S1-568, S1-1068, and S1-1568 insulation resistance testers

There are several reasons to select a test set with a high output current. Possibly the most important is that a high output current means that the item under test will be charged more quickly, which means that the test can be completed in a shorter time and also that there’s less risk that the readings will be taken before the test voltage has had time to stabilise properly. And, if you’re using the instrument’s guard terminal, don’t forget that a lot of output current may well be diverted via the surface leakage of the item under test. Unless the instrument has a high output current capability, this could mean that the output voltage will collapse, and the test results will not be valid.

That depends upon the size, complexity, and criticality of your equipment. Even identical units can differ in the required check periods; experience is your best guide. In general, however, working apparatus – such as motors and generators – are more likely to develop insulation weaknesses than wiring, insulators, and the like. A test schedule for working equipment should be established, varying from every 6 to 12 months, depending on the size of the equipment and the severity of the surrounding atmospheric conditions. For wiring and the like, tests once a year are generally sufficient unless the installation conditions are unusually severe.

These facilities are useful in a wide range of applications. For example, when testing a large item such as a power transformer, the instrument can be positioned on top of the asset near its terminals so that the test leads are kept short and operated from a much more convenient – and much safer – location, using the remote control option. Additionally, it’s sometimes necessary to carry out tests in hazardous areas, such as inside an energised substation. In these cases, once it has been connected, you can operate the test set and access your results outside the hazardous area, significantly increasing the operator’s safety. Finally, in production line test applications, it’s often desirable to control the test unit and retrieve the test results automatically. The remote control and remote downloading facilities offer a convenient way of achieving this and providing any safety interlocks that may be needed.

In cases of this type, the source of trouble is almost always induced noise in the measuring circuit. You can reduce noise pick-up on the test leads by keeping them as short as possible and using screened test leads. With screened leads, the screen is connected to the insulation test set guard terminal to divert the noise currents from the measuring circuits. However, if the noise is being picked up by the item under test rather than the test leads, these measures can’t help. In such cases, the only effective solution is to use an insulation test set with high noise immunity and effective filtering. The S1 has noise immunity of 8 mA, which ensures reliable operation in the harshest conditions, such as EHV substations. They also have adjustable long time constant filtering, which allows users to choose between faster operation when noise levels are only moderate, and slower operation but with enhanced noise rejection when working in the most challenging environments.

The electrical insulation should be top-notch when your plant’s electrical system and equipment are new. Furthermore, manufacturers of wire, cable, motors and other electrical equipment have continually improved their insulations for services in industry. Nevertheless, even today, insulation is subject to many effects which can cause it to fail – mechanical damage, vibration, excessive heat or cold, dirt, oil, corrosive vapours, moisture from processes, or just the humidity on a muggy day.

To various degrees, these enemies of insulation are at work as time goes on – combined with the electrical stresses that exist. As pin holes or cracks develop, moisture and foreign matter penetrate the insulation surfaces, providing a low resistance path for leakage current.

Once started, these attack agents often aid each other, permitting excessive current through the insulation.

Sometimes the drop in insulation resistance is sudden, as when equipment is flooded. Usually, however, it drops gradually, giving plenty of warning if checked periodically. Such checks permit planned reconditioning before service failure. If there are no checks, a motor with poor insulation, for example, may be dangerous to touch when voltage is applied and subject to burnout. Over time, what was good insulation has become a partial conductor.