Feature Article

Energy measurement test

The international standard for medical electrical equipment IEC 60601-1 incorporates different surge tests to verify that the device under test will continue to operate correctly if it is subjected to a defibrillation pulse. The tests are noted in IEC 60601-1, Figures 9, 10 and 11. Each of these tests requires a 5000-V supply sourcing 400 J, with approximately 360 J appearing at the output of the tester (worst case). The three figures in the standard document different delivery methods of this pulse to the equipment under test.

Two of the tests, which are shown in IEC 60601-1:2005 Figures 9 and 10, are common mode and differential mode tests, which check the isolation of signal input/output parts and patient connections. A divider network is used to allow monitoring of the change in voltage of the signal input/output parts when the 360-J pulse is applied.  

New to the 2005 edition of the General Requirements for Safety Standard, IEC 60601-1, is the energy measurement test, which has been brought into the general medical standard from IEC 60601-2-49, Requirements for the Safety of Multifunction Patient Monitoring Equipment. This new requirement stipulates that any device connected to the patient will reduce any defibrillator energy delivered through a 100-Ω load by a maximum of 10%.

All three of these tests, and many other surge tests in AAMI and IEC medical standards, use the same 5000-V/400-J engine, with different wave-shaping components and application techniques. Because of this similarity, test equipment manufacturers can provide one tester with the ability to test to many different standards. By checking the documentation of your IEC 60601 defibrillation tester, the user will be able to discern exactly which standards can be tested with the machine; connection information for each different test is offered.

Generation of the pulse delivered from the 5000-V/400-J supply is controlled by the IEC/AAMI standards in the same way—by providing circuit component values and tolerances, as shown by example in our Figure 1 (Figure 9 of IEC 60601-1:2005).

Figure 1: IEC 60601-1: 2005 Figure 9 – Application of test voltage to bridged patient connections for defibrillation-proof applied parts.

This figure shows in a shaded red box the circuits required to be supplied by a defibrillation-proof surge tester. The circuit in the lower part of the red box is the ubiquitous 5000-V/400-J surge supply. Below the circuit is the list of components, with tolerances; note that all tolerances for the surge supply are ±5%. Waveshaping components vary from standard to standard, but component tolerance values for the surge supply are ±5%, as well.

The component tolerance values are guaranteed by the calibration process. Users can rest assured that the defibrillation-proof surge tester is within these values when new and after a calibration cycle. However, short of opening the surge tester to recheck the component values, which would invalidate the calibration, there is no straightforward way to check proper operation between calibration cycles. In the next part of this article, we will discuss methods that can be used to check calibration without opening the surge tester.

Waveform comparison method

The pass/fail point of all three tests in IEC 60601-1:2005, which use the defib-proof surge tester (Figures 9, 10 and 11), are predicated on delivery of the pulse derived from the 5000-V/400-J supply to the device under test. However, with no referee data (except nonaccessible component values and tolerances), it is not possible to validate the output of the power supply using only the information given in IEC 60601-1:2005. Furthermore, since compliance of the surge tester with the requirements of IEC 60601-1:2005 involves only component tolerances, even an accredited calibration to ISO 17025 will only verify component tolerance.

Figure 2: IEC 60601 defib-proof waveform (typ) showing circuit and resulting waveforms.

Some manufacturers may provide waveforms of expected output for test setups contemplated by IEC 60601-1:2005. If you have these waveforms, we recommend that you check your defib-proof surge tester’s output to the waveforms generated when the tester was new, using the method described below. If the waveforms of your particular tester are not available, you may consider use of a modeled waveform using component values given in IEC 60601-1:2005, Figures 9 and 10 (shown in this article’s Figure 2.) Because of the tolerance limits noted in IEC 60601-1:2005, the modeled waveform may not exactly mimic the output of your particular defib-proof surge tester. If desired results are not obtained when evaluating output, other tests noted below in the Energy Measurement Test Accuracy Evaluation section are available, as well.

Waveform comparison test method
In this method, a new waveform from the defib-proof surge tester is compared with the as-received waveform included in the original calibration data package of the tester when new. If this information is not available, a generic modeled waveform is provided in Figure 2. This is a quick and easy method to verify correct surge tester output. Figure 3 of this article shows a sample referee as-received waveform for reference in your testing.

