IR Image Logging: Analyzing design or equipment failure with a thermal imager.

Ever receive a customer returned device only to realize that it got so badly burnt that you have run out of ideas as to what caused the damage in the first place? Upon receiving the failed device, failure analysis engineers will be scrambling to simulate the situation to find out whether this is pure misuse by the end user, or a design failure. Did the startup process cause a spike in the current and rise in temperature, causing it to burn? Was it a software-generated issue or a hardware issue?

Figure 1: Sample of a burnt circuit board

Some networking equipment designers and manufacturers use thermal imagers to help them identify hotspots in the circuit board. Often, the failure analysis engineer will capture images over a period of several hours, to see temperature distribution on the switch and server systems. A data logger will also be used at the same time to monitor temperature over certain pre-determined spots. In this specific case, this customer mentioned that their current solution, which is a video recording using a thermal imager is not a very good solution, because they could not perform video logging for a long duration.

One of the key features that enabled this customer to improve their failure analysis is the image logging capability. The image logging capability in the U5855A TrueIR thermal imager enabled this customer to log IR images over several hours while monitoring heat distribution over the switch and server systems. The images can be taken and saved, with minimum of 7 seconds interval (up to 3600 seconds of interval). The TrueIR thermal imager is one of the first imagers in the world to have this image logging capability!

For more information about the U5855A TrueIR thermal imager and its key features, go to: www.keysight.com/find/TrueIRimager

Last Mile

These communication boxes are popping up in a neighborhood near you, just like this one in a suburban parking lot.  In the battle to shorten the “last mile” of copper wiring to your wire line phone, telephone companies are putting more of these small remote terminals close to you, in place of larger terminals or central offices further away.  They contain all the electronics necessary for linking twisted copper pairs to fiber optics.  Since they reside outdoors, they must endure the elements, including having an operating temperature specification from -40^oC to +55^oC.

One of Keysight’s customers is an electronic designer for these remote terminals, amongst many other product lines that he is also responsible for.  Often, he has to set up verification tests for these terminals in their operating setting.  One common requirement would be testing with the door(s) closed.  

Keysight’s Remote Link solution is ideally suited for this type of testing.  Using Bluetooth communication to his Android tablet, our engineer is able to track at his desk testing data coming from inside the closed terminal box. 

Like all Keysight handheld multimeter, the U1272A he uses has an IR port in the back, besides the usual LCD display in front.  The IR to Bluetooth adaptor, U1177A, snapped over the IR port converts the IR link into wireless RF signal.  Our engineer received the RF signal on his Android Bluetooth port.  He even has a choice of two different free apps to manage and process the measurement information.  The Mobile Meter app captures and displays up to three live meters on screen.  The more complex Mobile Logger app further logs all three meter readings over time and presents the results in either graphical or numeric format.  The entire process takes only a few clicks to set up.

See a YouTube demo video here. 

http://www.youtube.com/watch?v=FiBuzDEOP4o&list=PL9F34A7C7C9DFCD10&index=1

TIPS: How do you measure temperature of an object with unknown emissivity?

Emissivity is the measure of effectiveness of a material in emitting thermal radiation. Objects or materials with high emissivity will produce a more accurate temperature measurement through thermal imager (with the right compensation and settings in the thermal camera). On the contrary, objects or materials with low emissivity have high reflectivity, making temperature measurement more challenging.
That said, it is very important to know the emissivity of the targeted object or material that we are measuring, to obtain an accurate temperature reading using thermal imagers. Most thermal imagers in the market today do come with a pre-defined emissivity table for common items that can be found around us.

Figure 1: Emissivity table found in U5855A TrueIR thermal imager

Together with this table, users also have the flexibility to input the emissivity values from 0.1 to 1.0. However, when the emissivity value or material of the object is not known, these steps can be taken to determine its emissivity: 

1. Clean up the surface of the targeted object to remove dusts or foreign materials.
2. Apply blackbody paint or electrical tape with known emissivity on the surface of the targeted object. See Figure 2.





