Computer Shocks Wife

I was a visiting friend in a developing country one evening when the wife screamed. She had touched the computer case and the modem's telephone line plug. Fortunately, she was more surprised than injured by the shock.

Out came the voltmeter and detective hat. Quickly the problem was found; the computer case was HOT (energized) because the case was tied via the green wire at the power supply to electrical ground. The culprit was a combination of a 120 volt two wire electrical system and the little UPS needed to ride thru the frequent electrical blinks that the developing nations know all too well.

Since the UPS is a separately derived source, internally it ties the output neutral and ground together. This is fine with polarized 3 pin plugs. In this case, a 3 to 2 plug adapter was in use and had been turned over (reverse polarity). This swapped the HOT and neutral into the UPS and it resulted in the HOT being tied to the outgoing "neutral" which was bonded to the equipment grounding conductor. This energized the UPS case and PC case!

The factory fix would be to use isolation input transformer, but that cost would be too much for the competitive mini-UPS market to bear, unless it was done industry wide.

Larry Bentley, Sr Power Engineer
Technical Facilities Engineering Standards
MCI WorldCom

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