How to Read Continuity Results on an Ohmmeter

CONTINUITY TESTS

Open circuits are those in which the flow of current is interrupted by a broken wire, defective switch, or any means by which the current cannot flow. The test used to detect open circuits (or to see if the circuit is complete or continuous) is continuity testing.

An ohmmeter (which contains its own batteries) is excellent for use in a continuity test. Normally, continuity tests are performed in circuits where the resistance is very low, such as the resistance of a copper conductor. An open is indicated in these circuits by a very high or infinite resistance between two continuously connected points.

Figure 3-20 shows a continuity test of a cable that connects two electronic units. Notice that both plugs are disconnected and the ohmmeter is in series with conductor D under test. The power should be off. When checking conductors A, B, and C (connection of ohmmeter to conductors not shown), the current from the ohmmeter flows through plug 2 (female) through conductor A, B, or C to plug 1 (female). From plug 1, current passes through the jumper to the chassis, which is "grounded" to the ship's structure. The metal structure serves as the return path to the chassis of unit 2 and completes the circuit through the series-connected ohmmeter. The ohmmeter indicates a low resistance because no break exists in conductors A, B, or C. However, checking conductor D reveals an open. The ohmmeter is shown indicating maximum resistance because current cannot flow in an open circuit. With an open circuit, the ohmmeter needle is all the way to the left since it is a series-type ohmmeter (reads right to left).

Figure 3-20. - Continuity test.

Where conditions are such that the ship's structure cannot be used as the return path, one of the other conductors (known to be good) may be used. For example, to check D, you can connect a jumper from pin D to pin A of plug 1 (female) and the ohmmeter leads to pins D and A of plug 2 (female). This technique will also reveal the open in the circuit.

TESTING FOR GROUNDS

Grounded circuits are caused by some conducting part of the circuit making contact either directly or indirectly with the metallic structure of the ship. Grounds can have many causes. The two most common are the fraying of insulation from a wire and moisture-soaked insulation. The fraying of insulation from a wire allows bare wire to come into contact with the metal ground. Moisture-soaked insulation causes reduced insulation resistance (also classified as a ground).

Grounds are usually indicated by blown fuses or tripped circuit breakers. Blown fuses or tripped circuit breakers, however, can also result from a short circuit other than a ground. A high-resistance ground can also occur when current is increased significantly but not enough to rupture the fuse or trip the circuit breaker.

Before testing any circuit, ensure the circuit under test has been de-energized and checked with a safety shorting probe

In testing for grounds, you may use a megger or an ohmmeter. Measuring the resistance to ground from points in a circuit determines if the point is grounded. Referring again to figure 3-20, you can see one possible means of testing a cable for grounds. If the jumper is removed from pin D of plug 1 (female), a test for ground can be made for each conductor in the cable. You can do this by connecting one meter lead to ground and the other to each of the pins of either of the plugs. A low resistance indicates that some part of that conductor or one of the plug assemblies is grounded. Both plugs must be removed from their units; if only one plug is removed, a false indication is possible because a conductor may be grounded through the unit.

TESTING FOR SHORTS

A short circuit, other than a grounded one, is one where two conductors touch each other directly or through another conducting element. Two conductors with frayed insulation may touch and cause a short. Too much solder on the pin of a connector may short to the adjacent pin. In a short circuit, enough current may or may not flow to blow a fuse or open a circuit breaker. A short may occur between two cables carrying signals but might not be indicated by a blown fuse.

Shorts occur in many components, such as transformers, motor windings, and capacitors. The major test method used to detect shorts in such components is to measure resistance. The indicated resistance is then compared with the resistance given on schematics or in the equipment technical manuals to determine whether the measured value is within specifications.

An ohmmeter is the device used to check for shorts. You can use the ohmmeter to detect a short between two conductors by measuring the resistance between them (be sure electrical power has been disconnected). A low resistance reading indicates a short. You can test the circuit in figure 3-20 for a short by first removing the jumper and disconnecting both plugs; you then measure the resistance between the two suspended conductors.

The following section discusses voltage measurements on live circuits. BE SURE YOU ALWAYS FOLLOW PRESCRIBED SAFETY RULES WHEN MEASURING VOLTAGES.

VOLTAGE TESTS

Voltage tests must be made with the power applied; therefore, the prescribed safety precautions must be followed to prevent injury to personnel and damage to the equipment. You will find in your maintenance work that the voltage test is of utmost importance. It is used not only in isolating casualties to major components but also in the maintenance of subassemblies, units, and circuits. Before checking a circuit voltage, you should check the voltage of the power source to be sure that the normal voltage is being applied to the circuit.

