Electrical equipment used in medical technology must not place patients or medical staff in danger. This, in turn, requires that designing safe equipment starts at the point where the power is supplied. Power connectors and power entry modules with or without a powerline filter must fulfill the requirements of the base standard for medical electrical equipment, IEC/UL 60601-1.
Medical technology uses a wide range of electrical equipment. During normal operation and in the event of a malfunction, it is imperative that the equipment does not pose any danger to patients or medical staff. A piece of equipment that causes a short circuit or residual current can trigger a protective system upstream and, in doing so, shut down other, possibly life sustaining equipment. Thus, it is necessary to pay special attention to how each unit is supplied with power.
Connectors and power entry modules used in the equipment are tested to component standards and, if in compliance with these standards, can generally be used in most equipment without additional testing. This is especially true for medical equipment, where extensive requirements regarding safety are in effect, as defined in the base standard for medical electrical equipment, IEC/UL 60601-1. In addition, there are a wide range of specific requirements for each category of equipment according to IEC 60601-x-xx. The IEC standards for medical electrical equipment are harmonized with UL so that the same requirements are also in effect for equipment in North America.
Medical devices can be connected directly to the mains or with a detachable power cord. Such plug connections must meet the requirements of Standard IEC 60320. Depending on the application, it is recommended to include a mechanism to protect against any unintentional removal of the plug from the equipment’s power socket. The most common type of protection against inadvertent disconnection is a cord-retaining bracket. Depending on the type of equipment and because of the many types and shapes of power sockets, it is important to select the correct bracket shape. There are cord-retaining brackets for most power sockets.
In addition, SCHURTER offers a VLock cord retaining solution, a simple alternative system to using brackets. In the V-Lock, the power socket is equipped with a notch that interlocks with a special latch on the connector on the power cord. The system prevents the cord from being pulled out of the socket unintentionally. The advantage of this is that no cord-retaining system specific to a unit’s power socket or retaining bracket must be adapted and attached. A “hospital grade” cord set is available for medical applications in North America. (See Figure 1)
Permanently installed medical electrical equipment comes with its own fuses. If there is a failure in a piece of equipment, a fuse prevents that unit from tripping the circuit breaker and disconnecting other, sometimes life-critical devices, from the mains. Equipment manufacturers must ensure that energized supply leads are protected by a fuse. A few power distribution networks, however, are not polarized. That means that the power plug can be inserted in such a way that the energized conductor can be on either the plug’s left or right pole (e.g., Schuko plugs in Germany). As a result, the equipment fuse could be protecting the neutral conductor, which results in no protection against a short circuit to earth. (See Figure 2)
Therefore, it is recommended that all medical electrical equipment be equipped with a double pole fuse holder. It must further be ensured that only authorized personnel can remove or replace fuses. The basic standard for medical electrical equipment, IEC/UL 60601-1, specifies fuse holders that can only be opened up with the help of a tool.
The line switch at the power feed has an important function. Just as with fuse holders, it must be ensured that the equipment is completely disconnected from the power network after being turned off. If only one pole in a nonpolarized power distribution network is interrupted, the equipment could still be live. Thus, it is preferable to use a switch that disconnects the power feed on two poles. (IEC 60601-1: 8.11.1)
Power Line Filters
The power line filter is a central protective element. It protects equipment from external high-frequency (HF) interference and reduces the HF interference radiated by the equipment. A power line filter is often necessary in order to fulfill the EMC standards for CE declaration of conformity. Because power line filters can be under constant voltage, they are subject to strict requirements:
Leakage currents. Capacitors wired between the power supply lines and earth within the power line filter are known as Y capacitors. They produce a leakage current to earth whose value depends on the component’s capacitance and the powerline frequency. This fault current can, if too large, trigger the fault interrupter whereby the power supply is shut off. Thus, touch leakage current for medical electrical equipment may not exceed 100 A under normal conditions, and for devices with direct patient contact, it even drops down to just 10 A. As a result, power line filters for medical electric equipment do not have any Y capacitors, which results in a maximum leakage current of 5 A, or they have only very small Y capacitors such as of 470 pF, which results in a peak leakage current of 80 A. Here it is important to note that additional components in the equipment can cause leakage currents, so the leakage current for the entire unit must always be measured. (IEC 60601-1: 8.7.3) Figure 3 illustrates a circuit diagram of a medical filter.