A final note: Although the 5% component tolerance stipulated in IEC 60601-1:2005 may seem tight, it has been our experience that some variation in output is to be expected, even if all tolerance criteria are met. If output data is not as expected, further testing should be done before any conclusions are reached regarding the current calibration status of your particular defib-proof surge tester. Depending on the tolerance and quality of the components used, the output peak voltage will be between 4500 and almost 5000 V, using a standard-compliant defib-proof tester.

1. Use a high-voltage, high-frequency oscilloscope probe. We have had very good results from the Tektronix P6015, available on the used market for a reasonable price.
2. Use a digital oscilloscope. Set the input for 1.00 kV/division vertically, and 500.0 µsec/division horizontally.
3. If your defib-proof surge tester has multiple wave-shaping networks, verify that it is set to 50 ohms, 500 µH.
4. Set the front panel meter to 5000 V, which indicates that the internal power supply is charged to 5000 V (output will be less; see Figure 2 of this article).
5. Fire the defib-proof surge tester into an open circuit output, with just the high-voltage probe connected.
6. Look at the oscilloscope display. Your peak output should be between 4500 and 4900 V (not 5 kV).
7. Find the 50% point of the V-peak waveform on the duration side of the waveform. It should be in agreement with the as-received waveform, in the neighbourhood of 2.42 msec.
8. If the peak value of the waveform is appreciably lower than expected, or if its duration is appreciably shorter than expected, the defib-proof surge tester may not be delivering the required 360 J. Further testing should be undertaken (see the Energy Measurement Test Accuracy Evaluation section below), or service should be scheduled.

Energy Measurement Test (IEC 60601-1, Figure 11): resistance measurement
The Energy Measurement Test requires two measurements to be conducted: a referee measurement to determine the exact output of the defib-proof surge tester and, secondly, a measurement with the device under test connected to the output of the defib-proof surge tester. The two measurements must be within 10% of each other for a passing result.

Figure 3: A sample referee as-received waveform for reference in testing the defib-proof surge tester.

The order of these tests is significant. As resistors heat, the resistance drops. As the resistance drops, the joules passed drops. This is significant because the change of resistance caused by heating cuts into the allowable 10% change in energy when the device under test is connected to the defib-proof surge tester. Therefore, it is extremely important to ensure that both tests are conducted using the same resistance value. Although manufacturers of defib-proof surge testers provide resistor banks that will stay within 5% of nominal value when the recommended duty cycle is followed, it may be beneficial to perform a measurement of resistor value between tests. It may take longer for the resistor to return to the value originally used for the referee test than stated in the manufacturer’s published duty cycle time.

Resistance value determination
1. Before conducting the Energy Measurement Test, obtain a cold resistance value of the 100-Ω resistor bank inside the defib-proof surge tester.
2. Ensure the defib-proof surge tester is not charged. This can be assured by following the manufacturer’s procedure. Generally, the device should be triggered, and the front panel voltage meter should read essentially 0 V.
3. Turn off the defib-proof surge tester.
4. Insert an ohmmeter between the energy measurement port and ground connection of the defib-proof surge tester. The value should be ±5% 100 Ω.
5. Conduct the Energy Measurement referee test; that is, in accordance with IEC 60601-1:2005, with nothing connected to the output of the defib-proof surge tester. Ascertain the joules output and record the results.
6. Repeat steps 2 and 3 above.
7. Insert an ohmmeter between the energy measurement port and ground connection of the defib-proof surge tester. If the value is appreciably lower than the value obtained in step 4, allow time for the resistor bank to cool and approach the value obtained in step 4.
8. Conduct the Energy Measurement test with the device under test connected to the output of the defib-proof surge tester. A passing result will require <10% difference between the two results.

Energy Measurement Test (IEC 60601-1 Figure 11): Accuracy evaluation
If your defib-proof surge tester is ready for testing to IEC 60601-1:2005, it is equipped with an energy port to allow the test in Figure 11 to be conducted. This port also can be used to ascertain the exact energy output of the defib-proof surge tester. The output should be above 360 J for any tests conducted that use the 5000-V supply and 32-µF capacitance (see Figure 1 of this article). This referee test is conducted in the same manner as the first part of the Energy Measurement Test. An abbreviated version is provided here, because transfer of the waveform data from the oscilloscope to the Excel spreadsheet will vary depending on the oscilloscope used. Also, the spreadsheet is available free on our website. The link to the website is provided at the end of the article.

