Designing for BF and CF classifications to deliver medical safety and performance

This article originally appeared in the Nov'23 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.

The IEC 60601-1 safety specification classifies medical electrical equipment as class I, class II, or class III. Class

I refers to products that have a reliable protective earth that prevents any accessible metal parts from becoming live in the event of a failure of basic insulation. This protects against electric shock and applies to low-risk devices such as bandages, handheld surgical equipment (e,g, scalpels) and non-electric wheelchairs. A class II product has no protective earth and relies on double or reinforced insulation, such as a plastic enclosure for protection. Class II devices are intermediate-risk devices such as tomography (CT) scanners, infusion pumps and patient monitors. Class III devices are high-risk devices, e.g. pacemakers and deep-brain stimulators.

Categories of patient contact

The specification also defines body contact categories. Type B (body), is when the applied part makes physical contact but is generally not conductive and can be immediately released from the patient, e.g. an ultrasound scanner probe.

Figure 1. Patient leakage current example

Type BF (body float)is where electrical contact is made, although not near the patient’s heart. Typical type BF examples include the electrodes of equipment such as electrocardiography, electroencephalography, electromyography, and transcutaneous electrical nerve stimulation (TENS) equipment.

Type CF (cardiac float) is the third category and is where the applied part is intended to make electrical contact close to the patient’s heart, for example cardiac catheter electrodes.

Essentially, type B applied parts may be connected to earth, while type BF and type CF are floating and must not be connected to the earth.

Power supply selection

To certify a power supply it must fulfil two requirements related to leakage current and isolation between its secondary output and protective earth.

In an AC-connected power supply, leakage currents are typically associated with capacitive coupling across power transformers and through Y-class filter capacitors, which are connected to satisfy EMC regulations.

The value of leakage currents permitted to flow from the equipment, through the patient, to earth are shown in Figure 1.

Current value is mostly determined by the main transformer capacitance and parasitic capacitances.

Figure 2. Patient leakage current via A connection

When designing patient-connected medical equipment, it is important to know whether applied parts should be floating or earthed to ensure regulatory compliance and optimise the price-performance ratio.

One thing to consider is if the equipment is intended to be used in isolation, especially if the requirement for an F class or type B is not explicitly stated in the IEC 60601-2-XX sub-standard. Type B applied parts can offer an acceptable level of safety in cases where the patient is not intended to be connected to other electrical equipment.

F type applied parts offer an advantage in situations where multiple pieces of medical electrical equipment are being used at the same time or if there it a possibility for the patient to come in contact with a device that is not medically approved. These are scenarios where there is a higher risk of the patient coming in contact with mains voltages.

Type B applied parts can offer an adequate level of safety where additional protection is provided in the electrical installation using isolated main supplies and residual current devices (RCDs). There may be some applications in which F type applied parts can be provided without technical difficulty or excessive cost.

Figure 3. Patient auxiliary current, or measurement current

Means of protection

There are two key aspects of the means of protection criteria - means of patient protection (MOPP) and means of operator protection (MOOP).

A basic principle is to provide two levels of protection in case one should fail. In medical equipment (2x MOOP + 2x MOPP).

These can be split across several aspects of the power architecture, such as the AC/ DC power supply and isolated DC/DC converters.

Medical power supplies should also be designed to ensure long-term reliability and may also be ruggedised to survive harsh operating conditions and be energy efficient to minimise reliance on active cooling using fans. IEC 60601 documents also include specifications on EMC which can be a particular challenge, because reducing leakage in line with BF and CF requirements may preclude connecting filters on inputs.

Due to so many regulations and design considerations, the design of many medical applications requires some level of power supply customisation through modification of standard parts, the use of modular, configurable units or the development of full, turnkey solutions.

Figure 4. Power supplies that support BF and CF ratings must also offer long-term reliability