Читать книгу Lightning Rod Conference - Various - Страница 15

From our close connection with the late Sir William Snow Harris, adviser to the Crown for upwards of twenty-five years in regard to lightning conductors for the navy, and having made lightning conductors our especial practical study for thirty-five years, we may be pardoned for making a few remarks on the protection of buildings from lightning.

We would, firstly, say that the system of conductors now fitted by us is based upon these past years of experience, and upon facts collected during this period, of accidents to buildings having the ordinary single line of conduction, as also from the practical success of the conductors in the navy.

The form of conductors used by us has been adopted after considerable experience, as being the most simple, solid, durable, and capacious form of conductor for the safe conduction of heavy strokes of lightning.

In place of insulators as fastenings, we use copper holdfasts, as we found the former dangerous and useless, as the glass, being non-conductive, the expansion and heat of the electric fluid, being confined, broke them, and caused an unsafe concussion; and it is also a disadvantage for a conductor to be away from the building, as nearly every material in nature assists, without detracting from, the safe discharge of the electric fluid through a good copper conductor. We find that the copper wire rope conductor, usually applied, is seldom more than ⅜th of an inch in diameter; but we did once remove, from the tower of St. Mary’s Church, Taunton, a copper wire rope conductor of ⅞th of an inch in diameter [A. 0·60 in.], said to be especially made to order—certainly the largest we ever came across; but it failed to give the necessary protection in a lightning storm, which did much damage to the tower and roof of the church. As capacity or weight of copper is the most important for safe conduction, copper wire rope is very deceptive in this respect, as will be seen by the following comparisons, viz.:—A copper wire rope conductor of ⅜ inch diameter [A. 0·11 in.] weighs 2¾ ounces per foot, not equal to a plain solid band ⅜ inch wide and ⅛ inch thick [A. 0·046 in.], which weighs 2·907 ounces per foot. A copper wire rope conductor of ½ inch diameter [A. 0·20 in.] weighs 5 ounces per foot, not equal to a solid band of ¾ inch wide and ⅛ inch thick [A. 0·092 in.], which weighs 5·814 ounces per foot. A copper wire rope conductor of ⅝ inch [A. 0·31 in.] weighs 9½ ounces per foot, not equal to a solid band of 1¼ inch and ⅛ inch thick [A. 0·153 in.], which weighs 9·690 ounces per foot. This is the largest size of wire rope conductor made or used.

From the above will be seen what protection can be given by conductors of such small capacities; and we may add that solid band conductors of the same weight, and superior in every way, can be fixed at less than half the cost of the wire rope, foot for foot.

Copper chains and copper wire bands, as conductors, answer in so uncertain a manner with the galvanometer, that they should never be used.

Iron in any form should be avoided, from its lower conducting power, and its utter uselessness when in a rusty and decayed state.

With regard to testing with the galvanometer, the mere testing of the conductors is no proof of the security of the building itself. We not only test the conductors, but also the building, to prove that it is under safe conduction in lightning storms.

In conclusion, we beg to state that our patent system of protection is the application of one or more main down and ground copper conductors and sizes, according to the height and area of the building, the fitting of the copper bands to each chimney-stack, and connecting the same, and the connecting of all the metals on the roofs thereto and to the main conductor, so that there shall be no circuit by which the lightning fluid would be likely to attack without having its exit to the main conductor.

For high working chimney-shafts we fit a copper band round the top, and four points thereon connected to main down conductor.

For further information, we earnestly solicit the careful perusal of our pamphlet and papers herewith.

J. W. GRAY & SON.

Chippendale Mews, Harrow Road.

1. Upper terminals pointed with one or more points, according to the nature of the building to be protected. Dimensions vary in like manner. Material—copper or brass, with electro-gilded points.

2. Conductor composed of copper or galvanized rope, according to height, &c., of building, &c., dimensions varying with resistance of the circuit.

3. The sectional area varies with the length.

4. Joints made, as far as possible metallically; where solder cannot be used, screw joints are made use of.

5. Attachment to building direct by metallic ties of requisite form.

6. Ground connection—When practicable, the end of conductor is metallically connected with gas or water main, otherwise a hole is dug deep enough to meet always moist earth. The end of conductor is either attached to an earth plate, or coiled up in a bundle and surrounded by coke.

