US3159240A

US3159240A - Aerial lifts for energized line working

Info

Publication number: US3159240A
Application number: US133200A
Authority: US
Inventors: Jr Charles J Miller
Original assignee: Ohio Brass Co
Current assignee: Ohio Brass Co
Priority date: 1961-08-22
Filing date: 1961-08-22
Publication date: 1964-12-01
Anticipated expiration: 1981-12-01

Description

Dec. 1, 1964 c, L JR 3,159,240

AERIAL LIFTS FOR ENERGIZED LINE WORKING Filed Aug. 22, 1961 4 Sheets-Sheet l INVENTORJV 'CHARLES J. MILLER JR.

mum

ATTORNEY Dec. 1, 1964 c. J. MILLER, JR

AERIAL LIFTS FOR ENERGIZED LINE WORKING 4 Sheets-Sheet 2- Filed Aug. 22. 1961 INVENTOR. CHARLES J. MILLER JR.

ATTORNEY Dec. 1, 1964 c. J. MILLER, JR 3,159,240

AERIAL LIFTS FOR ENERGIZED LINE WORKING Filed Aug. 22. 1961 4 Sheets-Sheet s I I I I f, I I I I L I I--- l 74 FIG? IN V EN TOR.

F G- CHARLES J. MILLER JR.

72 A m mm ATTORNEY Dec. 1, 1964 c. J. MILLER, JR

AERIAL LIFTS FOR ENERGIZED LINE WORKING Filed Aug. 22, 1961 I 4 Sheets-Sheet 4 mm mm INVENTOR. CHARLES J.MILLER JR. BY

m k) M ATTORNEY United rates Patent attests Fatented Dec. 1, i964 ice 3 15% 2 2i? A roan Liars non nirniicrzan nan 3G Charles .l. M ler, Era, Wadsworth, Ghio, assigncr to The Ghio Brass tlompany, Mansfield, a cop-oration of l lew .lersey Filed Aug. 22, 196i, Ser. No. 133,2tltl 16 claims. (6i. iSZ -ZZ) This invention relates to energized line working and more specifically to an insulated boom for aerial lifts.

A principal object of the invention is to provide means for supporting a personnel carrier in insulated relation to ground.

Another object of the invention is to provide an insulated boom for an aerial lift which functions at high voltages.

Still another object of the invention is to provide means for determining the condition of the insulation in an insulated boom during wet weather, presence of contaminating media, and the like.

There is disclosed in co-pending application, Serial No. 105,456, filed April 25, 1961, by H. L. Rorden, a new method of and means for energized line working, in which a workman is supported by a personnel carrier incorporating conductive members connected to and energized from the line conductor. The conductive members are arranged over the floor of the personnel carrier and about the body of the workman to energize the body of the workman to line potential and to constitute a shield means for receiving the electric field which would otherwise exist between the body of the workman and ground. The personnel carrier is part of an aerial lift which incorporates suitable insulating means so that the personnel carrier is electrically separated from ground to permit energization of the conductive members.

In the practice of the invention above referred to, the aerial lift may be constituted by a mechanical arm having two pivotally supported booms carried upon a pedestal. The booms are hydraulically operated and move under the control of the Workman to provide access to an elevated structure for working operations thereon.

In order to insulate the personnel carrier from ground at the line voltages involved, it is necessary to construct at least one of the two booms of structural insulating material along a substantial portion of its leng h. While the problems involved in the mechanical aspects of this apparatus have been largely solved, I have found that the known arrangements of insulated booms are not entirely satisfactory for the practice of the invention above referred to. Specifically, the insulating material of the boom, usually glass fiber reinforced resin, is apt to form leakage paths along the surface of the section. Moreover, the boom is usually in the form of a tubular beam and subject to failure under transverse stresses resulting from non-uniform electric field distribution along or about the insulating section.

I have found that excessive leakage currents, that is, currents flowing along the insulating section from the head end to the support end of the boom, may occur either by reason of the accumulation of contaminating media, soot particles and the like, or moisture, on the interior or exterior surfaces of the insulating section. Such effects may be minimized by the provision of means for closing the head end of the boom and by coating the interior and exterior surfaces of the insulating section with silicone greases.

However, it is highly desirable to determine the leakage current along and over the insulating section in order to detect the existence of an excessive conductivity along the insulating section which might endanger the safety of workmen or cause equipment failure while the boom is being used on high voltage lines. Specifically, l have devised and disclose herein suitable arrangements for measuring the leakage current wherein leakage current determinations may be made by the use of current meters suitable for use under field conditions.

