rapid-start electronic ballast  701  and the interface module  802 . The rapid-start electronic ballast  701  has two pairs of wires, each connecting to one of the two bi-pins  250  and  350  of the ballast-compatible LLT lamp  101 . When both ends are installed in the fixture, both shock protection switches  610  and  710  are actuated to turn on the connection between the rapid-start electronic ballast  701  and the ballast-compatible LLT lamp  101 . The interface module  802  further provides two electric current paths for the rapid-start electronic ballast to function properly in its filament preheat and arc discharge process and to end up with a rated current provided to the lamp  101  to continuously operate. As in the LLT lamps working with the instant-start electronic ballast, the interface module  802  further limits the AC current from the rapid-start electronic ballast  701  and opposes the AC current directly from the AC mains to flow into the voltage sensing and control module  805 . The two current paths provided in the interface module  802  are needed to avoid turn-on failure occurred in the ballast-compatible LLT lamp  101  working with the rapid-start electronic ballast  701 . Similar to an instant-start ballast which create a high start-up voltage, a rapid-start ballast produces a high preheat and arc discharge voltage. If no shock protection switch  710  is in place as shown in  FIG. 2 , this high voltage can energize the LED driving module  806  and the voltage sensing and control module  805  and appear between the bi-pin  350  and the earth ground—an electric shock hazard, if two pins of the bi-pin  350  in the lamp base  760  are not in the sockets of the fixture lamp holder  820 . Note that the ballast-compatible LLT lamps  100  and  101  in  FIGS. 1 and 2  may have the same internal configuration. In that case, the interface module  802 , the voltage sensing and control module  805 , and the double shock protection switches  610  and  710  adopted in such a ballast-compatible LLT lamp can work with either instant-start or rapid-start electronic ballast free of operational uncertainty and electric shock hazard. For illustration purpose, the shock protection switches  610  and  710  are both of contact type, which can be a snap switch, a push-button switch, a micro switch, a twist-on switch, a rotary switch, or any home-made switches that perform switch functions. In reality, the shock protection switch can be of a non-contact type, such as electro-mechanical, electromagnetic, optical, electro-optic, fiber-optic, infrared, or wireless based. Furthermore, the non-contact shock protection switch can be of a sensing type, having a proximity control with a sensing range up to 8 mm. 
       FIG. 3  is a preferred embodiment of a ballast-compatible LLT lamp employing double shock protection switches operable with a rapid-start electronic ballast according to the present invention. The interface module  802  in  FIG. 2  is embodied by two termination devices  803  and a frequency sensitive circuit  804 . The two termination devices  803  are embodied by two capacitors C 1  and C 2 , each connecting the two pins of the bi-pins  250  and  350  through the shock protection switches  610  and  710 . The two capacitors C 1  and C 2 , each terminated at each end of the lamp are used to provide filament preheat and arc discharge path ensuring proper electronic ballast functions. The frequency sensitive circuit  804  comprises two capacitors C 3  and C 4 , each connecting one pin of the bi-pins  250  and  350  through the shock protection switches  610  and  710  to the voltage sensing and control module  805 . The two capacitors C 3  and C 4  are a frequency sensitive component, of which the reactance is −1/(2πfC), where f is a frequency of the voltage across its terminals, and C is a capacitance. For a frequency of 40 KHz for an AC voltage from an electronic ballast, the impedance across each capacitor C 3  or C 4  with a specific capacitance is hundreds of ohms, which can be used to control input current flowing into the voltage sensing and control module  805 , thus conditioning power of LED arrays. For a frequency of 60 Hz of a voltage from the AC mains, the impedance can be as high as hundreds of kilo-ohms, thus significantly reducing the electric current to flow into the voltage sensing and control module  805 . In  FIG. 3 , the voltage sensing and control module  805  is embodied by four diodes used to guide electric current to flow by sensing electrical potentials. The four diodes are interconnected with four input/output ports  201 ,  202 ,  203 , and  204 . When an AC voltage from the bi-pins  250  or  350  appears between the input/output ports  201  and  203 , the diodes sense electrical potential difference between the neighboring input/output ports, for example,  201  and  202 ,  201  and  204 ,  203  and  204 , and  203  and  202 , and conduct the electric current only when forward biased. In this sense, the input/output port  202  always has a high electrical potential with respect to a low electrical potential at the input/output port  204 . Thus, the current flows into the high electrical potential port  808  of the LED driving module  806  and further delivers to LED arrays  807 . The current returns through the low electrical potential port  809  of the LED driving module  806  to the input/output port  204  of the voltage sensing and control module  805 . Then the diodes sense the electrical potential and guide the current to further flow out to the bi-pin  350  or  250 , completing delivery of power to the LED arrays  807 . As shown in  FIG. 3 , if the lamp base  660  is first installed in the fixture lamp holder  810  and if no shock protection switch  710  is in the lamp base  760 , the high preheat and arc discharge voltage from the rapid-start electronic ballast can energize the LED driving module  806  and the voltage sensing and control module  805  and appear between the bi-pin  350  and the earth ground—an electric shock hazard, if two pins of the bi-pin  350  in the lamp base  760  are not in the sockets of the fixture lamp holder  820 . 
