Biomedical Engineering Reference
In-Depth Information
In addition, SELV transformers for medical equipment usually have an electrostatic
shield that is tightly wound over the insulation of the primary windings. This shield
reduces capacitive coupling between primary and secondary windings, thus reducing leak-
age currents at the power line frequency. The shield is coated with reinforced insulation to
create a reinforced insulation barrier between the primary and secondary windings. The
core itself is isolated from the windings by supplementary or reinforced insulation.
Another convenient alternative for powering medical instruments is the use of batter-
ies. This substitution not only ensures inherently low leakage currents but can make the
equipment highly portable. Considering that you may need to travel all the way to South
America, Eastern Europe, or Asia to run your
first tests, the independence provided by a
battery-operated power supply is certainly a welcome blessing for an evaluation proto-
type. Whatever the choice in power supply, it is generally a good idea to purchase it as
an approved OEM (original equipment manufacturer) assembly. This helps you concen-
trate your e
fi
orts on the core of your instrument rather than having to deal with the
headaches of designing and constructing supplies that perform as required by the safety
standards.
Along the same philosophical lines, designing an instrument to make use of preap-
proved components can help considerably to receive and maintain safety approval once
you embark on the production and sale of a medical product. You can still use components
that have not been certi
ff
ed by a U.S. Nationally Recognized Testing Laboratory (NRTL,
or its equivalent in other countries); however, the assured continuity of safety performance
will have to be investigated for each device to be used. This is complicated further by the
fact that once you receive approval for your product, any change in any component will
require requali
fi
cation of the complete assembly. Finally, keep in mind that safety stan-
dards usually impose special performance characteristics for certain components, such as
power cords, switches, line
fi
fi
filters, fuse holders, optoisolators, CRTs and displays, and
printed circuit boards.
ADDITIONAL PROTECTION
Regardless of how carefully you designed your instrument, absolute safety cannot be guar-
anteed in the real world. Despite all the safety testing and evaluation required by the FDA,
medical device manufacturers still pay a premium for insurance to protect themselves from
exposure against liability. For this reason, it often happens that additional or redundant
hardware to ensure safety beyond the minimum requirements is cost-e
ff
ective, since it will
bring concomitant savings in insurance costs due to reduced risk.
Being extra conservative is especially important at the prototyping stage, since as an
entrepreneur you probably do not have the legal and
financial umbrella of a large cor-
poration to protect you against an unintentional mishap. Our personal preference is to
introduce, at the very least, an additional but independent layer of protection against
electrical shock at the patient interface. A very practical method to accomplish this is to
use Ohmic Instruments' Iso-Switch patient-lead fault interruptors. These devices, shown
in Figure 3.19, are two-lead semiconductor devices that can be placed, almost transpar-
ently, in series with every patient connection to break the patient circuit in case an over-
current fault develops.
As shown in the V-I plot of Figure 3.20, an Iso-Switch patient-lead fault interruptor
rated at
fi
resistor. Once the trip point of the Iso-Switch is exceeded,
the device presents a negative-slope resistance of magnitude equal to that of the positive
slope within the trip boundaries. The trip time under an overcurrent condition is very
fast, typically 10
10
A acts as a 40-k
s. Once the device trips, the resistance of the Iso-Switch increases to
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