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Among other things, cable testers are used to insure that cable assemblies do not have OPENS or SHORTS.
In simple terms it looks like this:
Cable testers measure the resistance between all test points to determine if there are, or are not connections. The big question is; at what resistance threshold are OPENS and SHORTS detected? Basic Go/No Go Continuity Tester Basic continuity testers generally have one fixed threshold (the resistance is usually not very accurate and can range from 100 to 10,000 ohms). Resistances measured under this threshold are seen, while resistances measured above this threshold are not seen. Whether they are OPENS or SHORTS is determined by whether they are intended connections or not as illustrated by this graphic.  Cable Testers with Selectable Resistance Thresholds Cable testers that have variable resistance thresholds allow you to set the measured resistance value that will determine pass/fail test status as illustrated by this drawing.  To set an appropriate Connection Resistance Threshold you must first determine the nominal resistance of the wires you are testing. For help determining standard wire resistance follow this link: http://www.easy-wire.com/testing/resistance/wirecalc.html
*Note: for very low resistance measurements use 4-wire Kelvin Testing. For more information on 4-wire Kelvin Testing follow this link: http://www.easy-wire.com/testing/resistance/fourwire.html
*Note: while a Low Voltage Insulation Resistance test does a better job of finding shorts, it is NOT as effective as HV insulation resistance testing. See http://www.easy-wire.com/testing/guidelines/voltinsl.htm for more information on HV insulation testing. A low voltage specification for Insulation Resistance is not important when HV test are performed. Cirris testers use dual-resistance thresholds. The lower threshold (components >) is used to verify that wires/points connected via diodes, resistors, and/or capacitors are tested for shorts. The higher threshold (IR) is used to verify that wires/points not connected to such components are tested at a more stringent LV Insulation Resistance value to all other such wires/points. For more info on testing components go here: http://www.easy-wire.com/testing/guidelines/testcomps.htm  Table of Cirris Testers with Adjustable Resistance Thresholds
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