DESC:
[001] The present invention relates generally to electrical testing apparatuses, and more particularly to a leak proof pressurized high voltage testing apparatus.

BACKGROUND OF THE INVENTION

[002] Stress cones are a means of preventing insulation failure at the termination of a shielded cable caused by the high concentration of flux and the high potential gradient that would otherwise exist between the shield termination and the cable conductor.

[003] A stress cone (also called stress relief cone) is made by increasing the separation between the shield termination and the conductor cable. Precautions must be taken to insure a termination which is free of dirt and foreign matter and insulating compounds and tapes must be applied in a prescribed manner. Potheads may also be used with the stress relief cone termination. A typical use for stress cones would be in transformers pluggable bushing, GIS termination, Outdoor dry / fluid filled cable termination, cable joint, high-voltage motor installations or in most applications were shielded cable is being terminated.

[004] High-voltage alternating current cable terminations are generally tested under the IEC standard test procedures. The procedure sets forth, inter alia, design tests to be performed by the manufacturer to obtain information on the performance of a high voltage cable termination.

[005] The design tests of the IEC procedure that are particularly useful in determining the effectiveness of a termination are a stress control arrangement include the “Partial Discharge Test,” the “Power Frequency Voltage Withstand Test” and the “Lightning Impulse Voltage Withstand Test.”

[006] In the Partial discharge (PD) test, electrical discharge in the termination is measured at specific applied voltages and has to be below specific values. Also the voltage at which the discharge extinguishes is measured and has to be above specific values.

[007] In the power frequency voltage withstand tests the specified voltage is applied to the cable and should be withstood without flashover or other dielectric breakdown.

[008] In the impulse voltage withstand test, impulses of specific value and waveform are applied to the cable and should be withstood without flashover or other dielectric breakdown. The voltage at which flashover occurs should be above specific values. The discharge, power frequency voltage and impulse voltage performance of the termination should meet the requirements set forth in the IEC Standard Test procedures.

[009] If the testing apparatus is operated in atmospheric pressure, then it will lose its electrical capability to withstand the applied voltage. For the test of stress cones, it has to be installed on a cable and put in a SF6 gas chamber and then the High Voltage (HV) is applied. Various methods known in the art are used to measure the PD and other parameters needed for the tests. Hence, it is required to operate the vessel at high pressure & take proper care for leak proof.

[0010] Since the HV cable consists of a strand of conductors at its core, so when the stress cone is installed on a piece of cable inside a pressurized chamber, the used gas may leak from in between the conductor strands.

[0011] The currently available HV testing apparatuses do not attempt to address this problem. In view of the limitations inherent in the available transformers and methods of cooling the transformer, there exists a need HV testing apparatuses, there exists a need for an improved apparatus for pressurized HV testing, which overcomes the disadvantages of the prior art and which can be used in a simple, cost effective, reliable, secure and environmental friendly manner.

[0012] The present invention fulfils this need and provides further advantages as described in the following summary.

SUMMARY OF THE INVENTION

[0013] In view of the foregoing disadvantages inherent in the prior arts, the general purpose of the present invention is to provide an improved combination of convenience and utility, to include the advantages of the prior art, and to overcome the drawbacks inherent therein.

[0014] A primary objective of the present invention is to provide an improved apparatus for pressurized HV testing without any leakages.

[0015] In one aspect, the present invention provides a leak proof high voltage testing apparatus that includes a cylindrical chamber having a cable entry side and a cover side, a cable disposed in the cylindrical chamber, the cable having a proximal end and a distal end, one entry seal plate for closing the cable entry side of the chamber, one cover plate for closing the cover side of the chamber, a cable entry side sealing arrangement mounted on the entry seal plate to prevent any passing of fluid across the entry seal plate from the proximal end of the cable and a conductor side sealing arrangement mounted on the distal end of the cable to prevent any passing of fluid across the chamber through the distal end of the cable.

