Meltdown

[cabo_sailor]

A couple of days ago my shorepower cord melted down at the connection to the boat. Only the positive lead (black) was involved. I see no signs of arcing to either the negative (white) lead or the ground lead (green). I used a multimeter to check for shorts in the shorepower cord and found none.

The boat had been plugged into shorepower at this point for a couple of months while I did maintenance. The cord was securely plugged into the boat's receptacle and no circuit breakers tripped, not on shore and not on the boat.

I will replace the end of the shorepower cord and install a new receptacle on the boat. However, I will not sleep well until I know and understand what happened. I know how fast fiberglass will burn and I'd just as soon not see it happen from inside the hull.

Does anyone have any insight as to what happened and how to stop it from happening again??

Not quite ready for a viking funeral,
Rich

[GordMay]

It could have been a (partly) broken pin on the shore power inlet, causing a very high resistance connection.
Any current high enough to cause a "meltdown", should have tripped the shore-side supply breaker.

[cabo_sailor]

Gord,

That is what concerns me a bit. How can I have the positive post meltdown without any of THREE circuit breakers tripping.

1) the one breaker on the dock supply
2) the breaker where it enters the boat ( 10 inches from the meltdown)
3) the circuit breaker on the main panel

I mean basic electricity is NOT rocket science but this one has me baffled.

As I mentioned I did a continuity check on all the power cord leads looking for a short somewhere in the cord but it turned up negative. Doesn't mean it's not there but maybe intermittent.

I see two choices: repair what I have and watch
if something happens again - replace the entire power cord.

Choice two, which pains my wallet, skip number one and just replace the whole power cord.

I'm not real happy doing either if I don't understand what the problem is.

[Robertcateran]

I agree with Gord, except that it was melted at the connection to the boat. A partially broken cable would concentrate all the current through a few wires which would heat up. Consider that we have toasters and ovens that provide plenty of heat without tripping circuits It doesn't need enormous current to melt if there is only a small bit of wire. I would repair the power cord but be fairly generous with what you cut off and check thoroughly your connection to the boat. It seems just possible that some arcing to the boat connection could generate some heat that transferred back to the cord. There is also the possibility that the heat could hve caused some damage to the conection on the boat side.

[Hampus]

The only breaker that should have tripped is the one on shore. The others are after the meltdown, aren't they? You could have a total short in the connector without the breaker on shore tripping, if the cable is long enough with enough resistance. We have this problem on some of the lighthouses I work with.

A bit simplified as there are more things involved here, like the characteristics of the fuse etc;
Let's say you have a 10 AMP fuse and a long cable with a resistance of 12 ohms on a 110 volt grid. Ohms law sais the U = R * I -> 110 = 12 * I -> I = 9.16 A.
So, a short in the connector, a long cable and a bit of bad luck and you'll have a lot of little electrons flying through your broken connector causing a lot of heat. Measure your cable to check resistance.

/Hampus

[Bill Burgette]

When you replace the female end on the shore power cord, take a razor knife and dissect it. You will probably find evidence of the problem starting there. The evidence will be brittle insulation on the conductors, discolored copper on the contacts, etc. The female end of the cord uses spring pressure to grip the male end. After heat cycles or just plugging in many times, the spring loses its gripping power causing a higher resistance. That, in turn, creates enough heat to send the cord to melt down. The resistance presents itself as a normal load and the breaker is just happy. Were you using more power than usual? Running A/C or heaters?

Because the power is seeing a resistive load the breaker will not trip. Take the cord apart. You've nothing to lose. Good luck and I hope this helps you to sleep better.

[btrayfors]

We see meltdowns frequently. They are the result of several underlying factors, and one proximate factor: high resistance at the contact point(s).

The underlying factors are:

1. dirty, corroded, or damaged contacts; and/or
2. high current loads.

These happen quite often, and it is rare that a breaker will prevent them. As someone said, the high-resistance which results in serious over-heating often does not involve sufficient current to trip a breaker.

Another factor: 30A 120VAC connectors are really not capable of handling that level of current continuously. Well, maybe on a cold day with brand new male and female connectors making excellent tight contact. But you can't depend on this. To be safe, you need to ensure that no more than about 24-25A are pulled thru these connectors.

Many boats we've seen are wired incorrectly as well, such that they can attempt to pull a lot more than 30A thru a 30A connection. This is a disaster waiting to happen.

What to do in your case?

I'd replace the whole cord and the male connector on the boat. This won't cost much more than just replacing the male and female connectors, since you can find good deals on shore cords.

Mostly, be absolutely sure that your connectors are clean and tight, and that you don't try to pull more than 24-24 amps thru a 30A cord.