Energy Measurement Test Procedure (abbreviated)
1. Remove any load from the output of the defib-proof surge tester.
2. If necessary, set the defib-proof surge tester to the 25-mH, 400-Ω settings.
3. Measure resistance of the 100-Ω resistor using the method provided in resistance measurement above.
4. Using a suitable high-voltage probe (for example, the Tektronix P6015), connect the oscilloscope across the 100-Ω resistance bank inside the tester. For most testers, this will be between the energy measurement output and tester ground.
5. Use a digital oscilloscope capable of transferring waveform data to an Excel spreadsheet. Set the oscilloscope to capture the entire waveform. Set the vertical axis to 1000 V/division and the horizontal axis to 2 msec/division.
6. Charge the defib-proof surge tester until its internal power supply charges to 5000 V. (In most cases, the power supply voltage appears on the front panel display.) Then, trigger the defib-proof surge tester.
7. The oscilloscope waveform should appear on the screen as a complete waveform, progressing from 0 to 0 V. The desired waveform presentation as part of the Excel spreadsheet can be viewed at http://www.compwest.com/Products/Downloads/Energy_calculation_TUV_EC13_1...). If the recommended settings do not provide a complete waveform, adjust them until the entire waveform appears on the oscilloscope screen.
8. Transfer the waveform data to the Excel spreadsheet. This step’s procedure will vary depending on the type of oscilloscope used.
9. Open the Excel spreadsheet. Delete waveform data before the waveform starts and after it completes. (The noise components of these data will erroneously add joules to the result.)
10. Allow the Excel spreadsheet to calculate the result. The result should be above 360 J.
If the result is substantially below 360 J, then the defib-proof surge tester should be examined and the cause for the low output should be corrected.

Defib-proof parts test (IEC 60601-1 Fig. 9 and 10): accuracy evaluation
All of the methods and discussion above have concerned the power supply of the defib-proof surge tester. A laboratory following the guidelines above will be assured of a properly operating tester which will be fit to perform the Energy Measurement Test. However, for the common mode and differential tests shown in IEC 60601-1:2005 Figures 9 and 10, proper operation of the voltage divider network, shown at the top of the red band in Figure 1 of this article, also must be verified.

One way to conduct this verification is to substitute a known value for the device under test; that is, for all the parts in white in Figure 1 of this article. This is the most effective way to perform this verification; additionally, it qualifies the entire test setup as correct, if the expected output is achieved.

Some manufacturers can provide a defib-proof pass/fail measurement tool for insertion into a test setup as described above. This pass/fail measurement tool is specifically paired with a particular defib-proof surge tester at the factory, because the tolerance specifications allowed by IEC 60601-1:2005 do not allow a single setting to suffice for all testers. However, use of a noncalibrated pass/fail measurement tool, if necessary, will still provide useful information and can still be used to verify test setups and general health of the defib-proof surge tester.

The defib-proof pass/fail measurement tool is calibrated with a specific tester to provide exactly 1 V on the display device (i.e., your oscilloscope). For a complete description of the entire procedure and a video showing details, please visit: http://www.compwest.com/Library/Defib-5%20Y1-Y2%20Measurement%20setup.pdf, and http://www.compwest.com/YouTubeDefib-5_Setup_and_Operation.html.

Conclusion
In this article we have provided information about the new surge tests in IEC 60601-1:2005, and provided procedures and suggestions to simplify testing and make sure test results are correct. In addition, we have provided some simple procedures to make sure your defib-proof surge tester is working correctly in between calibration cycles. These procedures will provide a general idea of the health of your defib-proof tester and ensure that the connections to the tester are valid. We hope these procedures will allow you to ensure that any tests are conducted with properly functioning equipment. 1

Jeff Lind
is President at Compliance West, 2120 Jimmy Durante Blvd., Suite 118, Del Mar, CA 92014, USA
tel. +1 858 481 6454
e-mail: sales@compwest.com
www.compwest.com