[0016] In another aspect of the present invention, the cable entry side sealing arrangement includes a sealing tube disposed in the through hole of the entry seal plate, a union screw adapted to engage with the sealing tube to close the cable entry side, a first O-ring seal disposed between the cable and the sealing tube and a second O-ring seal disposed between the cable and the union screw.

[0017] In a further aspect of the present invention, the conductor side sealing arrangement includes a shear head bolt connector adapted to enclose and snugly fit on the distal end of the conductor, a conducting tape mounted on the intersection of an open end of the shear head bolt connector and a cable conductive screen, a shear head bolt hole on the periphery of the shear head bolt connector, a shear head bolt disposed in the shear head bolt hole, such that when the cable is inserted in the shear head bolt connector, the shear head bolt is tightened to hold the cable and a spherical connector having a cylindrical bore with inner threads which are adapted to detachably engage with the outer threads of the shear head bolt connector.

[0018] In one aspect of the present invention, the distal end of the cable includes a plurality of exposed conductor strands and the proximal end of the cable includes an insulating sheath covered portion of the cable.

[0019] In another aspect of the present invention, the leak proof high voltage testing apparatus, further includes a self-curing silicone sealant applied over the shear head bolt after the head is sheared off to prevent any leakage of fluid across the shear head bolt.

[0020] In one aspect of the present invention, the sealing tube and the union screw are engaged using thread mechanism, such that the first O-ring seal and the second O-ring seal gets compressed to prevent any leakage.

[0021] In another aspect of the present invention, the leak proof high voltage testing apparatus is used to test a device mounted on the cable by pressurising the chamber with a dielectric gas and applying high voltage.

[0022] In yet another aspect of the present invention, the wherein the device tested is a stress cone and the dielectric gas one of SF6 and dry Nitrogen.

[0023] These together with other aspects of the invention, along with the various features of novelty that characterize the invention, are pointed out with particularity in the description annexed hereto and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be made to the accompanying drawings and descriptive matter in which there are illustrated exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The advantages and features of the present invention will become better understood with reference to the following more detailed description taken in conjunction with the accompanying drawings in which:

[0025] FIG. 1 illustrates a schematic view of a leak proof high voltage testing apparatus, according to one embodiment of the present invention;

[0026] FIG. 2 illustrates a schematic view of the cable entry side sealing arrangement, according to one embodiment of the present invention; and

[0027] FIG. 3 illustrates a schematic view of the conductor side sealing arrangement, according to one embodiment of the present invention.

[0028] Like reference numerals refer to like parts throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS

[0029] In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details.

[0030] As used herein, the term ‘plurality’ refers to the presence of more than one of the referenced item and the terms ‘a’, ‘an’, and ‘at least’ do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.

[0031] Reference herein to “one embodiment” or “another embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Further, the diagrams representing one or more embodiments of the invention do not inherently indicate any particular order nor imply any limitations in the invention.

[0032] Referring to FIG. 1 illustrates a schematic view of a leak proof high voltage testing apparatus 10, according to one embodiment of the present invention. The leak proof high voltage testing apparatus 10 comprises of a cylindrical chamber 12 having a cable entry side 14 and a cover side 16, a cable 18 disposed in the cylindrical chamber 12, at least one entry seal plate 20 for closing the cable entry side 14 of the chamber, at least one cover plate 22 for closing the cover side 16 of the chamber, a cable entry side sealing arrangement 200 mounted on the entry seal plate 20 to prevent any passing of fluid across the entry seal plate from the proximal end 18P of the cable and a conductor side sealing arrangement 300 mounted on the distal end 18D of the cable to prevent any passing of fluid across the chamber through the distal end 18D of the cable.

[0033] The cylindrical chamber 12 is a hollow cylinder open from both sides. The open side of the cylinder from where the cable 18 in entered into the chamber 12 is referred to as cable entry side 14 and other open side as cover side 16. The cover side 16 is closed by using the cover plate 22. In one embodiment of the present invention, the cover plate 22 includes a gas filing, recovery arrangement and protective and measuring instruments used in the test. In one embodiment of the present invention, the entry seal plate 20 and the cover plate 22 are attached to the cylindrical chamber 12 by nuts and bolts.

