The circuit is as follows: From the A. C. bus on the power panel, to the lower right terminal of switch 8, to the “operating board” terminal, to the A. C. lead, to the A. C. post on the signal block, to the white lamp and the resistance in parallel, to the A. C. jaw on the master block, to the testing switch T. S., to the “earth” post on the signal block, to the earth lead, to the D. C. earth, through earth, to the A. C. earth terminal on the power panel, through the choke coil, to switch 8, to the other A. C. bus on the power panel.
In this test it is imperative to see that all the automatic switches are up and all the firing switches are open.
7. Test of the power.—Insert two fuses in multiple across the fuse leads from the power panel. Put the fuses in a place prepared for the purpose outside of the casemate, so that there will be no danger from flying fragments. With all the switches on the power panel open, all the automatic switches up, and the firing switches on the master blocks open, energize the D. C. busses of the power panel, close switch No. 4 up (or down), and close the starting switch; close switch No. 9 up (or down); close switch No. 12 up (which connects the mine transformer secondary to the fuses); and, finally, close switch No. 11 up (which throws the A. C. power on the mine transformer primary). The fuses should explode.
If fuses are not available for this test, a low-voltage lamp or a short piece of fine wire may be heated to incandescence.
8. Test of grounds.—(a) “Separate” grounds shall be made for the A. C. power and the D. C. power on the power panel. The word “separate” as here used means actual connection to earth without metallic contact of the earth leads. A convenient method of making a ground is to connect to the armor of a cable running to salt water, a bond being made in case the armor of the cable in the casemate does not reach water before a joint is made. If a cable armor is used for one ground, the other ground lead must go to earth without contact with that armor. This may be accomplished by using the conductors of a cable, the ends of which are grounded to an earth plate in salt water.
(b) Neither of the grounds made should have more than 10 ohms resistance. To verify this, tests should be made as follows:
Close the double circuit breaker; close switch 7 up and plug the extension cord of the mil-ammeter lead of the power panel on the upper left-hand terminal of switch 8, the mil-ammeter extension cords for the operating boards being disconnected.
Ascertain the voltage across the mil-ammeter and lamp, and across the bus bars. Read the mil-ammeter.
From these readings the combined resistance of the grounds can be determined.
A table or chart may be prepared giving the resistances for various testing voltages and mil-ammeter readings.