I assume that these wall-warts are not serviceable; the ones I have used are certainly not so designed. I don't see a good technical reason to polarize the blades if the output is SELV. Ralph McDiarmid, AScT Compliance Engineering Group Xantrex Technology Inc.
________________________________ From: [email protected] [mailto:[email protected]] On Behalf Of Robert Johnson Sent: Thursday, August 10, 2006 8:44 AM To: [email protected] Subject: Re: Power Plug Blade Configuration There are a couple reasons for polarizing the phase and neutral pins. First, it help prevent electric shock of service persons servicing the innards of products with single pole switch or overcurrent protection who have relied on the switch or fuse to remove power. These service persons deserve electric shocks. Second, it provides ground fault protection for products with a single pole overcurrent protection, at least for wiring on the neutral side of the protection. Ground fault protection is generally provided by the branch circuit protection and not required in most products. The real reason is that electricians have for years wired branch circuits based on the above reasons and have extended those practices into appliances like lamps so you could unscrew edison based light bulbs while safely touching the screw shell. Since it has become a RULE, is is automatically applied to everything by professional inspectors. I grind the polarizing wings off the plugs I use and suggest you do the same since there is no engineering reason for them and they don't fit into many old receptacles and extension cords. Bob Johnson Price, Ed wrote: I recently bought a little Radio Shack wallwart power supply, equipped with two parallel-blade AC power terminals (with no ground blade) and a 6 VDC output wire pair. As you would expect, one of the blades is slightly wider than the other (implying that this is the neutral or return line), conforming to the convention of the typical NEMA 15 Amp female power receptacle. Since the AC power blades are located near one edge of the wallwart, you can plug the wallwart into a receptacle and the bulk of the wallwart body hangs to one side of the receptacle. Unfortunately, when you try to plug in a second wallwart, the bulk of the wallwart body prevents you from plugging it in, and the larger AC power blade prevents you from facing this second wallwart opposite the first one. My question is, why does this appliance have a polarized blade power connector? I am assuming that the input power pins go directly to the primary of a transformer, so why should we worry about which side is "high" or "return"? Could wallwarts be designed using identical power blades, allowing the option of a 180-degree rotation installation? Ed Price [email protected] WB6WSN NARTE Certified EMC Engineer & Technician Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 (Voice) 858-505-1583 (Fax) Military & Avionics EMC Is Our Specialty - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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