[0034] The entry seal plate 20 includes a through hole adapted to allow passing of the cable 18 into the chamber 12. The diameter of the through hole is larger than the cable 18 diameter at its biggest portion, such that the through hole holds the cable 18 and also facilitates mounting of the cable entry side sealing arrangement 200.

[0035] The cable 18 is having a proximal end 18P and a distal end 18D, such that the proximal end 18P is at the cable entry side 14 of the chamber. The cable 18 is the cable available nowadays which includes a plurality of bunched conductors, a conductive screen wrapped over the bunched conductors, XLPE, insulating sheath portion and outer sheath. Some cables may include further components.

[0036] In one embodiment of the present invention, the distal end 18D of the cable includes a plurality of exposed conductor strands and the proximal end 18Pof the cable includes an insulation portion of the cable. The cable 18 is disposed in the chamber 12 such that its distal end 18D is between the two open sides of the cylindrical chamber 12 and the proximal end 18P is located at the entry seal plate 20. The cable extends beyond the proximal end 18P outside the chamber 12 to make the required electrical connections.

[0037] Now referring to FIG. 2 that illustrates a schematic view of the cable entry side sealing arrangement 200, according to one embodiment of the present invention. The cable entry side sealing arrangement 200 is disposed in the through hole of the entry seal plate 20 with different components to seal the opening, so that the pressurized gas contained inside the chamber 12 does not leak from the gap between the outer periphery of the cable 18 and inner diameter of the through hole of the entry seal plate 20 from the proximal end 18P of the cable.

[0038] The cable entry side sealing arrangement 200 includes a sealing tube 202 disposed in the through hole of the entry seal plate 20, a union screw 204 adapted to engage with the sealing tube 202 to close the cable entry side, a first O-ring seal 206 disposed between the cable and the sealing tube 202 and a second O-ring seal 208 disposed between the cable and the union screw 204.

[0039] The sealing tube 202 is a cylindrical sleeve which circumvents the proximal end 18P of the cable. The outer diameter of the sealing tube 202 is almost equal to the inner diameter of the through hole of the entry seal plate 20, so that it fits tightly leaving no scope for any leakage of gas from the gap between the sealing tube outer surface and inner surface of the through hole of the entry seal plate 20. In one preferred embodiment of the present invention the sealing tube 202 is welded in the through hole of the entry seal plate 20. Similarly the inner diameter of the sealing tube 202 is almost equal to the outer diameter of the proximal end 18P of the cable which includes the insulation portion of the cable, thereby leaving no gap in between allowing any gas to escape from the chamber 12. The gap may further include some sealing lubricant or tape to prevent any gas leakage. The sealing tube 202 has a first portion which is disposed inside the chamber 12 and a second portion which is outside the chamber 12. The union screw 204 engages with the portion of the sealing tube 202 which is outside the chamber.

[0040] The union screw 204 has an opening to allow passing of the cable 12. The union screw a hollow cylinder with round shape from one side and a tapered conical shape at second side. The union screw 204 has two different diameters along its length. The first diameter is almost same to the outer diameter of the sealing tube 202, so as to accommodate the sealing tube 202 inside it along its length. The sealing tube 202 is disposed upto a predefined depth in the union screw 204 along its axis. Beyond the predefined depth, the second diameter starts which extends to the end of the conical shape portion of the union screw 204. The second diameter is same to the outer diameter of the proximal end 18P of the cable. The second diameter is less than the first diameter of the union screw creating a step along the hole, such that it prevents the sealing tube 202 to go inside the union screw 204 beyond the predefined depth. The first diameter, second diameter and the predefined depth of the union screw 204 is selected as per the engineering design requirements to seal the cable. A mechanical tolerance between the same diameters of the interacting components is also provided for installation and movement as per the knowledge of state of the art in mechanical engineering.

[0041] The sealing tube 202 is provided with threads on its outer surface and the union screw 204 is provided with corresponding threads on its inner surface, such that the sealing tube and the union screw engage and tightly connect with each other. The union screw nut 204 further encloses the sealing tube 202 over its top portion.