The present invention in concerned, more generally, with means for controlling the electrical stresses along the insulating part of the boom and, particularly, to prevent electric stresses transversely through the wall of the insulating section. I accomplish this by the provision of conductive members which extend circumferentially about the exterior and/ or the interior of the insulating section and function as control rings on the exterior or the interior of the section at both ends of the boom, or both. The conductive members are disposed so that the control rings constitute the most nearly adjacent of the metal parts along the central part of the insulated section, and so that the metal parts of the cable system which extends through the boom undergo translation at locations further distant than the control rings. With this arrangement, electrical stresses occur only in the longitudinal direction of the insulating member and there is no concentration of transverse stresses due to the presence of a contaminating media along one surface of the insulating member adjacent a conducting member of a different potential and which is separated from the contaminating media by the wall of the insulating member.

The invention, together with further objects, features, and advantages thereof, will be more fully understood by reference to the following detailed specification and claims, taken in connection with the appended drawings.

In the drawings:

FIG. 1 illustrates an aerial lift embodying the invention;

FIG. 2 is an elevation view in longitudinal section of the upper boom of the aerial lift of FIG. 1, and showing one embodiment of the apparatus of the invention;

Fl. 3 is a plan view taken in the direction 3-3 in FIG. 2 of the boom of FIG. 2;

FIG. 4 is an enlarged transverse sectional View of the boom of FIGS. 2 and 3, taken in the direction 4-4 in PEG. 5, particularly showing the concentric arrangement of the parts of the boom;

FIG. 5 is an enlarged sectional view of the boom of PlG. 2 adjacent the lower part of the boom;

FIG. 6 illustrates a device for measuring the leakage current along the boom;

FIG. 7 illustrates another embodiment of the apparatus of the invention having desirable leakage current measuring features;

FIG. 8 is an elevation View in longitudinal section of another embodiment of the apparatus of the invention;

FIG. 9 is an elevation view in longitudinal section of another embodiment of the apparatus of the invention;

FIG. 10 is an elevation View in longitudinal section of still another embodiment of the apparatus of the invent-ion and illustrates the practice of the invention in a boom having hydraulic power transmission; and

FIG. 11 shows a meter useful with the described embodiments of the apparatus.

in PEG. 1 the aerial lift It includes a mechanical arm 11 constituted by two booms i2 and 13, a pedestal 14, and a personnel carrier 15. The lower boom 12 is carried upon the pedestal l4 and supports the boom 13, which is connected at its inner end to the boom 12 by means of a pivotal joint or elbow 16. The personnel carrier 15 is supported on a head 17 at the outer end of the boom 13.

The pedestal 14- is carried upon a rotatable mount or turntable on a truck 18 so that the personnel carrier 15 may travel to any location above and about the truck by swinging movement in the horizontal plane and by pivotal movements of the two booms in the vertical plane. Suit- 3 able hydraulic apparatus is provided for turning the pedestal 14, and hydraulic cylinders (not shown) are provided for elevating the boom 12 with respect to the pedestal 14 and for extending and retracting the boom 13 with respect to the boom 12. The personnel carrier is maintained in the same position with respect tothe earth, whatever the position of the

booms

12 and 13, by means of a leveling system including a cable 19. Control of the movement of the arm 11 is accomplished from the personnel carrier 15 by means of a system of cables which extend through the interior of the

booms

12 and 13 after the manner of theleveling cable 19.

The boom 13 is constructed of insulating material along a portion of its length in order to insulate the personnel carrier 15 from the boom 12, the pedestal 14, and from ground. Thus, as shown in FIGS. 2, 3 and 4, the boom 13 is constituted by a hollow rectangular beam 29 of structural insulating material such as glass fiber reinforced resin (otherwise known as fiberglass). The beam is carried at its inner end by a metal end beam 21 having the same interior dimensions as the exterior of the beam 29 so that the insulating beam fits closely within the end beam. The beam 20 is suitably secured to the beam 21 by screw. fasteners, or the two beams are bonded or welded together along the adjoining surfaces. The beam 21 is carried at the outer end of the boom 12 by means of two arms or

plates

22 and 23 which pivot upon a shaft 24 as part of the elbow 16. The head 17 includes a metal bracket 25 which extends circumferentially about the interior of the beam 20 at the outer end thereof and is suitably fastened or bonded to the interior of the beam. The bracket 25 supports two

arms

26 and 27 which extend from the bracket 25 beyond the end of the beam 20 and carry a shaft 28 which supports a drum 29. The personnel carrier 15 is carried on the shaft 28 by suitable flange means (not shown) secured to the adjacent side Wall of the carrier 15. Turning move ment of the drum 29 produces an accompanying turning movement of the shaft 28 and of the carrier 15.