     In  FIG. 3 , although the electronic ballast  701  is of a rapid-start type, the ballast-compatible LLT lamp  101  can also work in an instant-start electronic ballast. The instant-start electronic ballast has only one pair of wires, each of the two wires connecting to one of the lamp sockets in the fixture lamp holders  810  and  820 . When the lamp sockets are shunted, the AC voltage provided by the electronic ballast can directly bypass the capacitors C 1  and C 2  and appear at the input ports of the frequency sensitive circuit  804 , operating the LED driving module  806  and powering the LED arrays  807 . If the lamp sockets are not shunted, and the two wires from the instant-start electronic ballast are, for example, connected to lower pins of the bi-pins  250  and  350 , the high frequency AC voltage provided by the electronic ballast can still pass the capacitors C 1  and C 2  due to the frequency-impedance feature of the capacitors used and reach the input ports of the frequency sensitive circuit  804 , thus operating the LED driving module  806  and powering the LED arrays  807 . 
       FIG. 4  is another embodiment of a ballast-compatible LLT lamp employing double shock protection switches operable with an instant-start electronic ballast according to the present invention. The interface module  802  is embodied by four capacitors  205 ,  206 ,  207 , and  208 , each connecting to one of the four pins in the bi-pin  250  and  350  via the shock protection switches  610  and  710 . The four capacitors  205 ,  206 ,  207 , and  208  in interface module  802  are frequency sensitive devices used to control AC current to flow into the voltage sensing and control module  805 . The voltage sensing and control module  805  is embodied by two bridge rectifiers  603  and  604 . The first bridge rectifier  603  has four diodes interconnecting at four input/output ports  301 ,  302 ,  303 , and  304 . Similarly, in the second bridge rectifier  604 , the four diodes are interconnected at four input/output ports  305 ,  306 ,  307 , and  308 . The two bridge rectifiers  603  and  604  are electrically connected in parallel such that the positive and the negative input/output ports  302  and  304  of the first bridge rectifier  603  respectively connect to the positive and the negative input/output ports  306  and  308  of the second bridge rectifier  604 . The common positive and negative input/output ports of the two bridge rectifiers  603  and  604  are then respectively connected to the two input/output ports  808  and  809  of the LED driving module  806 . Furthermore, the eight diodes in the two bridge rectifiers are partially paired to perform a full wave rectification of the AC voltage from the electronic ballast  700  in a lamp fixture. When an AC voltage from the instant-start electronic ballast is applied at bi-pins  250  and  350 , an electrical potential difference appears between the input/output ports  302  and  308  or between the input/output ports  306  and  304 , depending on the positive or the negative cycle of the AC voltage applied at the bi-pin  250  or the bi-pin  350 . If the positive cycle of the AC voltage appears at the bi-pin  250 , the two diodes of the first bridge rectifier  603  facing the second bridge rectifier  604  are forward biased and conduct the current while the two diodes of the second bridge rectifier  604  facing the first bridge rectifier  603  are reverse biased and prohibit the current flow across them. The current is then forced to flow into the high electrical potential port  808  of the LED driving module  806 , to the LED arrays  807 , returning to the low electrical potential port  809  of the LED driving module  806 . Since the positive cycle of the AC voltage is from the bi-pin  250 , the two diodes of the first bridge rectifier  603 , interconnected at the input/output ports  304 , are reverse biased and prohibit the current flow across them while the two diodes of the second bridge rectifier  604 , interconnected at the input/output ports  308 , are forward biased and conduct the current flowing to the bi-pin  350  through the shock protection switch  710 . Similarly for the negative cycle of the AC voltage, the current flows