A comprehensive 64 page guide to electrical insulation testing containing helpful diagrams, value tables, product information and much more!. over a period of time, and the location and importance of the equipment megohmmeter is connected to the insulation to be tested and is operated for a short. The Dielectric Absorption (or Time-Resistance) test is an extension of the Megger, “A stitch in time – The Complete Guide to Electrical Insulation Testing”, a free.
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Equipment Rating AC Testing vs. Every electric wire in your plant whether it s in a motor, generator, cable, switch, transformer, etc. The wire itself is usually copper or aluminum, which is known to be a good conductor of the electric current that powers your equipment. The insulation must be just the opposite from a conductor: To understand insulation testing you really don t need to go into the mathematics of electricity, but one simple equation Ohm s law can be very helpful in appreciating many aspects.
Even if you ve been exposed to this law before, it may be a good idea to review it in the light of insulation testing. The purpose of insulation around a conductor is much like that of a pipe carrying water, and Ohm s law of electricity can be more easily understood by a comparison with water flow.
Pressure on water from a pump causes flow along the pipe Fig. If the pipe were to spring a leak, you d waste water and lose some water pressure. With electricity, voltage is like the pump pressure, causing electricity to flow along the copper wire Fig. As in a water pipe, there is some resistance to flow, but it is much less along the wire than it is through the insulation. Figure 1 Comparison of water flow a with electric current b. Also, the lower the resistance of the wire, the more current for the same voltage.
Actually, this is Ohm s law, which is expressed this way in equation form: Such a current may only be a millionth of an ampere one microampere but it is the basis of insulation testing equipment.
Note also that a higher voltage tends to cause more current through the insulation. This small amount of current would not, of course, harm good insulation but would be a problem if the insulation has deteriorated.
Now, to sum up our answer to the question what is good insulation? We have seen that, essentially, good means a relatively high resistance to current.
Used to describe an insulation material, good would also mean the ability to keep a high resistance. So, a suitable way of measuring resistance can tell you how good the insulation is. Also, if you take measurements at regular periods, you can check trends toward its deterioration more on this later. What Makes Insulation Go Bad? When your plant electrical system and equipment are new, the electrical insulation should be in top notch shape.
Furthermore, manufacturers of wire, cable, motors, and so on 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 vapors, moisture from processes, or just the humidity on a muggy day. In 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 surfaces of the insulation, providing a low resistance path for leakage current. 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 not only be dangerous to touch when voltage is applied, but also be subject to burn out.
What was good insulation has become a partial conductor. How Insulation Resistance is Measured You have seen that good insulation has high resistance; poor insulation, relatively low resistance. The actual resistance values can be higher or lower, depending upon such factors as the temperature or moisture content of the insulation resistance decreases in temperature or moisture.
With a little record-keeping and common sense, however, you can get a good picture of the insulation condition from values that are only relative. The Megger insulation tester is a small, portable instrument that gives you a direct reading of insulation resistance in ohms or megohms.
For good insulation, the resistance usually reads in the megohm range. The Megger insulation tester is essentially a high-range resistance meter ohmmeter with a built-in direct-current generator. This meter is of special construction with both current and voltage coils, enabling true ohms to be read directly, independent of the actual voltage applied.
This method is nondestructive; that is, it does not cause deterioration of the insulation.
Figure 2 Typical Megger test instrument hook-up to measure insulation resistance. This current usually at an applied voltage of volts or more is measured by the ohmmeter, which has an indicating scale.
Figure 3 Typical scale on the Megger insulation tester.
A Stitch In Time | Megger Insights
How to Interpret Resistance Readings As previously mentioned, insulation resistance readings should be considered relative. They can be quite different for one motor or machine tested three days in a row, yet not mean bad insulation.
What really matters is the trend in readings over a time period, showing lessening resistance and warning of coming problems.
Periodic testing is, therefore, your best approach to preventive maintenance of electrical equipment, using record cards as shown stutch Fig. Figure 4 Typical record of insulation resistance of a mill motor. Curve A shows test values as measured; Curve B shows same values corrected to 20 C tlme page 22giving a definite downward trend toward an unsafe condition. Reverse side of card at right is used to record the test data.