The cable 19 is reeved about the drum 29 and extends over and across an idler pulley 30, carried on a shaft 31 and supported by the bracket 25, through the boom 13, over and across an idler pulley 32 carried on a shaft 33 and supported by the

side pieces

22 and 23, over an idler drum 34 carried on the shaft 24, and through the boom 12 to a drum 35 fixed upon a shaft 36pivot for the boom 12. With this arrangement the personnel carrier 15 is maintained in'a level position with respect to ground by longitudinal movements of the cable 19 as the

booms

12 and 13 are turned on the

shafts

24 and 36.

The cable 19 is separated into two parts 37 g and 38 by means of two insulating

spacers

39 and 40 which extend along the central part of the interior of the beam 20. Thus, mechanical movements are transmitted between the part 37 of the cable and the part 38 of the cable with the head 17 maintained in electrically insulated relation to the beam 21. The insulating

spacers

39 and 40 are suitably in the form of rigid rods of structural insulating material connected to the ends of the cables 37 and- 38 by

metal ferrules

41 and 42 and 43 and 44 respectively. The

' spacers '39 and 40 are maintained in a state of tension with the cable 19 and may be suitably formed of insulating rope, strap or other material, although the fiber reinforced rigid rods are desirable from the standpoint of durability and electrical insulating strength. Insulating strengths of upwards of 500,000 volts may be achievedv in practical structures.

Use of the lift 10 for energized line working includes energizing the conductive shield members on the interior of the personnel carrier'15 (indicated as a wire mesh in FIG. 1) from the line conductor. The head 17 is energized to line voltage either through the shaft 28 or by means of an auxiliary cable 45 which is connected to the bracket 25 and tothe conductive members of the personnel carrier, or to the energized line conductor. Ac-

cordingly, a voltage difference equal to the voltage from line to ground is'established between the head 17 and the metal beam 21. It will be seen that the entire line-toground voltage is effective to stress the insulating material of the insulating beam 20 between the bracket 25 and the metal beam 21. Even in the absence of a conductive connection between the head 17 and the shield members of the personnel carrier 15, the head 17 floats at substantially the same potential as the shield members so that the same condition prevails.

According to the invention, control of electrical stresses along the insulating beam 20 between the metal end beam 21 and the bracket 25 is accomplished by two control rings 45 and 46, positioned adjacent the end of the metal beam 21 and spaced from the bracket 25, respectively. The control ring 45 comprises a metal member conformed to and extending about the interior'surface of the insulating beam 20 in the transverse direction thereof and having an end face flush with, or projecting slightly beyond, the end face 47 of the metal: beam 21. The control ring 46 is similarly constituted by a metal member conformed to and extending about the interior surface of the in sulating beam 20, in the transverse direction thereof, and held in spaced and electrically connected relation to the bracket 25 by

metal bars

48 and 49.

The control rings 45 and 46 are each more closely adjacent the other than the associated electrically conducting parts, e.g., of the beam 21 and the head 17. Accordingly, the voltages existing between the beam 21 and the head 17 are impressed upon the insulating beam 20 as a voltage gradient along the interior surface of the beam 20 and between the

rings

45 and 46. The electric field is distributed along and about the interior surface of the beam 20 substantially without transverse stresses through the wall of the beam.

The arrangement of the parts of thecable 19 is such that the proximate relation of the

rings

45 and 46 exists without regard for the movement and disposition of the parts 37 and 38 of the cable. This is accomplished by so proportioning the

spacers

39 and 40 and the elements of the cable system and by locating the

rings

45 and 46 so that the ferrules 41 to 44 do not pass beyond the control rings 45 and 46, to the central part of the beam 2i between the control rings'45 and 46, for any position of the booms 11 and 12 or any movement of the cables of the control system.