7. The area protected is supposed to be a radius equal to the height of conductor.

8. If more than one terminal is attached to one conductor, the size of the latter is increased, except under certain conditions.

F. RUSSELL & CO.

137, Princess Street, Manchester.

1. A copper tube 1¼ inch diameter or 1 inch diameter, finished at the upper end, with a forged copper point or cone, connected with the tube by a cast copper (or gun-metal) coupling, into which coupling are also screwed three or more smaller points round the larger central one. At the lower end the tube is screwed into a somewhat similar coupling, to receive also the brazed and screwed end of the conductor. Or a solid copper rod ½ inch diameter [A. 0·20 in.], or wrought iron rod 1 inch diameter [A. 0·79 in.] (where iron conductors are used) the rod in either case forged to a blunt point, and screwed at the lower end, like the tube first described, to fit the coupling.

2. (a). Copper wire rope of 7 strands each, No. 10 Birmingham wire gauge, or in specified cases of No. 8 or 7 wire gauge, making, when spun, a rope with a sectional area varying from 7/16 to 11/16.

(b). Solid copper rods ½ inch diameter [A. 0·20 in.].

Solid iron rods 1 inch diameter [A. 0·79 in.].

(c). Copper band or “tape” of sizes from ¾ × ⅛ to 2 or 3 × 3/16 inches [A. 0·09 to 0·38 or 0·56 in.].

(d). Copper tube ⅝ inch diameter outside, and ⅛ inch thick [A. 0·20 in.]

3. Although no definite rule exists for the proportional sizes of the conductor, it is usual and prudent in a large building to employ for the main conductors, which should come from the highest and most exposed points to the earth in the most direct way, a larger conductor than would be required for a small building, and the branches or connections to this main conductor may be smaller in sectional area than the principal one. Thus, a church tower with four angle pinnacles may be protected by four finials or points, one to each pinnacle, and these four parts fitted to rope of 7 wires No. 10 gauge [A. 0·10 in.], to be united to a continuous band round the parapet, from whence a rope of 7 wires No. 8 gauge [A. 0·15 in.] should descend into the earth; or an infirmary or workhouse built with wings would have, perhaps, three direct rod conductors, one to each chimney stack, and connections with the water spouts, or lead flashing made of small copper tape ¾ × ⅛ [A. 0·09 in.] soldered to the lead and worked round the rods.

4. The fewer joints the safer, and for this reason—the copper rope or tape is better than the rod or tube, as the former is made conveniently any required length, and the danger of a fault or break in the continuity is avoided. Of the necessary joints the rope requires one at its junction with the top rod or tube; this is made by brazing a small ring of brass (or copper) round the rope; the solid end thus formed being chased with a deep male thread, which fits the prepared base of the rod. The branch conductors or connections, with adjacent constructive or decorative iron work—as beams, girders; cresting, vanes, &c., are made by threading a bead with a similar ring to receive the branch, as that already described. Where the branch reaches its object a ring or solid coupling should be “tapped” into the girder or cresting, to ensure thorough metallic connection, if the destination of the branch be the lead flashing, the seven wires must be opened like a fan, and each wire strongly soldered with common plumbers’ solder to the lead—

(b). Copper or iron rods are made continuous by couplings of either metal, as the case may be, which should exceed the diameter of the rods by enough metal to allow of a good thread. These couplings should be hexagonal or octagonal in plan, to allow the workman a certain grip; and the thread should be of the kind called right and left, so that while screwing one length he may not unscrew the other. These conductors require very careful, steady workmen, as a great element of danger exists in these numerous joints.

5. The various natures of the buildings provided with conductors require separate, and often different treatment: but the principle in all cases is the same, viz., to attach the conductor closely to the fabric, and the more the conductor is made an integral part, as it were, the more efficacious it will be. Any attempts at so called isolation are opposed to the theory of protection by conductors. The mechanical means of fixing are best illustrated by diagrams, the chief objects to be considered are—

(e). Permanence or strength and durability.