The voltmeter is used for voltage tests. In using the voltmeter, make certain that the meter used is designed for the type of current (ac or dc) to be tested and has a scale with a suitable range. Since defective parts in a circuit can cause higher than normal voltages to be present at the point of test, the highest voltmeter range available should be used first. Once you have obtained a reading, determine if a lower scale can be used that will cause no damage to the meter movement. If so, use the lower scale. This provides a more accurate reading.

Another consideration in the circuit voltage test is the resistance and current in the circuit. A low resistance in a high-current circuit could result in considerable voltage drop, whereas the same resistance in a low-current circuit may be minimal. Abnormal resistance in part of a circuit can be checked with either an ohmmeter or a voltmeter. Where practical, an ohmmeter should be used because the test is then carried out with a "dead" circuit.

The majority of the electronic circuits you will encounter in equipment will be low-current circuits, and most voltage readings will be direct current. Also, many of the schematics will indicate the voltages at various test points. Therefore, if you suspect that a certain stage is defective, you can check the voltage by connecting a voltmeter from the test point to ground. If the suspected stage is not defective, the voltmeter readings should match the voltages given on the schematic.

Some technical manuals also contain voltage charts on which all the voltage measurements are tabulated. These charts usually indicate the sensitivity of the meter (for example, 20,000 ohms/volt) used to obtain the voltage readings for the chart. To obtain comparable results, you must use a voltmeter of the same sensitivity (or greater) as that specified. Make certain that the voltmeter is not "loading" the circuit while taking a measurement. If the meter resistance is not considerably higher than the circuit resistance, the reading will be markedly lower than the true circuit voltage because of the voltmeter's loading effect. (To calculate meter resistance, multiply the rated ohms-per-volt sensitivity value of the meter by the scale in use. For example, a 1,000-ohms-per-volt meter set to the 300-volt scale will have a resistance of 300,000 ohms.)

RESISTANCE TESTS

Before checking the resistance of a circuit or of a part, make certain that the power has been turned off. Also make sure capacitors in an associated circuit are fully discharged. To check continuity, always use the lowest ohmmeter range. If the highest range is used, the meter may indicate zero, even though appreciable resistance is present in the circuit. Conversely, to check a high resistance, use the highest scale since the lower range scale may indicate infinity, even though the resistance is less than a megohm. In making resistance tests, you must remember that even though the external ohmmeter leads are connected in parallel with the circuit to be measured, the internal meter circuitry is electrically connected in series.

In making resistance tests, take into account that other circuits containing resistances and capacitances may be in parallel with the circuit to be measured. Erroneous conclusions may be drawn from readings obtained in such cases. Remember, a capacitor blocks the dc flow from the ohmmeter. To obtain an accurate reading when other parts are connected across the suspected circuit, disconnect one end of the circuit to be measured from the equipment. For example, many of the resistors in major components and subassemblies are connected across transformer windings. To obtain a valid resistance measurement, you must isolate the resistors to be measured from the shunt resistances of the coils of the transformers.

Resistance tests are also used to check a component for grounds.

In these tests, the component to be tested should be disconnected from the rest of the circuit so that no normal circuit ground will exist. Dismounting the component to be checked is not necessary. The ohmmeter is set for a high-resistance range. Then the ohmmeter is connected between ground and each electrically separate circuit of the component being tested. Any resistance reading less than infinity indicates at least a partial ground. You can also check capacitors suspected of being short-circuited by measuring the resistance. To check a capacitor suspected of being open, temporarily shunt a known good capacitor across it; then recheck the performance of the circuit.

To avoid possible damage to equipment during resistance tests, observe the following precautions:

• Always connect an ammeter in series - never in parallel.
• Connect a voltmeter in parallel.
• Never connect an ohmmeter to a live circuit.
• Observe polarity when using a dc ammeter or a dc voltmeter.

View meters directly from the front. When viewed from an angle off to the side, an incorrect reading will result because of OPTICAL PARALLAX. (Parallax was covered in NEETS, Module 3, Introduction to Circuit Protection, Control, and Measurement.)

• Always choose an instrument suitable for the measurement desired.
• Select the highest range first and then switch to the proper range.
• In using a meter, choose a scale that will result in an indication as near midscale as possible.
• Do not mount or use instruments in the presence of a strong magnetic field.
• Remember, a low internal resistance voltmeter (low sensitivity) may shunt the circuit being measured and result in incorrect readings.

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Source: https://www.tpub.com/neets/book16/68h.htm

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