Megger “A Stitch in Time...” / The Complete Guide to Electrical Insulation Testing
Year: 2006 Language: english Genre: Guide Format: PDF Quality: eBook Number of pages: 67 Description: WHAT IS “Good” insulation? Every electric wire in your plant – whether it’s in a motor, generator, cable, switch, transformer, etc. – is carefully covered with some form of electrical insulation. 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: it should resist current and keep the current in its path along the 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. In Fig. 1 we show this comparison. Pressure on water from a pump causes flow along the pipe (Fig. 1a). 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. 1b). 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. Note, however, that no insulation is perfect (that is, has infinite resistance) so some electricity does flow along the insulation or through it to ground. 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).
Content
WHAT IS “Good” insulati on?......................................................................... 3 What Makes Insulati on Go Bad?................................................................. 4 How Insulati on Resista nce is Measured................................................... 5 How to Interpret Resista nce Readings..................................................... 6 FACTORS AFFECTING INSULATION RESISTANCE READINGS............................... 8 Types of Insulati on Resista nce Tests........................................................ 10 test Voltag e vs. Equipment Rati ng........................................................... 16 AC Testing vs. DC............................................................................................. 17 USE OF DC DIELECTRIC TEST SET....................................................................... 18 TESTS DURING DRYING OUT OF EQUIPMENT................................................... 18 EFFECT OF TEMPERATURE ON insulati on Resista nce................................ 21 Effects of humidity....................................................................................... 23 Preparati on of Apparatus to test............................................................ 24 Safety Precautions....................................................................................... 26 Connections for testing insulati on resista nce of electrical equipment................................................................................... 27 Additional Notes About using A Megger Insulati on Tester............ 33 Interpretati on-Minimum Values............................................................... 36 Minimum Values for Insulati on Resista nce........................................... 38 Tests Using Multi-Voltag e Megger Insulati on Testers....................... 42 Step-Voltag e Method................................................................................... 48 Use of a Guard Terminal............................................................................. 50 Bushings, Potheads and Insulat ors........................................................ 54 Outdoor Oil Circuit Breakers.................................................................... 57 Setting up a Maintenance program........................................................ 60 How Often Should You Test?..................................................................... 60 MEGGER 5 AND 10 KV INSULATION TESTERS................................................... 62
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Megger “A Stitch in Time...” / The Complete Guide to Electrical Insulation Testing
Language: english
Genre: Guide
Format: PDF
Quality: eBook
Number of pages: 67
Description:
WHAT IS “Good” insulation?
Every electric wire in your plant – whether it’s in a motor, generator, cable, switch, transformer, etc. – is carefully covered with some form of electrical insulation. 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: it should resist current and keep the current in its path along the 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. In Fig. 1 we show this comparison. Pressure on water from a pump causes flow along the pipe (Fig. 1a). 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. 1b). 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.
Note, however, that no insulation is perfect (that is, has infinite resistance) so some electricity does flow along the insulation or through it to ground. 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).
Content
WHAT IS “Good” insulati on?......................................................................... 3What Makes Insulati on Go Bad?................................................................. 4
How Insulati on Resista nce is Measured................................................... 5
How to Interpret Resista nce Readings..................................................... 6
FACTORS AFFECTING INSULATION RESISTANCE READINGS............................... 8
Types of Insulati on Resista nce Tests........................................................ 10
test Voltag e vs. Equipment Rati ng........................................................... 16
AC Testing vs. DC............................................................................................. 17
USE OF DC DIELECTRIC TEST SET....................................................................... 18
TESTS DURING DRYING OUT OF EQUIPMENT................................................... 18
EFFECT OF TEMPERATURE ON insulati on Resista nce................................ 21
Effects of humidity....................................................................................... 23
Preparati on of Apparatus to test............................................................ 24
Safety Precautions....................................................................................... 26
Connections for testing insulati on resista nce of
electrical equipment................................................................................... 27
Additional Notes About using A Megger Insulati on Tester............ 33
Interpretati on-Minimum Values............................................................... 36
Minimum Values for Insulati on Resista nce........................................... 38
Tests Using Multi-Voltag e Megger Insulati on Testers....................... 42
Step-Voltag e Method................................................................................... 48
Use of a Guard Terminal............................................................................. 50
Bushings, Potheads and Insulat ors........................................................ 54
Outdoor Oil Circuit Breakers.................................................................... 57
Setting up a Maintenance program........................................................ 60
How Often Should You Test?..................................................................... 60
MEGGER 5 AND 10 KV INSULATION TESTERS................................................... 62
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