[0042] The assembly of the sealing tube 202 and union screw 204 over the cable 18 provides good leakage prevention, however to further make it better two O-ring seals are provided. A ‘first O-ring seal’ 206 is disposed between the cable 18 and the sealing tube 202. The sealing tube 202 includes a groove on its inner surface located at the end which engages with the union screw 204 at the predefined depth. The ‘first O-ring seal’ 206 is disposed in the groove and prevents any gas leakage from the gap between the outer surface of the cable 18 and the inner surface of the sealing tube 202.

[0043] Similarly a ‘second O-ring seal’ 208 is disposed between the cable and the union screw 204. The union screw 204 also includes a groove on its inside surface located after the predefined depth and between its end of conical portion such that the ‘second O-ring seal’ 208 fills in the groove and prevents any gas leakage from the gap between the outer surface of the cable 18 and the inner surface of the union screw 204.

[0044] In one embodiment of the present invention, the sealing tube 202 and the union screw 204 are engaged using thread mechanism, such that the ‘first O-ring seal’ 206 and the ‘second O-ring seal’ 208 gets compressed to prevent any leakage. The ‘first O-ring seal’ 206 is compressed when the union screw 204 is tightened over the sealing tube 202 and provides primary sealing. The ‘second O-ring seal’ 208 also gets compressed in the radial direction on the cable 18 and hence provide secondary sealing.

[0045] The cable entry side sealing arrangement 200 prevents gas leakage from the potential gap from the cable entry side 14, however the pressurized gas may leak from the distal end 18D which is conductor strand side as there is a possibility that gas will escape from the gap between conductor strands. To prevent this, the conductor side sealing arrangement 300 is provided as shown in FIG. 3.

[0046] Referring to FIG. 3, that illustrates a schematic view of the conductor side sealing arrangement 300, according to one embodiment of the present invention. The conductor side sealing arrangement 300 includes a shear head bolt connector 302 adapted to enclose and snugly fit on the distal end 18D of the conductor, an conducting tape 304 mounted on the intersection of an open end of the shear head bolt connector and a cable conductive screen 18C of the cable, and a spherical connector 310 having a cylindrical bore with inner threads which are adapted to detachably engage with the outer threads of the shear head bolt connector.

[0047] The shear head bolt connector 302 is a hollow cylindrical metallic sleeve closed from one side having threads on its outer surface. It receives the distal end 18D of the cable which has the plurality bunched exposed conductor strands of the cable from its open end. The shear head bolt connector 302 includes a shear head bolt hole 306 on the periphery of the shear head bolt connector and a shear head bolt 308 disposed in the shear head bolt hole 306, such that when the cable is inserted in the shear head bolt connector 302, the shear head bolt is tightened to hold the cable.

[0048] The shear head bolt hole 306 is located towards the closed end of the shear head bolt connector 302 and a head of the shear head bolt 308 shears off on tightening beyond a pre-defined amount of torque. This makes sure that the shear head bolt 308 is tightened to the required torque for properly holding the cable 18. The dimensions and pre-defined torques of the shear head bolt 308 is designed as per the engineering design requirements of the cable 18 and the testing apparatus 10. In one embodiment, the present invention further includes a self-curing silicone sealant 312 applied over the shear head bolt 308 after the head is sheared off to prevent any leakage of fluid from the shear head bolt hole 306.

[0049] After the shear head bolt connector 302 is secured over the cable, the conducting tape 304 is wrapped over the intersection of the open end of the shear head bolt connector 302 extending to a portion of the cable conductive screen 18C of the cable. The conducting tape 304 may be a self-amalgamating adhesive or tape made from a material available in the state of the art to provide sealing. The cable conductive screen 18C also prevents any leakage and the conducting tape 304 further adds to the functionality and prevents any leakage from the distal end 18D of the cable.