Figure 2: Object with unknown emissivity




3. If the targeted object is radiating heat, do allow some time for the surface of blackbody paint or tape to reach equilibrium with the targeted object. 
4. Perform the reflected temperature calibration (RT cal) and set the known emissivity (Ɛ) value of the blackbody paint or tape on your thermal imager. Measure the temperature of area (A) whereby the blackbody paint or tape was applied. Ensure the thermal imager is focused correctly and perpendicular to the targeted object to minimize emittance effect. Record the average temperature of that focused area.
5. Measure the temperature of area (B) as shown in Figure 2. Manually lower the emissivity (Ɛ) value on the thermal imager until the temperature of area (B) is equal to the temperature measured at area (A). The emissivity reading indicated now will be the emissivity value of the targeted object.

All in all, by knowing the emissivity of the object or material we will be able to get the right temperature measurement, or better known as a quantitative measurement; measurement of data that can be put into numbers. However, another method is to do a qualitative measurement; where data is purely comparative and not numerical. For example, when checking a three-phase electrical system, it is merely comparing the hotspot among the three phases. 

Figure 3: IR image of a three-phase circuit breaker taken using U5855A TrueIR thermal imager

There are advantages and disadvantages to both qualitative and quantitative measurements, depending on whether or not further statistical analysis is needed or just a need to know the general feel of how the target object is.

Servicing a jumbo jet

Nowadays, very few of us get to see the controls of a wide body jet.  But for one of Keysight’s customers working in the aerospace industry as a design engineer, this is a common work space.  He would make changes at the control here and verify that the proper results followed.  Many times the results are simple and can be measured with a handheld multimeter.  The unfortunate challenge is that the test points mostly reside not on the flight deck, but in the lower electronics bay underneath.  However, getting there is not so simple or easy.

Here is a picture of a wide body jet.  One can see the passenger door on its left front.  There is a service door on its right, almost a mirror image of the passenger door.  One gets into the lower electronics bay from a smaller door, located below the service door.  Literally it is impossible to go back and forth between the controls on the flight deck and the test points in the electronics bay.  He must have a helper or rig up a close circuit TV to allow him to read the meter readings.  He did both, depending on the occasion.  Not a fun or efficient part of his work.

Keysight’s Remote Link Solution is ideally suited for this type of testing.  Using Bluetooth communication to his Android tablet, our engineer is now able to track his testing data on the flight deck.  The meter is left attached at the proper test points in the lower electronics bay.

Like all Keysight handheld multimeter, the U1272A he uses has an IR port in the back, besides the usual LCD display in front.  The Bluetooth adaptor, U1177A, snapped over the IR port converts the IR link into wireless RF signal.  Our engineer received the RF signal on his Android Bluetooth port.  He even has a choice of two different free apps to manage and process the measurement information.  The Mobile Meter app captures and displays up to three live meters on screen.  The more complex Mobile Logger app further logs all three meter readings over time and presents the results in either graphical or numeric format.  The entire process takes only a few clicks to set up.

http://www.youtube.com/watch?v=FiBuzDEOP4o&list=PL9F34A7C7C9DFCD10&index=1

What is fine resolution?

The technology used to enhance the resolution of a digital image is known as super resolution. Over the course of digital imaging system development, many different techniques of super resolution have been developed; each with its advantages and disadvantages. A thermal imag­ing system assimilates the resolution enhancement technology used in digital imaging system to improve on its resolution. The TrueIR thermal imager uses a specific multi-frame super resolution technique and algorithms that is known as fine resolution, which enhances the resolution of a thermal image by four times.

So what is fine resolution and how does it work?

It can be summarized to 3 main processes – Acquisition, Super-position, Reconstruction.

Each of the above mentioned processes have different tasks on its own.

Acquisition:

Whenever the trigger button is pressed, the thermal imager will automatically capture multiple images of the same scene continuously. It also assumes that each of the images or frames taken is shifted, due to natural hand movement of the thermographer. Also, during the acquisition, all the frames are automatically up-scaled to higher resolution images through interpolation technique – predicting image pixels using data from adjacent pixels that has been captured initially. The technique is very similar to a curve fitting mathematic function. The new pixels are predictive values instead of measured values.

The interpolation process is fast, hence, the high resolution inter­polated images are real time. Instead of showing the real time low resolution image on the display, the interpolated high resolution images are displayed on the LCD, which serves as a view finder for the thermal image, just like a digital camera.

Super-Position:

As mentioned, each of the frames taken through multi-frame acquisition process is slightly shifted. Therefore, overlapping the frames without any intelligence will not work. In this process, feature points from each of the frames needs to be identified, positioned and aligned together before being superimposed to form a high resolution image. Figure 1 below illustrates this process.

