Using Y rated capacitors in medical applications
Mandatory for medical power equipment
The medical standard IEC60601-1 is mandatory for medical equipment and requires manufacturers to review the isolation system employed within their products. One of the more radical changes in the 3rd edition standard is the definition of requirements when using Y capacitors to bridge isolation barriers.
Amendment 1 of the standard, released in 2012, re-addresses this issue and provides clear information regarding the use of Y rated capacitors to bridge safety isolation barriers in medical applications.
Y capacitors must meet the requirements of the IEC60384.4.14 and are rated as Y1 or Y2. The difference between Y1 and Y2 capacitors is the maximum isolation voltage of the component. Y2 capacitors can be used for barriers with up to 1500VAC test voltage, Y1 rated capacitors can be used for barriers up to 3000VAC test voltage.
MOPP & MOOP
With the differentiation between Means of Patient Protection (MOPP) and Means of Operator Protection (MOOP) comes the restriction that Y2 capacitors, which are rated to an isolation voltage of 1500VAC, may not be used to bridge a 2 MOPP isolation barrier which protects the patient from electric shock.
This is because the test voltage for this isolation barrier is 4000VAC for a working voltage of 250VAC and two Y2 capacitors connected in series can only support 3000VAC. In this instance two Y1 capacitors installed in series are required supporting an isolation voltage of up to 6000VAC. Using Y capacitors in series requires that the same type and value are employed to ensure that the test voltage is divided equally between the parts.
Y2 rated capacitors will provide sufficient protection to an operator in a 2 x MOOP isolation barrier as this will be tested at 3000VAC a single Y1 capacitor would also be adequate in this scenario. If the working voltage of the isolation barrier is lower than 42,4VAC or 60VDC a solitary Y1 cap is sufficient to provide 2 MOPP.
The details discussed in this article can be found in IEC, EN & ANSI/AAMI ES60601-1+A1 in chapter 8.5.1.