The insulating beam 20 and the-metal end beam 21 are shown in enlarged partial view in FIGS. 4 and 5. As is shown, the inner end of the beam 20 fits within the beam 21 and, conforms closely to the interior surface of that vbeam about the entire periphery thereof.- Likewise, the

control ring 45 is fitted Within the insulating beam 20 and conforms closely to the interior surface of that beam. The ring 45 is formed as a narrow rectangular strip or band of metal and contacts the interior surface of the insulating beam 2% along its surface. The control ring 45' is held in place 'by screw fasteners 50 which extend through the

beams

20 and 21 and simultaneously form an electrical connection between the control ring 45 and the metal beam 21. Electrical connection between the control ring 45 and the beam 21 is provided by a wire lead 51, as an alternate connecting means. The heads of the screw fasteners 50 are flush with the beam 21, and the corners of the control ring 45 and the end of the metal beam 21 at the face 47 are smooth and rounded to prevent corona formation. Contact between the control ring 45 and the interior wall of the beam 20 may be insured by means of a coat 52 of electrically conducting paint applied to the interior surface of the beam 20 to avoid any possibility of arcing between the control ring and the surface of the beam.

The construction of the control ring 45 is not critical and the same function may be performed by a plurality of strips extending along the fiat sides of'the insulating beam or by a coat of conducting paint applied along and about the interior surface of the insulating beam.

The lower control ring 45, by reason of its connection to the metal beam 21, is grounded or is substantially at ground potential. This is so since the metal beam 21 is connected to the truck 18 through the boom 12 and the turret l4- and the truck is, according to preferred practice, grounded through a suitable earth connection. Even in the absence of an earth connection, the capacity of the truck and lower boom to ground is large compared to the capacity of the conducting members at the head of the boom 13 and the control ring 45 is effectively grounded.

The apparatus of FIG. 6 comprises an arrangement for measuring leakage current along the insulating beam 2%, and includes means for connecting a current meter in series between the metal beam and the associated control rin Thus, a metal control ring 55 extends about the interior of an insulating beam 56 carried by a metal beam 57. The end face 53 of the ring 55 projects slightly beyond the end face 5? of the beam 57 and is held in insulated relation to the beam :37 by the walls of the beam 56 and by rivets 6b of insulating material. A metal pin 61 carried by the ring 55 extends through the wall of the beam 5b and through an opening 62 in the Wall of the beam 57, and is adapted to be contacted by a coaxial connecter which is threaded into the opening 62 to connect a microammeter in series between the pin til and the metal beam 57. A ground plug 63, shown in P18. 6, connects the pin 51 to the beam 57 so that the control ring performs its normal function, as described above, when the meter is not utilized.

In the apparatus of PEG. 7, an insulating beam 65 is carried by a metal end beam as and voltage grading and stress control is accomplished at the head end of the insulating beam 65 by means of a control ring 67, substantially as described in connection with FIGS. 2 and 3. Voltage grading and stress control at the lower end of the insulatin beam es is accomplished by means of two metal control rings 63 and 65 which extend circumferentially about the exterior and interior surfaces of the beam as and are connected by means of conductive rivets 7 The

rings

68 and 69 are snaced a short distance from the end face 71 of the metal beam 66, l to 3 inches in an exemplary embodiment. The control rings (:8 and 59* are connected to a meter 72, shown in FIG. 11, by means of a coaxial cable 73, having the center conductor connected to the control rings and the outer conductor connected to the metal beam es and to ground, and a coaxial cable '74. The cable 73 may be terminated in a coaxial connector arranged, for example, along the boom 12 so that leakage current readings may be taken conveniently while the

booms

12 and 13 are extended and the shield means of the ,ersonnel carrier 15 is connected to a line conductor.

The arrangement of PEG. 7 is advantageous in that larger current readings are obtained for any given condition of the insulating beam than with the arrangement of FIG. 6. This is so since the control rings 68 and 6') intercept substantially all of the electrostatic liux along the insulating section 65 whereas in the arrangement of FIG. 6 a substantial part of the electrostatic flux along the insulating section 56 is intercepted by the metal end beam 57, and is thereby shunted past the current meter. The arrangement of FIG. 7, in addition to giving a true measure of the condition of the insulating section as along both the interior and exterior surfaces of the member, enables the use of a meter 72 of standard construction which is sufficiently durable to Withstand field usage. A moving coil meter having a range of ll-2Gtl micro-amperes has been used satisfactorily.

