It’s annoying when you are working at home and the power goes out for a couple of minutes, perhaps causing you to lose a document you were working on, But if you are a patient in a hospital the effect of a power loss, even for a minute, is more than annoying., It could be fatal. For that reason in hospitals and other medical facilities which control machines keeping people alive require a medical grade UPS (uninterruptible power supply). A medical grade UPS will maintain power voltage and frequency variations in electrical equipment not only in the event of complete power failure, but also as a result of fluctuations in the power supply like those caused by surges, brown-outs and storms and car accidents that take down power lines.
Commercially available power supplies are generally manufactured to the IEC-950 or UL-1950 standards. But because it may operate near patients medical-grade power supplies differ from the standard commercial variety and must comply with IEC601-1 or UL2601-1 relating to design and test criteria that seek to eliminate the shock hazard to patients and medical attendants. For electrical equipment operated from the AC mains, the IEC standard focuses on two issues: high voltage breakdown and current leakage. Current leakage directly affects the patient, while breakdown can indirectly affect the patient by increasing leakage in their vicinity.
For UPS product standard commercial UPS systems can have earth leakage of no more than 3mA, and with EMC filters employed UPS systems have earth leakage in the 1 to 2mA region. Medical standards, on the other hand call for earth leakage to be no more than 300 microamps, about one-tenth of the levels allowed by specifications aimed at commercial-grade supplies
Sophisticated medical equipment such as ultrasound systems, laboratory equipment, and other health care devices also need optimal power protection from noise, sags and surges. A medical grade UPS should be able to withstand an electrostatic discharge compliant to ENC61000-4-2, level 4 relating to the immunity of electrical and electronic equipment to repetitive electrical fast transient s without damage and without affecting the connected load.
All switching power supplies draw current from the line in narrow, high-amplitude pulses instead of a smooth, sinusoidal flow from the ac mains. As such medical-grade supplies also must satisfy standards for power factor correction. IEC 60601-1-2 requires PFC in power supplies above 75 W. By way of review power factor is the ratio of real power (Pr) to apparent power (Pa). The power factor of a typical switching power supply is 0.6 to 0.7. The power factor of a corrected power supply is typically 0.95 or greater.
