Ahh...Insulation Resistance (IR) testing! I love this topic, particularly when it comes to wires only. I understand that it's performed all the time to ensure the cables and wires are good after install, but does it really tell you anything? That's the real question. If a wire (not a cable) has some type of shielding on it, an IR test will give you a definitive result. The reason is you are applying voltage to a conductive material (like the metal conductor) and measuring the voltage on another conductive material (like the shielding) through the insulation. What if the wire has a jacket over the insulation? Well the shielding MUST be beneath the jacket so you are ONLY measuring the insulation and not the jacket as well. But what if it's a typical building wire like THWN or XHHW? Well there is no shielding on those typically thus you don't have that second conductive plane to measure against. So people often measure to a conduit or tray or other wires. Well to measure properly you must have the second plane in constant contact with the insulation for the entire length. I'm willing to bet that wires run in conduit don't always lay perfectly flat and will cross over one another inside that conduit......which makes the IR test useless. Or if you measure between 2 wires then you are actually measuring more than 1 insulation and again they probably aren't touching each other through the circuit either. The NETA standard even tells us this and is the gold standard for insulation resistance testing. It will refer you to table 100.1 for values of passing results of certain voltage ratings. Look carefully at the notes in that table, as it will tell you that unshielded cables will not give pass or fail results and a result should be used as a data-point for a trending analysis. Which essentially means the test won't give a good or bad result and you would need to test several times (with the EXACT same test set up, humidity and temperature). Theoretically possible to do that, but not realistic at all. So shielded cables CAN be tested accurately, but unshielded cables cannot and you are simply wasting time, spending money on a result that means nothing. You will get better information simply checking for shorts with an Ohmmeter.
