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gives a multiplying power of 20, which is equivalent to the deflection produced by the battery current through a standard resistance of one megohm. The true deflection thus obtained, or, in other words, the observed deflection multiplied by 20, gives us the insulation "constant, ” which term has been already explained. The plug is then withdrawn and re-inserted in the opening marked INSULn., when the deflection due to the cable, with or without a shunt, will be obtained. It remains, therefore, to divide the latter deflection into the former to ascertain the insulation resistance of the cable under test in megohms.
The key on the left may, if all the values in the proportional arms of the bridge be plugged up, be employed as a short circuit key across the terminals of the galvanometer, and, in this capacity, is useful for checking the oscillations of the needle.
By employing the maximum shunt, i.e., the shunt giving a multiplying power of 100, a larger testing voltage than that described above may be employed with the same result, except that the constant will then be through an equivalent of five megohms instead of one, which fact must be taken into consideration in calculating results.
The following additional rules concerning this test are also excerpted from the manufacturers' instructions, and should be rigidly adhered to, if satisfactory results are to be obtained.
(1) Too much care cannot be taken in preparing the ends of the cable. Since we are measuring a very small current of electricity passing from the conductor to the outside sheathing, through the insulated covering, it is clear that our results will be entirely misleading if any current be allowed to pass over a dirty surface at the ends where the conductor is exposed. These ends should be looked to before testing, and in the case of india-rubber or other firm material, the section of the insulator should be pared all over with a sharp and perfectly clean knife.
(2) Care should be taken not to short-circuit the battery, which may easily occur in two ways.
One is by allowing the two battery plugs to touch one another, when the other ends of the leads are attached to the
battery terminals; and another is by allowing the lead attached to the earth terminal to touch that attached to the insulation terminal.
“In both cases the battery of small cells will be for a time much over
verworked, and in the second the needle may become bent or demagnetised.
(3) Another point that may be noticed is that in deducing the insulation resistance per statute mile from a test on any given length, the result obtained from a test on the latter is to be multiplied by the length of the piece in miles, and not divided by it."
Such a combination of instruments and apparatus as the one just described is admirably suited for installation work such as falls under the duties of mains inspector, wiring contractor, etc., etc., and, in fact, in all cases where a convenient portable testing set is required and
Norman Testing Switch for Insulation and Capacity Tests on
Electric Cables and Circuits.
extreme accuracy is not essential, but, as its range is
A switch and shunt combination which has been de-
A general view of the switch is shown in the accompanying block, to which is appended a general plan dia
to the latter diagram, a and b are two fan-shaped sheets of ebonite forming the vertical sides of the switch, and around the inner circumference of which are fixed a series of metallic contacts, al, a2, a3, a4, all, 61, 62, 63, and b4. The (a) contacts are severally connected with the (6) contacts by the successive movements of the lever c, the sole office of which is to thus form a path of negligible resistance between these points; in other words, the lever forms a circuit key severally connecting them.
Of the external connections, A and B are the galvanometer terminals seen to the front of the complete illustration, of which A also provides a means for connecting the standard high resistance for insulation testing, the standard condenser for capacity measurement, and subsequently the extremity of the cable itself under test, re spectively. G is the galvanometer, and S a set of shunts, wound with platinoid wire, and contained in the cylindrical ebonite covers seen in the front of the complete switch above. These shunts are arranged on the “Universal” principle, the various points of contact being connected to the studs a4, a5, ab, and a7, yielding 1-10,000th, 1-1,000th, 1-100th, and 1-10th shunts respectively, and, being constructed on the above principle, the switch is available for use with any galvanometer, without altering the respective multiplying powers of these shunts. E and El are the batteries for insulation and capacity measurement respectively, whil
the earth connections are arranged as shown. A subsidiary short-circuiting device upon the switch lever, with corresponding galvanometer contacts upon the inner surface of the front of the switch, serves to short-circuit the galvanometer during the initial charging period in insulation testing, the short circuit being removed as the lever moves forward over successive shunts, thus allowing the electrification readings to be taken.
Taking the various a and b contacts in turn, we will proceed to discover what happens when the connecting lever is moved over the range of the switch. Firstly, when connecting al and bl, the terminal A, and in consequence the cable or condenser connected between it and earth, is charged electrostatically from the battery El; on moving the lever so that it connects a2 and 62, nothing happens unless a shunt be required in taking the
capacity throw, in which case the shunt required is brought into circuit by two small ebonite-handled plugs connected by a short length of flexible wire, one of which is inserted in the hole on the exterior face of the switch, corresponding to the shunt required, and the other in the hole corresponding to contact a2, thus completing the circuit indicated by the dotted line in the diagram of connections.
If no shunt be required in taking the capacity throw, the lever is moved so as to connect a3 and 62, thus discharging the cable or condenser. When the lever is next moved to connect a3 and 63, one pole of the insulation testing battery E is connected to A, and consequently to the cable if connected, the opposite pole being earthed; simultaneously the galvanometer is short-circuited by the device before alluded to, and so protected from injury due to the rush of current, on moving further to positions a4, a5, a6, and a7, connecting them respectively with 63, the 1-10,000th, 1-1,000th, 1-100th, and 1-10th shunts are successively brought into circuit, and the short circuit is at the same time removed from the galvanometer terminals. The connection of a8 and 63 gives the position of no shunt;” then, the electrification readings having been duly noted, the lever is next moved to connect a9 and 64, when the cable will commence to discharge itself. Then, the initial rush of the discharge current being over, the lever may be moved to connect al0 and 64, when the necessary shunts for taking the “ earth readings ” may be introduced as in the case previously mentioned, when describing the capacity test with this apparatus, by inserting one of the plugs in the hole corresponding to the shunt required, and the other in that of alo. Finally, with all and 64 connected, the cable discharges itself in the "no shunt” position.
The capacity battery El is connected to the switch contact through a small plug switch at the back, so that it can be disconnected, if desired, by simply withdrawing the plug.
In brief, the mode of using this switch for capacity and insulation testing is as follows :—The testing batteries, galvanometer, and earth connections having been made as shown in the figure, the switch lever is moved over to the “
no shunt” insulation position, and the leakage deflection, if any, noted; if used under ordinary