What is Good Insulation in Electrical Systems? [PDF]

In every electrical system, whether it powers motors, generators, cables, switches, or transformers, electrical wires are coated with insulation to ensure safe and efficient operation. While the wires themselves, typically made of copper or aluminum, are excellent conductors of electric current, the insulation’s role is the opposite – it must resist the flow of electric current and maintain it within the conductor’s path.

To grasp the concept of insulation testing, you don’t need an in-depth understanding of electrical mathematics. Instead, a fundamental equation known as Ohm’s law can greatly assist in appreciating many aspects of insulation. Even if you’ve encountered this law before, reviewing it in the context of insulation testing can be beneficial.

Ohm’s Law and the Comparison to Water Flow:

Imagine that electricity behaves somewhat like water flowing through a pipe. In this analogy, the pressure exerted by a pump on water corresponds to voltage in electricity (Fig. 1a). Just as a leak in a water pipe causes water loss and reduced pressure, in electricity, there is some resistance to current flow – it’s much less within the conductor but significantly higher through the insulation (Fig. 1b).

This relationship is encapsulated in Ohm’s law, expressed as:
E = I x R
Where:

• E = Voltage in volts
• I = Current in amperes
• R = Resistance in ohms

In essence, the higher the voltage, the greater the current flow, provided that the wire’s resistance remains low.

However, it’s important to note that no insulation is perfect; there is always some finite resistance. As a result, some electricity flows along or through the insulation to the ground. This current is typically minuscule, often just a millionth of an ampere (one microampere), but it forms the basis for insulation testing equipment. Moreover, higher voltage levels can induce more current through the insulation, potentially becoming problematic if the insulation has deteriorated.

What Is “Good” Insulation?

In the context of electrical systems, “good” insulation essentially implies a relatively high resistance to current. When describing an insulation material as “good,” it signifies its ability to maintain a high resistance over time. Consequently, an effective way to assess insulation quality is by measuring its resistance. Regular measurements over time can help monitor any trends toward deterioration.

In summary, “good” insulation refers to its capability to resist current effectively. Understanding this principle and employing suitable testing methods are essential for ensuring the safety and reliability of electrical systems.

Related Articles:

What is Good insulation?
What Makes Insulation Go Bad?
How Insulation Resistance is Measured.
How to Interpret Resistance Readings.
Factors Affecting Insulation Resistance Readings.
Types of Insulation Resistance Tests.
Test Voltage vs. Equipment Rating.
AC Testing vs. DC.
Use of DC Dielectric Test Set.
Tests During Drying out of Equipment.
Effect of Temperature on insulation Resistance.
Effects of humidity.
Preparation of Apparatus to test.
Safety Precautions.
Connections for testing insulation resistance of electrical equipment.
Additional Notes About using A Megger Insulation Tester.
Interpretation-Minimum Values.
Minimum Values for Insulation Resistance.
Tests Using Multi-Voltage Megger Insulation Testers.
Step-Voltage Method.
Use of a Guard Terminal.
Outdoor Oil Circuit Breaker.