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Practical Engineering: Capacitor Safety Considerations

Welcome to my sixth monthly “Practical Engineering” blog post for In Compliance Magazine! I have over twenty-seven years of experience developing complex measurement devices for use in harsh industrial environments. Much of what I know about compliance engineering has been learned the hard way, and I want the readers of In Compliance to learn from my mistakes. For these reasons, I am honored and excited to bring to our readers short articles related to compliance engineering that address either something I have personally experienced or witnessed and that I believe would be of great interest to our readers. I intend to cover a broad range of topics, including product safety, standards and regulations, electromagnetic compatibility design and test, measurement uncertainty, training, leadership, signal integrity, etc. If there is a topic you are interested in and would like me to address, please let me know, and if it is something I am familiar with, I will cover it in a future blog post. This sixth blog post addresses an issue pulled from the field of product safety engineering.

Capacitor Safety Considerations

In this post, I will cover an issue I have recently witnessed regarding the proper specification of capacitors used in safety applications. Specific manufacturer names are not provided as this is unimportant to this conversation.

Here is the specification of the capacitor as stated on its datasheet:
Ceramic AC Capacitors Class X1, 760 VAC/Class Y1, 500 VAC

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From the above, notice the datasheet indicates that these capacitors have both X1 and Y1 safety ratings. The datasheet also indicates that both X1 and Y1 ratings have approval from a National Recognized Testing Laboratory (NRTL). The proper NRTL approval of this capacitor (considered a safety critical component) is required to obtain NRTL approval of the end-product in which the capacitor is used.

If this capacitor is used within its ratings and in a location that requires Y1 or line-to-ground isolation, then everything should be good, right? Not so fast.

For this issue, nobody checked the NRTL’s online certification directory (here is a link to an example of one such directory: to confirm both X1 and Y1 ratings for this part were properly listed. Right or wrong, the online directory indicated that the part had approval for only the X1 rating and no indication that it was approved for Y1 applications!

This is an issue because an X1-only rated capacitor cannot be used where a Y1 part is required. Subsequently, when it was time to work on the certification for the end product, this part was flagged as not suitable since it was missing the correct Y1 rating. Not having the correct Y1 rating for this part caused unnecessary churn within the development organization and held up NRTL approval of the end product until it was resolved.

The moral of this story is that if you are involved in product safety for an end-product that involves the use of safety-rated capacitors, do not trust what the specifications on the datasheet say if they are related to safety. Early in the development cycle, take the extra step of looking at what is listed on the NRTL’s online certification directory. If you are surprised at what you discover, then by checking early, you will have time for a plan B or C. Plan B could be finding an alternate supplier for the part or working with the current supplier to resolve the issue. Plan C could be doing both activities.

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In the case that brought about the idea for this post, it turns out that the supplier of the capacitor was able to provide a certificate of compliance from the NRTL in question. The certificate showed that the capacitor had the proper X1 and Y1 safety ratings. The capacitor supplier worked with the NRTL to correct their online directory for this part.

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