from the bi-pin  350  to the bi-pin  250 , thus completing a power delivery to the LED arrays  807 . In  FIG. 4 , each of the capacitors  205 ,  206 ,  207 , and  208  in the interface module  802  is connected between one of two sets of the electrical contacts of each shock protection switch and one of the input/output ports of each bridge rectifier. The embodiment of the two bridge rectifiers  603  and  604  enables the ballast-compatible lamp to receive power from any two pins of the bi-pins  250  and  350  and to operate the lamp normally. As other embodiments mentioned in this patent application, if the lamp base  660  is first installed in the fixture lamp holder  810  and if no shock protection switch  710  is in the lamp base  760 , the high start-up voltage from the instant-start electronic ballast  700  can energize the LED driving module  806  and the bridge rectifiers  603  and  604  and appear between the bi-pin  350  and the earth ground—an electric shock hazard, if the two pins of the bi-pin  350  in the lamp base  760  are not in the sockets of the fixture lamp holder  820 . 
     In the embodiment shown in  FIG. 3 , each of the capacitors C 1 , C 2 , C 3 , and C 4  may have a resistor connected in parallel as a snubber circuit for stable operation of the ballast-compatible LLT lamp and may comprise two or more capacitors. Each diode may comprise two or more diodes connected in series. In  FIG. 4 , each of the capacitors  205 ,  206 ,  207 , and  208  in the interface module  802  may comprise two or more capacitors, and the capacitors  205  and  206  or the capacitors  207  and  208  may be removed from the interface module  802 . All of these and other modifications are possible art without departing from the scope of this invention. Furthermore, although embodied by a bridge rectifier and passive electronic components, the voltage sensing and control module  805  and the interface module  802  in the LED driver  801  can be of non-hardware type, implemented by an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or a microcontroller. 
     Whereas preferred embodiments of the invention have been shown and described, it will be realized that any other alterations, modifications, and improvements may be made thereto without departing from the scope of the following claims. Another interface module and voltage sensing and control mechanism in a ballast-compatible LED linear tube lamp using various kinds of combinations to accomplish the same or different objectives could be easily adapted for use from the present invention. Accordingly, the foregoing description and attached drawings are by way of example only, and are not intended to be limiting. 
     In the present invention, double shock protection switches are incorporated into a ballast-compatible LLT lamp for operating with an electronic ballast. Because leakage current flowing from a live bi-pin, through the LED driver, to an exposed bi-pin is interrupted by the double shock protection switches, the ballast-compatible LLT lamp is electric shock hazard-free. 
     Additional and Alternative Implementation Notes 
     Although the techniques have been described in language specific to certain applications, it is to be understood that the appended claims are not necessarily limited to the specific features or applications described herein. Rather, the specific features and examples are disclosed as non-limiting exemplary forms of implementing such techniques. 
     As used in this application, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or.” That is, unless specified otherwise or clear from context, “X employs A or B” is intended to mean any of the natural inclusive permutations. That is, if X employs A; X employs B; or X employs both A and B, then “X employs A or B” is satisfied under any of the foregoing instances. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more,” unless specified otherwise or clear from context to be directed to a singular form. 
     For the purposes of this disclosure and the claims that follow, the terms “coupled” and “connected” may have been used to describe how various elements interface. Such described interfacing of various elements may be either direct or indirect.