[0050] The spherical connector 310 is like a sphere with a cylindrical bore into it. The cylindrical bore includes inner threads which are adapted to engage with the outer threads of the shear head bolt connector 302 tightly. The shape and size of the cylindrical bore is also derived from the shape and size of the shear head bolt connector 302. The spherical connector 310 in addition to tightly gripping the cable 18 via the shear head bolt connector 302, further prevents formation of sharp edges at the cable end 18 which can result occurring of partial discharge in the test situation when high voltage is applied on the cable 18.

[0051] In one embodiment, the leak proof high voltage testing apparatus 10 of the present invention is used to test a device mounted on the cable 18 by pressurising the chamber 12 with a dielectric gas and applying high voltage.

[0052] For using the apparatus 10 for testing a device, the cable 18 is inserted from the cable entry side 14 of the chamber 12. While preparing the cable 18, the conductive screen 18C of the cable is removed from over the plurality of bunched conductors upto a length which is supposed to be enclosed in the shear head bolt connector 302. The conductive screen 18C is retained beyond this length. This will ensure gas will not escape from the XLPE and cable conductor joint , a gap created due to shrinkage of XLPE.

[0053] Now the distal end 18D with the conductor strands is inserted in the shear head bolt connector 302 and the shear head bolt 308 is tightened till its head breaks, ensuring the correct amount of tightening achieved. The self-curing silicone sealant 312 is applied over the shear head bolt 308 after the head is broken. Now the conducting tape 304 is wrapped over the joint of shear head bolt connector and cable covering a portion of the conductive screen 18C on the cable as shown in FIG. 3.

[0054] Now after this, the device to be tested is mounted on the cable 18 from the distal end 18D by passing over the installed shear head bolt connector 202 and is secured at a required location. The dimensions of the shear head bolt connector are such that it allows the device to be tested to pass over it on the cable 18 even when the shear head bolt connector 302 is installed over the cable 18. The shear head bolt connector 302 also prevents damage to the device during its mounting or dis mounting with cable 18. This gives repeatability to the apparatus for testing different devices on same cable.

[0055] In next step, the spherical connector 310 is brought over the shear head bolt connector 302 and tightened to make the whole joint complete leak proof. Now the entry seal plate 20 along with cable entry side sealing arrangement 200 and the cover plate 22 are mounted to create the leak proof high voltage testing apparatus 10. The apparatus 10 is further provided with other instruments known in the art to prevent any leakage of gas from it and also maintaining the pressure inside it during the test. The apparatus further includes HV supplying means and designed to withstand the HV without any damage to itself and other components connected to it including the apparatus 10 of the present invention. The insulation design of the apparatus chamber 12 is done in such that it will be able to withstand test voltage required for testing upto 220 kV test object.

[0056] Now the gas is filled in the chamber to the required pressure and then required high voltage is applied for testing the device. The apparatus 10 further includes various sensors and measuring instruments to measure and monitor the parameters as per the test requirements.

[0057] In one embodiment of the present invention, the device to be tested is a stress cone and the dielectric gas is at least one from a SF6, a dry Nitrogen. The stress cone is inserted and mounted on the cable 18 after peeling the outer sheath leaving the conductive screen 18C of the cable exposed. Standard procedure to mount the stress cone over the power cable known in the art is used here.

[0058] The apparatus 10 of the present invention, provides following advantages:

• User friendly test chamber for the High Voltage (HV) & Partial Discharge (PD) testing of the stresscone (Outdoor fluid filled termination application). The HV and PD test to be performed based upon the specification given in IEC60840 & IEC62067
• Can be employed for testing of stress cones of different voltage rating
• Can be employed for different sizes of stress cones of similar voltage rating
• Leakage proof during its application with compressed gas environment
• Safe operation
• User friendly operation
• Repeatability of test without damaging stress cones during mounting/ un mounting on the cable
• Scalable to any size of the cable.

[0059] The apparatus 10 of the present invention may also be used for routine testing of fluid filled out door stress cone and in industries who manufacture HV test equipment. For example, this apparatus can also be a cost effective alternative solution in place of reusable water termination used in cable industry / cable accessories industries for routine test purpose

[0060] Although a particular exemplary embodiment of the invention has been disclosed in detail for illustrative purposes, it will be recognized to those skilled in the art that variations or modifications of the disclosed invention, including the rearrangement in the configurations of the parts, changes in steps and their sequences may be possible. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as may fall within the spirit and scope of the present invention.

