Safeguarding Medical Equipment with Medical-Grade Power Supplies

Medical systems require various suitability ratings (isolation voltage ratings) to be met by the power supplies being integrated into those systems. These suitability ratings range from least to most stringent. Least being “B” suitable and “CF” suitable being the most stringent.

Let's begin by first covering the standard that defines the parameters. The IEC60601-1 medical standard identifies the term “Applied Part” – which refers to a part of the medical device that has the potential to come into direct physical contact with a patient during normal operating conditions. There are three classifications with their own respective restrictions due to safety concerns. The three classifications are as follows: Type B “Body”, Type BF “Body Floating” and Type CF “Cardiac Floating”. These classifications are with respect to the type of contact that can be expected with the patient body.

Understanding the Differences in Power Supply Classifications

The IEC 60601-1 standard introduces classifications based on the level of patient contact and associated safety requirements: Type B, Type BF, and Type CF. Each type represents a different level of isolation and protection, crucial in preventing electrical shocks.

Type B (Body) – This classification is the least stringent of the three. Type B suitable power supplies may be connected to an earth ground (non-floating), but are intended for applications where the device being powered does not have conductive contact with the patient and can be released immediately should the situation require it. Some examples include: medical laser equipment, MRI machines and hospital beds.

Type BF (Body Floating) – This classification is the most common because it covers many applications for medical devices in reference to patient safety. Type BF suitable power supplies are not connected to earth ground. These power supplies are considered to be “floating.” Type BF suitable power supplies will also have increased isolation voltage requirements to provide additional levels of protection against electrical shock. Type BF suitable power supplies will be powering devices that will have conductive contact with the patient, or where the duration can be for prolonged periods of time. Examples of equipment requiring BF suitable power supplies are: EKG monitors, ultrasound imaging devices, and digital blood pressure cuffs.

Type CF (Cardiac Floating) – This classification is the most stringent as CF suitable power supplies are powering devices that come into direct contact with cardiac tissue and/or the bloodstream. Type CF suitable power supplies are not connected to earth ground. These power supplies are considered to be “floating”. Type CF suitable power supplies will also have increased isolation voltage requirements to provide an additional layer of protection against electrical shock. Type CF suitable power supplies will be powering devices such as: surgical equipment, defibrillators, heart-supporting systems and dialysis machines.

Now that we have briefly touched on the three suitability classifications – we will discuss the shock protection classifications and leakage current thresholds. 

Navigating the Means of Protection

When exploring medical power supplies – you will likely come across designations defining Means of Protection (MOP). These designations refer to the protective isolation that prevents the equipment operator or patient from coming into contact with lethal mains voltages of the power supply.

The medical standard not only defines isolation parameters, but also creepage distances (the shortest distance along the surface of the insulating medium between two conductive parts).

Two distinct protection classifications with respect to the Means of Protection relate to the intended protected asset. They are Means of Patient Protection (MOPP) and Means of Operator Protection (MOOP). A MOPP rating is to protect the patient and MOOP rating is to protect the operator. MOPP classifications are more stringent than their MOOP counterparts – however – they are intended for different applications.

The following are the IEC/EN 60601-1 isolation voltages, creepage distances and insulation requirements for the following MOOP and MOPP classification

Means of Protection Versus Isolation/Creepage

Applied Parts Classification Verses Isolation and Insulation

The Role of Leakage Current

When exploring a medical power supply for your application – leakage current will be a critical parameter to pay particular attention to. It is important to understand that leakage current is caused by the capacitive coupling as a result of the transformers and filtering capacitors used in the power supply to attenuate electromagnetic interference. The following chart defines the levels of leakage current for both normal conditions (NC) and single fault conditions (SFC).

Explanation of Different Leakage Current Types

Earth Leakage Current: The current that flows from the protective earth of the medical equipment through the patient (in a test environment – the measuring device) back through to the protective earth of the power connection.

Enclosure Leakage Current: Also known as “touch current.” The current that flows from an exposed conductive surface of the device enclosure through a conductor that is not the protective earth conductor.

Patient Leakage Current: The current that flows through a patient connected to an “applied part (or parts)” to earth. This can also be the current that flows through the patient from a high potential source to earth.

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