No. 2A201-519EN*A
38
8.1.3
Ground fault interrupter
8.1.3.1 Leakage
current
(1) Leakage current based on the IEC standards
The leakage current test was conducted according to the IEC standards and it was
confirmed that the results completely satisfy the standards.
International standards and
test methods
IEC60601-1 Medical electrical equipment
19 Continuous leakage current and patient auxiliary
currents
Allowable value
Earth leakage current (note 3) 5 mA
Conditions
After the humidity preconditioning treatment, the
leakage current during scanning was measured at an
input voltage of 200 V
±
10%.
This test is designed to simulate the worst conditions, i.e., the case where all the
leakage currents flow through the body of a person touching the system with the ground
cable disconnected.
When the leakage current measurement test is performed, the circuit shown in the
figure below is added to the grounding cable of the system.
The circuit simulates the resistance of the human body and its insensitivity to high-
frequency electromagnetic radiation. As the result, the high frequency of 10 kHz is
reduced by approximately a factor of ten.
Therefore, the current may differ from that running through the ground cable in normal
use.
R1 (10k)
R2 (1k)
C1 (0.015 F)
CT system
Ground cable
Measuring device
(2) Leakage current in normal use
In addition to the standard test, the current of the ground cable was measured during
normal use. It was seen that the current waveform included a noise waveform.
However, frequency analysis showed that the level near the frequency of 50 Hz, which
is of most relevance to safety considerations, was less than 10 mA; and the main
frequency element was a high frequency element of more than 10 kHz.
CT systems include a large-capacity inverter and the operation frequency of the inverter
is of the order of tens of kHz. Therefore, noise with a frequency of the same order
occurs within the inverter. To prevent this noise element from coming out, a noise filter
is installed. As this noise filter transfers the noise element to the ground cable, the high-
frequency leakage current increases.