(f). Room for expansion of the conductor.

(g). Facility in fixing without cutting or breaking the conductor.

(h). Neatness in appearance.

These objects are gained by a careful consideration of the materials to which the conductors are fixed by “holdfasts,” for stone, slate or tiles, wood, and iron. It is important that sharp bends be avoided. A string course, for instance, should be drilled, and the rod or rope passed straight through. Also, that any metal bodies in the line of the conductor should be connected with it by staples screwed into such bodies. It is most necessary that the ends of vane bolts or rods should be joined to the conductor, or, where this is impossible, should be fitted with an independent wire or rod to the earth.

6. The connection with the ground is of special importance, as the object of the conductor is to provide a free passage between the two currents, and if this be not done, a lateral discharge is pretty sure to result. A building provided with suitable conductors, properly fixed, should at all conditions of the atmosphere, allow a free course to the electricity, and be in all its parts electrically equivalent, and with this intention the several parts (as mentioned in answer to question 3) are brought into connection with each other or with the ground. The actual length of the ground conductor is fixed by the nature of the subsoil, as it is obvious that dry sandy soil is unsuitable for a termination. We therefore continue the rope or tape until a good damp earth is reached, if possible, a spring or open water—generally speaking, about 5 to 10 yards will be sufficient in most localities. The conductor is then buried 5 to 10 feet, or upwards, in the damp earth or water. If a rope, the several strands are unravelled and opened out: if a rod or tape, a discharging fork is usually attached to the end to promote the easy discharge, for which purpose it is also usual to fill the trench with charcoal. The trench must be dug with a slight fall from the building downward.

7. The extent of area supposed to be protected by the conductor is estimated by many as included in a radius of double the height of the conductor from the base line; but the immunity from accident enjoyed by many buildings situated at a greater distance from a number of tall factory chimneys; or to take an opposite example, in a city where there are many lofty spires or towers, would go to show that a number of conductors attached to tail objects, serve to obviate the dangers arising from lightning by providing, at many different points, a direct communication between the positive and negative currents which exist in the clouds and earth. We have never known a church spire, when the conductor was fixed in accordance with ordinary skill, injured by lightning; and the tall factory chimneys of our manufacturing towns afford strong corroborative evidence of the value of conductors, and this in two ways—first, because those to which conductors are fixed, do not get struck; and, second, because those unprovided with conductors, do get destroyed from time to time.

8. A reference to the answer to No. 3 question, will show that we consider that when several terminals are used, an increased diameter is advisable in the main or principal conductor; but it must be remembered that either of the conductors referred to in the answer to question 2, is greatly in excess of what many eminent electricians consider necessary. A single wire being thought sufficient of 3/32 inch diameter (A. 0·06 in.) for any ordinary current of electricity. But both the English and French Governments have thought it prudent to specify a copper body, with a sectional area of ½ inch in English, or 1 centimetre in French (0·40 in.)—partly to provide against corrosion, which would rapidly deteriorate a thin wire, and partly to obviate the danger of the melting of the smaller conductor under the continued force of an unusually strong shock of lightning. We, therefore, respectfully follow the decision of such experts as have, by careful experiment and considerable diligence, acquired the knowledge they possess—both as to the substance, the form, and the treatment of this subject; and have only to add the fact, that any small experience we have practically had, goes to support the conclusions already arrived at by these authorities.

FREEMAN & COLLIER.

24 & 26, Lever Street, Manchester.

1. Our upper terminals are made of copper or brass, plain spike or ball with spike at top, and three radiating from it, or four or five spikes radiating from the ball. Attached to the ball (screwed into it) is a solid rod of copper, to which the conductor is fastened, as explained below.

2. Conductor is made of good quality copper wire strand 7 ply: ⅜ inch [A. 0·11 in.] to 7/16 inch [A. 0·15 in.] diameter.

4. Joints of the strand not usually permitted, as we spin it any reasonable length.

The end of the conductor is knotted and drawn through a cup-shaped ring of metal one end, the top of which is screwed into the bottom of the solid rod of the terminal. This makes a good connection.