If you find your electrical needs to be close to the maximum sustainable, consider upgrading with another 30A circuit OR converting to a 50A circuit upgrade.

Bill

You most certainly can meltdown a powercable WITHOUT blowing a circuit breaker. It is not just current which is the problem, as Bill and Gord have both suggested RESISTANCE is the real problem.

The about of power generated in a circuit is equal to the Current(Squared) times the Resistance. Let's think about this for a minute...

Imagine you are pulling 25 amps through your shore power cord. The connection between the cord and the recepticale has a resistance of 0.2 ohms.

25 Amps^2 * 0.2Ohms = 125Watts! Imagine how hot your power plug would get if you had a 125 watt light blub buried inside! We aren't anywhere close to the tripping point of any of the breakers in the system.

Low resistance connections are VITAL in shore power connections. Clean, shiny, and tight.

This time of year sees a sudden increase in melted power cords as people turn on electric heaters in their boats. I think Bill is a bit pesimistic about power cords in good condition being able to handle their full 30 AMP rating, but I know we would agree that the key word there is GOOD condition. Most people never check them, and just wait until they melt to see the problem. An annual disassembly and cleaning woudl go a long way to reducing electrical fire hazards. The problem isn't often in the cord itslef, but in the connection between the cord and recepticale. The cord is just the "weak link" when things heat up.

Bill K

[btrayfors]

Yep. Bill K nailed it; thanks for the explanation of how meltdowns can happen without blowing breakers.

Another problem, "splitter cables".

Walk down the docks in any marina and you'll see a profusion of "splitter cables". These plug into 220V 50A outlets on the dock, and split off two 30A 120V connections. This is the way many boats feed their two 30A circuits onboard.

Problem is, these splitters are protected only by the 50A breaker upstream, so that it's possible to pull up to 50A thru EACH of the two 30A connections without causing the breaker on the dock to pop. (This is because each of the single-phase 120V circuits are actually different phases of a multiphase system on the dock.)

These damned things are downright dangerous, IMO. I really don't see how they can be sold without CPDs installed in each of the two split legs, but they are and they're everywhere. BE ESPECIALLY CAUTIOUS when using them.

Bill

[cabo_sailor]

Thanks to all of you,

I now believe I have the culprit. Recently I was using a new heat gun to strip varnish on the boat. Not being totally stupid, I made sure to turn off all other AC circuits. Nevertheless, this new gun is more powerful than the one I used in the past and is most likely responsible. For future use I'll either go back to my old heat gun or not crank up the power on the new one.

I had suspected it might be the heat gun but was led astray by it not popping any breakers. Thanks again for the education.

Rich

[witzgall]

Cabo;
I had this happen while I was living on an old wodden cabin cruiser in College. It did not just melt, it caught fire. If I was not there the boat, and maybe others on the dock, would have been lost. I was running a ceramic heater 24x7 at the time, plus the fridge.

The heat gun you have most likely won't draw more than 12-13 amps, I have never seen one that draws more. So I would not blame the heat gun. How old was the cable and the shoreside connector?

[cabo_sailor]

The cable and shorepower receptacle are not more than a year or two old. During a thunderstorm the whole dock took a power surge. I was below at the time and all the lights got REAL bright for a moment. I smelled smoke and very quick inspection found that the shore power plug and the receptacle had almost literally melted into each other. Several other boats on the dock had the same problem. So, to cut to the chase, neither are that old.

Note that I also used my multimeter to see if one or more of the wires in the cable were shorted with one another. Nada, infinite resistance between all pair combos.

[Cowboy Sailer]

This former rocket scientist electrical engineer is sitting here just shaking his head.

Merry Christmas!

[cabo_sailor]

Well Cowboy Sailor,

That has to be one of the more intelligent and useful replies I've ever seen. I'm afraid I was never a rocket scientist or an electrical engineer but I was an earth scientist specializing in statistical hydrology. Pardon the rest of us for breathing on your planet.

[Bill Burgette]

This former rocket scientist electrical engineer is sitting here just shaking his head

Well Jack, don't get a sore neck. We're just trying to help. It's easier to assist then wobble your head.

Cabo Sailor, I'll stick by my previous answer. Your multimeter is probably powered by a 9 volt battery. To really check for continuity you'll need to use a megger. It applies a real world voltage to the cable and acts as a high voltage multimeter......kinda. The push supplied by 9 volts may not be enough to cause a breakdown in the insulation or the contacts. The push supplied by a megger (actually a megohm meter) can be the same voltage you are using. Don't touch the leads or you will find out. Most have 250 volts as the lowest setting.

I didn't really want to get this technical, but I'm not retired and am a current electrical/electronic engineer. Now if I could just learn to work on rockets.

Good luck!! --Bill--