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Old 07-30-2013, 11:00 PM   #99
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Great you got the grey matter in gear and dusted off those old books. It is a bad day when I don't learn or relearn something. Mostly now since I am in my mid 70's I forgot what I knew or don't know what I forgot if that makes sense.

I said it has been 39 years since I taught that stuff, actually it is more like 47 years after I reflected on it a bit but most of the basics are not burried too deep in my old memory banks.

Came accross my slide rule a couple of weeks ago. I remembered some stuff but I scratched my head a few times on some of the tougher things. So many formulas you need in your head to get proper results. Getting a scientific calculator was a huge leap forward back in the day wherein it enabled actual results vs close within the 3 decimal points you could get with a slide rule.

Good forum for stimulating the brain and boy do I need stimulating.
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Old 07-30-2013, 11:12 PM   #100
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Originally Posted by steveclv View Post
It's a bloody ac circuit!!! Not dc
Treat resistance as Impedance. Then calculate the impedence. It is.simple if you do that and ohms law still applies. And your point is? ???

As long as the load in AC is resistive you do not need to convert to impedence.

Impedance being the squareroot of the squared resultant vector of the XL, XC + the squared resistance vector. Calculating the Xl and Xc involve other factors such as frequency and capacitive and inductive components. But I am not even going to go there.

Perhaps steveclv you were not really following all of the discussion on this, it was not specifically relating to an AC circuit from a general formula discussion point so I will give you slack and consider you comment as constructive. Thanks for pointing that out.
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Old 07-31-2013, 04:50 PM   #101
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With all of this belly bumping... me among yea'all. How much flux (inductance) is produced by a twisted pair?
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Old 07-31-2013, 05:28 PM   #102
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With all of this belly bumping... me among yea'all. How much flux (inductance) is produced by a twisted pair?
Ideally zero. The purpose of using twisted pair cable is to minimize electromagnetic induction and can be calculated using the wire size, insulation thickness, and dielectric constant of the insulation.

Not relevant to any motor home power supply cables and certainly not relevant to the OP however...
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Old 08-01-2013, 06:05 AM   #103
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Not completely true in relevance, Bryan. All of our shore cables have some twist in them and on 50A a balanced neutral should have minimum inductance in the cable as I understand it. I am out of school too many years to remember testing this in the lab.

I think poor connection resistance and the misuse of extension cords is a great threat to RV's than a coil in the power bay. I never recall heat buildup on my coil, though it is never tight nor many loops. Any heat problems I have had are always at the plug or circuit breaker. Poor connection, week contacts, heat under load. Too much heat and parts fail. We all agree on this.

Twisting wires is a science of its own!
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Old 08-01-2013, 09:56 AM   #104
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Thank you for this information. Good stuff to carry in the Electrical Bay.
BTW, this stuff is GREAT for all sorts of electrical contact problems. I have used it on my truck steering wheel radio controls, fantastic fan thermostat, and most of all on park cable TV connections - gets the interference out!
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Old 08-01-2013, 10:08 AM   #105
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I am not an Electrical Engineer, just a lowly baggy assed mechanic but I can tell you that if it is getting hot, you have resistance and it Will cause a problem eventually.

Just as a matter of good practice, one should be checking the shore connectors each time before use. If they LOOK Bad, junk em and replace. Don't try and save money by making your own. This stuff is not really expensive. Try to look for the UL or CSA sticker that hopefully will tell you it is not some "Made in China Junk"
BTW, as stated by the experts, make all your connections Outside of the Electrical Bay and if there is a problem it won't burn your rig down.

Not Rocket Science, just "Good Old Common Sense"
Most people do not realize that electrical connections getting hot is part of an endless loop syndrome that usually ends in connection failure. Here's how it goes: A little bit of resistance and a healthy current flow will generate some heat, by Ohm's Power Law - Power(Heat) is equal to the resistance times amount of current squared (RxIxI). This heat and the oxygen in the air (and other airborne contaminants) causes corrosion of the contact surfaces and more resistance. More resistance gives more heat. And on and on the cycle goes until something smokes.
The best connections are gas tight metal-to-metal. I like crimped and soldered connections best. Unfortunately, you can't do this with stuff that needs to be connected-disconnected frequently.

