When low is the right way to go!
Many of the latest instruments for testing electrical installations offer a low voltage insulation test option – 100 V is typical. But when is it appropriate to use such a low test voltage? Peter Wade of Megger, explains.
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.
A 100 V test is unlikely to cause damage to either the circuit or the connected devices and, if it gives an unexpectedly low value for insulation resistance, this is a clear indication that further investigations must be carried out before proceeding with tests at higher voltages.
A second case where 100 V testing is needed is when working on equipment or cables rated to operate at between 24 V and 50 V. With the spread of electronic equipment in homes and businesses, this is becoming a common situation, and the use of test voltages higher that 100 V in such instances carries a high risk of damage. Indeed, some manufacturers and suppliers explicitly state that the insulation resistance of their products must not be tested at voltages higher than 100 V and these stipulations should always be observed.
A third instance where 100 V insulation testing is invaluable is when work is being carried out on data cabling and telecommunications installations. In many cases these are likely to be damaged by testing at the “usual” voltages of 250 V and above, whereas the lower voltage significantly reduces the risk of damage while still providing confirmation that the insulation is in good condition. Nevertheless, before testing installation of this type at 100 V, especially if it is impossible to disconnect all devices from the cabling, the manufacturer’s instructions should be consulted in case testing at an even lower voltage is recommended.
For those instances where even lower test voltages are specified, it is worth knowing that insulation testers are now available that allow users to set any required test voltage from 10 V to 1,000 V in 1 V steps. These instruments are also ideal for use where the equipment manufacturer stipulates that insulation testing must be carried out at a specific non-standard voltage.
So far, when talking about insulation test voltages, we’ve made an important assumption – that the test voltage selected by the user is the voltage that the instrument actually applies to the circuit under test. Unfortunately this isn’t always true.
In fact, the IET Wiring Regulations allow a variation of – 0% + 20% on nominal insulation test voltages. This means that when you think you’re carrying out a test at 500 V for example, the actual test voltage could be anywhere between 500 V and 600 V, and similarly for other test voltages.
Often this doesn’t matter too much but sometimes, especially when testing delicate electronic equipment, it’s important to know the actual test voltage much more precisely. Fortunately, many modern instruments make this possible. One technique, typically seen in the best MFTs, is to provide a secondary display that shows the measured test voltage at the same time as the insulation resistance is shown on the primary display.
Another approach, which has recently started to appear on the latest dedicated insulation testers, is not only to provide a secondary voltage display but also to incorporate circuitry that stabilises the test voltage so that it is guaranteed to be within 2% of the nominal value under all normal operating conditions.
At first sight, it may seem that it is unnecessary for instruments to offer a choice of insulation test voltages, especially when this choice includes 100 V, an option that isn’t explicitly mentioned in the Wiring Regulations. However, as we’ve seen, having a choice of test voltages – and the means to ensure that the instrument accurately generates these voltages – are decided benefits in many applications. In fact, when it comes to insulation test voltages, low can often be the right way to go!
For instruments offering the 100 V insulation test, look at the MFT1721, and the MIT410/2