Figure 1: Super-position

Reconstruction:

Superposition process above might incur some noise into the thermal image, causing it to be fuzzy. Hence, at this stage, many mathematical models and image processing techniques are used, such as an averaging algorithm for noise reductions and an edge enhancement algorithm for image sharpening.

The Result: 

Here’s a simple lab test to prove the point. We simply measure the temperature of a slim 1-mm vertival bar at a fixed distance. Figure 2 illustrates the ability of the TrueIR detector’s array to capture the thermal image of the slim bar. Due to limitation of the physical iFOV discussed earlier, if a single frame is used (in this example that means only Frame 1 is used), only the average temperature of the bar is recorded, which is inaccurate.

Figure 2: Detector's pixel arrays

Through multi-frame acquisitions, Fine Resolution is able to recover sub-pixel information. Figure 3 shows the test result comparison between using a detector with 160 x 120 pixels versus the results obtained using a fine resolution imager which generates an image with 320 x 240 pixels. Fine resolution provides 1.5x better iFOV, thus resulting in 1.5x more accurate temperature measurement. So you get to reap the benefit of a 320 x 240 pixels thermal imager for just a fraction of the cost!

Figure 3: Lab test resuts (1-mm bar)

Sample Fine Resolution IR images taken with Keysight's U5855A TrueIR thermal imager

To read more about Fine Resolution capability, click here. 

The U5855A TrueIR thermal imager

Or to learn more about U5855A TrueIR thermal imager, go to Keysight U5855A TrueIR thermal imager

Preventive Maintenance test with Insulation Resistance Test, Part 3

Part 1 provides an overview on the insulation resistance test in preventive maintenance. To read part 1, click here.
Part 2 covers the insulation resistance test methods. To read part 2, click here.

Test Voltage Selection
As the insulation resistance test consists of the high DC voltage, the appropriate test voltage has to be selected to avoid over stressing the insulation, which may lead to insulation failure. The test voltage applied to should be based on the product/equipment manufacturer recommendations. If the test voltage is not specified, industrial standards and practices may be applied.   The following guideline for rotating machinery shown in Table 2 may be adopted in the absent of the manufacturer’s data.

Table 2 Guidelines for DC voltage to be applied during insulation resistance test (extracted from IEEE Std 43-2000)

Winding rated voltage (V)1	Insulation resistance test direct voltage (V)
< 1000	500
1000 - 2500	500 - 1000
2501 - 5000	1000 - 2500
5001 – 12000	2500 – 5000
> 12000	5000 - 10000

¹ Rated line-to-line voltage for three-phase AC machines, line-to-ground voltage for single-phase machines, and rated direct voltage for DC machines or field windings.

Table 3 insulation Resistance Test Values Electrical Apparatus and System (extracted from NETA ATS-2007 Acceptance Testing Specifications for Electrical Power Distribution Equipment and Systems)

In the absence of consensus standards dealing with insulation-resistance tests, the Standards Review Council suggests the above representative values.

Test results are dependent on the temperature of the insulating material and the humidity of the surrounding environment at the time of the test. Insulation-resistance test data may be used to establish a trending pattern. Deviations from the baseline information permit evaluation of the insulation.

The test voltage may vary according to the international standards. Consulting the product/equipment manufacturer for the proper test voltage values is recommended.

Determination of Minimum Insulation Resistance

The IEEE Std 43-2000 indicates that the minimum insulation resistance for AC and DC machine stator windings and rotor windings can be determined by:

R_m = kV + 1

Where,

• R_m is the recommended minimum insulation resistance in MΩ at 40 °C of the entire machine winding, and
• kV is the rated machine terminal-to-terminal voltage in kV unit

Table 4 Recommended minimum insulation resistance values at 40 °C (extracted from IEEE Std 43-2000)

Minimum insulation resistance (MΩ)	Test specimen
IR1 min = kV + 1	For most windings made before about 1970, all field windings, and others not described below
IR1 min = 100	For most dc armature and ac windings built after about 1970 (form-wound coils)
IR1 min = 5	For most machines with random-wound stator coils and form-wound coils rated below 1 kV

Safety consideration

As insulation resistance testing involves high DC voltage application, the following safety precautions should be taken:

• Make sure that the device under test is discharged.
• Conduct the test at the de-energized condition to ensure that no test voltage other than that from the insulation resistance tester is applied.
• Restrict personal access when high voltage testing is being conducted.
• Use of personal protective equipment (e.g. protective gloves) where applicable. 
• Ensure suitable test leads are used and that they are in good condition. Using unsuitable test leads not only contributes to errors in readings, they may be hazardous.

