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were intended to act as tell-tales, and, which by virtue of the holes punctured by the discharges, gave me some record as to what was going on. Some of these tell-tale papers are shown, slightly reduced, in Fig. 8. It will thus be seen that although the discharges that caused these holes were larger than are ordinarily obtained with a battery of Leyden jars, yet they are not to be compared with the holes that one would expect from the lightning flashes that are so often said to "strike the lines." The fact that these tell-tale papers were not burned is sufficient evidence that the dynamo current did not follow the discharge.
There is, however, quite another side to this matter of protection against lightning, one which has been little discussed and which is almost invariably overlooked by those to whom interruptions due to lightning are a matter of vital importance. I refer to the insulation of the apparatus to be protected. When a lightning arrester fails. to protect, it is condemned; the general opinion being that the failure is due to some inherent fault in the lightning arrester. But we have already learned that a lightning arrester is nothing more than a spark gap. It would be difficult, then, to conceive of anything fundamentally wrong with a lightning arrester, so far as offering an opportunity for discharge is concerned. We have also learned that disruptive discharges do not always embrace the opportunity for discharge that is offered by a spark gap lightning arrester. This fact is one reason, and a very frequent one, why lightning arresters sometimes fail. Another, and all too frequent, reason is defective or improperly applied insulating material. In a certain sense, a lightning arrester is a safety valve.
would not expect to protect a defective or weak boiler with a safety valve set to blow at or near the bursting strain of the boiler; no more should we expect a spark gap lightning arrester to protect weak or defective insulation.
Defective insulation results either from weak insulating material or a faulty application of the insulating material. Good insulating material is generally used. Faulty application may result in two ways: (A) through improper design; (B) through carelessness or ignorance. Examples under A are : First, exposed surfaces, offering opportunities for "surface discharge;" and this is not an infrequent occurrence in connection with insulating materials that would otherwise stand very high voltage; and, second, insufficient allowance for a proper margin of safety. The effects of rough handling, heat and cold, damp and dry atmospheres, dirt and grit (this latter having a particular attraction for electrical apparatus), etc., demand a margin of safety that is not always appreciated even by the designer. Under B might be mentioned. a long list of details, such as bruises, cracks,
pin-holes, cuts, open joints, bits of metal imbedded in the insulation, sharp corners, etc., etc., which will tend to lower the insulation strength fifty, seventy-five or even one hundred per cent. And right here it must be observed that the weakest point in the insulation of a given piece of apparatus (it may be a pin-hole or minute crack invisible to the naked eye), is the measure of its insulation strength.
Repair work in shops of local and power companies is liable to be defective, and but few such companies are provided.
with testing sets. Repaired armatures and converters are placed in service, and might stand indefinitely the normal electro-motive force of the circuit to which they are connected; but field discharges, lightning, rise of potential and proximity to other circuits carrying high potentials, demand a margin of safety which cannot be assured unless insulation be actually tested with an electro-motive force from four to six times the normal.
Even though the insulation of apparatus may be perfect and have its proper margin of safety when installed, yet deterioration may occur from various causes, principal among which would be moisture and overheating. In general, it may be stated that if we undertake, by means of spark gaps, to offer absolute protection against static disruptive discharges, the insulation strength must bear such a relation to the spark gaps as to place it beyond the limit of selection. Ordinarily, however, absolute conditions do not occur in practice; we can but approximate them, and then philosophically accept a reasonable percentage of failures as inevitable.
The failure of lightning arresters is too often due to careless installation. It may be instructive to note several examples:
One plant is reported as having, for better protection, connected two arresters in series. This was probably done with the idea that if a little was good, more would be better. 2. A large bank of station arresters was grounded to an iron bolt about two feet long driven into dry sand.
3. Line arresters were grounded by pushing the ground wires into the earth.
4. Line arresters were grounded on iron poles, which were themselves set in Portland cement.
5. An annual inspection of automatic lightning arresters developed the fact that the arresters were nearly all burned out; in other words, that the line was left unprotected.
6. The ground plate of a bank of arresters was thrown into a neighboring stream, which subsequently changed its course, leaving the ground plate high and dry.
7. The ground plate of a bank of station arresters was laid on the rock bottom of a neighboring stream.
8. In a large number of cases, a portion of the ground wire is wound into a fancy coil (choke coil).
And SO on; the list might be indefinitely extended, each such case forming a source of complaint that the arresters "fail to protect." But when these curious mistakes are located and properly remedied, the complaints cease.
Overhead wires become
charged. They are discharged through lightning through lightning arresters, which are spark gaps. Shifting points of high and low pressure are formed along the line, so that the discharge does not necessarily occur over the shortest or easiest path; that is, the discharge is selective. Lightning arresters offer opportunities for discharge. Coils protect. A liberal distribution of line arresters offers the only practical means of protecting widely distributed apparatus.
Lightning arresters fail to protect: First, because of the shifting of high and low pressure points; or, in other words, for lack of a sufficient number of line arresters; second, because insulation is defective; and third, because lightning arresters are not properly installed.
ORDER OF BUSINESS.
THURSDAY, FEBRUARY 28th, 1895.
FIFTH SESSION, IO. 15 A. M.
I. Discussion of Dr. Bell's Paper (Continued).
Report. Committee on Data. H. M. SWETLAND,
3. Paper. "Arc Carbons and the Rating of Arc Lamps." By L. B. MARKS.
Report. Committee on Rules for Safe Wiring.