By Michael Stuart, Electrical Products Manager, Fluke Corporation, michael.stuart@fluke.com

 Electrical insulation includes the entire complex system of cable insulation, bushings, and spacers within conduit, motors and general equipment. Low insulation resistance indicates that current is leaking through the insulation. That can cause heat build up, which in turn causes the insulation to degrade even faster and eventually fail. Leaking current can cause over-current protection devices to trip and motors and transformers to overheat, and is just plain unsafe and inefficient.

Insulation problems are usually caused by improper installation, environmental contamination, mechanical stress or age. Insulation testing can easily be combined in with regular maintenance, to identify degradation before failure, and during installation procedures to verify system safety and performance. When troubleshooting, insulation resistance testing can be the missing link that enables you to get a unit back into operation the easy way, by simply replacing a cable.

Insulation testers apply a dc voltage across an insulation system and measure the resulting current. This allows them to calculate and display the resistance of the insulation. Typically, the test verifies high insulation resistance between a conductor and ground or high insulation resistance between adjacent conductors. Two common examples include testing motor windings for insulation from the motor frame and checking phase conductors for resistance from bonded conduit and enclosures.

The best solution is to conduct insulation resistance testing on a regular basis, so that you can spot insulation degradation before it causes a failure. Many electricians are now adding this test to their maintenance contracts. Plan to test motors, transformers, compressors and conductors every three years, unless the equipment is essential to operations and/or exposed to harsh environments. Test this type of equipment at least once a year.

 

Troubleshooting Applications for Insulation Resistance Testing

Check for conductor leakage to bonded conduit Check for leakage between conductors sharing a conduit or raceway Check for motor winding leakage to bonded frame Check for leakage current in transformers

What’s a good insulation resistance reading?

The bad news: An insulation resistance measurement, in itself, is not necessarily indicative of anything in particular. As a general rule, the One-Megohm Rule (1MΩ per 1000 volts) is the widely-accepted absolute minimum insulation resistance value for normal electrical equipment.

However, actual minimum acceptable insulation resistance values are based upon the type of insulating material being used, and on the physical and electrical characteristics of the equipment being evaluated.

The good news: When compared against a previously recorded baseline measurement, or a manufacturer's factory specification, an insulation resistance measurement may be able to assist you in both troubleshooting and preventive maintenance.

Persistent downward trends usually indicate there’s something occurring which can prevent optimum performance. It’s a fair warning of impending trouble.

 

• With a less than 25% difference, the equipment is probably still operating acceptably.
• If there is a 25%-50% difference, most professionals recommend additional testing** and inspection to verify proper operation. (Check for potential environmental contamination.)
• At more than 50% difference, while equipment may still operate for some time, most professionals would view this much change as indicative of potential problems somewhere in the system. Additional testing** and diagnostics are highly recommended.

** Additional testing could include: additional insulation resistance testing, inspecting for the presence of moisture, oil, dirt, or other environmental contaminates, temperature measurement, loop impedance testing, advanced motor circuit testing, or other manufacturer's recommended diagnostics.

Example: Motor insulation resistance testing

While any failure is problematic, motors present additional challenges — because a specific motor failure or its cause isn't necessarily obvious, especially if insulation is at fault. Follow these guidelines for testing the insulation resistance on key points of a failed motor, using an insulation tester.

1. Visual inspection

First, look for a reason NOT to energize. Remove power from the motor and starter, following lockout/tagout procedures, and disengage the motor from the load.

• Conduct a visual, smell, and heat inspection, interview the client and check the nameplate. Look for loose connections at the starter and check all fasteners.
• Use a DMM to check the supply voltage, then the voltage starter contacts.

Don’t risk a fire from a possibly shorted motor. If the supply is good, then there's a motor problem.

 2. Control contacts check
Next, check the control contacts for quality of contact:
1. Lockout and tagout the disconnect to the starter.
2. Manually engage the starter, so the contacts close.
3. Set the insulation tester to the low ohms range.
4. Measure the resistance across each set of contacts.
5. The reading should be nearly zero. If it's higher than 0.1 ohms, that set of contacts needs to be replaced.

 3. Resistance of line and load circuits to ground
Then, measure the insulation resistance of the line and load circuits to ground.

However, before doing ANY insulation resistance testing, you MUST isolate any electronic controls and other devices from the circuit under test. Then:
1. Lockout and tagout the disconnect to the starter.
2. Set the insulation tester to the appropriate test voltage (250, 500 or 1000 V).
3. Identify the resistance between these points:

• Line side of starter to ground
• Load side of starter to ground

To pass these tests, the line and load circuits need to show high resistance. As a general rule, AC devices need a minimum 2 megohms to ground and DC devices need 1 megohm to ground to ensure safe operation.

Note: Different companies have different threshold minimums for insulation resistance on used equipment, ranging from 1 to 10 megohms. Resistance on new equipment should test much higher — from 100 to 200 megohms.

If the load side resistance values are acceptable then proceed to the next test. If they aren't, then start tracing the problem: is the insulation breakdown in the load side of the starter, the cables, or the motor?

 4. Winding resistance phase to phase and phase to ground
Take insulation resistance measurements phase to phase and phase to ground.

Good results:

• Balanced comparative low resistance values on all three stator phases
• High resistance values on the phase to ground insulation test

Problems:

• Gross resistance deficiencies, such as a phase on phase short.
• Any winding to winding resistance imbalance. If the readings differ by more than a few percent, the motor is probably unsafe to energize.

Remember: There's no magic bullet test that will tell you whether your motor is good or bad. Every test by itself is inconclusive until you do them all. Just because you have a good reading doesn't mean something isn't still wrong. But a bad reading means something is definitely wrong.