The effect of high currents on disconnecting switches: With special reference to the mechanical stresses resulting

In the early days of the electrical industry, disconnecting switches adequately performed their functions without the use of locking devices, except perhaps in a few isolated cases, where the blades opened downward, and some mechanism was provided to hold the blade in against the action of gravity (when subjected to jars, vibration, etc.). The generating capacity of central stations at this time was relatively small. Hence, the short-circuit currents obtaining were relatively low, and the forces resulting were insufficient to overcome the friction and other resistance offered by the blade and to cause opening. With the increase in generating capacity came a formidable increase in the short-circuit currents, to such an extent that it was not uncommon for a disconnecting switch to open, causing considerable damage, with consequent demoralization of operation. The result was that there were attempts made to attach locks to switches already installed, and to design new switches of which the lock was an integral part. Many of these locks were found to be inadequate, as opening occurred in many instances. In an attempt to prevent the possible recurrence of such unfortunate incidents the tests described in the following paper were planned; it was hoped thereby to improve the class of service rendered the public and safe-guard the lives of our employees. Specifically, it was desired in addition to a general study of the subject, to attempt to improve the locks already in use on our system and to provide locks for the switches located at dangerous points, i. e., points where short-circuit currents are likely to obtain which might open a given switch. One or more of the various types of switches in use on our system were tested, and in addition, a number of types which were considered for replacement of the obsolescent types now in use. As a result of these tests it was possible by a very simple expedient to raise the opening point of one of our switches from about 40,000 peak amperes to 180,000 peak amperes. A very simple lock was added to another switch largely used on the system which opened at about 51,000 peak amperes so that it would withstand the mechanical forces exerted by 143,000 peak amperes. The tests clearly demonstrated that some effective form of lock should be provided. This seems to have been generally recognized, and most manufacturers have attempted to take care of this in some way or other. Switches have been constructed (without locks) in which the current through the switch parts does not tend to open the blade. Such switches are satisfactory when used under almost ideal conditions, but certain unfavorable arrangements of the bus and leads usually found in practise, exert magnetic forces which might open the blade under short-circuit conditions, thus completely nullifying the principle of design. Although locking devices are provided they are not always effective, the various reasons being mechanical weakness of lock, current flow through lock, etc. It was noted from a study of the oscillograms that a switch seldom required more than one cycle to open, one-half cycle usually being required. It was generally recognized that there were outward forces on the blade of a switch due to the passage of high currents; but it was not generally recognized that there were also outward forces on the jaws and insulators tending to spread them apart. These forces must be recognized and dealt with by substantial design as a number of lock failures may be attributed to this. Insulators may fail due to these outward forces, and this can be remedied only by strengthening or properly supporting the insulators.