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Addressing patient leakage current issues

11 Feb 2015  | Dante De Guzman

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IEC 60601 is the generally accepted standard for medical electrical and electronic equipment, required for the commercialisation of this type of equipment in many countries. To guard against electric shock, the specification defines maximum leakage currents for equipment with an applied part, that is, a part of the equipment that comes into contact with the patient under normal operating conditions.

There are four types of applied part as defined in the standard. Type B (body) means applied parts that are not normally conductive and can be immediately released from the patient, including everything from MRI scanners to hospital beds and lighting. Type B is the least stringent classification. Type BF (body floating) and CF (cardiac floating) are floating with respect to earth. Type BF parts have conductive contact with the patient, or medium to long term contact with the patient, like ultrasound equipment and blood pressure monitors. Type CF parts have the most stringent classification as they are parts that may come into direct contact with the heart, like dialysis machines.

There are also three types of leakage current, each with their own defined limit in the standard: earth leakage current, enclosure leakage current, patient leakage current and patient auxiliary current. The leakage current type that is typically the hardest to meet is the patient leakage current, that is, the current flowing from the applied part via the patient to earth as a result of an unintended voltage from an external source on the patient. For the third edition of the standard, the limit is 100 uA for B & BF and 10 uA for CF.

In power supplies, the patient leakage current is defined as the amount of leakage current flowing from the output through a 1 kΩ impedance to ground (which represents the patient). This is directly proportional to amount of capacitance between the mains and the output (input to output capacitance), which is dependent on two things: the interwinding capacitance of the transformer and the Y-capacitors bridging the primary side to the secondary side (bridging capacitors). Reducing the input to output capacitance is therefore desirable, but it's not possible to remove it entirely because it is required for patient safety by the standard.

Input to output capacitance
Let's consider the input to output capacitance of a typical medical power supply. Figure 1 is a simple representation of such a device, with a leakage current path shown by the dotted line. The capacitance between the primary side and the secondary side of the transformer comprises the bridging capacitance (C4) and the transformer's interwinding capacitance. If C4 increases, the impedance of this current path decreases; if the impedance decreases, more current will flow through point G2. This current constitutes the patient leakage current. Note that C1 and C3 also create a path for current leakage – this is the earth leakage current rather than the patient leakage current.

Figure 1: A simple representation of a medical power supply. In the absence of any bridging capacitors, the dominant path for the patient leakage current is through the transformer (marked by dotted line).

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