Instrument for testing three-wire electrical outlets

Claim 1

An instrument for testing wiring connection patterns in a three-wire AC electrical outlet which includes a line voltage terminal, a neutral terminal, and a ground terminal, comprising: a three-wire plug adapted for insertion into an outlet to be tested comprising a line probe, a neutral probe, and a ground probe; AC electrical meter means including a pair of leads and a needle deflectable over a scale including a plurality of zones marked with indicia identifying different wiring connection patterns which may exist in the outlet; and test circuit means connecting said plug and said meter means, comprising: a first circuit connected between said line probe and a central terminal, said first branch including first resistance means; a second circuit branch connected between said ground probe and said central terminal, said second branch including second resistance means, and a third circuit branch connected between said neutral probe and said central terminal, said third circuit branch including third resistance means, said first, second, and third resistance means having mutually different values, and means for connecting said leads of said meter means in one of said circuit branches such that the deflection of said meter needle is indicative of the absolute current flow in said one branch and in said test circuit means, different wiring connection patterns in a tested outlet producing different series-parallel connections of said first, second, and third resistance means and different current flows in said one branch, each of said zones being positioned on said scale such that a needle deflection to a particular zone indicates that a particular wiring connection pattern exists in the outlet, the location of said needle in a particular zone being indicative of the absolute line voltage on the line voltage terminal in the outlet.