Leakage Currents: A Critical Parameter in Medical Power Supplies

By Thomas O’Brien, Field Application Engineer at Advanced Energy

Though invisible, electricity is ubiquitous in every facet of life and nowhere more critical than the field of healthcare, where research is accelerating, and more advanced devices and medical equipment are launching to market at a rapid pace. To ensure patient safety, medical devices have some of the tightest power supply requirements of any industry to minimize the hazard of leakage current—the flow of electric current in an unwanted conductive path under normal operating conditions.

The human body is extremely sensitive to the effects of leakage current, which can result in several adverse physiological effects depending on the nature of the current, its intensity and duration of contact. These range from muscle contraction, cramps and burns to respiratory arrest or ventricular fibrillation. Since the consequences of leakage current came to the forefront, IEC 60601-1 has served as the technical standard for the safety and effectiveness of medical electrical equipment. First published in 1977 by the International Electrotechnical Commission (IEC), IEC 60601-1 consists of a general standard, approximately 10 collateral standards and about 60 specific standards that have been updated and restructured over time. The current standards, which have been updated, define maximum leakage current under normal and single-fault conditions that can occur on the finished design of medical equipment used in emergency, hospital and operating rooms, as well as patient homes. 

Means of Protection (MOP)

Medical equipment that is powered by a mains voltage will of course have a hazardous voltage somewhere in the system. Isolation barriers are physical separations between electrical circuits that prevent the transmission of DC and unwanted AC between two circuits. They are used to separate hazardous voltages from surfaces that may come in contact with the patient during treatment (referred to as accessible parts) or must come in contact with the patient (applied parts). 

60601-1 sets certain minimum requirements for the physical separation of isolation barriers related to creepage, clearance and dielectric strength. When these three requirements are met, the barrier can be considered a Means of Protection (MOP).

There are two types of MOP defined in 60601-1:

1. Means of Operator Protection (MOOP) are designed to protect the operator from hazardous voltages.
2. Means of Patient Protection (MOPP) are designed to protect the patient from hazardous voltages. 

The requirements of a MOPP are more stringent than MOOP given that a patient is considered weaker and at greater risk of harm than the operator of the equipment. For medical power supplies, isolation barriers are usually designed to comply with the requirements of a MOPP so that both the operator and patient are protected.

MOPs and Leakage Current

The reason why MOPs are important in the leakage current discussion is detailed in the fundamental principles of electrical safety, which states that medical equipment must be safe both in Normal Condition (NC) and Single Fault Condition. Understanding what both mean is critical when it comes to safety testing:

• Normal Condition (NC): For Normal Condition testing, the safety engineer is permitted to breach any barrier that does not meet the requirements of a MOP, and the system must meet the Leakage Current limits for Normal Condition operation.

• Single Fault Condition (SFC): For Single Fault Condition testing, the safety engineer is permitted to breach any single barrier (but only one) that meets the requirements of a MOP, and the system must meet the Leakage current limits for Single Fault Conditions.

Single Fault testing means that medical equipment must always have two MOP between hazardous voltages and accessible parts so that even if a single MOP fails, there is always a backup to keep the operator or patient safe from hazardous voltages and leakage currents. These two MOPs can be implemented as two separate isolation barriers but are usually implemented with a single isolation barrier whose physical separation is large enough to represent the same risk to the patient or operator as two MOPs. This is why many power supply data sheets include an isolation rating of 4 kVac from input to output. 4 kVac is the dielectric strength needed for 2 MOPs for a working voltage of 240 Vac (Mains Voltage).

Types of Leakage Currents

60601-1 defines four types of leakage current (defined by the path the leakage current may take) and sets acceptable limits on them:

• Earth Leakage Current – Current that flows from the mains connected part (or primary) to Earth through the protective earth conductor of earthed equipment. 

• Touch Current – Current that could flow from the enclosure through an external path (other than the protective earth conductor) to Earth or another part of the enclosure.

• Patient Leakage Current – Current that flows through a patient connection or applied part to Earth but plays no role in the medical treatment.

For Type BF and Type CF floating applied parts, we must also consider the possibility that the patient is connected to multiple medical devices, any of which may fail to result in mains voltage coming in contact with the patient. A Single Fault Patient Leakage current limit is also specified for this eventuality.

• Auxiliary Patient Leakage Current A non-functional current that flows through multiple applied parts in medical equipment (flows out one applied part, through the patient, and back through another applied part). 

Safety testing varies with each type of leakage current. Studies have shown that the risk of ventricular fibrillation is highest for AC current frequencies from 10 to 200 Hz. The risk is slightly reduced at 1000 Hz, and then rapidly decreases for frequencies higher than 1000 Hz. When Leakage Currents are measured, a special measurement device that gives a higher weighting to lower AC current frequencies is used to represent the human body.

Given the increased use of sensitive analog electronics, wireless technologies and microprocessors in medical devices, it is critical that, during the manufacturing process, power supplies meet customer design specifics and essential safety standards, including electromagnetic compatibility (EMC) compliance, and appropriate isolation barriers. For example, providers of cosmetic procedures using medical-grade lasers will likely require high-power density and reliability incorporated into a portable power supply that has longevity, as well as black box recording capabilities and connectivity and firmware that is easily upgradable. In order to avoid electrical shock, manufacturers must deliver a power supply that satisfies all requirements.

Advances in power supplies alone have been instrumental in improving not just the safety of surgical procedures but the results as well. Take knee replacements, for example. Since the late 1960s, when the first surgery was performed, knee replacements have become one of the most successful medical procedures. According to Ortho Info, more than 600,000 are performed each year in the United States, with the numbers expected to dramatically increase. Many surgeons now operate with the help of 3D models that are fed into the power supply. If the surgeon is on the verge of removing too much or too little of the knee in relation to the 3D model, the power supply can be programmed to shut off immediately. This approach has boosted success from 50 percent to 90 percent or higher. Advanced power supplies have also enabled life-saving thermal solutions such as helmets for newborns that have automatic and interval temperature adjustment capabilities to prevent brain damage. 

According to Precedence Research, the global medical devices market size is expected to reach US$671.49 billion by 2027 (a CAGR of 5.2% from 2020 to 2027). As more advanced devices and equipment launch to market, flexibility, control and performance in power supplies—from radio frequency (RF) generators for electrosurgical equipment, thermal solutions for heating and cooling, precision imaging equipment delivering detailed and real-time images, and lasers for surgery and aesthetic treatments—will be key to reducing cost, complexity and human error.

About the author: Tommy O’Brien is the Lead Field Applications Engineer for Advanced Energy with over a decade of successful experience working alongside design engineers to support Advanced Energy power solutions in medical and industrial applications.

Tommy holds a degree in Electronic Engineering from University College Galway and a Masters degree in Sustainable Energy from University College Cork.