DESIGN OF SAFE MEDICAL DEVICE PROTOTYPES - Design and Development of Medical Electronic Instrumentation - page 126

Figure 3.19 Ohmic Instruments' Iso-Switch patient-lead fault interruptors are two-lead semicon-
ductor devices that can be placed, almost transparently, in series with every patient connection to
break the patient circuit in case an overcurrent fault develops. Various models are available for
different applications, with trip currents ranging from 1 A to 100 A. Various operating volt-
age ranges are also offered: 325 V for 115-V ac instruments, 720 V for 220-V ac equipment, and
the DP (defibrillation-proof ) series that withstands pulses of 5 kV for 10 ms.

IF

(µΑ)
9.496

1.582
/div

0

-6.330

0

-2.000 2.000

VF: 0.4V/ div.

Figure 3.20 V-I plot of an IS-10 Iso-Switch patient-lead fault interruptor. Between 7.9 and
7.9 A (rated 10 A), the device acts as a 40-k resistor. Beyond the trip point, the resistance

of the Iso-Switch increases to approximately 1000 M at the maximum absolute operating voltage
of 325 V.

approximately 1000 M at the maximum absolute operating voltage for the device. Once

the overload is removed, the device resets itself automatically.
Various Iso-Switch models are available for di ff erent applications, with trip currents

ranging from 1 A to 100 A. Various operating voltage ranges are also o ff ered:

325 V for 115-V ac instruments and 720 V for 220-V ac equipment; and the DP series