Biomedical Engineering Reference
In-Depth Information
The frequency-response characteristics of the AAMI load have been selected to approx-
imate the inverse of the risk current curve as a function of frequency. In turn, this risk cur-
rent curve was derived from strength/frequency data for perceptible and lethal currents.
These data showed that between 1 and 100 kHz, the current necessary to pose the same
risk increases proportionately to 100 times the risk current between dc and 1 kHz. Since
insu
cient data exist above 100 kHz, AAMI decided not to extrapolate beyond 100 kHz,
but rather, to maintain the same risk current level corresponding to 100 kHz.
Actual current measurements should be conducted after preconditioning the device
under test in a humidity cabinet. For this treatment, all access covers that can be removed
without the use of a tool must be opened and detached. Humidity-sensitive components
which in themselves do not contribute signi
cantly to the risk of electrocution may also be
removed. Next, the equipment is placed in the humidity cabinet containing air with a rela-
tive humidity of 91 to 95% and a temperature
t
within the range
fi
32°C for 48 hours
(or 7 days if the instrument is supposed to be drip-proof or splash-proof ). Prior to placing
it in the humidity cabinet, however, the equipment must be warmed to a temperature
between
t
and
t
20 to
4°C.
The measurement should then be carried out 1 hour after the end of the humidity pre-
conditioning treatment. Throughout this waiting period and during testing, the same tem-
perature
t
must be maintained, but the relative humidity of the environment must only
be 45 to 65%. Testing should be performed with the equipment's on-off
switch in both
conditions while connected to a power supply set at 110% of the maximum rated supply
voltage. When operational, the maximum rated load must be used. As mentioned in the
fi
ff
first part of this chapter, allowable patient leakage and auxiliary currents are de
fi
ned for
both normal and single-fault conditions.
Single-fault conditions
are de
ned as conditions
in which a single means of protection against a safety hazard in the equipment is defective
or a single external abnormal condition is present. Speci
fi
c single-fault conditions that
must be simulated during testing include interruption of the supply by opening the neutral
conductor as well as the interruption of the protective ground conductor. Patient leakage
current between an F-type applied part and ground assumes that an external voltage equal
to 110% of the maximum rated supply voltage is in direct connection with the applied part.
For battery-powered equipment, the external voltage that is assumed to be connected to the
F-type applied part is 250 V.
Leakage current tests are conducted as shown in Figure 3.24
a
-
d
, with the device's
power switch in the on and off
fi
ff
conditions and creating the single-fault conditions speci
fi
ed
in the
figure. If the enclosure or a part thereof is made of insulating material, a piece of
metal foil 20 cm
fi
10 cm applied to the nonconductive part of the enclosure must be used
as the protectively grounded enclosure connection. The metal foil is wrapped on the sur-
face of the insulating enclosure, simulating the way in which a human hand could act as a
capacitively coupled electrode.
The connections for measuring patient auxiliary currents are shown in Figure 3.24
e
.
Here, the current
flowing between each patient connection and every other patient con-
nection is measured under normal and single-fault conditions. For this test, the measuring
instrument should be capable of di
fl
erentiating the ac components from the dc components
of the RMS current reading. As you can see from Table 3.2, di
ff
ff
erent ac and dc auxiliary
current levels are permitted to
fl
flow through the patient, depending on the use intended for
the equipment.
Versatile Microammeter
Figures 3.25 to 3.28 present the schematic diagrams of a versatile instrument for the meas-
urement of leakage and auxiliary currents. The core of the circuit is an AAMI load that
converts a leakage or auxiliary current into a voltage waveform with a factor of 1 V/mA
Search WWH ::
Custom Search