141 / 2663 CIRCUIT DESCRIPTION
Art. No.: 80116-201
09 / 2004
an inductivity of the coils results which limits the high-frequency leakage current with this inductive
impedance.
Monitoring the high-frequency leakage currents
As intended, the high-frequency current should only be used for cutting or coagulating tissue. To do this, it
must flow to the active electrode in a small area and with a high current density, and flow back again from
the neutral electrode over a large area and with a low current density.
However, if the body of the patient is in contact with another electrically conductive object, the current may
also take an undesirable path and, for example, cause a burn there. These undesirable currents are known as
“leakage currents”. It is therefore important to know whether such a leakage current is flowing and how
large it is.
Such leakage current cannot simply disappear, but must instead always flow back to the generator. The
largest proportion of these high-frequency leakage currents flow back to the generator via the power cable
and the grounded conductor.
Since all power lines and the grounded conductor are directed through the ring core of the coil L1, a
corresponding high frequency voltage is induced if HF leakage current is present in the coil L1, which is
rectified with diode D1, restricted with D6, D7 and smoothed in capacitor C13. For further processing, this
DC voltage is fed to the processor system via the isolation amplifier IC12.
Changeover between flowing and capacitance-grounded output
High-frequency surgical units are generally available either in “capacitance ground of the neutral electrode”
or “floating neutral electrode” connection systems, whereby no components are integrated between the
connection to the neutral electrode and chassis in the latter connection system.
Both connection systems have advantages and disadvantages.
The advantage of capacitance ground is that the feed line from the neutral electrode to the patient is grounded
at a high frequency via the capacitor and the risk of burns is reduced. However, it is a disadvantage that
impermissible low-frequency leakage currents may flow through this capacitor during cardiac operations.
If the neutral electrode is operated as floating, on the other hand, the low-frequency leakage currents can be
kept very easily below the limit value of 10 or 50 mA during cardiac operations, but the risk of burns is
increased in this connection system since the line from the neutral electrode is about half the high frequency
voltage of the generator.
With the ICC high-frequency surgical units, the low-frequency leakage current to the patient is measured.
At the first malfunction, this must not exceed 50 mA. Once this condition is ensured, the unit is permitted
to carry the CF designation (Cardiac Floating). If the possible low-frequency leakage current is under 500
mA in the first case of malfunction, the unit is therefore not suitable for operations on the heart and has the
designation BF (Body Floating).
Initially the ICC is capacitance-grounded via capacitors C30 and C31 and the relay contact REL 2. The
advantages of capacitance ground are fully exploited. The low-frequency leakage current is measured via
the prescribed simulation R25, C28. If for any reason a low-frequency leakage current should appear, it is
then evaluated. If it exceeds the limit of 50 mA, the relay contacts REL 2 are then opened and thus a low-
frequency leakage current can no longer flow. The ICC thus fulfills the CF requirement for cardiac surgery
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