Electrical measuring devices and instrumentation

DOE-HDBK-1011 Vol.4 §14-1

Test instruments for distribution maintenance Maintaining a distribution system depends on correct use of test instruments. The multimeter (analog VOM or digital DMM) measures voltage (connected in parallel across the points, high input resistance to avoid loading), current (connected in series, or via a clamp/CT for large currents, low insertion resistance), and resistance (on de-energized, isolated circuits only, using the meter's own source). The wattmeter reads true power directly by combining a current coil and a voltage coil, so it accounts for power factor, and on switchboards it is fed through CTs and PTs. The megohmmeter (megger) applies a high DC test voltage (commonly 500-1000 V) to read insulation resistance in megohms, the primary test for motor, generator, cable, and switchg…

NAVEDTRA 14104 Ch. 11 — Voltmeters

Both dc and ac voltmeters determine voltage the same way. They both measure the current that the voltage is able to force through a high resistance. This resistance is connected in series with the indicating mechanism or element. Voltmeters installed in switchboards and control consoles (fig. 11-16) all have a fixed resistance value. Portable voltmeters, used as test equipment, usually have a variable resistance. For both installed and portable voltmeters, resistances are calibrated to the different ranges that the meters will display. The normal range for the switchboard and electric plant meters is 0 to 600 volts. Figure 11-17.—An ac ammeter. Figure 11-18.—Frequency meter.

NEETS Mod. 3 §1-2

Ammeters and voltmeters — connection and range extension An ammeter measures current and must be connected in series so all the circuit current passes through it; the circuit must be opened to insert it. An ammeter has very low resistance to avoid disturbing the circuit, and its range is extended by a shunt — a low-value resistor in parallel with the movement that carries most of the current while the movement reads a fixed fraction. Never connect an ammeter across a voltage source: its low resistance would draw a huge current and be destroyed. For AC and for high currents, a clamp-on ammeter or a current transformer reads current without breaking the circuit. A voltmeter measures potential difference and is connected in parallel (across) the component to be measured. It has high resistan…

NEETS Mod. 3 §1-4

Wattmeters, frequency meters, and instrument accuracy Power in an AC circuit is measured with a wattmeter, which uses two coils: a low-resistance current coil in series with the load and a high-resistance voltage (potential) coil across it. The instrument's deflection is proportional to the product of voltage, current, and the cosine of the phase angle, so it reads true power in watts directly, automatically accounting for power factor — something a simple volt-times-amp calculation cannot do. On large switchboards, current and potential transformers feed the wattmeter safely from high-current, high-voltage buses. Frequency is monitored with a frequency meter (vibrating-reed or pointer type) to keep generators at the correct 50/60 Hz, which is essential before paralleling machines. Power-…

NEETS Module 16 §1.5.2.2

Other Instrument Precautions Precautions to be observed to prevent instrument damage include the following: • Keep in mind that the coils of wattmeters, frequency meters, and power meters may be carrying large quantities of current even when the meter pointer is on scale. • Never open secondaries of current transformers when the primary is energized. • Never short-circuit secondaries of potential transformers the primary is energized. • Never leave an instrument connected with its pointer off-scale or deflected in the wrong direction. • Ensure that meters in motor circuits can handle the motor starting current. This may be as high as six to eight times the normal running current. • Never attempt to measure the internal resistance of a meter movement with an ohmmeter since the movement may…

NEETS Module 16 §1.6

BASIC MEASUREMENTS Electronic measurements involve the fundamental electrical quantities of voltage and current and the inherent characteristics of resistance, capacitance, and inductance. In circuits being tested, voltage and current are dependent upon resistance, capacitance, and inductance for their distribution; therefore, voltage and current measurements are valuable aids in determining circuit component conditions and in the evaluation of symptoms. Practically any reading obtained from the use of test equipment will depend on these basic measured quantities of resistance, capacitance, and inductance.