The radio dealer (Apr-Sept 1922)

June, 1922 THE RADIO DEALER 61 Lightning Protection for Radio Equipment By L. S. BRACH The experience of the writer for sixteen years in the development, manufacture and sale of lightning protective apparatus has given him wide opportunity to observe the effects of various forms of lightning protective apparatus under actual service conditions. In most cases the wires which we have been called upon to protect have extended over a long distance, and under these conditions the danger of being affected by lightning disturbances is greater than in the case of radio antennas. Our records indicate that we have furnished over two million lightning arresters to factories, railroads, for signal-line protection, telephone companies, police and fire alarm systems, circuits and other forms of low voltage wires, and in no case where our arresters are in use have we any record of lightning currents getting into the building or having resulted in fires in building or in any of these lines. It is possible that fires may have occurred, but in that case it would have been a very rare instance. We have numerous records of the operation of lightning arresters proving that their use has undoubtedly prevented fires from starting. The purpose of this explanation is to off-set any fear that may be in any reader's mind as to what possibility there is of the radio antenna conducting lightning currents into the house. Generation of lightning is the building up of small charges of electricity which accumulate on moisture. These charges combine as the moisture forms into fog, clouds and raindrops, and subsequently becomes so heavy as to discharge to other clouds or to the earth. It is the presence of moisture in the air that permits the conductance of the discharge between the clouds and the earth. The damage done by lightning may be caused in either of two ways : that by the direct stroke of the discharge striking a wire or by an inductive charge being built up in the wire by reason of the flash of lightning passed between two clouds or between cloud and the earth. All lightning discharges have an area of electrification, and wires located within this zone are subject to having the induced potential created in the wires which would, if not properly sidetracked into the earth, enter into the apparatus and result in damage. Di rect discharges are always too violent to be protected against except by the antenna wire itself, which generally melts and breaks the circuit. Even a heavy switch will not cause suitable protection, but as stated above, the possibility of such a condition occurring would be no greater with the antenna than it would be without. The strength of the induced charges in lines depends upon the nearness of the direct discharge, length of line and its position in the electrified zone. It is safe to say that the longer the antenna circuits are the more subject they are to being affected by passing storms. These tests reveal that discharges occur in two different ways : one is the brush discharge, in which lightning passes through insulating material readily and is harmless in passing; the other occurs in a line of thread-like discharge and with it heat is produced sufficiently to cause damage to the parts between which the discharges occur. In the designing of arresters we treat only with the latter type of discharge, and it is essential that protectors must carry such discharges freely and without injury to themselves. The most efficient arrester is, therefore, one that will instantaneously and repeatedly dissipate the largest amount of energy without being affected. The potential at which an arrester should discharge should be determined by the insulation of the apparatus or circuits that are to be protected. For example: If the apparatus is tested to withstand 500 volts between its windings and other parts, the protector should discharge at a potential of approximately 375 to 400 volts, that would insure an operation of the arrester in preference to the damage of the insulation. It is possible to design arresters that will discharge at different potentials to a certain degree, but in this we are limited in getting below 375 volts in practical types of arresters. Another test which will reveal the sensitiveness of' an arrester to static current is the comparing of the efficiency of the arrester with an adjustable air-gap, thereby getting an air-gap equivalent. The process in doing this is to have two needle points supported in a way that a micrometer adjustment may be had. These are to be held in multiple with the arrester and an electric generator. The separation at which the needle gap is adjusted when the arrester will start to assume the discharge in place of its passing across the needle point would give a value to the arrester as in the air-gap equivalent. Therefore, when we say a certain arrester has an air-gap equivalent of one or two thousandths of an inch, we mean that the arrester will start to discharge the current from a generator when held in multiple circuit with a needle gap held that distance apart. There are three principal designs of lightning arresters, the air-gap, vacuum and high resistance types. In addition to this there is the choke coil, but choke coils, when used, are generally found in combination with one of the three types mentioned. The air-gap design depends on its efficiently bringing as close together two electrodes having one connected to the line and connected to the ground. The air between the two electrodes acts as the insulation for the normal operating currents. Airgap arresters are generally made having the conducting medium forming the electrodes. The vacuum arrester consists in having electrodes held in a fixed position in a sealed chamber from which the air has been exhausted, and through this thin air we find that inductive currents readily pass, even when the electrodes are held much farther apart than in the air-gap types, and equally good results obtained. The vacuum types are practically free from the fusing together of electrodes or from collection of moisture or dust on the operating parts. It is a well-known fact that discharges will occur at a lower potential between conductors at a given separation in a vacuum than in air, and this fact has been taken advantage of in the designing of protectors so as to provide protectors of low voltage potential discharge value having a fairly high carrying capacity. The high resistance medium arrester consists in placing between the line and ground a composition block, generally a mixture of carborundum or silicon with a clay binder. The carborundum has the property of conduction and the clay binder acts as an insulator. The binder being porous and the conductive material being in very small particles, it is found that when mixed together we have an insulating mass with small conductive particles that arrange themselves in a way that the total mass is of exceptionally high resistance, but static current will pass from particle to particle through the binder and then discharge itself.