The arrangement of control rings in spaced relation from the end 71 of the beam 65 is advantageous for current measurement and the rings or 69 may be used individually or together for that purpose. Where only one of the rings is utilized with a single control ring at the outer end of the boom, the rings should both be on the 6 interior or exterior of the boom, as herein described. In the arrangement of FIG. 7, the exterior surface of the insulating beam must be kept free of contaminants.

In the apparatus of FIG. 8, an insulating beam 75 is carried by a metal end beam 76 and supports a head "F7 in an arrangement similar to that illustrated in FIG. 2 and H6. 3. Voltage grading and stress control is accomplished along the exterior of the insulating beam 75 by means of a control ring 78 which cooperates with the end face '7? of the beam '76. The control ring 78 comprises a fiat metal piece which extends circumferential-1y about the insulating beam 75 in contact with the exterior surface thereof. The control ring 78 is connected to the head 77 and maintained at the same voltage as the head by means of a lead 8% which is connected to the bracket 81 of the head '17. The control ring "78 and the end face 79 are each more closely adiacent the other than the nearest approach of the ferrules of the insulating spacers 8.2 and 33, as described in connection with the apparatus of FIG. 2 and FIG. 3.

In the apparatus of FIG. 9, an insulating beam 35 is carried by a metal end beam 86 and supports a head 8? in arrangement also similar to that illustrated in FIG. 2 and FIG. 3. Voltage grading and stress control is accomplished along the exterior of the insulating beam by means of a control ring 88 which cooperates with the end face $9 of the metal beam $6 and, along the interior or" the insulating beam, by means of a control ring 5h) concentric with the control ring 88 and a control ring @1 concentric with the end part of the beam 86. The control ring 9% is mechanically attached and electrically connected to the bracket 92 of the head 87 by means of metal bars 93 and 94-, similar to the

bars

48 and 49 in FIG. 2. The control ring 88 is secured and electrically connected to the ring 9%? by means of rivets which extend through the wall of the beam 555. The control ring 91 is arranged at the end of the beam as, as described in FIGS. 2 to 5 inclusive. The control rings 88, 9b and 91 comprise fiat metal pieces extending circumferentially about the beam 85 and conforming closely to the exterior and interior stufaces of the beam, as heretofore described.

The apparatus of PEG. 8 and FIG. 9 is adapted for use in boom designs in which the insulating beam extends between two lengths of hollow metal beam. Thus, the ring '78 or the ring 83 may constitute the end part of a etal beam which constitutes the outer end of the boom.

The various aspects of the functioning of the apparatus of the invention may be understood by reference to the embodiments of FIGS. 8 and 9 with the embodiments illustrated particularly in FIGS. 2 and 3. It will be evident that the conductive members which perform a control function as, e.g., the control rings 45 and .6 in FIGS. 2 and 3, the end part of the beam 76 and the control ring 78 in FIG. 8, and the beam 36 and control rings 88, 9'19 and 91 in FIG. 9, all serve to distribute the electrical stresses circumferentially about the insulating beam 26 '75 and 85 respectively, and that the electrical stressing of the insulating material of the beam is in the longitudinal direction between the longitudinally opposed members. Accordingly, there is no tendency toward a concentration of electrical stresses between any of the members or between any of the members and the ferrules of the insulating spacers of the cable 19. Further, the control rings prevent transverse stressing of the Wall of the insulating beam by reason of the fact that a layer of conductive contaminating media on the exterior of the insulating beam, for example, on the beam '75, can have no effect in producing a concentration of stresses at any place along the beam. Likewise, in the insulating beam $5 of FIG. 9 conductive contaminating media on either the interior or exterior surfaces of the insulating beam has no effect in producing transverse stressing of the wall of the beam. In the arrangements of FIGS. 2, 3 and 6, there is a possibility of heavy con- 7 tamination on the exterior of the beam producing trans verse stressing adjacent the

control ring

46 or 67 so that the exterior surface of the insulating beam must be kept clean and free from such contaminants. The exterior control ring corresponding to the control ring 88 of'FIG. 9 is omitted in the embodiments of FIGS. 2, 3 and 7 only as a practical expedient to avoid the possibility of short circuit during working operations. This is possible because the condition of the exterior surface of the insulating beam may be visually determined at any time and because the exterior surface may be readily cleaned.

