- { plunger type;
- a. Moving iron instruments { inclined coil type;
- { magnetic vane type;
- b. Hot wire instruments;
- c. Induction instruments { shielded pole type;
- { repulsion type;
- d. Dynamometers;
- e. Instrument transformers;
- { commutator type;
- f. Watthour meters { induction type;
- { Faraday disc type;
- { synchronous motor type;
- g. Frequency indicators { resonance type;
- { induction type;
- { lamp type;
- h. Synchronism indicators { voltmeter type;
- { resonance type;
- { rotating field type;
- i. Power factor indicators { wattmeter type;
- { rotating field type;
- j. Ground detectors;
- k. Earth leakage cut outs;
- l. Oscillographs.
CHAPTER LVII
SWITCHING DEVICES
A switch is a piece of apparatus for making, breaking, or changing the connections in an electric circuit.
The particular form and construction of any switch is governed by the electrical conditions under which it must operate.
Since the electric current cannot be stopped instantly when the circuit in which it is flowing, is broken, an arc is formed as the switch contacts separate; this tends to burn the contacts, and to short circuit, the severity of such action depending on the voltage and the proximity of the switch terminals. Accordingly in switch design, provision must be made to counteract these tendencies. Thus,
- 1. The contacts should separate along their entire length, rather than at a point;
- 2. The terminals should be far enough apart and properly protected to prevent short circuiting of the arcs;
- 3. The break should be quick;
- 4. The gap should be surrounded by the proper medium (air or oil) to meet the requirements of the electrical conditions.
A great variety of switches have been introduced to suit the different requirements. Knife switches are used for low pressure service, the multiple break form being used where it is desired to reduce the arcing distance.