Electrical Measuring Devices and Instrumentation

What the rule requires

How the instrument enters the circuit decides more exam questions on this topic than anything else, because that is what the distractors are built from. An ammeter is always connected in series with the circuit under test, and its resistance must be much less than the load resistance — in series it adds resistance and lowers circuit current, so a high-resistance ammeter falsifies its own reading NEETS Module 3 — Learning Objectives ¶6. A voltmeter is always connected in parallel, and it must have a high resistance compared with the circuit it is across, that resistance being expressed as sensitivity in ohms per volt. The loading effect is the error a voltmeter introduces into the circuit it is measuring, and it is minimized by internal resistance many times higher than the circuit resistance NEETS Module 3 — Appendix a. As a working figure, the voltmeter's input impedance should exceed the impedance of the circuit under test by at least 10 to 1 NEETS Module 21 — Learning Objectives ¶1.

Ranges come from resistors arranged in opposite ways in the two instruments: range resistors in series with the movement for a voltmeter NEETS Module 3 — Resistance Sensitivity Voltage Full ¶1; shunt resistors in parallel with the movement for an ammeter, internal for ranges below 50 amperes and external above that, the external shunt being connected in series with the circuit and in parallel with the ammeter .

Resistance measurement is different in kind. The ohmmeter supplies its own dc potential NEETS Module 16 §3.5, so the circuit has to be dead: de-energize and discharge completely before connecting, and never apply power while measuring NEETS Module 3 — Resistance ¶4. Discharge any capacitors in the circuit first, and note any point that has no bleeder resistor or discharge path NEETS Module 16 §1.5.2.2.

Range discipline

Excess current through the movement is what destroys an analog instrument, which is why an unidentified quantity is approached from the top of the scale downward and never the other way. If the quantity can be read on several ranges, use the one that puts the indication near the middle of the scale, where an analog movement is most accurate NEETS Module 21 — Learning Objectives ¶2. The stop rule catches candidates: when the value indicated is equal to or greater than the next range down, do not switch to that range .

Observe polarity on dc instruments, never use a dc ammeter or voltmeter on ac, be certain a multimeter is switched to ac before measuring ac circuits and read the result on the ac scale, and re-zero the ohms scale after every change of resistance range . When finished, switch the multimeter OFF; if there is no OFF position, leave it on the highest ac voltage range.

The wattmeter hides its own overload

A wattmeter is an electrodynamic (electrodynamometer) instrument with a pair of fixed current coils of a few turns of comparatively large conductor in series with the load, and a movable potential coil of many turns of fine wire across the line NEETS Module 16 §3.3.5. Deflection is proportional to voltage, current and the cosine of the phase angle, so the instrument reads true power in watts directly and accounts for power factor, which a volts-times-amps calculation cannot NEETS Mod. 3 §1-4.

That same dependence on power factor is the trap. A wattmeter is rated not in watts but in volts and amperes, and the pointer gives no warning of overheating: a low power-factor circuit gives a very low reading even when both the current and potential circuits are loaded to the maximum safe limit and their insulation is burning NEETS Module 3 — Resistance ¶2. The coils of wattmeters, frequency meters and power meters may be carrying large quantities of current with the pointer well on scale .

Switchboard instrumentation and paralleling

Instrument transformers keep the meter out of the high-current, high-voltage path: current and potential transformers feed the wattmeter safely from the bus. Frequency is held at the correct 50/60 Hz with a frequency meter, which matters most before machines are paralleled — the operator works the frequency meter and synchroscope together to match frequency and phase, closing the breaker only as the synchroscope creeps slowly through its in-phase twelve o'clock position, and a wattmeter that swings negative afterward is reporting reverse power, the generator being motored by the bus and requiring a trip to protect its prime mover .

Two prohibitions on transformer-fed instruments are absolute — never open the secondary of a current transformer while the primary is energized, and never short-circuit the secondary of a potential transformer while the primary is energized — and meters installed in motor circuits must be able to handle the starting current, which may be six to eight times normal running current .

The ammeter on an energized 440 V feeder reads through a current transformer. You need to remove the meter for calibration. What must you do first, and why?

Do not open the CT secondary with the primary energized. The feeder must be secured, or the secondary shorted by the means provided, before the instrument comes out. Note also that "pointer on scale" tells you nothing about what the coils are carrying.

Movements, and what the scale is telling you

Most meters run on the permanent-magnet moving-coil arrangement, the d'Arsonval movement, which reacts to dc; a rectifier converts ac to pulsating dc so that a dc movement can be used on ac . What the movement responds to is then the average value of the ac, while the scale is calibrated to indicate the effective (rms) value NEETS Module 3 — Learning Objectives ¶1.

Three movements read ac or dc without any rectifier :

• Electrodynamic — fixed field coils and a moving coil; the usual wattmeter movement.
• Moving-vane (moving-iron) — magnetic repulsion of like poles in iron vanes; the most common movement for ac meters, and it will measure current or voltage.
• Hot-wire and thermocouple — both measure current only.

The electrostatic movement is the odd one out. It uses the electrostatic repulsion of two sets of charged plates, one fixed and one movable, so it reacts to voltage rather than to current, and it is used to measure high voltage .

An instrument is only as trustworthy as its last calibration, which is why critical switchboard readings are cross-checked with a portable calibrated meter . For reference on accuracy questions: analog voltmeters using d'Arsonval movements run about ±2% of full-scale reading, while a digital multimeter gives at least ±0.1% and, being direct-reading, removes parallax altogether .

Telling it apart

Two questions settle which instrument a stem is describing: what quantity is wanted, and whether the circuit may be alive when the leads go on.

• Ammeter — current, in series, circuit alive. A stem describing a split-core clamp is not describing a series ammeter; see the hook-on entry below.
• Voltmeter — voltage, in parallel, circuit alive. It goes wrong in high-resistance circuits, where a low-sensitivity voltmeter on a low range may disturb the circuit or give a false indication NEETS Module 21 — Learning Objectives ¶1.
• Ohmmeter — resistance and continuity, in series with the resistance, circuit dead. Never use one to measure a meter movement's internal resistance; the ohmmeter's own current output may damage the movement NEETS Module 16 §1.5.2.2.
• Megger (megohmmeter) — very large resistance, chiefly insulation, on an isolated and dead circuit. Isolate the item from other circuitry, connect the leads, turn the hand crank, and note the indication; infinity is the normal indication for sound insulation. Meggers are for high-resistance measurement only, and the test leads are never touched while the handle is being cranked NEETS Module 3 — Resistance ¶4.
• Wattmeter — power, in watts, reading true power.
• Watt-hour meter — energy. It is a small motor whose instantaneous speed is proportional to the power passing through it, so total revolutions over a period are proportional to watt-hours. Read the dials left to right and take each hand as the figure it has last passed, not the one it is approaching NEETS Module 3 — Resistance ¶2.
• Hook-on voltammeter — current, without disconnecting anything. A split-core current transformer clamps around the conductor, which acts as the transformer primary NEETS Module 3 — Resistance ¶1.
• Frequency meter — frequency of an ac signal, in two types: the vibrating-reed type, usually an in-circuit meter, and the moving-disk type, usually out-of-circuit for spot checks . A vibrating-reed meter is read at the scale value opposite the reed showing the greatest vibration .

Behind that list sits a simpler split the exam also uses: in-circuit meters are permanently installed to monitor the equipment they are built into, and out-of-circuit meters are portable and self-contained, which is why they are the more valuable in locating the cause of a malfunction NEETS Module 3 — Learning Objectives ¶2.

Working a question

You are asked to measure a voltage of unknown value in a panel, with a voltmeter offering 10, 50, 250 and 1000-volt ranges. The steps below follow the method the exam expects, and the trap is in the last one NEETS Module 3 — Resistance Sensitivity Voltage Full ¶1.

1. Confirm the connection and the function before touching anything: voltmeter in parallel, switched to ac if the circuit is ac, polarity observed if it is dc.
2. Select 1000 volts, the highest range. Excess current through the movement is what destroys it, and the highest range is the only setting that cannot be exceeded by a voltage you have not yet identified.
3. Read. The pointer sits barely above zero. That indication cannot be interpreted accurately, so the range is wrong for the job, not the meter.
4. Switch to 250 volts. The pointer now allows the voltage to be approximated as 20 volts. Since 20 is well below the next range down, switching again is justified.
5. Switch to 50 volts. The reading is now 22 volts, taken as close to midscale as this instrument will give.
6. Stop. The next range is 10 volts, and 22 volts exceeds it. Switching down would drive excess current through the movement. The 50-volt range is the final answer.

The same logic runs on an ammeter, where the stop rule bites harder: a reading of 5 milliamperes on the 10-milliampere range is not switched down to the 5-milliampere range, because the indicated value equals that range NEETS Module 3 — Learning Objectives ¶6.

Where candidates lose the point

• Parallel sounds like the gentler, less intrusive connection, and candidates put the ammeter across the load. An ammeter has a small resistance compared with the load, so across it the meter carries very high current and is damaged NEETS Module 3 — Resistance ¶4.
• A pointer sitting on scale is no evidence that a wattmeter is within its rating. The instrument is rated in volts and amperes, and at low power factor it reads low while both coil circuits are overloaded NEETS Module 3 — Resistance ¶2.
• Where switchboard volts times amps disagrees with the wattmeter, the arithmetic is what is wrong. Deflection includes the cosine of the phase angle, so on any load below unity power factor the instrument reads lower than volts × amps, and that is the instrument working correctly NEETS Mod. 3 §1-4.
• Infinity on a megger is not a fault; it is the normal indication for sound insulation. Nor is an ordinary ohmmeter the instrument for that measurement — meggers are used for high-resistance work only, with the item disconnected from other circuitry first .
• Two scale errors travel together. An ac measurement read on the dc scale goes wrong because the two scales differ, the movement responding to average value while the effective value is wanted NEETS Module 3 — Resistance ¶5. A meter read from off-axis goes wrong too: on a mirrored scale, if you can see the pointer's image, the reading is in error NEETS Module 3 — Resistance ¶1.

Check yourself

You must check the insulation of a de-energized 440 V motor feeder cable. Which instrument, what is the procedure, and what reading do you want?

A megger. Isolate the item from other circuitry, connect the leads, turn the hand crank, and note the indication; infinity is the normal reading for good insulation. Never touch the test leads while the handle is being cranked NEETS Module 3 — Resistance ¶4.

A wattmeter pointer is sitting at one-third scale on a heavily loaded, low power-factor circuit. Is the instrument safe?

Not necessarily, and this is the question the exam likes. Both the current and potential circuits may be overloaded to the point of burning insulation while the pointer is only part way up the scale, because pointer position depends on power factor as well as on voltage and current. Work to the volts and amperes rating on the instrument face, not to the watts scale NEETS Module 3 — Resistance ¶2.

An ammeter on its 10-milliampere range indicates 5 milliamperes. Do you switch to the 5-milliampere range for a better reading?

No. The indicated value equals the next range, and the rule is to stop once the reading is equal to or greater than that range — going further drives excess current through the movement NEETS Module 3 — Learning Objectives ¶6.

The generator switchboard shows 450 volts and 100 amperes on a single-phase feeder, but the wattmeter reads well below 45 kW. Which instrument is at fault?

None of them. Wattmeter deflection is proportional to voltage, current and the cosine of the phase angle, so any load at less than unity power factor reads below the volts-times-amps product NEETS Mod. 3 §1-4. Reading true power directly is precisely what the wattmeter does that the calculation cannot.

You need the running current of a motor feeder without breaking into the wiring. Which instrument, and how does it work?

A hook-on type voltammeter — essentially a current transformer with a split core and a rectifier-type instrument on the secondary. The conductor being measured is the primary, and the split core lets the instrument be hooked on without disconnecting the conductor NEETS Module 3 — Resistance ¶1.

Shortly after a generator is paralleled, its wattmeter pointer swings to the left of zero. What is happening, and what is the required action?

Reverse power: the generator is being motored by the bus. Trip it to protect the prime mover .

Your voltage readings in a high-resistance circuit disagree with the equipment technical manual's chart. What do you suspect about the meter before you suspect the circuit?

Its sensitivity. A voltmeter with low sensitivity used on a low range may disturb the circuit under test or give a false indication, and the chart was prepared with a particular instrument. Aim for input impedance at least 10 times the circuit impedance NEETS Module 21 — Learning Objectives ¶1.