In the apparatus of FIG. 10, the insulating beam 95 constitutes substantially all of the boom and is carried upon a short metal part or end beam 96 (which may constitute a part of the elbow 16 in FIG. 1) and supports a head 87. Voltage grading and stress control is accomplished along the interior of the beam 95 by means of a control ring 98 associated with the head 97 and a control ring 99 associated with the beam 96. The control ring 93 is mechanically attached and electrically connected to the bracket 1% of the head 97 by means of metal bars 191 and 192, and the control ring 99 is mechanically attached and electrically connected to the beam 96 by means of metal bars 193 and 104. The control rings 98 and 99 are each more closely adjacent the other than the ferrules of an insulating spacer 195.

The apparatus of FIG. 10 illustrates the practice of the invention in connection with aerial lift apparatus. in which hydraulic means are utilized for the transmission of power or control movements between the pedestal 14 and the head 17. Thus, the element referred to as the insulating spacer M is incorporated as part of a hydraulic line 1% for transmitting hydraulic pressure to hydraulic leveling and/ or control apparatus (not shown) in the head 97. The insulating spacer 105 comprises a conduit of insulating material and is connected to metal conduits and fittings 197 and 108 in the inner and outer end parts of the boom. The stress control rings 98 and 99 function in the same way as in the prior described embodiments although the hydraulic line 1% is fiXed in the boom.

While the several embodiments shown in FIGS. 2, 3, 6, 8, 9 and are useful in achieving the objects and purposes of the invention relating to voltage grading and stress control, the embodiment of FIG. 7 is particularly desirable because of the leakage current measurement capability. The arrangement of the control rings 68 and 69 adjacent the metal beam 66 of FIG. 7 may be advantageously utilized with any of the outer end control arrangements, but particularly with the dual control rings 88 and 90 of FIG. 9. The apparatus of the invention is useful in connection with'voltage grading and stress control in any elongate insulating member utilized in an aerial lift, ladder, or the like, suitable for energized line working. 1

It is to be understood that the foregoing description is not intended to restrict the scope of the invention and that various rearrangements of the parts and modifications of the design may be resorted to. The following claims are directed to combinations of elements which embody the invention or inventions of this application.

. I claim: 1

1. In an aerial lift in which a boom is carried, at

tributing and confining electric fields along the insulating beam comprising a conductive member adjacent the said.

one end of the beam electrically connected to the said support means and a cooperative conductive member adjacent the said remaining end of the beam electrically connected to the said personnel carrier and the said conductive members, each being more closely spaced to the remaining conductive member than any other metal part at the end of the beam with which it is associated.

2. The invention in accordance with claim 1 in which the said movement translating means comprises a cable arranged for movement along the length of the boom, the said cable comprising, at least in part thereof, an insulating spacer extending along the central part of the beam with terminal members rthereof connected to the said cable, and the said stress control means at the support end of the beam being positioned along the beam more closely adjacent the head end of the beam than the terminal members of the spacer at the said one end of the beam at the point of closest approach to the head end of the beam thereof and the stress control means at the said head end of the beam being positioned more closely adjacent the said support end of the beam than the terminal member of the spacer at the point of closest approach to the support and of the beam thereof.

3. The invention in accordance with claim 1 in which the said movement translating means comprises a hydraulic line, an insulated section in the said hydraulic line extending along the central part of the beam, and metal fittings for the said insulated section at each end thereof, each of the said stress control means being disposed more closely adjacent the remaining end of the beam than the metal fitting associated therewith.

4. The invention in accordance with claim 1 in which the said conductive members comprise metal rings extending along and conforming to one of the interior and exterior surfaces of the insulating beam, all for directing electric fields along the length of the insulating beam and preventing transverse stressing of the wall of the beam.

5. The invention in accordance with claim 4 in which the insulating beam is carried by a coextending metal beam at the support end thereof, in which the metal rings are spaced apart along the interior of the insulating beam and the metal ring at the support end of the beam is coaxially within and at least in alignment with the end of the metal beam, all for directing electric fields along the length of the insulating beam and preventing transverse stressing of the wall of the beam.

one end, upon a support means and supports a personnel carrier at the remaining end thereof, in which the boom comprises a hollow beam of structural insulating material for electrically separating the personnel carrier from the support means, and in which means for translating functional movements between the said support means and the said personnel carrier extends longitudinally through the insulating beam, and an insulating part in the translating means extending along the central part of the beam in a longitudinal direction thereof, that im- 6. The invention in accordance with claim 1 in which the insulating beam is carried by a coextending metal beam at the support end thereof, and the first named conductive member of the stress control means comprises control rings in the form of coaxial metal pieces extending along the interior and exterior surfaces of the insulating beam and spaced from the end of the metal beam.

