HyTEPS conducted research into immunity and emissions to ensure patient safety and the reliable operation of medical equipment.
The Amphia hospital is the largest hospital in the Dutch province of Brabant, with 550 beds, more than 340 specialists, 4500 employees and 340 volunteers. Like all hospitals, Amphia makes use of equipment that needs to meet strict requirements. This requires a very high quality of mains power supply for reliable and safe operation.
When medical equipment fails due to poor Power Quality, the consequences can be significant, especially if the equipment in question is intended to keep patients alive or monitor their vital signs. Problems can have various causes: grounding, transformers, network connections, or interference, for example. Power Quality, immunity, and performance testing, and ensuring compliance with standards are essential when it comes to preventing such issues. Control and reporting are of great importance in this respect.
Preventive measures
Amphia wanted to make sure that the harmonic distortion – THDu – in the installation would remain within limits because it can have an effect on all devices connected to the installation. These can fail, not work as intended, or their operational lifetime may be shortened.
Amphia asked HyTEPS to investigate what level of harmonics and voltage dips several medical devices were immune to. They also wanted to know whether the harmonics that certain devices were feeding back into the installation (emissions) met the international IEC 60601 standard for medical devices. Several medical devices were tested according to the limits specified in IEC 60601 and EN 50160, regarding voltage dips, harmonic immunity and emissions, and supraharmonic current emissions. The total harmonic distortion in the voltage (THDu) was also investigated. This enabled the hospital to take preventive measures and upgrade selected devices.
To simulate all PQ phenomena, a programmable 1.5 kVA voltage source was used. This can inject any possible waveform into the circuit. Voltage dips were tested against levels as defined in IEC 60601. Harmonic emission was tested by injecting a perfect 50 Hz, 230 V sine wave and then measuring harmonic current. This was subsequently compared to the limits in the medical standard. Harmonic immunity regarding pollution in the hospital was determined by measuring the THDu during a week and then injecting it into the installation. The injected THDu contains the actual spectrum of THDu present in the hospital.
Voltage dips, which can be caused by starting up larger devices, or by short circuits in the installation, were also tested. Voltage dips that exceed a prescribed ‘safe’ standard can lead to equipment failure.
Results
In a controlled environment it was confirmed that certain devices were not immune to a certain THDu level or did not meet the medical standard for voltage dips. The THDu limit value of the equipment confirmed that relatively high harmonic distortion in the hospital was indeed causing problems. Optimizing the power quality in the electrical installation was vital. Installing permanent PQ filters, a proven method of solving PQ risks, turned out to be an ideal solution in this case. As a result, the hospital is investigating the possibility of strengthening the grid by installing a third DRUPS – a dynamic UPS with integrated diesel engine.
Permanent Power Quality monitoring
In general, hospitals are advised to permanently monitor Power Quality in their electrical installation, so that any (potential) issues can be identified and investigated at a very early stage. During a hospital visit, people are generally unaware of the number of staff involved and the work that goes on behind the scenes to ensure patient care. This involves various parties, such as HyTEPS. That is why it was a great experience for me to provide proof that Power Quality in a hospital installation directly affects the operation of devices.
Medical devices examined for Power Quality immunity
- Modular patient monitor, which measures the patient’s electrocardiographic (ECG) signal
- A humidifier, which keeps the humidity of air intended for patients above 90%.
- A medical ultrasound device
Parameters examined
Voltage dips
This is a voltage reduction of 1% to 90% of the rated voltage, where the duration of the reduction is less than 1 minute. If the duration exceeds 1 minute, the reduction is classified as ‘undervoltage’. Voltage dips are caused by a large current passing through a conductor. This current can result from short circuits, starting up of large devices, and associated inrush currents, or overload.
Harmonic pollution
Harmonics are multiples of the fundamental frequency (50 Hz). These higher frequencies, caused by nonlinear loads, are added to the fundamental frequency, creating harmonic distortion. Harmonics can have negative consequences, such as overloading the neutral conductor, lowering the power factor due to increased harmonic power, creating additional rotor winding currents and, as a result, higher losses in drives, but also capacitor overloading.
Standards for immunity and emissions
- EN 50160 defines the limits of the quality of the voltage at the Point of Connection (POC) to the grid. This standard defines the maximum number of harmonics in the voltage.
- IEC 61000-4-11 describes the procedure for immunity tests related to voltage dips.
- IEC 61000-3-2 describes emission standards relating to harmonics. This defines limits of harmonics in the current as less than 16 A per phase injected into the grid.
- Standards for high-frequency current emissions exist (>2 kHz), but these have their limitations.
- For frequencies above 150 kHz, several standards exist, such as CISPR-15 and IEC 61000-3-8.
- Limits between 2 kHz and 150 kHz are poorly defined and still under development.
