EMI Mayhem: Smart Phone Switches on a Maytag Oven

Awhile back the NYT ran an article about a guy’s smart phone starting up his Maytag Model CGR1425ADW oven (plus a couple of his neighbor’s units) in a Brooklyn apartment building. Read the full text: Hello Oven? It’s Phone.  Now Let’s Start Cooking!

What makes the phenomenon particularly distressing is the oven’s tendency to turn itself on to the highest setting.

A spokeswoman for Maytag said, “In our experience, this situation is highly unusual.” She went on to say that all of Maytag’s appliances are tested and meet safety standards set by Underwriters Laboratory and the American National Standards Institute.

With the finger of blame pointing strongly at EMI, we wondered what requirements for EMI/EMC apply to kichen appliances. So we turned to EMI Guru Daryl Gerke, co-owner of Kimmel-Gerke Associates, an EMI/EMC consulting company.

“Ah, the law of unintended consequences catching up with us. Yup, radio transmitters (such as cell phones) certainly can, and do, affect electronic control systems. This problem alone helped put two kids through college here. I’ve chased down field problems with embedded controllers, medical devices, alarm systems and more. In one case, I even used a cell phone to force the problem, and kept adding fixes until I could no longer trigger the device with the cell phone right next to it.

Thanks to some good lobbying by the appliance industry in the past, appliances are exempt for any EMI/EMC requirements in the US — both emissions and immunity. In the article, the manufacturers tried to hide behind the UL requirements, but these are for safety (shock and fire), not EMI/EMC. The bottom line — these appliances are not subject to any rules in the US, and were likely never tested for EMI/EMC effects.

Europe is a different story. After 1996, appliance controllers (like virtually all electronics) became subject to EMI/EMC requirements and testing for products sold in the European Community. The normal RF immunity test level is 3 Volts/meter, but was recently increased to 10 Volts/meter for “protective electronic circuits.” My guess is that turning on the oven would be classified as a “protective electronic circuit” failure, but some might argue with that.

Even passing these tests, however, might not be enough for a nearby cell phone. Using our tried and true but quick and dirty field level estimator (see below), I come up with a field level of 3 Volts/meter at a distance of about 1.5 meters, and a field level of 10 Volts/meter at 0.5 meters (based on a nominal power level of 600 milliWatts.) Thus, a cell phone at about 18 inches or less might still exceed the more stringent safety levels the Europeans now require for appliances.

Here is the formula I used: E=(5.5 sq rt(p))/d, where
E= Electric field in Volts/meter
p = transmitter power at antenna in watts
d = distance from the antenna to the victim circuit in meters

This simple formula is widely used in the EMI/EMC community, and is pretty accurate for these conditions.

Finally, I’ve seen evidence of this in my own office. No, I don’t turn on appliances, but I do hear the cell phone “ticking” (when communicating with the local towers) in my computer speakers. Yea, the are really expensive speakers, too–I probably paid less than $10, so I’m sure the manufacturer didn’t want to waste any money on RF filtering.

I could go on and on with these tales, but a particularly funny one occurred years ago when I was active with ham radio. One day my next door neighbor commented how his kids enjoyed listening to me talk on the “pig radio.” Seems the RF was getting into their electronic organ. Fortunately, my next door neighbor was another engineer, so he was not upset and simply tolerated it. However, when he explained to his kids (about 4 and 5) they were hearing a ‘ham radio’ they redubbed it a ‘pig radio.’”