Mistakes were made, even in a simple 3-wire AC hookup

Article By : Bill Schweber

The many ways of making basic mistakes never fails to amaze, as this engineer saw in a basic 3-wire AC connection.

We recently had a new dishwasher installed to replace a 15-year-old model that had failed and was not worth repairing (and we were never really happy with it, anyway). For a modest fee, the appliance store sent over a licensed electrician who had a sideline job of removing old units, installing new ones, doing the water lines, and hooking up the AC power.

When it came time for him to do the AC connection, I had what I’ll call a Proust madeleine micro-moment. In his monumental work “In Search of Lost Time” (also known as “Remembrance of Things Past”), author Marcel Proust eats a madeleine cookie that triggers a wave of memories, leading to his seven-volume, 1000+ page remembrance (see “How Proust’s ‘madeleine moment’ changed the world forever”). I had a much smaller flashback to the installation of this now-defunct unit.

That dishwasher “died” the second time we used it. At the time, I was torn between contacting the unit’s manufacturer (“Your new product is junk”) or the plumbers who installed it (they would likely say, “Hey, it wasn’t us, it was working fine when we left”). I decided to do neither but instead do a little basic troubleshooting.

Since the unit was dead, with no panel lights at all, I checked the obvious first and found the AC line leading to the unit was fine. So the problem, whatever it was, was within the dishwasher. I took off the dishwasher’s kick panel to check the primary wiring, which consists of just three wires – hot, neutral, and ground – that “simply” needed to be connected to their corresponding wires on the AC supply line using supplied wire nuts.

What I saw next both stunned and shocked me (and might have done so literally, at some point). When I removed the panel, I saw the connectors and wires were simply floating in air behind that kick panel. The two wires for the non-ground AC connections (the black and white pairs) were not inserted INTO their wire nuts. Instead, only one wire of each pair was actually in its nut, the other wire was wrapped around some bare copper outside the nut.

What had happened, I surmised, was that one of the “wraparound” wires loosened from normal vibration, and that’s why the dishwasher died. Incidentally, whether you put the wires in the wire nut straight or twisted is a matter of personal taste, as both are OK by the electrical code. Either way, the two wires do have to go into their nuts to stay connected and any copper for those hot and neutral wires showing outside the nut is dangerous and can short to the dishwasher and user.

Worse, the safety ground wire was not under the screw – it was dangling in the general area of the ground screw, but the screw was loose and had not been tightened at all! That ground wire is there for a good reason. The same not-attached ground wire was looped counterclockwise under the screw head, even though a basic rule is that a wire goes clockwise around screw, not counterclockwise.

It doesn’t stop there. The protective safety cover over the AC connections was not installed, it was on the floor under the middle of the dishwasher, even though it takes less than a minute to clip it on and screw it down. Finally, the large nut around the power-wiring conduit entry was not tightened at all, it was loose (this was the least of the problems). I re-did the wiring and the dishwasher worked fine from then until its final “passing.”

So, when the new dishwasher was about to be installed, I mentioned this series of mistakes to the electrician. He said not to worry, for two reasons: a) he was a licensed electrician, not a plumber, and knew how to do AC wiring (that was somewhat reassuring, but still), and b) they no longer allowed “free-floating” AC wiring with wire nuts. Instead, the dishwasher comes with a protected, substantial junction box with strain relief on the AC line side, along with detailed wiring instructions that mount at the back of the undersink cabinet next to the dishwasher (Figure 1).

Dishwasher AC line box BoschFigure 1 The three wires for the AC line to the dishwasher power are clearly called out and in a well-designed box. (Source: Bosch; Fig 9)

Next, the vendor-supplied appliance-facing AC cord has a connector that snaps into the back of the dishwasher. It’s all nice, clean, protected, and rugged (Figure 2). The only possibility of error is in the wiring of the three line AC connections, but the directions are clear even to anyone who isn’t an electrician, but paying attention, one step at a time.

Dishwasher AC connection BoschFigure 2 The fully enclosed connection box goes in the cabinet next to the dishwasher and has a secure snap-in AC connection to the dishwasher. (Source: Bosch; Fig 10)

If you add it up, we had five mistakes with a three-wire connection of the now-replaced dishwasher, which is truly an amazing accomplishment and ratio (that’s 1.7 errors per wire). I’ll always keep in mind that you should never assume that because something is simple, there aren’t a lot of ways to screw it up through sloppiness and negligence.

Have you ever seen or committed one or more simple, “stupid” mistakes that wasted debug time, sent you looking down the wrong paths, or were risky to you and others? How did you figure it out and what did you do?

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

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