A brief discussion on the electrical design of large medical equipment

Abstract: Taking the PLA General Hospital project as an example, this paper introduces some experience in the in-depth design of electrical equipment for large medical equipment, pointing out that the key to design lies in communication and exchange with professional engineers from equipment manufacturers, correct understanding of English materials, and true mastery of the installation process and working principle of the equipment.

After two years of construction, the Tumor Center Building of the General Hospital of the Chinese People's Liberation Army was finally completed and put into use. It is currently the most advanced tumor treatment center in China and even in Asia. It has 26 sets of large medical equipment (cyclotron, tomographic accelerator, DSA, PET/CT, DR, high-frequency thermal therapy machine, simulation positioning machine, after-installation, etc.) in a single building with a construction area of ​​35,000 square meters, with two underground floors and 15 floors above ground. Among them, 10 equipment are introduced for the first time in China. As the main designer of the electrical major of this project, each equipment is accompanied by the steps of cooperation, communication, design, confirmation, redesign, drawing construction, cooperation, and debugging with many international equipment manufacturers. Although the process is difficult, it is fruitful. Seeing the scenes of equipment being put into use one after another and receiving patients to serve the society, the feeling is beyond words. I just want to summarize some of my technical gains in cooperating with the in-depth design for reference by my peers.

The electrical design requirements of large medical equipment will vary from manufacturer to manufacturer. The key to the design lies in communication with the professional engineers of the equipment manufacturer, correct understanding of English materials, and a true understanding of the installation process and working principle of the equipment. The following is a discussion of the technical key points and difficulties in the design.

1. On-site distribution box

The equipment host must be equipped with a dedicated on-site distribution box, which, as well as the on-off switch (EAT) and emergency switch (AT) connected to it, must be installed in place before installation. The power supply for auxiliary equipment and other electrical facilities (such as lighting, air conditioning, etc.) should be provided by the hospital's public power grid. Their control switches and protection circuits can be installed in the on-site distribution box, but the on-site distribution box is absolutely not allowed to supply power to them.

The following is the circuit principle of the host power supply circuit in the DSA equipment distribution box of the tumor center:

The system power supply complies with national standards, with a voltage of 380V and a maximum deviation of no more than 10%. The frequency is 50Hz and the maximum deviation is no more than 0.5Hz. The maximum deviation between phase voltages shall not exceed 2% of the minimum phase voltage. The maximum power of the equipment is 171KVA, the continuous power is 20KVA, and the power factor is 0.9; the maximum instantaneous peak current of the equipment is 289A, and the continuous current is 31A. It can be protected by a circuit breaker with a rated current of 160A. The equipment requires a dedicated power supply. The three-phase line is marked with phase sequence and introduced into the distribution cabinet together with the PE line. The incoming cable must be a multi-strand copper core wire connected to the circuit breaker with a rated current of 160A in the cabinet, and the cable color and circuit breaker specifications must comply with the provisions of the standard electrical installation manual. The distribution cabinet must have an anti-opening cover locking function to ensure the need for electrical safety operations. The emergency power-off button of the distribution cabinet must be installed on the wall next to the operating table in the operation room to facilitate the operator to cut off the system power supply in an emergency.

2. System power supply cable

It is best to configure a dedicated power cable from the transformer to the on-site distribution box, and it must be a copper core cable. High-power inductive loads such as air conditioners, elevators, refrigerators, etc. are not allowed to be connected to this power cable to avoid interference caused by the start and stop of the equipment. In order to ensure that the internal resistance of the power line meets the requirements in the above table, the minimum cross-sectional area of ​​the laid cable must be calculated based on the actual laying length of the cable, the internal resistance value of the transformer output end, and the internal resistance value of the on-site distribution box. The longer the laying length and the higher the internal resistance value of the transformer output end and the on-site distribution box, the larger the cable cross-sectional area is required. The cable cross-sectional area selection can be calculated according to the following empirical formula:

Cable cross-sectional area A = (2 × laying length) / (56 × cable’s own resistance).

For example: It is measured that the actual laying length of the cable is 100m, the internal resistance value of the transformer output end is 0.060 kiloohms , and the internal resistance value of the on-site distribution box itself is 0.030 kiloohms.

Then: the maximum allowable resistance of the cable itself = 0.11-0.06-0.03 = 0.02. The cable cross-sectional area A = (2×100)/(56×0.02) = 178.5, so the cable cross-sectional area should be at least 185mm2.

3. Lighting requirements

The best system is a combination of constant fluorescent lighting and adjustable incandescent lighting to meet the patient's comfort and facilitate the staff's operation. Taking the tumor center DSA as an example, the following lighting solution is recommended:

4. Use a common grounding system and equipotential connection

In the early stage of design, equipment manufacturers will require separate grounding. We should understand that the grounding line from the equipment to the grounding steel plate should not be connected to other equipment to avoid interference. Instead of a separate grounding electrode. When we encounter some manufacturers who stubbornly require a separate grounding electrode and do not adopt our shared grounding, we should analyze and explain from the following aspects.

1. International standard IEC61312-1. US Electrical Code (NEC). my country's lightning protection regulations (Articles 6.3.3 and 6.3.4) both stipulate the use of a common grounding system and equipotential connection.

2. It is difficult to set up separate grounding for electronic equipment in a building with a shared grounding system. This is because the equipment housing, the metal components of the entire building, the steel bars, and the electrical protection grounding system are all connected to the shared grounding system. It is difficult to lead the (functional) grounding wire from the electronic equipment to a separate outdoor grounding electrode and completely isolate it from the shared grounding system to avoid reverse overvoltage.

3. The impedance of a dedicated grounding wire is very large at high frequencies (the operating frequency of information equipment), so it is meaningless to require a small resistance. Because the impedance of the dedicated grounding wire is infinite under self-resonance conditions, it is equivalent to being disconnected and becoming an antenna, which in turn causes the equipment to receive interference signals.

4. Single grounding is not conducive to overvoltage protection.

5. In the case of TN-S system, there is no need to worry about power frequency current noise interfering with electronic equipment (PE and N are separated, and PE does not carry current normally). In places where there are many electronic equipment, steel pipes and closed steel cable ducts should be used to lay wires to prevent single-phase grounding short-circuit current from flowing through the shared grounding system and causing interference to electronic equipment.

Therefore, in engineering design, the practice of some foreign and domestic manufacturers that still use the old standard to require separate grounding should be corrected. The above is an experience exchange on the electrical in-depth design of large medical equipment rooms. If there are any imperfections, please correct them!

References

1. <> JGJ/T16-92

2. Equipment information provided by GE Medical Equipment Division, USA

3.IEC60364-7-710  INTERNATIONAL STANDARD