The detection of transient arcs in metal-clad fusible cut-outs

This report gives directions for the employment of a neon lamp as a substitute for an oscillograph in the detection of transient low-current arcs between the case and poles of metal-clad fusible cut-outs operating under short-circuit conditions. Such arcs can only be detected by special means, but previous tests have shown that they are the precursors of others of much greater magnitude and duration when the short-circuit current is increased, and invariably lead to complete destruction of the cut-out. The characteristic of the neon lamp of which advantage is taken is that there is a difference between the minimum voltage which will cause the lamp to glow and the value (about 20 volts less in commercial lamps) to which the voltage may subsequently be lowered before the lamp is extinguished. In the method described herein, the transient voltage across a resistance arising from the passage of a transient arc causes the lamp to glow, and in virtue of the above characteristic it then remains glowing indefinitely although the arc has ceased. Directions are given for obtaining maximum sensitivity (for present purposes) from any given lamp, and a simple method of selecting the most sensitive lamp from a stock is described. It is shown that the ordinary commercial neon lamp costing a few shillings can readily be made to detect transient arcs of about 20 amperes flowing for 0.002 sec., which is all that is required in practice, while a similar lamp specially selected by the methods described responded to the passage of a current of 3 amperes for 0.0005 sec. The only drawback to the ordinary commercial neon lamp is that, owing to impurities in the gas, etc., its characteristics may vary with time, although in the tests no appreciable variation was noted after 100 experiments. Constant characteristics can be obtained by suitable precautions in manufacture, and the General Electric Co.´s. Research Laboratories have kindly signified their willingness to make up special lamps of- this type for the above purpose at a cost of less than 20s. each.