By Mike Sokol
I found this picture of a 50-amp shore power plug meltdown last week, and posted it on my RVelectricity Facebook page asking for guesses as to how it happened. The comments and guesses went on for 2 days, and I finally posted my conclusions, which I’ll share with you below.
If you want to read the entire thread (you don’t have to join Facebook to see comments, only if you want to make a comment yourself), please click HERE.
What am I looking at?
It’s a stock replacement 50-amp shore power plug that was probably installed by an RV owner. Generally, factory-built shore power plugs are molded at the factory so you can’t see inside of them. But in this case we have a clear view of both the inside and outside of the replacement plug.
Is this normal?
While I have dozens of examples of 30-amp shore power plugs that are melted down like this, overheated 50-amp shore power plugs are much more rare. I think that’s because a 30-amp shore power connection is typically run much closer to its maximum amperage limits (and 3,600 watts of power). However, a 50-amp shore power connection (which is actually 2 legs with 50-amps each for 100-amps total) isn’t run nearly as close to its 12,000 watts of power.
A properly assembled and maintained shore power plug connected to a properly installed and maintained pedestal outlet should be able to maintain full rated amperage indefinitely.
Why did this happen?
I used Occam’s Razor (one of my favorite troubleshooting tools) to strip away all the complicated explanations and come up with the simplest possible answer. But I’ll give you the bullet list of the highlights which could help you avoid a similar shore power plug meltdown.
What it wasn’t…
- It wasn’t caused by low voltage, since contrary to urban myth that does not increase current draw, except for the air conditioner compressor motor at under 100 volts.
- It wasn’t caused by a loose or worn pedestal outlet, because there’s more heat damage on the inside of the plug than the outside contacts.
- It’s not caused by plugging in and out under load since that would have created a high resistance connection on the outside contacts, but the inside of the plug is more overheated than the exterior.
- It wasn’t caused by oxidation of the contacts causing a high resistance connection. Yes, there is a lot of oxidation on the screws, but this is likely a side effect of the hot air drawing in moisture either from convection air currents, or the bellows effect of heating and cooling drawing in more moist (and maybe even salt) air.
- It wasn’t caused by some kind of “surge” since this took sustained heating to create this level of melting, and by definition a “surge” typically lasts a fraction of the second.
- It wasn’t caused by a bootleg/miswired single-pole 50-amp outlet, since that would burn up the neutral, not one of the hot legs.
What it most likely was…
My best guess is that the terminal screw on the black wire of the shore power plug wasn’t properly tightened, which resulted in the high-resistance connection which began heating up. Once that begins to occur then it will soon go into cascade failure, where more heating begets more resistance, which causes more heat, which results in more resistance, etc., etc.
What can I do to prevent this?
Well, any screw terminal connection on an RV is subject to loosening from road vibration, and it’s possible this wasn’t properly torqued when it was assembled. So I think that yearly periodic inspection and maintenance of these types of electrical connections are a great idea.
And yes, you’ll really want a torque limiting screwdriver since over-torquing a screw terminal can be just as damaging as under-torquing it, or allowing it to loosen over time. I like this inexpensive one since it stores all the drive bits in the handle. Get it on Amazon HERE.
Feel the heat….
Also, I’m a big fan of infrared thermometers – which can show you if something is beginning to overheat before damage occurs. Then you can take evasive action before meltdown. These thermometers are also great at finding a dragging brake shoe or pad, wheel bearing that’s beginning to overheat, or even a tire that’s running hot.
I have several different ones on my bench ranging from very expensive ones from Fluke, to really cheap ones for your home oven. But I really like this one from Southwire for use on the road. Get it at Lowe’s or from Amazon for less than $40 HERE.
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 50+ 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.
For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign.