Module 3 −− development to Circuit Protection, Control, and Measurement

Pages i,1−1,1−11,1−21,1−31,1−41,1−51,1−61,1−71,2−1,2−11,1−21,2−31,2−41,3−1,3−11,3−21,3−31,AI−1,AII−1,AIII−1,IV−1,Index


Figure 1-30. - reading a voltmeter at miscellaneous ranges.

You are watching: What precaution should be taken when using an ohmmeter

In number 1-30(A) the meter is in the 1000-volt range. The guideline is barely over the 0 position. That is not feasible to accurately read this voltage. In figure 1-30(B) the meter is switched come the 250 volt range. Indigenous the tip position the is possible to almost right the voltage as 20 volts. Because this is well below the next range, the meter is switched, as in figure 1-30(C). Through the meter in the 50-volt range, the is feasible to review the voltage as 22 volts. Since this is an ext than the next selection of the meter (10 volts), the meter would certainly not it is in switched come the following (lower) scale.

Q34. How is it feasible to use a present sensitive meter activity to measure up voltage?

Q35. What is voltmeter sensitivity?

Q36. What an approach is offered to enable a voltmeter to have several ranges?

Q37. Why have to you constantly use the highest selection when connecting a voltmeter come a circuit?


The final meter movement covered in this thing is the ELECTROSTATIC METER MOVEMENT. The various other meter movements you have studied all reaction to current, the electrostatic meter movement reacts to voltage.

The device is based upon the repulsion of choose charges ~ above the key of a capacitor. The electrostatic meter motion is actually a large variable capacitor in i m sorry one set of bowl is enabled to


move. The motion of the key is opposed by a feather attached come the plates. A guideline that shows the worth of the voltage is enclosed to this movable plates. Together the voltage increases, the plates develop more torque. To develop sufficient torque, the plates must be huge and carefully spaced. A really high voltage is crucial to administer movement, therefore, electrostatic voltmeters are provided only for High Voltage measurement.

VOLTMETER safety PRECautionS

Just as with ammeters, voltmeters call for safety precautions to prevent injury to personnel and also damage to the voltmeter or equipment. The adhering to is a list of the MINIMUM safety precautions for making use of a voltmeter.

· constantly connect voltmeters in parallel.

· constantly start v the highest variety of a voltmeter.

· Deenergize and also discharge the circuit fully before connecting or disconnecting the voltmeter.

· In dc voltmeters, observe the proper circuit polarity to prevent damage to the meter.

· never use a dc voltmeter to measure ac voltage.

· observe the basic safety precautions of electrical and electronic devices.

Q38. What type of meter activity reacts come voltage rather than current?

Q39. What is the just use for the voltage sensitive meter movement?

Q40. Perform the six security precautions because that the use of voltmeters.


The two instruments most frequently used to examine the continuous (a finish circuit), or to measure the resistance the a circuit or circuit element, are the OHMMETER and the MEGGER (megohm meter). The ohmmeter is widely supplied to measure up resistance and check the continually of electric circuits and also devices. Its selection usually expand to just a few megohms. The megger is widely provided for measuring insulation resistance, together as in between a wire and also the outer surface of the insulation, and insulation resistance that cables and insulators. The range of a megger may prolong to more than 1,000 megohms.

The ohmmeter is composed of a dc ammeter, with a few added features. The added features are:

1. A dc source of potential (usually a 3-volt battery)

2. One or an ext resistors (one of which is variable)

3. A an easy ohmmeter circuit is shown in number 1-31.

The ohmmeter"s pointer deflection is regulated by the amount of battery present passing with the moving coil. Before measuring the resistance of one unknown resistor or electric circuit, the test leads of the ohmmeter are first shorted together, as presented in number 1-31. Through the leader shorted, the meter is calibrated for ideal operation top top the selected range. When the leads are shorted, meter current is maximum and the guideline deflects a maximum amount, somewhere near the zero position on the ohms


scale. As such current with the meter through the leader shorted, that is necessary to eliminate the check leads once you are finished utilizing the ohmmeter. If the leads were left connected, they could come in call with every other and discharge the ohmmeter battery. Once the variable resistor (rheostat) is readjusted properly, with the leader shorted, the tip of the meter will come to rest specifically on the zero position. This shows ZERO Resistance in between the test leads, which, in fact, space shorted together. The zero analysis of a series-type ohmmeter is on the right-hand side of the scale, where as the

zero analysis for one ammeter or a voltmeter is usually to the left-hand side of the scale. (There is another type of ohmmeter which is disputed a tiny later top top in this chapter.) when the check leads of one ohmmeter space separated, the reminder of the meter will return to the left side of the scale. The interruption of current and the spring stress and anxiety act ~ above the movable coil assembly, relocating the tip to the left next (∞) that the scale.


Figure 1-31. - a simple ohmmeter circuit.


After the ohmmeter is readjusted for zero reading, the is ready to be linked in a circuit to measure resistance. A typical circuit and also ohmmeter arrangement is displayed in number 1-32.



Figure 1-32. - measuring circuit resistance through an ohmmeter.

The power switch of the circuit to it is in measured should constantly be in the off position. This avoids the resource voltage that the circuit from gift applied throughout the meter, which could reason damage come the meter movement.

The test leads of the ohmmeter are connected in series with the circuit to it is in measured (fig. 1-32). This causes the current created by the 3-volt battery the the meter to flow through the circuit being tested. Assume that the meter test leads are linked at clues a and b of figure 1-32. The lot of current that flows v the meter coil will rely on the full resistance that resistors R1 and R2, and the resistance the the meter. Due to the fact that the meter has been preadjusted (zeroed), the amount of coil motion now depends solely on the resistance the R1and R2. The inclusion of R1 and R2 raises the total collection resistance, diminish the current, and also thus to decrease the guideline deflection. The tip will now pertained to rest in ~ a scale figure indicating the merged resistance the R1 and also R2. If R1 or R2, or both, were replaced with a resistor(s) having a bigger value, the current flow in the moving coil the the meter would be lessened further. The deflection would additionally be additional decreased, and the range indication would check out a still greater circuit resistance. Motion of the moving coil is proportional to the quantity of existing flow.


The amount of circuit resistance to be measured may vary end a wide range. In some instances it might be only a couple of ohms, and also in others it may be as great as 1,000,000 ohms (1 megohm). To allow the meter come indicate any value being measured, v the the very least error, range multiplication features are used in most ohmmeters. For example, a typical meter will certainly have four test lead jacks-Common, R x 1, R x 10, and also R x 100. The jack significant Common is connected internally v the battery come one side of the relocating coil that the ohmmeter. The jacks marked R x 1, R x 10, and also R x 100 are associated to three different size resistors situated within the ohmmeter. This is displayed in figure 1-33.



Figure 1-33. - an ohmmeter v multiplication jacks.

Some ohmmeters room equipped through a selector move for choosing the multiplication scale desired, so just two test command jacks room necessary. Various other meters have a different jack because that each range, as shown in number 1-33. The selection to be used in measure any particular unknown resistance (Rx in figure 1-33) depends on the approximate value of the unknown resistance. For instance, assume the ohmmeter in number 1-33 is calibrated in departments from 0 to 1,000. If Rx is better than 1,000 ohms, and also the R x 1 variety is gift used, the ohmmeter cannot measure it. This occurs due to the fact that the combined collection resistance of resistor R x 1 and also Rx is too great to allow sufficient battery current to circulation to direction the tip away from infinity (∞). (Infinity is a quantity bigger than the biggest quantity you have the right to measure.) The test command would need to be plugged right into the following range, R x 10. Through this done, i think the pointer deflects to suggest 375 ohms. This would suggest that Rx has 375 ohms x 10, or 3,750 ohms resistance. The adjust of range caused the deflection since resistor R x 10 has around 1/10 the resistance the resistor R x 1. Thus, picking the smaller series resistance allowed a battery present of sufficient amount to reason a helpful pointer deflection. If the R x 100 range were supplied to measure up the very same 3,750-ohm resistor, the guideline would direction still further, to the 37.5-ohm position. This boosted deflection would occur since resistor R x 100 has about 1/10 the resistance the resistor R x 10.

The foregoing circuit arrangement permits the exact same amount of present to flow through the meter"s relocating coil even if it is the meter steps 10,000 ohms on the R x 10 scale, or 100,000 ohms ~ above the R x 100 scale.

It constantly takes the exact same amount of present to direction the reminder to a specific position on the range (midscale position for example), regardless of the multiplication factor being used. Because the multiplier resistors space of various values, the is vital to constantly "zero" adjust the meter for each multiplication truth or selected.

