Shock hazard: filtering on input power lines

Article By : John Dunn

Compliance with EMI requirements usually involves the use of filtering on input power lines, which can pose dangerous issues.

This is another shock hazard story.

Compliance with electromagnetic interference (EMI) requirements usually involves the use of filtering on input power lines. Such filters, from the simplest to the most elaborate, are configured pretty much along the following lines (no pun intended).

Figure 1 A typical EMI filter arrangement

There is one, sometimes overlooked, issue with using filters such as this, which is that someone will neglect to make the illustrated connection to ground. In that case, the two capacitances shown here as C2 and C3 become a capacitance voltage divider, which for an input line of 120 volts AC, can put the case of the product at half the line voltage. Clearly, this can be very dangerous.

The Thevenin impedance of that capacitive voltage divider would probably vary versus filter design details. The more C2 and C3 capacitance there might be, the lower that Thevenin impedance would be and the more likely an inadvertent shock could be dangerous if not outright lethal.

When my son entered college in Boston and set up his PC in the dormitory, he touched the PC case and a nearby radiator at the same time and got an electrical shock. I measured the voltage between the two and it was 50 volts.

Among a raft of other dormitory safety issues, the building’s wiring did not provide for ground at any of the electrical outlets. To read how that played out, how the dormitory owners were unresponsive,  how municipal agencies in Boston were uncooperative, and what I finally had to do to force the building wiring to be updated to modern code to keep my son from accidentally being killed in there, please see my article “Grandfathering.”

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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