Three Phase Insulation Resistance Test

Considering that we have multiple phases to test we need to carry out the test in the following sequence:

L1 - L2

L1 - L3

L2 - L3

N - L1

N - L2

N - L3

E - L1

E - L2

E - L3

E - N

Insulation Resistance 3 Phase

In a perfect installation these results should all be greater that 1000Mohms

Any wires which make up switch wires should also be tested by activating the switches

When performing insulation resistance testing, you must maintain consistency. Why? Because electrical insulation will exhibit dynamic behavior during the course of your test; whether the dielectric is "good" or "bad." To evaluate a number of test results on the same piece of equipment, you have to conduct the test the same way and under the relatively same environmental parameters, each and every time.

Can you remember what a dielectric?

When carrying out the test consider.......

Before carrying out any test......

After you make your connections, you apply the test voltage for 1 min. (This is a standard industry parameter that allows you to make relatively accurate comparisons of readings from past tests done by other technicians.)

Almost forgot you need to remove all the resistance relays and anything that could mess up your results

Any Questions

Work must be carried out in a safe manner. Things to consider when developing safe working practices include:

• control of risks while working;
• control of test areas;
• use of suitable tools and clothing;
• use of suitable insulated barriers;
• adequate information;
• adequate accompaniment;
• adequate space, access and lighting;
• precautions to prevent people not carrying out the testing coming into contact with exposed live parts.

During this interval, the resistance reading should drop or remain relatively steady. Larger insulation systems will show a steady decrease; smaller systems will remain steady because the capacitive and absorption currents drop to zero faster than on larger systems. After 1 min, you should read and record the resistance value.

Three Phase Insulation Resistance Test

So last week we looked at single phase insulation resistance test.

This week we are looking at Three Phase Insulation resistance testing.

• This is a dead test.
• Carried out using a Insulation Resistance tester
• Tester needs to be set to 500v
• Test carried out between all conductors
• Results should be >1000Mohms

Three phase insulation resistance test

A three phase system consists of three single phase AC supplies sharing its return path (neutral conductor). Each of the three phases is separated by 120° (shown below), giving the effect of constant power over each cycle.

Your resistance measurement readings will also change with time. This is because electrical insulation materials exhibit capacitance and will charge during the course of the test. This can be somewhat frustrating to a novice. However, it becomes a useful tool to a seasoned technician.

This results in an overall line voltage (VL) of √3VP, where VP is the voltage of a single phase.

Given that the voltage of single phase (VP) is 240 V, VL = √3 x 240 = 415 V. Therefore the running voltage of a three phase system is 415 V.

As you gain more skills, you'll become familiar with this behavior and be able to make maximum use of it in evaluating your test results.