Take a look at many of the latest insulation testers and multifunction installation testers (MFTs) and you’ll see that they offer a choice of DC insulation test voltages. Almost all offer testing at 250 V, 500 V and 1,000 V but increasingly an option for testing at 100 V is also available.
To see where this might be useful, let’s start with a quick refresher on the choice of voltage for insulation tests on low voltage installations. Note that the information given in this article is only an overview – before you start testing make sure you're familiar with the full requirements and provisos detailed in the IET Wiring Regulations.
For most circuits with a nominal operating voltage of 500 V or less, the insulation test voltage should be 500 V. Before testing is carried out, the loads must be disconnected along with surge protection devices and any electronic equipment that might be damaged by the test. If it is not possible to disconnect these devices, it is permissible to test at 250 V.
When working on SELV and PELV systems, insulation resistance tests between the SELV and PELV circuits and other live circuits should be performed at 500 V. However, tests between the SELV and PELV conductors themselves, as well as tests between PELV conductors and PELV protective conductors, should be performed at 250 V.
For low-voltage circuits with a nominal operating voltage greater than 500 V, an insulation test voltage of 1,000 V should normally be used.
Now for the 100 V tests! It will be clear from what has already been said that, unlike the tests at higher voltages, these are not prescribed by the IET Wiring Regulations. Nevertheless, there are many cases where 100 V tests are very useful, not least to carry out a preliminary check on a circuit before performing a test at the higher voltage prescribed in the regulations.
Many contractors find such preliminary checks to be particularly useful when working on existing circuits where it is difficult to be absolutely certain that all of the loads, surge protection devices and the like have been disconnected.