Kilohm range: what is it?

31 July 2015

Virtually all hand-held insulation testers offer two discrete test functions: insulation and continuity. The core function, of course, is the insulation test, a high-voltage measurement of the quality of insulating material in electrical equipment. The unit of measurement is the Megohm (MΩ) and these testers can correctly be termed megohmmeters as well as insulation testers. A corollary function present in nearly all hand-held 1 kV models is the continuity test. This is at the other end of the resistance spectrum. The unit is the Ohm (Ω) and it is usually measured to the tenth or hundredth. Only a few volts are required. These two tests are all that are required for a vast number of electrical jobs, such as proofing final construction or troubleshooting building wiring. Has a nail been driven through the Romex? The insulation test will fail. Has a junction box been left unconnected? The continuity range will catch that. Accordingly, the purchase of a relatively economical basic-function meter may be sufficient to fulfill everything an electrician or maintenance person needs to do. But insulation testers can offer much more in order to meet higher and/or more varied demands. What is this kΩ selection? This is the kilohm range, a low voltage test that measures resistance in thousands (kilohms) rather than millions (Megohms). It has a number of useful, even indispensable functions including:

  • Closing the gap between continuity and insulation. With a kilohm range, the tester can measure from single Ohms (possibly even a milli-Ohm) to Megohms (or even Tera-Ohms) continuously, with no break in the range.
  • It helps proof insulation tests that have failed, or indicated breakdown. A test item that has shorted out will run off the low end of the Megohm scale. Nice to be able to confirm this (and enter an actual reading in a test report) by dropping down to the thousands (the continuity test will likely just over-range).
  • Sub-assemblies and other internal components of electrical equipment often require insulation only in the kilohm range, as they will be further isolated from ground by the additional insulation of the casework. Motor windings are also sometimes rated in this fashion.
  • It is a highly useful tool in repair and restoration, such as flooded equipment. While insulation tests are non-destructive of good insulation, putting a thousand volts across something that has been under water for days isn’t a good idea. Better to start with a kilohm test (typically about 5 V), get a reading, and then gradually ramp up into the Megohm range as cleaning and drying take effect.

So next time you are preparing to work on an electrical job, keep in mind this versatile and handy tool. It can really be worth the investment in the tool as your work becomes more sophisticated and diverse.