[0061] The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching.

[0062] The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the appended claims.

,CLAIMS:
1. A leak proof high voltage testing apparatus, comprising:
- a cylindrical chamber having a cable entry side and a cover side;
- a cable disposed in the cylindrical chamber, the cable having a proximal end
- and a distal end, such that the proximal end is at the cable entry side of the chamber;
- at least one entry seal plate for closing the cable entry side of the chamber, the entry seal plate including a through hole adapted to allow passing of the cable into the chamber;
- at least one cover plate for closing the cover side of the chamber;
- a cable entry side sealing arrangement mounted on the entry seal plate to prevent any passing of fluid across the entry seal plate from the proximal end of the cable; and
- a conductor side sealing arrangement mounted on the distal end of the cable to prevent any passing of fluid across the chamber through the distal end of the cable.

2. The leak proof high voltage testing apparatus according to claim 1, wherein the cable entry side sealing arrangement includes:
- a sealing tube disposed in the through hole of the entry seal plate, the sealing tube is a cylindrical sleeve which circumvents the proximal end of the cable;
- a union screw having an opening to allow passing of the cable, the union screw is adapted to engage with the sealing tube to close the cable entry side;
- a first O-ring seal disposed between the cable and the sealing tube; and
- a second O-ring seal disposed between the cable and the union screw.

3. The leak proof high voltage testing apparatus according to claim 1, wherein the conductor side sealing arrangement includes:
- a shear head bolt connector adapted to enclose and snugly fit on the distal end of the conductor, the shear head bolt connector is a hollow cylindrical metallic sleeve closed from one side having threads on its outer surface;
- a conducting tape mounted on the intersection of an open end of the shear head bolt connector and a cable conductive screen of the cable; and
- a spherical connector having a cylindrical bore with inner threads which are adapted to detachably engage with the outer threads of the shear head bolt connector.

4. The leak proof high voltage testing apparatus according to claim 1, wherein the distal end of the cable includes a plurality of exposed conductor strands.

5. The leak proof high voltage testing apparatus according to claim 1, wherein the proximal end of the cable includes an insulation portion of the cable.

6. The leak proof high voltage testing apparatus according to claim 3, wherein the shear head bolt connector includes:
- a shear head bolt hole on the periphery of the shear head bolt connector, the shear head bolt hole is located towards the closed end of the shear head bolt connector; and
- a shear head bolt disposed in the shear head bolt hole, such that when the cable is inserted in the shear head bolt connector, the shear head bolt is tightened to hold the cable;

7. The leak proof high voltage testing apparatus according to claim 6, wherein a head of the shear head bolt shears off on tightening beyond a pre-defined amount of torque.

8. The leak proof high voltage testing apparatus according to claim 7, further includes a self-curing silicone sealant applied over the shear head bolt after the head is sheared off to prevent any leakage of fluid from the shear head bolt hole.

9. The leak proof high voltage testing apparatus according to claim 1, wherein the cover plate includes a gas filing, recovery arrangement and a protective and a measuring instruments.

10. The leak proof high voltage testing apparatus according to claim 2, wherein the sealing tube and the union screw are engaged using thread mechanism, such that the first O-ring seal and the second O-ring seal gets compressed to prevent any leakage.

11. The leak proof high voltage testing apparatus according to claim 1, wherein the entry seal plate and the cover plate are attached to the cylindrical chamber by nuts and bolts.

12. The leak proof high voltage testing apparatus according to claim 1, is used to test a device mounted on the cable by pressurising the chamber with a dielectric gas and applying high voltage.

13. The leak proof high voltage testing apparatus according to claim 12, wherein the device is a stress cone.

14. The leak proof high voltage testing apparatus according to claim 12, wherein the dielectric gas is at least one from a SF6, a dry Nitrogen.