Electrical measuring devices and instrumentation

DOE-HDBK-1011 Vol.4 §13-1

DOE-HDBK-1011 Vol.4 §13-1 — Three-phase connections and distribution transformers Transformers change AC voltage by mutual induction in the ratio of their turns (Ep/Es = Np/Ns) and are the reason AC is used for distribution. Three-phase transformation uses either a single three-phase transformer or a bank of three single-phase units, connected in one of four combinations of delta and wye on primary and secondary: delta-delta, wye-wye, delta-wye, and wye-delta. A wye (star) connection provides a neutral point and a line voltage that is √3 (1.732) times the phase (winding) voltage, with line and phase currents equal; a delta connection has line voltage equal to phase voltage but line current √3 times the phase current. Delta-wye is common for stepping distribution voltage down while providi…

DOE-HDBK-1011 Vol.4 §14-1

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, g…

DOE-HDBK-1011 Vol.4 §15-3

DOE-HDBK-1011 Vol.4 §15-3 — System grounding and ground detection Grounding connects parts of the electrical system, or equipment enclosures, to the ship's hull/earth reference for safety and for controlled operation of protection. Two distinct concepts apply. Equipment grounding bonds all non-current-carrying metal (motor frames, enclosures, conduit) together and to ground so that a winding-to-frame fault cannot leave the frame energized at a dangerous voltage; the fault current instead returns through the ground path and trips protection, and the bonding also drains static charge. System grounding concerns whether a current-carrying conductor (typically the neutral) is intentionally connected to ground. Many shipboard power systems are operated ungrounded (an ungrounded delta), so that …

NEETS Mod. 2 §1-2

NEETS Mod. 2 §1-2 — Peak, RMS, average values and phase Because a sine wave is constantly changing, several values describe it. The effective, or root-mean-square (RMS), value is the most important: it is the DC value that would produce the same heating in a resistance, and it is what AC voltmeters and ammeters read and what "120 V" or "440 V" ratings mean. For a sine wave, RMS = 0.707 × peak, and conversely peak = 1.414 × RMS. The average value of one half-cycle is 0.637 × peak (the full-cycle average is zero). Instantaneous value is the amplitude at a specific instant. Phase describes the time relationship between two sine waves of the same frequency, measured in degrees of the 360° cycle. Waves that reach corresponding points together are in phase; otherwise one leads or lags the other…

NEETS Mod. 3 §1-1

NEETS Mod. 3 §1-1 — Meter movements and analog metering Electrical measurement quantifies voltage, current, resistance, power, and frequency to verify operation and troubleshoot faults. The classic analog indicator is the D'Arsonval (permanent-magnet moving-coil) movement: current through a pivoted coil in a magnet's field produces a torque proportional to the current, deflecting a pointer against a hairspring. The movement itself is very sensitive (full-scale of microamps to milliamps) and is the heart of analog ammeters, voltmeters, and ohmmeters. Because the bare movement handles only tiny currents, ranges are extended with resistors. Meter accuracy is stated as a percentage of full-scale deflection, so readings are most accurate in the upper two-thirds of the scale — select a range ac…

NEETS Mod. 3 §1-2

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. …