Component continuity and insulation resistance testing

A variety of devices need to be checked for continuity, including cable assemblies, printed circuit boards, and connectors to ensure that these components have the expected continuous path. When setting up a continuity test, engineers must specify the maximum resistance at which the device is usable. For example, if the measured resistance is 1Ω or less, the device is good. Continuity checks require measuring low resistance, so they are usually measured using a 4-wire ohmmeter to eliminate the measurement line and switch resistance from the measurement value.

In addition to continuity testing, isolation resistance or insulation resistance testing is often performed. This is especially true for multi-core cables, where each conductor forms a continuous path between the two ends and each conductor is required to be isolated from all other conductors.

Figure 1, Continuity Test System

Since continuity testing often involves multi-conductor installations, it is useful to have a switching system that automatically connects the ohmmeter to each conductor.

Figure 1 shows a typical continuity test circuit. Use two sets of double-pole switches to make 4-wire resistance measurements on 20 conductors. In the Model 2700 Multimeter/DAQ System, to measure the resistance of conductor 1, close Channel 1 (Ch. 1). In the 4-wire Ohms mode, Channel 21 is automatically closed. Repeat for each conductor.

To measure 20 conductors, a Model 2700 with a Model 7702 40-channel differential multiplexer module is required. If more than 40 conductors are to be measured simultaneously, a Model 2750 Multimeter/Switch System with a Model 7702 module is required.

Insulation resistance test

DC insulation resistance (IR) is the ratio of the voltage applied between two conductors separated by insulation to the total current flowing between the two conductors. The test voltage is applied for a certain period of time before the current is measured. The measured current is usually very small, so it is often necessary to use a picoammeter or electrometer to measure it.

Sometimes the insulation resistance of a sample is measured simply to confirm that it is greater than a specified minimum value. For example, any resistance value greater than 10 MΩ is considered acceptable. The accuracy of the measurement is not critical; what is important is that the measured resistance value is greater than a specified value.

Examples of measuring insulation resistance include measuring the resistance of a path between traces on a printed circuit board, or between conductors in a multi-conductor cable. Since IR measurements often involve multiple conductors, a switching system is usually required to switch the picoammeter and source to all conductors in the test circuit.

The design and type of switch cards used in an IR test system depends on several factors, including test voltage, resistance magnitude, accuracy, common connections, etc. The following sections describe two IR test systems.

Figure 2 shows a test system for measuring IR, which measures the insulation resistance between each terminal and all other terminals in a multi-core connector through a 7111-S 40-channel C-type switch card in a 7001 switch mainframe. In the de-energized position, the voltage source is connected to all pins under test. When any specified channel is selected, the leakage current from that pin to all other pins is measured. The offset current specification of the 7111-S switch card is <100 pA. When the test voltage is 100 V, the leakage resistance is 1 TΩ. When the system is connected to the actual circuit, leakage resistance greater than 10 GΩ can be easily detected.

Figure 2, measuring the insulation resistance (IR) from any pin to all other pins in a multi-core connector

Figure 3 shows a system where a test voltage can be applied to one or more pins while measuring the current from one or more pins. Note that each pin has two independent sets of switches connected to it. One set connects the test voltage to the pin while the other measures the leakage current. Thus, the IR can be measured between any pin and any other pin or all pins. Note that at some point in the test cycle, all switches will be subject to the test voltage. Therefore, both sets of switches must be able to withstand the expected test voltage and should have good channel-to-channel isolation to prevent degradation of the test signal. [page]

In Figure 3, the insulation resistance can be measured at relatively high voltages (up to 500 V) using the Model 7169A 20-channel Form C Switch Card (for voltages up to 1300 V, the Model 7153 4×5 High Voltage Low Current Matrix Card can be used). The Model 7169A Switch Card has position-sensitive relays and can only be used in the Model 7002 Switch Mainframe.

The two busses on the Model 7169A switch card can be configured as switches or multiplexers. To measure the insulation resistance between pins 1 and 2, close channels 1 and 22.

Figure 3, Testing the insulation resistance (IR) between any two terminals

Resistors (R) limit the charging current through the relay. These resistors replace the factory-installed jumpers on the switch card and minimize the cable capacitance charging and discharging currents. Typical R values ​​are 100 kΩ.

Continuity and insulation resistance combined test

Some multi-pin installations need to measure the resistance or continuity of the path through each conductor (low resistance) and the insulation resistance between the conductors (high resistance). Test systems need to switch and measure both low resistance (< 1Ω) and very high resistance values ​​(> 109Ω).

The test system can be used for various devices such as connectors, switches, multi-core cables and printed circuit boards.

Switch Configuration

Figure 4 shows a combined continuity and IR measurement system for testing multi-core cables using a 4-wire DMM or source meter. Resistors R1 to R20 represent the wire resistance. To measure the resistance R1 of wire 1, channels 1 and 21 need to be closed. Resistors Ra and Rb represent the leakage resistance between wires. The leakage resistance between any two or more wires can be measured. To measure the leakage resistance Ra, channels 1 and 22 need to be closed. This is actually the leakage resistance between wires 1 and 2, and Ra is much larger than R1.

Figure 4: Continuity and IR test system

A single Model 2700 Multimeter/Data Acquisition System with a Model 7702 40-Channel Differential Amplifier can test up to 20 conductors. When measuring leakage resistance with a DMM, the maximum voltage applied is usually less than 15 V. In addition, the maximum resistance measured is often no higher than 100 MΩ. To test IR at a specified test voltage, a test configuration such as a Model 2400 SourceMeter and a Model 7011 1×10 Multiplexer Card in a Model 7001 or 7002 Switch Mainframe can be used.

Figure 5: Extended continuity/IR test system

If a higher test voltage is required or a higher leakage resistance must be measured, the circuit shown in Figure 5 can be used. In this figure, two Model 7154 High Voltage Scanner Cards are used to switch the Model 2410 SourceMeter and Model 2010 DMM to eight conductors. The system can measure conductor resistances as low as 0.1 mΩ and leakage resistances as high as 300 GΩ at test voltages up to 1000 V. Note that the Model 2410 and Model 2010 are not connected to the switch card outputs, but to the designated channels of the plug-in card. The outputs of the plug-in card are only used to expand the system to measure a larger number of conductors. To measure the value of resistor R1, close channels 1, 10, 11, and 20. This connects the Model 2010 across R1. To measure Ra, the leakage resistance between R1 and R2, close channels 1, 9, 12, and 19. This connects the Model 2410 across the leakage resistance (Ra).