THE COLLIERY GUARDIAN AND JOURNAL OF THE COAL AND IRON TRADES. Vol. CXV. FRIDAY, MARCH 15, 1918. No. 2985. Useful Instruments for Colliery Power Plants.* By H. W. RAVENSHAW. It has been of great advantage to the electrical industry that for many years simple instruments have been available for measuring volts, amperes, ohms, watts, and watt-hours, thus enabling tests and records to be obtained more readily than in any other branch of engineering. Several instruments that do not appear to be widely known—for instance, leakage recorders—have been in use to a certain extent for some years, but quite a number of engineers who have seen them for the first time had never previously heard of them. The continuous indicator has been in use for a number of years, and many interesting records have been obtained with it from winding engines, rolling mills, and gas engines, showing variations of speed and Fig, 1—Connections to D.-C. Leakage Recorder. H KEY RESISTANCE XeSISTANCE h Key f RECORDER 10 4L 1!L 21 21 4-1 411 Fig. 2.—C.-C. Leakage Recorder. pressure and other useful data. The diagrams (figs. 6 to 8) are intended only to give instances of its appli- cation, and are not put forward as typical of the class of engine tested. Direct-Current Leakage Recorder. The “ Special Rules for the Installation and Use of Electricity in Mines ’’ under the Coal Mines Regulation Act state that “ efficient means shall be provided for indicating any defect in the insulation of a system ”— Rule 124 (d). The memorandum accompanying the rules also states that “ in cases where no point of the system is connected to earth, an electrostatic voltmeter con- nected between each line or phase and earth may be used to indicate any defect in the insulation of the system.” The arrangement of electrostatic voltmeter suggested has the serious fault that it shows only the difference of potential between the particular line to which it is connected and earth, and gives no measure of the insu- lation resistance. In fact, where the insulation resist- ances of both lines oT all phases are about equal, the readings will be the same whether the insulation is high or low. A form of instrument for direct-current circuits was, therefore, put on the market by several firms, which gave an indication when there was low insulation on one pole, and, by pressing first one and then the other of two keys, readings could be taken which, when com- pared with a table (which was supplied with the instru- ment) gave the insulation resistance of each pole in ohms. This instrument was largely used, but did not leave any record of the state of the insulation except that which would be booked at intervals by the attendant. * Paper read before Midland Institute of Mining, Civil and Mechanical Engineers. At the suggestion of the writer, Messrs. Evershed and Vignoles designed and made a recording leakage indicator, which has a roll of paper worked by a clock, and the pen normally marks the reading as if neither key was depressed. At intervals, however, of about five minutes, contact is automatically broken, first on one pole and then on the other, in the same way as would be done by pressing the keys, and a record is obtained of the deflection first on one pole and then on the other. The actual insulation is obtained from a similar table to that for the indicating instrument. This recorder gives most interesting and valuable results, as the actual condition of the insulation is given at frequent intervals both night and day. In the early instruments some trouble arose with the contact-breaker, especially where heavy leaks occurred. This part of the apparatus has, however, lately been greatly improved. Fig. 1 shows the connections of this instrument; and fig. 2 is a record from a defective circuit. Alternating Current Leakage Recorder. This instrument is a very convenient one, as it can be arranged to read the total insulation resistance of the circuit in ohms. It is based on the principle that a low- resistance choking coil can be made to act as if there was a high resistance in an alternating circuit, while it only imposes a low resistance to a direct-current circuit; also that a permanent magnet ammeter gives no deflec- tion with an alternating current. A battery or a small special dynamo is connected to the alternating ciicuit to earth through a choking coil and a low-reading permanent magnet ammeter. The result is that the ammeter gives a reading proportional to the voltage of the battery and inversely proportional to the resistance to earth of the circuit, plus the resistance of the choking coil and ammeter. If the direct-current voltage remains constant, the ammeter can be calibrated in ohms, and by means of a recording apparatus the state of the insulation of the circuit can be shown continuously on a roll of paper. Fig, 3.—Connections to Alternating-current Leakage Recorder. BATTERY LfAK B LEAK RECORDER C EARTH INDUCTIVE resistance "a I Fig. 4.—Continuous Steam Engine Indicator. Flexible PAPER BELT CCARED DftUM SIGNAL MAGNET INDICATOR ] CLOCKWORK Fig. 5.—Continuous Steam Engine Indicator. This is an excellent instrument, as it shows both night and day any variation of insulation resistance, and particularly those leakages which come on and off for short intervals and usually develop into serious faults. The writer has several of these recorders in use on 3,000-volt and 500-volt circuits. In addition to the recording instrument, various safe- guards are employed to give a signal, and to cut out the recorder so as to prevent it from being damaged in case of a dead earth. A somewhat similar arrangement is used with an earthed neutral, the instrument and battery being in series with the neutral connection to earth. This instru- ment, which is made by Messrs. Nalder Brothers and Thompson, requires some attention, as it is important that the battery should be in good order, and regular readings of the battery voltage should be taken. Where possible, it is better to use a small direct-current dynamo similar to that which is now being used in many places for shaft signals. All recorders require to be carefully looked after, as the heat of the engine-room, especially in the summer, dries up the ink, and there is room for improvement in that direction. Fig. 3 shows the connections of this instrument. Over-Volt Vacuum Breaker. Although ail turbines are fitted with some form of geai' to prevent possible runaway, mishaps have occurred at times, owing to the failure of the runaway governor to act. This danger arises particularly in cases where turbines have to run for many days on end without stopping, as the runaway gear cannot in that case be frequently tested. Again, the very large valves of low- pressure turbines are more liable to stick than those controlling high-pressure steam. Fig. 6.—Readings from Continuous Diagram from Cylin dro-conical Drum Winding Engine. MEANPRESSUR SECONDS 30 REVOLUTIONS Ail So 70 to 50 AO 30 20 100