' 7. A boom for an aerial lift comprising an insulating beam constituted by an elongate member of structural insulating material, means for carrying the said insulating beam in movable relation to a support means including a metal part at one end of the insulating beam, means carried by the said insulating beam at the remaining end thereof including a metal part, and stress control means spaced apart along the insulating beam between the ends thereof for distributing and controlling an electric field along the beam due to voltage differences impressed between the said metal parts.

8. Apparatus in accordance with claim 7 in which the stress control means are spaced apart along a central part of the insulating beam between the metal parts, and the insulating beam being free of conductive parts between the control means.

9. A boom for an aerial lifit comprising an insulating beam constituted by an elongate tubular member of structural insulating material, means for carrying the said insulatingbeam in movable relation to a support means including at least a metal part extending along the insulat ing beam at one end thereof, means for carrying a head at the remaining end of the said insulating beam including at least a metal part extending along the beam, and stress control means spaced apart at the central pant of the said insulating beam for distributing and controlling electrical stresses due to electric voltage differences impressed between the metal parts of the said support means and the said head means and including a conductive member extending along one surface of the beam adjacent the said one end of the beam and a second conductive member extending along the same surface as the first named conductive member, the said two conductive members being more closely adjacent each other than the said metal parts at the two ends of the beam, associated therewith, respectively, and adjacent surfaces extending in the transverse direction of the beam.

10. The invention in accordance with claim 9 in which each of the said conductive members comprises a metal ring extending peripherally about the beam along one surface thereof.

11. The invention in accordance with claim 9 in which the said stress control means comprises a metal ring extending peripherally about the insulating beam adjacent the head end thereof and an adjacent end part of the said support means.

12. The invention in accordance with claim 9 in which the said stress control means comprises, in each instance, coaxial metal parts extending peripherally about the insulating beam.

13. The invention in accordance with claim 9 in which the first named means comprises a tubular metal beam coextensive with the insulating beam at the said one end thereof, and the control means at the head end of the insulating beam is carried upon metal rods extending longitudinally along the beam from the head end thereof.

14. The invention in accordance with claim 9 with mechanical movement transmitting means extending through the boom from the support to the head end thereof, and insulating spacer means extending along the central part of the mechanical movement transmitting means with metal parts at the ends of the said spacer means and the control means are spaced apart along the central portion of the insulating beam at points closer together than the said metal pants.

15. In an aerial lift, a personnel carrier having conductive, shielding and energizing means, extensible elongate members carrying the personnel carrier at the upper end of an upper one of the elongate members with the lower one of the elongate members supporting, at its upper end, the lower end of the upper elongate member, a section of structural insulating material in one of the elongate members for electrically separating the personnel carrier from ground, stress control rings spaced apart along the said insulating section, an electrical connection between the upper one of the control rings and the conductive means of the personnel carrier, and means effectively grounding the lower one of the said control rings.

16. Apparatus in accordance with claim 15, in which there are insulated means for movement translation extending along the said insulating section, and the said insulated means and the said insulating section being free of conductive pants between the control rings.

References Cited in the file of this patent UNITED STATES PATENTS 1,905,412 Kasson Apr. 25, 1933 2,422,644 Martenet June 17, 1947 3,043,394 Hall July 10, 1962

Claims

7. A BOOM FOR AN AERIAL LIFT COMPRISING AN INSULATING BEAM CONSTITUTED BY AN ELONGATE MEMBER OF STRUCTURAL INSULATING MATERIAL, MEANS FOR CARRYING THE SAID INSULATING BEAM IN MOVABLE RELATION TO A SUPPORT MEANS INCLUDING A METAL PART AT ONE END OF THE INSULATING BEAM, MEANS CARRIED BY THE SAID INSULATING BEAM AT THE REMAINING END THEREOF INCLUDING A METAL PART, AND STRESS CONTROL MEANS SPACED APART ALONG THE INSULATING BEAM BETWEEN THE ENDS THEREOF FOR DISTRIBUTING AND CONTROLLING AN ELECTRIC FIELD ALONG THE BEAM DUE TO VOLTAGE DIFFERENCES IMPRESSED BETWEEN THE SAID MEATAL PARTS.