The leakage current is the current flowing into the ground through the earth conductor. In ungrounded situations, if there is a conductive path (such as human body), the current may flow from the surface of the conductive component or the non-conductive part to the ground. An external current is always present in the safety grounding conductor. Usually, the upper limit of the leakage current of medical power is 1/10 of the common power supply.
All power leakage current indicators of the IEC601 standard are far more stringent than non medical power supplies. The technical specifications define several different and critical leakage currents, such as the drain current (along the ground into the ground) and the leakage current of the casing. In the IEC601 standard, the maximum leakage current of the medical power supply of the three main types of equipment is defined differently:
Class B: equipment that is not in contact with the patient, such as a laser therapy apparatus.
Class BF: equipment that need to contact the patient, such as ultrasound, various monitors (including EGC equipment), and operating table.
Class CF: equipment to contact the patient’s heart, such as a cardiac puncture monitor.
It is often misunderstood that leakage current indicators are different for these devices. In fact, the leakage currents allowed for these devices are the same. The North American index requires more stringent leakage currents than the European EN60601-1 provides. Europe, for example, allows 0.5mA, while the United States and Canada allow only 0.3mA. As a result, medical device designers need to be aware of where their products will be sold.
And BF or CF devices (commonly known as “contact with the body” equipment) also require additional insulation measures to enable the patient and the ground, signal port and power output insulation. This is to protect the patient from accidental failures of the equipment and to keep the leakage current within the standard limits. This insulation can also be achieved through other parts of the final equipment, such as plastic probes or bushings with sufficient insulation. But in the case of the need to electrify the patient, one of the treatment methods is to use the DC/DC power supply according to the IEC601-1 standard to supply power to add second level insulation protection. Therefore, the DC/DC converter must be carefully selected to ensure that the insulation requirements are met.
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