5. Copper holdfasts fasten the rod to the building.

6. Ground end is coiled loosely in damp earth or a well.

RICHARD JOHNSON, CLAPHAM, & MORRIS.

180, Rottemore, Glasgow.

We beg to reply to your queries on the material, system, and fitting of lightning conductors, as practiced by us for over 25 years, during which time we have never had a building injured in which we have been engaged, and have fitted from 15,000 to 20,000 feet a year, without advertisement.

1. Uniformly solid copper, consisting of 1 centre concave point, about 14 inches long, presenting 8 sharp angles = 3½ inch surface; this is surrounded by 4 smaller points of same construction. These all terminate or spring out of a hollow copper ball, which is screwed on a copper tube ¾ inch diameter inside, and from 4 inches to 5 inches long, according to requirement. The copper cable is passed through this tube, is knotted inside of the ball, and the points are all screwed against it, which forms the point of contact, and thoroughly fixes the cable at the top; but the fixing of the top or terminal rod is fashioned in accordance with the requirements of the building or material to be fixed to.

2. Uniformly copper cable constructed of 49 strands, hard drawn square copper wire Nos. 17, 18, or 19 w.g.

3. We never use less than 6–inch surface, i.e., measuring the circumference of each wire, we contending that surface is the only power of the conductor. Up to 150 ft. we use No. 19 (= ½ inch diam.) [A. 0·20 in.],¾ inch for a longer length of cable (i.e., 17 or 18) [A. 0·44 in.].

4. Usually with a gun-metal screwed coupler.

5. With brass holdfasts, lined with porcelain, glass, or guttapercha.

6. Spread out end of strands of cable like a fan, and bury it in the moist earth a few feet deep, in an oblique way tending from building.

7. 30 to 40 yards.

8. We invariably run one cable from each terminal or top rod: but in spires we commonly take a connection from the bottom of the vane rod, and connect it to the main conductor, which goes to the highest point of vane or final: if the former, we fix a copper bush or disc to the vane rod at foot of vane, which is fast to cable, and a corresponding one on vane, with cable at highest point, when the cable is fringed out, presenting its 49 points, and by these discs the vane revolves with that portion of the conductor attached, and the point of contact is given by the discs.

C. H. PENNYCOOK & CO.

All Saints’ Works, Derby.

1. Form for upper terminals:—A straight copper tube,¾ inch diameter; thickness of metal, 15 B.W.G. [A. 0·15 in.], with solid copper point (no branches); the point is soldered and rivetted into the tube; or a solid copper rod,½ inch diameter [A. 0·20 in.], tapering towards the top.

2. Material and dimensions of conductor:—Either a copper band of 2½ inches wide and No. 16 B.W.G. thickness [A. 0·16 in.]; or a copper wire rope,½ inch diameter, of 6 strands, each strand containing 6 wires [A. 0·20 in.].

3. Proportion between length and sectional area of conductor:—The ½ inch copper rope [A. 0·20 in.], or 2½ × 16 B.W.G. band [A. 0·16 in.], is used for heights not over 120 feet; for higher buildings, a ¾ inch rope [A. 0·44 in.] or band, 2½ × 12 B.W.G. [A. 0·27 in.] should be used.

4. Joint, how made:—Joint is made between band and copper rod with a brass screwed socket, the rod is soldered and rivetted into socket, and the band is soldered round socket, then soldered and rivetted. When the copper rope is used, a hole is drilled into socket, same diameter as rope, at the lower end, and turned out conical shape; the rope is then passed through the socket, the ends spread out, and the spaces filled up with solder.

5. Attachment to building:—The conductor is fixed close to building without insulators, and is brought into close contact with the spouting; is closely attached to chimney and walls by means of copper straps and copper nails driven into the masonry.

6. Ground connection:—Should a good, permanent drain be near, the conductor is brought to it and bound round and firmly fixed.

If there should be an open drain or brook, the conductor is brought under it at sufficient depth that if the stream be dry at any time there will be sufficient moisture to carry away the charge without disruption. Should there be neither drain pipes nor brook sufficiently near, the conductor is taken from 12 to 20 feet below the surface to the clay, where it is certain to be always damp, even in seasons of the greatest drought ever known.

