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Power Quality Case Study: the failure of medical devices

    Measurement tools: Fluke 43B Power Quality Analyzer.
    Fluke 87 True RMS digital multimeter,booster cables Fluke 27 digital multimeter.
    Operator: Electrical contractor.
    Features used: True-RMS voltage, average voltage, voltage waveform.
    Description of the problem.

    This is the importance of the system to solve the problem as a typical example. battery clip It involves a collaboration with several high-tech manufacturer of electrical contractors.

 tow rope ;   The case first began to visit a building manager, the manager is subject to its electrical system problems. Statement by the manager in his building, there is no electrical equipment will work, ratchet tie dowm and no one is sure what is the reason or propose a solution. The manager said he had resigned three electrician, and now he was in real trouble.

    The contractor asked many questions - trying to get more detailed description of the problem. Such efforts resulted in nothing, so the situation was brought to the contractor building the worst part. He followed a principle: "When in doubt, the worst load from the start."

    In the far corner, a National Taiwan University's medical equipment is undergoing rigorous testing procedures. Instrument equipped with a large display, a keyboard and a few cables and hoses to connect to other devices in the control panel. The operator's display shows the test program is running.

    And medical equipment for the repair of an adjacent circuit board table. The table has a soldering iron, a magnifying glass, and a fan. Bench power supply board and medical devices using the same power outlet. When contractors are observed, the table side who opened the fan. At this time, the operation became a blank screen, and then appeared in large font display "Program Reset" (program reset) message.

    Measurement method.

    Contractor for the two devices measured the voltage power supply socket. His Fluke 87 True RMS digital multimeter to measure the value of 115V. Building manager to use his Fluke 27 digital multimeter average response repeated measurement, display voltage is 118V. Why True RMS multimeter reading will be lower it?

    True RMS measurement equipment will be calibrated, but in measuring the square wave square wave or similar waveform, the reading is lower than the average response equipment. Contractors connected to the Fluke 43B to display the voltage waveform. Readings, waveform (Figure 1) at the top of the chopper was a serious - and so is more like a sine wave than a square wave. The peak measured only 135V, far below the expected 162V.
    As a result, the contractor drew a single-line diagram of the system. Diagram shows the power transformer in the test area relative to the other side of the building - about 500 feet away (Figure 2). Most of the transformer load is nonlinear, the peak voltage, they are absorbed high peak current. Peak current and the final combination of high impedance, the end of the circuit voltage chopper had a serious - just a test area location.

    Reasoning and analysis.

    Since the work of the internal circuitry of medical devices in low voltage DC power supply, the internal power supply will have a diode / capacitor input circuit, the circuit requires a minimum to maintain the normal operation of the peak voltage. Medical equipment, display equipment required on the nameplate AC voltage between 100 to 135V RMS. Instruments and fixed-plate design engineers assume that the supply voltage is a sine wave, then the minimum peak voltage should be 141V (100 × 1.41). The measurement of the peak input voltage is only 135V, operating voltage of the instrument than the absolute minimum required for low voltage 6V. When the fan is turned on, the fan caused by the inrush current reduces the voltage at the instrument irregular power supply. This is what led to equipment reset reasons.


    In the high-tech buildings, voltage peak chopper (flat) is a common problem. Many buildings now in use are not considered in the design of today's typical computer and nonlinear loads.

    In this case, in order to avoid the voltage between the transformer and the load loss, large-scale re-wiring is necessary. Another solution is to move most of the inductive loads near the end of the transformer.

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