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The Day of the Living Dead Variac

September 14th, 2010


At my old company in one corner of the engineering lab there were a few tables with a 25 amp inverter setup for testing. Over the years I had laid claim to this area for my embedded controls development. For the company's main product line I had designed one universal control board which we used for all the models from a few amps at 120 volts single phase all the way up to 2400 amps at 480 volts three phase. The single-phase, 120 volt, 25 amp unit was perfect for development. It was big enough that it sort of behaved like the larger units but small enough that when it exploded it didn't make your ears ring.

I spent most of my lab time banging away on the keyboards of my development machines in front of that 25 amp test system. So much time that I got a bit too familiar with the setup honestly. I became a bit of a cowboy when it came to operation of the unit and reconfiguring it.

The topology of the product was such that the inverter was normally series connected with an input source voltage. I used a 120 volt manually operated variac to act as my test input source voltage. One day I was working on a proof of concept idea where we reconfigured the inverter dynamically from series to parallel connection and back. There were a few technical issues I was working through in attempting to do that. I was trying out lots of ideas at that time, one of which was adding some series resistance to a power connection to damp out some voltage ringing that was occurring during the series to parallel transitions.

The unit was connected to the variac with the standard two wires: line and neutral, just like the common two wire power cord for household electronics. I had a separate ground wire for the unit directly connected to our lab electrical grounding system. I can't remember the exact reasoning behind the connection methodology but I remember wiring in the damping resistance from the inverter neutral to ground.

Anyway, I was working away with the damping resistance installed and nothing seemed out of the ordinary. I felt like a taking a soda break so I turned off the input variac using its built-in on/off switch and walked out of the lab over to the vending machines in the break room. I headed back to the lab with my Diet Dew and walked into some sort of commotion in the vicinity of my test setup.

One of the other engineers (Don) was standing over my test setup looking rather agitated. Evidently within seconds of me leaving the lab, smoke started pouring out. I had a Plexiglas shield placed over my unit which acted like an upside down fish tank, collecting the smoke as it rolled off the inverter. That penned up smoked made it impossible to see what was getting hot inside the unit.

Don had been in the lab working on a different test setup. When he noticed the smoke and me nowhere to be found he ran over to my bench and frantically looked for the off switch. To my credit I had it clearly labeled with a sign and red electrical tape. However he quickly found the switch was already in the off position. Don, who I always admired for keeping his cool in situations like this, quickly located the upstream connection of the source variac and yanked out the cord. He confirmed the smoke was in remission, then started checking my setup to verify the power really was off. That was when I walked back in the lab.

When Don noticed I was back he told me what happened. I had just started telling him that he must be confused because my test setup was switched off when I left. Then I got a whiff of the burnt electronics smell and noticed the blackened resistor in my test setup, the same damping resistor I had just wired in about ten minutes ago. Not only was it completely charred but it had started to roast all the components around it. I think I set a personal record that day on how quickly I had to eat my own words. Explosions and burning parts are par for the course in a power electronics lab but what was disconcerting in this instance was the variac was switched off but the power was obviously still flowing into the unit. It was like a zombie variac: supposedly dead but still very much alive.

Once Don knew the situation was back under control he went back to his setup and left me to figure out what happened. It took me a few minutes but I eventually figured out that I had miss connected line and neutral after I rewired the unit to add the damping resistor. If you remember I had placed the resistor neutral to ground. Normally those two connections are at the same voltage potential because they are bonded together at the breaker box in the electric service to the lab. However I had flipped neutral and line.

On top of that, the off switch I was using was not a full two-pole disconnect, but instead a simple one-pole on/off switch. The switch was meant to open up  the line connection but the neutral always remained connected. In my miss-wired state the switch was opening the neutral with the line connection remaining connected to the unit. All of this meant the damping resistor I added ended up with the full source input voltage across it. I had sized the resistor for a few volts but inadvertently applied 100  times that. Joule's combined with Ohm's Law: voltage squared times resistance equal power or in my case smoke.

In retrospect I was surprised the resistor didn't instantly burst into flames. I tend to use over-sized components during development, which is why it probably held on till I left for my soda break unfortunately. From that day on whenever I left the lab I yanked the cord of the varic before leaving.

The whole episode reminded me of a quote by Mark Twain that went something like "loaded guns are perfectly safe but unloaded guns are the most deadly and unerring things that have ever been created by man." That sentiment is timeless.
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