DESIGN OF SAFE MEDICAL DEVICE PROTOTYPES - Design and Development of Medical Electronic Instrumentation - page 139

TABLE 3.3 Normal and Fault Conditions Detected by Three Neon Lamps Used in the Circuit of
Figure 3.27 to Verify That the AC Plug from Which Power Is Obtained Is Wired Correctly
Indicator Light

Condition

LP1 (Hot-Neutral) LP2 (Hot-Ground) LP3 (Neutral-Ground)

Off Off Off Instrument off or open hot

Off On Off Open neutral

Off On On Hot-ground reversed

On Off Off Open ground

On Off On Hot-neutral reversed

On On Off Correct, or ground-neutral reversed

may result in enclosure of the device under test becoming live, threatening anyone who
would come in touch with it accidentally. In addition, since power line-level voltages can
be injected into patient connections and the associated power system, never conduct these
tests in the vicinity of a patient or on a power system branch that is used to power medical
electronic instruments connected to patients.

HiPot Testing

Once compliance with current leakage limits is established, high-potential application test-
ing, commonly known as hiPot testing , is used to assess the suitability of the insulation bar-
riers between isolated parts of a medical instrument. In essence, very high voltage is applied
di erentially between the parts separated by the isolation barrier under test. As shown in

Table 3.4, the test voltage is dependent on the voltage U to which the barrier is subjected
under normal operating conditions at the rated supply voltage. While high voltage is applied,
current is monitored to ensure that no arc breakdown occurs. HiPot testers have internal cir-
cuitry that automatically disconnects the high-voltage supply across the insulation under test
whenever current exceeds a preset threshold value.
Although slight corona discharges are allowed by the standards, excessive RMS leak-
age current measurement is not su

ff

cient for reliable detection of dielectric breakdown.
Rather, milliampere-level current spikes or pulses should be monitored, since these are
indications of the type of arc breakdown that occurs on insulation prior to catastrophic and
destructive breakdown. Here again, the test voltage is supposed to be within the rated
operating frequency for the instrument under test, and measured magnitudes refer to their
ac RMS values. Despite this, the technique is sometimes modi ed by applying the dc

equivalent to the peak-to-peak amplitude of the ac RMS voltage required. This obviously
reduces current leakage between parts, since capacitive and inductive coupling disappear,
leaving a current signal that directly conveys information about insulation breakdown
processes at the peak of the dielectric stress.
As an example of applying this technique, if the highest-rated supply voltage for
an instrument is 125 V ac, the standard requires that testing of basic insulation be con-
ducted at 1000 V RMS . The peak-to-peak voltage of the ac test signal required would thus
be 1000 V RMS

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1410 V P-P . As such, 1410 V dc would be applied, for example,
between a wire connecting the hot and neutral of the power cord and another wire attached
to the protective ground connection of the instrument. Similarly, the insulation barrier
between an F-type applied part and any other point of the instrument of the example is
required to be tested at 3000 V RMS , which corresponds to a 4230 V dc voltage for the
modi

1.41

ed test. Breakdown of the insulation would then be indicated by current spikes that
would appear while the high voltage is applied between a point that ties together all patient

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