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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Electricity Investigation 20A We use electricity every day, nearly every minute!
Or, time-resistance curves may meggdr taken over a period of 10 to 30 minutes or longer. Rosamund Owens 2 years ago Views: Current Leakage at Switches Jn apparatus is shut down for the insulation resistance test, make sure that the readings are not affected by leakage over or through switches or fuse blocks, etc. Measuring the capacitance of the test object 4. Never use the tester or any of its leads or accessories for any purpose not described in this book.
Terms volts, amps, ohms, watts, positive, negative, AC, DC 2. Periodic testing is, therefore, your best approach to preventive maintenance of electrical equipment, using record cards as shown in Fig. This is very important, because the more equipment that is included in a test, the lower the reading will be, and the true insulation resistance of the apparatus in question may be masked by that of the associated equipment.
Electricity provides us with light, heat, refrigeration, communication, elevators, More information.
The Complete Guide to Electrical Insulation Testing A STITCH IN TIME 1
As explained previously, in DC testing, this current drops rapidly meggre the initial charging period. Proof Test Voltages for Rotating Equipment: These tests are in addition to acceptance tests specified. Insulation and resistance measurement Contents: Many testers have an additional test range measuring in kilohms kw. The following is a general procedure More information. Test for foreign or induced voltages.
If an AC test voltage has been used and you want to use DC tests as an alternative, you will need to increase the maximum DC test voltage somewhat to obtain equivalent results. Alternator Knowledge The Repository of Alternator Knowledge As part of our commitment to provide service and expertise, we ve developed this ebook to give our customers a single resource for a wide range of alternator related More information.
Troubleshooting Submersible and Jet Pumps A. Megger invented More information. In some cases, AC testing may be more suitable for proof testing of equipment that is, seeing that the equipment meets prescribed standards. Note that good insulation shows a continual increase in resistance less current see curve A over a period of time in the order of 5 to 10 minutes. 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.
Unified requirements for systems with voltages above 1 kv up to 15 kv Rev. The curves of Fig.
Take a reading at the end of the first 30 seconds and another reading at the end of the minute. Likewise the armature can be tested by itself. It can raise or lower the More information.
As the test voltage increases, both the cost and weight of AC equipment go up much faster than with comparable DC test equipment. Make future tests with the same length of operating time. Megger supplies graph paper which makes rime easy to plot megohms of insulation resistance from your voltage and current readings. You can, therefore, compare several motors and establish standards for new ones, regardless of their horsepower ratings.
More recent studies of the problem, however, have resulted in formulas for minimum values of insulation resistance that are based on the kind of insulating material used and the electrical and physical dimensions of the types of equipment under consideration.
Megger ‘A Stitch in Time’ Information Booklet on Insulation Testing
Calculation of winding temperature. Cable fault location in power cables Sstitch classification: Here again, records of insulation resistance tests over a period of time reveal changes which may justify investigation.
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. Such leakage may mask the tiime insulation resistance of the apparatus under test. What was good insulation has become a partial conductor. For every 10 C tlme in temperature, halve the resistance; w, for every 10 C decrease, double the resistance. This small amount of current would not, of course, harm good insulation but would be a problem if the insulation has deteriorated.
When testing cable, it is usually best to disconnect at both ends in order to test the cable by itself, and to avoid error due to leakage across sittch through switchboards or panelboards. 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.
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. If the insulation contains much moisture or contaminants, the absorption effect is masked by a high sttitch current which stays at a fairly constant value, keeping the resistance reading low remember: Also, the lower the resistance of the wire, the more current for the same voltage.
The Complete Guide to Electrical Insulation Testing A STITCH IN TIME 1 – PDF
Also, if you take measurements at regular periods, you can check trends toward its deterioration more on this later. Table I gives values of the ratios and corresponding relative conditions of the insulation that they indicate. The rule may be stated: If you want to make reliable comparisons between readings, you should correct the readings to a base temperature, such as 20 C, or take all your readings at approximately the same temperature usually not difficult to do.
For equipment rated above volts the one megohm rule is usually stated as a minimum of one megohm per thousand volts. For example, let s say the spot reading on a synchronous motor was 10 megohms. Capacitance effects are most noticeable in large generators, in power and communication cable more than a few hundred feet in length, and in capacitors.
Capacitance causes the pointer to swing towards zero while the instrument is being brought up to speed, and to swing off scale beyond infinity when the generator is slowing down. It is normal for a heat pump to have a build up of white frost on the outside coil during cold damp weather.
Larger units with more capacitance will take longer to be charged.