After the test, make sure the device is fully discharged. This can be done by shorting the terminal with a suitable resistor. A minimum discharge time of four times the applied voltage duration is recommended. Some insulation resistance testers may have the built in self discharge circuit to ensure a safe discharge after the test. Testers with this feature ensure devices are safely discharged after every test.

Planning for a Maintenance Program

When planning for a maintenance program, equipment that needs maintenance needs to be identified, and priorities set accordingly. A motor or machine that supports the whole line should be a high priority. The frequency of checks to be conducted should also be defined. The frequency can be varied from unit to unit depending on the criticalness of the unit in the environment. Past history will be a good guide for determining when the next maintenance activities will be needed.
The maintenance record should cover the following:

1. Date of the test
2. Test voltage and current
3. Test time
4. Insulation resistance value
5. Temperature of winding/equipment
6. Identification of the equipment/device under test
7. Parts or equipment that were included in the test
8. Relative humidity

As with every preventive maintenance program, record keeping and plotting of consecutive readings can identify trends and enable you to predict and plan for the next action.

Conclusion

Periodic testing is the best approach for preventive maintenance of electrical equipment and charting result values helps in monitoring the trend of the insulation resistance, which helps predict the future need for action.

Preventive Maintenance test with Insulation Resistance Test, Part 2

Part 1 covers the introduction of the insulation resistance test in preventive maintenance can be found here.

What are the test methods for insulation resistance test?
There are three types of tests for measuring insulation resistance.

• Spot reading
• Time-resistance
• Step voltage

Each test applies its own methodology that focuses on a specific insulating property of the devices being tested. Users need to choose the one that best fits the test requirements.

Spot test

A test voltage is applied for a short interval until a stable reading is achieved, or for a fixed period of time, normally 60 seconds or less. The reading is collected at the end of the test. This test is normally performed for Go/NoGo testing or historical records. Temperature and humidity variations may affect the readings and have to be compensated for if necessary.

For the historical record, a curve is plotted based on the history of the readings. Observation of the trend is taken over a period of time, normally over years or months. 

Figure 3 For an effective monitoring the equipment insulation resistance, the insulation resistance values collected at each test interval should be plotted at the graph to track it’s trend

This test is suitable for a device with a small or negligible capacitance effect, e.g. short wiring run.


Time-resistance test

Successive readings are taken at a specific time, typically every few minutes, and difference in readings compared. Good insulation will show a continual increase in the resistance value. If the reading is stagnant and it does not increase as expected, the insulation may be weak and attention may be needed. Moist and contaminated insulation may lower resistance readings since they will increase the leakage current during testing. The temperature influence on this test is negligible as long as there is no significant temperature change in the device under test.

This test is suitable for the predictive and preventive maintenance of rotating machines.

The polarization index (PI) and dielectric absorption ratio (DAR) are commonly used to quantify the time-resistance test result.

Figure 2 Curve plot of a time-resistance test made on a motor winding with Keysight Handheld Meter Logger software. Good insulation shows a continual increase in resistance, the trend line should be in inverse exponential

Polarization index (PI)

The polarization index is defined as the ratio of the 10 minute resistance value to the 1 minute resistance value. The interpretation of the value is shown in Table 1. The IEEE Std 43-2000 recommends the minimum value of PI for AC and DC rotating machinery in thermal class B, F and H as 2.0, and the minimum PI value for class A equipment is 1.5.

NOTE: Some new insulation systems have a faster response to the insulation test. They usually start with test result at GΩ range yielding a PI between 1 and 2. In these cases, the PI calculation may be disregard. According to the IEEE Std 43-2000, if the 1 minute insulation resistance is above 5 GΩ, the calculated PI may not be meaningful.

Dielectric absorption ratio (DAR)

Dielectric absorption ratio is referred to the ratio of the 60 second resistance value to the 30 second resistance value. The interpretation of the value is shown in Table 1.