Capacitors. The capacitors used in power-line filters must meet strict criteria. They are wired directly between the energized conductors (X capacitors) or between the energized conductors and earth (Y capacitors). In addition, they are constantly under line voltage without any additional protection. Common equipment standards such as IEC 60950 (IT equipment) require that capacitors between a phase conductor and the neutral conductor be at least Class X2, and capacitors between phases/neutral and the earth conductor be at least Class Y2. For medical electrical equipment without direct patient contact (MOOP), Class Y2 capacitors to earth are sufficient; for equipment with direct patient contact (MOPP), Y1 capacitors are required. (IEC 60601-1: 8.5.1)
Capacitors store energy corresponding to their value of capacitance. If you pull a power cable out of a power socket, the stored voltage from the X capacitors is still present on the power pins. If touched, they can result in an electrical shock. Thus, medical electrical equipment with detachable power connectors may only have a maximum of 60V on the touchable plug pins one second after the plug has been pulled. To achieve this, a bleeder resistor is switched in between the phase and neutral conductors, and it discharges the capacitor in a very short time. (IEC 60601-1: 8.4.3)
Dielectric Strength. Increased dielectric strength is required for medical applications. An important safety aspect for achieving high dielectric strength consists of the distances between voltages. The base standard for medical electrical equipment, IEC/UL 60601-1, specifies air and creepage distances of at least 3 mm between energized parts of different polarities and 4 mm between energized conductors and earth. Care is required here because various UL standards allow a smaller separation in these distances than does the IEC standards.
A power socket compliant with IEC 60320 fulfills medical requirements, while sockets compliant with UL 498 do not have sufficient air and creepage distances to meet the requirements of IEC. The UL 498 standard for plugs and sockets specifies only a 1.2 mm separation between energized parts of different polarities at a rated voltage of 250 VAC max. In contrast, for the same components, IEC 60320 requires 3 mm as does the base standard for medical electrical equipment IEC/UL 60601-1. Power sockets as well as power entry modules with and without a filter fulfill the IEC/UL 60601- 1 requirements, making them qualified for use in medical applications. This means that by meeting the requirements of the standards, the components can generally be used without any additional testing.
In the case of power line filters, voltage separations inside the devices must also be observed. Here, too, the base standard for medical electrical devices specifies 3 mm between the energized phase conductor (L) and the neutral conductor (N), and it must be 4 mm to earth. According to base standards for medical electrical equipment, these air and creepage distances can be reduced to some extent if the filter is potted. (IEC 60601-1: 188.8.131.52) Figure 4 shows a diagram of minimum air creepage.
As a safety test, the base standard for medical electrical equipment specifies a high-voltage test of 1.5 kVAC between L and N as well as between L/N and earth (PE). To PE, the 1.5 kVAC corresponds to the values of the filter standard and can be met without any problem. However, a filter between L and N should not be tested with 1.5 kVAC because its capacitors could be damaged. Therefore, here IEC / UL 60601-1 makes an exception for interference suppression capacitors, which can be removed for a high-voltage test if they have been tested to IEC 60384-14. If capacitors in power line filters are tested to IEC 60384-14, then no additional high-voltage testing is necessary. (IEC 60601-1: 8.8.3)
Following fabrication, SCHURTER subjects its power line filters to a rigorous high-voltage test of at least 1.5 kVAC between L/N and PE; filters of Protection Class 2 are even tested to levels of 4 kVDC between the energized conductors L/N and the housing. Such filters and connectors can be found in medical electrical equipment intended for home use that have power cables without an earth conductor, such as specified in IEC 60601-1-11.
Electrical equipment used in medical technology must fulfill high requirements in terms of safety. This protects both the patient and medical staff. This protection begins with the safe supply of electric power.
This article was written by Diane Cupples, Vice President of Marketing, SCHURTER, Inc., Santa Rosa, CA. For more information, Click Here