A High Resistance to Current is Key for 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 using a unit like a Megger MIT525 or MIT1025 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.
Figure 1–Comparison of water flow (a) with electric current (b).
Common sense tells us that the more voltage we have, the more current there’ll be. 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 where:
E = I x R
E = voltage in volts
I = current in amperes
R = resistance in ohms
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).
Download the entire article on “The Complete Guide to Electrical Insulation Testing”
Megger 5-kV and 10-kV Insulation Resistance Testers – MIT525, MIT1025
- Industry best guard terminal accuracy
- Compact and lightweight for easy transport and use
- PI, DAR, DD, SV and ramp test
- Unique dual case design provides additional user protection
- Lithium-ion battery – extended capacity, rapid charge
- Advanced memory with time/date stamp
- CAT IV 600 V safety rating on all terminals
The new range of Megger insulation resistance testers consists of three models: two 5 kV units (MIT515 and MIT525) and a 10 kV unit (MIT1025). Resistance measurement is available up to 10 TΩ for the 5 kV models and 20 TΩ for the 10 kV model. The new instruments are smaller and lighter than previous models yet offer advanced features and rapid charge capability. A key productivity feature is the ability to take measurements when connected to line power/mains with a dead battery. Intelligent battery charging ensures the optimum charge rate as a function of battery level, resulting in minimum charge times.
The rugged, unique dual case design provides the ultimate protection for a portable instrument and a clip-on lead pouch ensures that leads remain with the instrument at all times. The case lid is removable for improved terminal access. IP rating is IP65 with the case closed preventing water/dust ingress. High reliability and safety are built in; all models are safety rated to CATIV 600 V and are double insulated.
Five preset voltage ranges are provided in insulation test mode, plus a user settable lock voltage range. Any selectable test voltage may be locked and restored via the selector switch, thereby increasing efficiency of commissioning and repetitive tests. Preconfigured diagnostic tests include Polarization Index (PI), Dielectric Absorption Ratio (DAR), dielectric discharge (DD), Step Voltage (SV) and ramp test.
The ramp function gradually increases voltage up to a selected level while graphing current vs. voltage (graph downloadable). Graphs can be compared to example curves in IEEE 95-2002 to reveal a variety of faults difficult to detect otherwise. Small defects can be easily detected without risking the sudden large voltage increments produced by a Step Voltage test. Monitoring the developing graph during test enables the operator to terminate prior to breakdown, thereby reducing the possibility of damage to already flawed insulation. These units are particularly informative on polyester, asphalt and epoxy-mica insulations. They can also test voltage suppression devices.
Simplicity of operation is achieved with two rotary switches and the large backlit display enables multiple results to be displayed simultaneously. Advanced memory storage includes time/date stamping of results, logging of data and recall of results to screen. A fully isolated USB device interface (type B) is used for safe transfer of data to Megger’s PowerDB asset management software.
Typical end users include:
- Electrical contractors
- Testing and service companies
- Wind farm and solar generation operators
- Power generation and distribution companies
- Industrial companies
- Rail companies
JM Test Systems has the Megger MIT525 & MIT1025 for purchase.
Call us today for a quote at 800-353-3411 or check us out on the web at www.jmtestsystems.com
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