DAR is suitable for devices with insulation materials in which the absorption current decreases quickly.

Table 1. PI and DAR test result interpretation

Insulation condition	PI value	DAR value
Insufficient	< 2	< 1.25
OK	2 to 4	< 1.6
Excellent	> 4	> 1.6

Step Voltage Test

Different voltage levels are applied in steps to the device under test. The recommended ratio of the test voltage is 1:5. The test at each step is same length, usually 60 seconds, and goes from low to high. This test is normally used at test voltages lower than the rated voltage of the equipment. The rapid increase of the test voltage level creates additional stress on the insulation and causes the weak point to fail, subsequently leading to a lower resistance value.

This test is particularly useful when the rated voltage of the equipment is higher than the available test voltage generated by the insulation resistance tester.

In the last article (Part 3), I will cover the test voltage selection, minimum insulation resistance, as well as the safety consideration for the insulation resistance test.

Preventive Maintenance test with Insulation Resistance Test, Part 1

Preventive maintenance is a predetermined task performed based on a schedule and its objective is to keep equipment in good condition to avoid breakdowns.

Insulation resistance testing is commonly performed as part of electrical testing in a preventive maintenance program for rotating machines, cables, switches, transformers, and electrical machinery where insulating integrity is needed. Insulation resistance testing in the preventive maintenance program helps identify potential electrical issues to reduce unpredictable, premature equipment repair and replacement cost.

With properly scheduled monitoring and data collection, this testing can be very useful in analyzing and predicting the current and future behavior of equipment. Early problem detection helps avoid major repairs, resulting in cost savings when compared to a run-to-failure maintenance practice. Preventive maintenance has the added benefit of pre-planning for necessary parts and resources.

This first of three articles will describe what is insulation resistance testing, how it plays a part in preventive maintenance and factors that affect insulation resistance. The second article will focus on the methods of insulation resistance testing¹  while the last article will detail test voltage selection guidelines and safety considerations.

1 Consulting the original product/equipment manufacturer for more detailed information is recommended.
 

What is Insulation Resistance Testing?
Insulation resistance is used to verify the integrity of the insulation material. It can be the cable insulation or motor/generator winding insulation. Insulation resistance testing is carried out by applying a constant voltage to the equipment under test while measuring the any flowing current. High DC voltages are used causing a small current to flow through the insulator surface. The total current consists of three components: capacitance charging current, absorption current, and leakage current (refer to Figure 1.)

• Capacitance charging current is relatively high upon start-up and drop exponentially within a few seconds to a few ten seconds. It is normally negligible when the reading is taken.
• Absorption current decays at a decreasing rate. It may require up to a few minutes to reach zero depending on the insulation materials.
• Leakage current is constant over time.

Figure 1 Components of test current

How Insulation Resistance Testing Helps in Preventive Maintenance
For an effective test, results should be regularly recorded over a period of time and compared with earlier recorded values taken when the equipment was new and in good condition. The trend of the readings over a period of time will help identify the presence of anomalies. Insulation resistance values that are consistent over time indicate that the equipment’s insulation properties are good. If the resistance values are decreasing, it indicates that potential issues can occur sometime in the future and more thorough preventive maintenance should be scheduled soon.

Factors That Affect the Insulation Resistance

The factors that commonly affect the insulation resistance are:

• Surface condition. For example oil or carbon dust on the equipment’s surface that can lower the insulation resistance.
• Moisture. If the equipment’s surface temperature is at, or below, the dew point of the ambient air, a film of moisture forms on its surface would, lowering the equipment’s resistance value.
• Temperature. The insulation resistance value may vary inversely with the change of the temperature. Its influence on readings can be mitigated by performing preventive maintenance testing at the same temperature each time. If the temperature cannot be controlled, normalizing to a base temperature such as 40 °C is recommended. This is commonly done using the estimation rule, “Every 10 °C increase in temperature halves the insulation resistance, while a 10 °C reduction doubles the resistance”.  As different materials may have different degrees of resistance change due to temperature, for more precise temperature correction, some may adopt a temperature correction factor the measurement reading should be multiplied by multiplying the measurement reading with the temperature correction factor at the corresponding temperature.

In the next article (Part 2), I will cover the insulation resistance test methods.