RV Electricity – What causes transfer switch burnt neutrals?

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By Mike Sokol

Dear Mike,
Last fall I had an electrical short in the transfer switch of my motorhome which burned several wires and melted the box and the fan motor in the convertor. I was told that because the wiring in the transfer switch gets hot and cools, the lugs can loosen, and therefore the lugs should be checked for tightness once a year. Is this something that RV owners are doing? —Fred TremoseDear Fred,
I’m going to use your question to help correct an inaccurate statement I hear all the time about electrical problems. That word is “short” as in “short circuit.” Most likely (especially looking at the pictures you graciously sent) you have a high-resistance connection on the neutral wire in the center of the picture. And a high-resistance or open connection is the opposite of the short circuit. But more on this in my advanced classes.

However, your point that the terminating screws (or lugs) in the transfer switch might have loosened up is spot on. But your point about heating and cooling causing the loosening may not be totally correct. In a properly assembled transfer switch there might be some heating and cooling of the connection that would eventually contribute to the loosening of the screws. However, it’s more likely the initial failure was due to road vibration that began the process of the screws loosening up, increasing the resistance, which causes more heat, which causes more loosening/resistance/heat, and eventually causes the failure you had.

So, yes, it’s a great idea to tighten the screws in all of your circuit breakers and transfer switch connectors at least once a year/season. But no, I doubt that very many RV owners (or even technicians) are aware of these connections loosening up over time.

However, you should really be using a torque limiting screwdriver (left) for the screw terminals in your transfer switch so you don’t break off the terminals on your contactors/relays. I wrote that up on my RVelectricity newsletter last Sunday which you can read HERE about halfway down the main page.

Tthere’s one more non-obvious possible cause of your neutral burning up, and that’s a pedestal 50-amp outlet that’s been improperly wired with a single pole (phase) from the campground’s main electrical panel. When that occurs you’ll measure 0 volts between the two hot legs of the outlet, not the 240 volts expected. But you’ll still measure 120 volts between the neutral and either of the hot legs. This single-pole connection allows current in each leg to be additive on the neutral, instead of subtractive.

That means that on a properly wired pedestal outlet with 40 amps on Hot 1 and 30 amps on Hot 2, you should only have 10 amps of current on the neutral. But on an improperly wired outlet (Hot 1 and  Hot 2 jumped together) that neutral current will work out to 30 amps + 40 amps = 70 amperes of current, which is way above the 50-amp rating of the connector and wiring. That’s certainly enough to create the type of overheating damage we see in your picture.

Interestingly, the newest Intelligent/EMS surge protectors from Southwire/Surge Guard, have a sensor on the neutral and will shut down the power if it exceeds a safe level (70 amps). This is just one more reason you should meter all pedestals before plugging in, and not accept a campsite with incorrect power.

Okay, that’s about it for this week. See you in the Monday newsletter for my next JAM Session on “100-amp campsite service.”

In the meantime, let’s play safe out there….

 

Mike Sokol is an electrical and professional sound expert with 40+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

##RVT900


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Abe Loughin
Abe Loughin

Mike, I am by nature skeptical when I hear someone proclaimed to be “an expert” in any field but you have won me over. I’m formerly an RVIA certified expert technician and consider myself proficient with electricity. I have to admit that in the past year or so I have learned more than one thing from you.
I currently work camp, mainly in maintenance and tend to end up being one of the go to guys for electrical issues and have used knowledge you’ve passed on on more than one occasion.
Keep up the good work, thanks for doing what you do.