Checking Outlets

An electrical outlet tester is an invaluable tool for any homeowner, do-it-yourself-er, or professional working in the electrical or energy field. There are many different types of devices that all qualify as "outlet testers" - and here we'll review the capabilities (and limitations) of the various devices.

The basic outlet tester (~$5 from home improvement and big box stores) provides a quick check of outlet wiring, and the presence of voltage on an outlet.

It can spot an open ground, open neutral, reversal of hot / neutral, or hot / ground. And it gives a quick "OK" check. Perfect for a quick check of all outlets in a space, checking on a repair, or confirming power at outlets, plug strips, etc.

For a few dollars more (~$10-15) one can get this device with a GFCI test button - pressing the button places a small (~ 20 mA) load between hot and ground. A properly wired and functioning GFCI outlet or circuit breaker should trip. Again, a good "quick check" for GFCI outlets in kitchens, lavatories, exteriors, garages, and other wet locations. It's especially handy if (for instance) a string of outlets is supposedly wired from a single GFCI outlet - one can check each outlet to confirm it's actually fed from the GFCI outlet and wired correctly.

Now, both of these devices are fairly limited in capabilities. They do not place a significant load on the source, so cannot identify loose or corroded connections, nor determine voltage drop for distant outlets. They cannot discriminate between the grounded (neutral / white wire) conductor and the ground (green wire / safety) conductor. Mixing or intermingling neutral and ground connections can cause electrical noise, safety concerns, and magnetic fields.

More advanced outlet testers are available to provide more thorough inspection and analysis of electrical outlets.

The first feature is the addition of an electrical load to provide some indication of voltage drop. Typically measured in percent based on rated load - the National Electrical Code suggests that "5% voltage drop under rated load" is "adequate for good service". Voltage drops exceeding 10% are possible - depending upon conductor size, run length, and condition of connections / junctions. If one is (for instance) using such an outlet for a home office in the far end of the building, with a laser jet printer, space heater, or mini-fridge, light flicker, low voltage, transients, and other problems may ensue.

A quick survey of outlet testers that measure voltage drop include:

• Suretest (Ideal Industries) - Various models, $300 - $400
• Extech Instruments - CT70 / CT80 AC Circuit Analyzer, $150 - $250
• Amprobe - INSP-3 Wiring Inspector Circuit Tester, $300 - $400
• General Specialty Tools & Instruments - CA10-AC Circuit Analyzer, $100 (great value, especially as it's advertised to measure neutral and ground conductor impedance!)

These devices incorporate a variety of options and features (digital metering, testing of neutral and ground impedance, PC interface, etc.) 

As a power quality consultant, I carry a small outlet tester with me to any site that might involve 120 VAC AC power issues. Such a device permits a rapid survey of all receptacles in an area or space, often spotlighting the problem area quickly - my report includes a table of all outlets tested, eliminating the need to open up each and every outlet, and allowing me to focus on specific problems.

In addition, grounding issues (especially intermingling of neutral and ground wires) can cause ground loops, ground noise, and magnetic fields. So if I'm looking for noise issues on a higher voltage or dedicated electrical system, checking out all of the nearby AC outlets is a good way to be sure there's no sneak path for ground currents or noise.