Overcoming patient leakage current issues

January 27, 2015 //By Dante De Guzman
Overcoming patient leakage current issues
Dante De Guzman of XP Power explains the meaning of patient leakage current for medical power supplies and describes several ways of reducing this leakage current for the most stringent medical applications.

IEC 60601 is the generally accepted standard for medical electrical and electronic equipment, required for the commercialization 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:

Design category: 

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