Wiring Harnes News Nov-Dec 2021 - Page 23

Figure 2 . Continuity Testing

TTI + the unknown resistance is the device under test ( DUT ).

When performing a continuity test , the quality of the connection is evaluated by its resistance with the acceptance criteria set in Ohms or fractions of an Ohm . Generally , the lower the resistance of a connection , the better . Therefore , the test passes if the measured value is less than or equal to the specified value . Sometimes , this is referred to as a “ pass-ifunder ” test .
The tester performs continuity tests two points at a time , regardless of the size of the circuit ( more commonly called a network , or simply net ) by connecting a current source to one point in the net while holding one other point in the net at ground and floating all remaining points by connecting them neither to the current source nor to ground . Some testers use the same source point for each continuity test in a net while others step through the net using the ground point in the original test as the source point for the subsequent test , as shown in the simple example below ( Figure 2 ). Either method is acceptable .
To perform each step , the tester applies a steady , calibrated current to the DUT then measures the voltage drop from the source to ground
through the circuit . The resistance is calculated using Ohm ’ s law by dividing the measured voltage drop by the known current ( R = V / I ). The result is compared to the specified threshold to determine the pass or fail condition .
When performing low voltage isolation testing to ensure no unintended connections ( shorts ) exist , the acceptance condition is again set as a resistance value , except the pass / fail threshold is set in kilohms or megohms with the test passing if the measurement exceeds the threshold . The “ pass-if-over ” test is used as the greater the resistance between isolated nets the better .
To ensure that no shorts exist , all nets must be tested against all others . The exact process can vary , but for simplicity ’ s sake , we ’ ll assume that the tester applies the current source to one or more points in each net while it floats others in the net ( connects them neither to the current source or ground ) and holds all other points in the DUT at ground ( Figure 3 ). The voltage drop from source to ground is measured and Ohm ’ s law is again used to calculate the resistance . It steps through this process by applying the source to one net at a time . If any isolation tests fail , the tester
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