Hi all,
We've been having a bit of a philosophical(?) debate in my office about proper classification of switch-mode power adapters. I'm hesitant to call it a philosophical debate because I feel there has to be a right and wrong answer here.
The company I work for manufacturers switch-mode power adapters...predominantly wall-plug in type.
Many people are probably familiar with the standard classification of appliances which plug into mains:
Class I: The transformer (and primary-secondary isolation barrier, in general) incorporates a single mean of protection against electric shock. A protective earth wire connection to output negative (and the metal enclosure, if there is one) forms two means of protection.
Class II: Two means of protection directly built into the insulation between primary and secondary, thereby making the Earth wire unnecessary to meet global safety standards.
These days, the vast majority of switching power supplies, are designed with "double insulated" transformers, as triple-insulated wire (TIW) has made it possible to design a double-insulated transformer which is hardly any bigger than a hypothetical equivalent single-insulated version.
So the question is: If you attach an earth wire to the output negative in a power supply which has a Class II insulation rating (based on transformer construction and other creepages/clearances), what class does the power supply become? Class I? Still Class II?
The company I work for calls it Class I, but my problem with that is: Class I implies an inherent need for the earth conductor to be safe. By my logic, the addition of the earth wire should only make the power supply more safe. If I dare say, I might even call it a "triple insulated" power supply in this configuration. Am I wrong? Is it somehow possible that defeating the earth isolation somehow degrades the power supply to a true Class I designation?
It's interesting that, for the most stringent medical applications, earthed power supplies are not allowed, and in fact, must be floating. (These classifications are called Type-BF, for body floating and Type-CF, for cardiac floating.) The medical standard IEC60601 notes that earthing a patient (via power supply earth connection) opens up the possibility that an unintended external voltage could be applied to the patient, causing a current to flow through the body to earth.
Anyone have any experience with this?
Thanks,
Tim