As some of you may remember from previous columns I've written, I volunteer regularly at a local Children's Museum. Many of the programs we offer teach kids about various aspects of electricity, such as static electricity, and simple battery operated electromagnet or light bulb circuits. All of these are quite safe for the kids to operate by themselves, or to recreate at home.
Recently, I challenged myself to tackle the task of using a fun, yet memorable demonstration to teach children from five to twelve years old (and even adults, for that matter) about electrical safety in the home. I intended for it to be useful for the older kids, such as scouts working on electrical merit badges.
I finally decided to demonstrate the safety benefits of a Ground Fault Circuit Interrupter (GFCI) receptacle such as you should find in your kitchen or bathroom at home. And, because of the target audience, I needed an attention-getting (but safe) experiment.
Introducing: The Deli Pickle Light
A few months ago, I found a rather startling experiment in a YouTube video. Actually, I found several – and it's not at all clear with most of these that safety has been considered. The experiment uses a deli pickle connected to an ordinary extension cord through a pair of spikes. It's connected to a 120 volt outlet to cook the pickle. It's quite impressive – the pickle smokes and sizzles, and it lights up in a yellow green flashing pattern as fluids begin to drip off the bottom. It also smells terrible. Few, if any, of the experimenters discuss the dangerous, and possibly lethal, aspects of playing with 120 volts directly from an outlet.
I wanted to show how electricity from an unprotected outlet in a bathroom could create a dangerous situation when a defective or dirty or wet hair dryer (for example) is used near a grounded (via metal piping) faucet in the sink. I figured the pickle light could be a good substitute for a defective or wet hair dryer in my demonstration.
The procedure consists of impaling the pickle on a pair of spikes, one connected to the hot conductor and one to the neutral return in a power cord. I also have a test lead with a conductive probe, mounted in an insulated handle, connected to the hot spike powering the pickle. The probe contains a 2 watt current limiting resistor of about 10 kohms.
Plugging this pickle light into an unprotected socket causes nasty things to happen (flashing, smelly, smoking and noisy.) I touch the probe, representing a person's finger, to a sink faucet. With the unprotected receptacle, nothing changes – the pickle keeps sizzling.
Then, I repeat the experiment with the demo plugged into a GFCI receptacle. Again, the pickle sizzles and lights – until I make the probe connection to the faucet. In this case, the GFCI relay in the receptacle pops and the pickle stops sizzling. This is because the current returning via the ground exceeds 6 mA, the threshold for the GFCI protection circuit. The pickle has been de-energized.
So, just how does a GFCI circuit work anyway?
A GFCI receptacle or breaker senses the current in both the 'hot' and neutral conductors of a circuit. If everything is working correctly, and the load does not provide an additional (fault) path to ground from the hot side, then all of the current flowing from the receptacle to the load should return via the neutral to the panel. If some of the current (over 6 mA) flows elsewhere – to a grounded point such as a faucet – then the sensing circuit in the GFCI device sees the unbalance and trips a relay, removing energy from the faulty device or cord. Note that this works even if there is no ground pin on the power cord to the load device.
GFCI protected circuits should be tested periodically. GFCI receptacles have Test and Reset pushbuttons that anyone can push to see if the circuit trip mechanism is working. Is that enough? Not really, because it's possible that the receptacle is mis-wired, and even if the circuit trips, a device plugged into the receptacle may still be energized. You could verify that the circuit is properly wired by pressing the test button while a light fixture or DMM is connected. If the circuit is not wired correctly, the light will remain on or the meter will continue to indicate voltage after the test button is pressed.
The most complete test requires the use of an electrical tester calibrated to draw only 6-10 mA, such as the Fluke T+ tester, available for less than $100. When connected between the narrow slot (live phase conductor) and the round ground pin on the receptacle. The GFCI should trip immediately.
Although not calibrated as precisely, the Lo-Z feature of either the Fluke 117 DMM or the 289 DMM will also trip one of these outlets when connected as indicated above.
How GFCIs Work
The best electric pickle demo I found:
Electric Pickle - YouTube