You should choose the multiplication factor (range) the will result in the guideline coming to rest as close to as possible to the midpoint the the scale. This allows you to check out the resistance more accurately, due to the fact that the scale readings are much more easily taken at or near midpoint.


Q41. What electrical quantity is measure up by an ohmmeter?

Q42. What various other measurement can an ohmmeter make?

Q43. Just how is a series-type ohmmeter linked to the circuit being measured?

Q44. What is used to administer the ohmmeter with number of ranges?

Q45. What area of one ohmmeter scale must be offered when measure circuits?


The ohmmeter described to this allude is well-known as a series ohmmeter, due to the fact that the resistance to be measured is in collection with the interior resistors and the meter motion of the ohmmeter. Another form of ohmmeter is the Shunt OHMMETER. In the shunt ohmmeter, the resistance to it is in measured shunts (is in parallel with) the meter movement of the ohmmeter. The most obvious way to phone call the difference in between the collection and shunt ohmmeters is through the range of the meter. Figure 1-34 shows the scale of a collection ohmmeter and also the scale of a shunt ohmmeter.


Figure 1-34. - series and shunt ohmmeter scales.

Figure 1-34(A) is the range of a collection ohmmeter. Notification "0" is top top the right and "∞" is on the left. Number 1-34(B) is the range of a shunt ohmmeter. In the shunt ohmmeter "∞" is on the right and also "0" is on the left. A shunt ohmmeter circuit is shown in number 1-35.

In number 1-35, R1 is a rheostat offered to change the ∞ reading of the meter (full-scale deflection). R2, R3, and also R4 are provided to carry out the R x 1, R x 10, and R x 100 ranges. Points a and B stand for the meter leads. Through no resistance connected between point out a and also B the meter has actually full-scale current and also indicates


∞. If a resistance is connected between point out a and also B, that shunts few of the present from the meter movement and also the meter activity reacts to this reduced current. Due to the fact that the scale of the meter is significant in ohms, the resistance that the shunting resistor (between clues a and B) is indicated. An alert that the switch has an turn off position, as well as positions because that R x 1, R x 10, and also R x 100. This is provided to stop current flow and prevents the battery from gift discharged if the meter is no being used.


Figure 1-35. - a shunt ohmmeter v internal range resistors.

The shunt ohmmeter is associated to the circuit to be measured in the same method the series ohmmeter is connected. The only difference is that on the shunt ohmmeter the ∞ reading is adjusted, while on the series ohmmeter the 0 reading is adjusted. Shunt ohmmeters space not frequently used since they are minimal generally come measuring resistances native 5 ohms come 400 ohms. If you usage a shunt ohmmeter, be certain to switch it come the turn off position as soon as you room finished utilizing it.

Q46. What are the two species of ohmmeters?

Q47. What is the most noticeable difference between the two species of ohmmeters?

Q48. Perform the four safety precautions observed once using ohmmeters.

OHMMETER security PRECautionS

The adhering to safety precautions and operating procedures for ohmmeters space the MINIMUM important to stop injury and also damage.

· Be particular the circuit is deenergized and discharged prior to connecting an ohmmeter.

· perform not apply power to a circuit while measuring resistance.

· as soon as you room finished making use of an ohmmeter, move it to the ofF position if one is detailed and remove the leader from the meter.

· Always readjust the ohmmeter for 0 (or ∞ in shunt ohmmeter) after you change ranges prior to making the resistance measurement.



An ordinary ohmmeter can not be supplied for measure resistance the multimillions that ohms, such as in conductor insulation. To adequately test for insulation break down, that is essential to use a much higher potential than is furnished by the battery of one ohmmeter. This potential is placed in between the conductor and the external surface that the insulation.

An instrument called a MEGOHMMETER (MEGGER) is supplied for these tests. The megger (fig. 1-36) is a portable tool consisting of two primary elements: (1) a hand-driven dc generator, G, which gives the high voltage because that making the measurement, and also (2) the instrument portion, which shows the value of the resistance gift measured. The instrument section is of the opposed-coil type, as shown in number 1-36(A). Coils a and also b are mounted on the movable member c with a fixed connection to each other, and also are free to revolve as a unit in a magnetic field. Coil b often tends to move the pointer counterclockwise, and also coil a often tends to relocate the guideline clockwise.


Figure 1-36. - a megger inner circuit.