For example, a small pump motor or a short control cable may be vital to a process in your plant. sticth
Experience is the best teacher in setting up the scheduled periods for your equipment. You should make these periodic tests in the same way each time. That is, stktch the same test connections and with the same test voltage applied for the same length of time. Also you should make tests at about the same temperature, or correct them to the same temperature.
A Stitch In Time
A record of the relative humidity near the equipment at the time of the test is also helpful in evaluating the reading and trend. Later sections cover temperature correction and humidity effects. In summary, here are some general observations about how you can interpret periodic insulation resistance tests, and what you should do megver the result: Condition a Fair to high values and well maintained. What To Do no cause for concern. Condition is probably all right, but cause of low values should be checked.
Clean, dry out, or otherwise raise the values before placing equipment in stitdh. Test wet equipment while drying out.
Megger ‘A Stitch in Time’ Information Booklet on Insulation Testing
Make tests at frequent intervals until the cause of low values is located and remedied; or until the values have become steady at a lower level but safe for operation; or until values become so low that it is unsafe to keep the equipment in operation.
There are a number of things that affect current, including temperature of the insulation and humidity, as mentioned in the previous section. Right now, let s just consider the nature of current through insulation and the effect of how long voltage is applied. Current through and along insulation is made up partly of a relatively steady current in leakage paths over the insulation surface.
Electricity also flows through the volume of the insulation. Actually, as shown in Fig. Capacitance Charging Current Current that starts out high and drops after the insulation has been charged to full voltage much like water flow in a garden hose when you first turn on the spigot.
Absorption Current Also an initially high current which then drops for reasons discussed under the section Time-Resistance Method.
Conduction or Leakage Current A small essentially steady current both through and over the insulation. As shown in Fig. In practice, as you will see in the test methods described below, you read a value that is the apparent resistance a useful value to diagnose troubles, which is what you want to do.
Note also in Fig. Larger units with more capacitance will take longer to be charged. This current also is the stored energy initially discharged after your test, by short-circuiting and grounding the insulation. You can see further from Fig. This stored energy, too, must be released at the end of a test, and requires a longer time than the capacitance charging current about four times as long as the voltage was applied.
With good insulation, the conduction or leakage current should build up to a steady value that is constant for the applied voltage, as shown in Fig. Any increase of leakage current with time is a warning of trouble, as discussed in the tests described in the following section. Types of Insulation Resistance Tests Short-Time or Spot-Reading Test In this method, you simply connect the Megger instrument across the insulation to be tested and operate it for a short, specific time period 60 seconds is usually recommended.
As shown schematically in Fig. Bear in mind also that temperature and humidity, as well as condition of your insulation affect your reading. Figure 6 Typical curve of insulation resistance in megohms with time for the short time or spot-reading test method.
However, most equipment is capacitive and so your very first spot reading on equipment in your plant, with no prior tests, can be only a rough guide as to how good or bad the insulation is. For many years, maintenance professionals have used the one-megohm rule to establish the allowable lower limit for insulation resistance. The rule may be stated: Insulation resistance should be approximately one megohm for each 1, volts of operating voltage, with a minimum value of one megohm.
For example, a motor rated at 2, volts should have a minimum insulation resistance of 2. In practice, megohm readings normally are considerably above this minimum value in new equipment or when insulation is in good condition.
By taking readings periodically and recording them, you have a better basis of judging the actual insulation condition. Any persistent downward trend is usually fair warning of trouble ahead, even though the readings may be higher than the suggested minimum safe values. Equally true, as long as your periodic readings are consistent, they may be ok, even though lower than the recommended minimum values. The curves of Fig. The curves were plotted from spot readings taken with a Megger instrument over a period of months.
It is based on the absorption effect of good insulation compared to that of moist or contaminated insulation. You simply take successive readings at specific times and note the differences in readings see curves, Fig.