In no case should the earth connection be taken into a closed tank or well.

If a band be used, it should be cut into strips about 18 inches long and laid in different directions; rope should be unwrapped and spread in a similar manner.

7. Supposed area protected:—It is impossible to determine exactly the area the conductor protects. It is erroneously supposed that the rod will protect buildings within its radius, but experience will not bear out this axiom. Many instances may be related of buildings being struck much within the radius of well-protected churches or chimneys.

The protection a conductor affords depends to a great extent on the relative positions of the electric discharge and the objects that it may meet in its course. As a general rule, a church with a high spire with a proper conductor may be considered to protect the remainder of the edifice; but a low, straggling building should have several conductors at the outside highest points.

8. If there is more than one terminal is the size of conductor increased?—No; as sufficient material should always be used to carry off without disruption the heaviest known charge, it is unnecessary to increase the size of conductor. Should two or more upper terminals be connected with the main conductor, the size of material need not be increased; for if two or more terminals receive the charge simultaneously it necessarily follows that it is sub-divided; therefore the conductor will have no more work than if one point only had been struck.

Note.—We quite agree with Snow Harris regarding insulators, that if there be anything in insulators they are a disadvantage, for if the building be struck in any other part than the conductor, the current cannot easily find its way to the conductor. The current will take the line of least resistance; therefore it is reasonable to assume that the building is more certain to escape the disruptive force of lighting when the conductor is in close proximity with the building.

JOHN DAVIS & SON.

Bigg Market, Newcastle-on-Tyne.

1. For upper terminals I generally use ½ inch diameter solid copper rod [A. 0·20 in.], or ¾ inch diameter tube [A. 0·24] with four points, and I fix them 4 or 6 feet above the building they are intended to protect. I always endeavour to get the upper terminal as near the size of the conductor as is consistent with strength. I make my points of the best copper tipped with platinum.

2. For the conductor I use ½ inch diameter copper wire rope [A. 0·20 in.], which is (in my opinion) the best and most applicable conductor used, as it appears to be an open question, at present, whether it is surface or mass which conducts. If it is mass, then a tube conductor is insufficient. If it is surface, then a solid rod is superfluous. The copper tape conductor I consider the worst form of any, as it bends too easily round sharp corners, projections, &c., of buildings, which is a thing to be avoided as much as possible. A conductor should be brought to earth as direct as possible, and with no bends if they can be avoided. The copper wire rope conductor has both surface and mass conduction, and can be led about roofs and other difficult places better than any other form of conductor that I know of.

3. None; I imagine it is not necessary.

4. I avoid joints as much as possible; but, when they must be made, I scrape the ends of my wire bright, and then splice or interlace them together, covering the whole with thin sheet lead—I object to solder, as I think it must interfere with surface conduction; the wire is fastened to the upper terminal, with a Matthew Walker knot let into a hollow cup, and the terminal screwed down on it.

5. I attach my wire to the building with a brass or gun-metal holdfast 4 inches long, having a ⅝ hole, the inner edge being flush with the wall of the building, so as to allow the conductor to touch the wall of the building all the way up, and still allow plenty of room for the free passage of the electric fluid. I do not approve of insulators, nor yet of that kind of holdfast that is driven in tight on to the wire, for I think that must interfere with the clear passage of the electric fluid.

6. I cut a trench some 15 or 20 feet long, gradually deepening from 1 foot at the commencement to 4 ft. at the termination, which I fill with pounded charcoal and bury the wire in it. Earth-plates are not necessary when this is done.

7. It is calculated that a conductor will protect a surface in the shape of a cone, the diameter of the base of which is equal to the height of the conductor. Thus, if a conductor were 100 feet high, the space protected would be represented by a straight line drawn from a radius of 50 feet from the base of conductor, to a radius of 8 or 10 feet from its highest point.

8. I consider, if there are two terminals, there should also be two wires, or the wire should be of sufficient capacity to carry off a double charge, in case both terminals should be struck at one time. I think the conductors should certainly be of sufficient capacity to carry off any charge that might be received by the terminals, be they few or many.