Coil a is linked in collection with R3 and the unknown resistance, Rx, to it is in measured. The mix of coil, R3, and also Rx develops a direct series path in between the optimistic (+) and an unfavorable (-) brushes of the dc generator. Coil b is associated in series with R2 and this mix is additionally connected throughout the generator. There are no restraining springs on the movable member that the instrument portion of the megger. Therefore, once the generator is not operated, the guideline floats freely and also may concerned rest at any position top top the scale.


The safety ring intercepts leakage current. Any type of leakage currents intercepted are shunted to the an unfavorable side the the generator. They do not flow through coil a; therefore, they execute not impact the meter reading.

If the check leads are open-circuited, no current flows in coil a. However, present flows internally with coil b, and deflects the tip to infinity, which indicates a resistance too huge to measure. As soon as a resistance such as Rx is linked between the check leads, current additionally flows in coil a, tending to move the guideline clockwise. At the exact same time, coil b still tends to move the tip counterclockwise. Therefore, the moving element, composed of both coils and also the pointer, comes to rest in a place at which the two pressures are balanced. This position depends ~ above the worth of the exterior resistance, i beg your pardon controls the family member amount of existing in coil a. Due to the fact that changes in voltage impact both coil a and also coil b in the exact same proportion, the place of the moving device is live independence of the voltage. If the check leads are short-circuited, the pointer rests in ~ zero because the current in coil a is relatively large. The tool is no damaged under these circumstances because the current is restricted by R3.

The outside view that one type of megger is shown in number 1-36(B).

Navy meggers are usually rated at 500 volts. To avoid excessive test voltages, many meggers are equipped v friction clutches. When the generator is cranked much faster than its rated speed, the clutch slips and also the generator speed and also output voltage space not allowed to exceed your rated values. When incredibly high resistances-for example, 10,000 megohms or more-are to it is in measured, a high voltage is essential to reason sufficient present flow to actuate the meter movement. For expanded ranges, a 1,000-volt generator is available.

When a megger is used, the generator voltage is current on the test leads. This voltage could be hazardous to you or come the tools you space checking. Therefore, never TOUCH The check LEADS if The MEGGER Is being USED and isolate the items you space checking indigenous the equipment prior to using the megger.

Using the Megger

To usage a megger to examine wiring insulation, connect one test cause the insulation and also the other test result in the conductor, after ~ isolating the wiring from the equipment. Rotate the hand crank until the slip clutch just starts to slip and also note the meter reading. Regular insulations need to read infinity. Any small resistance reading indicates the insulation is break down.

Megger safety and security Precautions

When you use a megger, you might be injured or damages equipment you are working ~ above if the adhering to MINIMUM safety and security precautions room not observed.

· use meggers top top high-resistance dimensions only (such together insulation measurements or to inspect two different conductors ~ above a cable).

· never ever touch the test leads when the manage is gift cranked.

· Deenergize and discharge the circuit fully before connecting a megger.

· Disconnect the article being checked from other circuitry, if possible, before using a megger.

Q49. What is the main use the a megger?


Q50. What is the procedure for making use of a megger to inspect the insulation the a conductor?

Q51. What is a normal indication ~ above a megger once checking insulation?

Q52. Perform the four safety precautions observed as soon as using a megger.


A MULTIMETER is the most common measuring maker used in the Navy. The surname multimeter originates from MULTIple METER, and that is specifically what a multimeter is. The is a dc ammeter, a dc voltmeter, one ac voltmeter, and an ohmmeter, every in one package. Figure 1-37 is a snapshot of a usual multimeter.


Figure 1-37. - a typical multimeter.


NEETS Modules
-Matter, Energy, and Direct Current
-Alternating Current and also Transformers
-Circuit Protection, Control, and Measurement
-Electrical Conductors, Wiring Techniques, and Schematic Reading
-Generators and Motors
-Electronic Emission, Tubes, and also Power Supplies
-Solid-State Devices and also Power Supplies
-Wave-Generation and Wave-Shaping Circuits
-Wave Propagation, transmission Lines, and Antennas
-Microwave Principles
-Modulation Principles
-Introduction come Number Systems and also Logic Circuits
-- development to Microelectronics
-Principles the Synchros, Servos, and Gyros
-Introduction to test Equipment
-Radio-Frequency interactions Principles
-Radar Principles
-The Technician"s Handbook, master Glossary
-Test Methods and also Practices
-Introduction come Digital Computers
-Magnetic Recording
-Introduction come Fiber Optics
Note: navy Electricity and also Electronics Training series (NEETS) contents is U.S.Navy property in the general public domain.

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