All of this discussion about inductance may have some bearing, but the bottom line is: Where do the cables fail? At the connection ends, not in the middle (unless they are abused).
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Old 08-01-2013, 11:50 AM   #106
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Treat resistance as Impedance. Then calculate the impedence. It is.simple if you do that and ohms law still applies. And your point is? ???

As long as the load in AC is resistive you do not need to convert to impedence.

Impedance being the squareroot of the squared resultant vector of the XL, XC + the squared resistance vector. Calculating the Xl and Xc involve other factors such as frequency and capacitive and inductive components. But I am not even going to go there.

Perhaps steveclv you were not really following all of the discussion on this, it was not specifically relating to an AC circuit from a general formula discussion point so I will give you slack and consider you comment as constructive. Thanks for pointing that out.
You cheater LOL No AC circuit is purely resistive - they all have at least an inductive component.

The thread evolved into a discussion regarding the power cables from the RV to the Pedestal and then everyone started throwing in DC theory.

As we were also discussing looped cables, there is a significant inductive component in there too and I suspect with a 4 wire 220V 50amp cable that capacitance also plays a part - so I was just reminding everyone who was contributing their DC theory that we should be considering AC.

It was meant a bit tongue in cheek tho
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Old 08-01-2013, 12:01 PM   #107
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I for one am more concerned that I remember to unplug the cable before I drive off.
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Old 08-01-2013, 12:35 PM   #108
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STEVECLV - COOL no sweat. few things in this world are 100% except death and taxes perhaps and if anyone can figure out a way around these it would be worth millions.
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Old 08-01-2013, 01:18 PM   #109
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I for one am more concerned that I remember to unplug the cable before I drive off.
Hear, hear.
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Old 08-01-2013, 01:39 PM   #110
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D & T

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STEVECLV - COOL no sweat. few things in this world are 100% except death and taxes perhaps and if anyone can figure out a way around these it would be worth millions.
If you participate in the first, you can get around the second (at least for yourself).

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Old 08-01-2013, 06:53 PM   #111
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I was camping with our local WIT club this weekend. One of our club members had a fire in his 2000 Itasca Meridian at about 12:30am Saturday morning.

Apparently the 30A to 50A adapter shorted while in his electrical bay, resulting in a ball of fire inside the compartment.

They were in bed, but heard a loud 'pop'. The coach quickly filled with smoke, and they immediately got out of the motorhome. The owner used a fire extinguisher and got the flames down, and the fire truck shortly arrived. Their smoke alarm did go off.

Next door neighbor was a retired sheriff deputy, and quickly helped out. Thanks to him, they are both safe.

First picture shows the shorted 30A to 50A adapter which apparently caused the fire.

Second picture shows the damage to the electrical compartment. There was significant damage to the coach, and of course smoke damage inside.

Here are my learnings:

1) Always use the shortest length of shore supply cord to the power pedestal, starting with your 50A shore power cord, to run to the power post. If necessary, use the 30A to 50A adapter at the pedestal; DO NOT keep it coiled up inside your coach, and simply run a 30A extension cord to the power post.

Use the adapter outside of the coach. If it does short out, apparently as happened in this situation, it will happen outside of the coach.

Don't keep extra lengths of extension cords on your shore cord if you don't need them. 30A plugs have a habit of getting hot and melting the connections. The longer the run of wire, the more resistance, potentially causing heat build-up at the connections.

2) 30-to-50A adapters cost about $14. Replace them every few years.
Thanks to everyone for all the posts on electrical theory, etc., possible theories on causing this fire.

If you look at the first picture I posted, you can zoom in. It clearly shows (to me at least) that the 30A male plug on the 30-50A adapter is cooked. And no, it's not modified.

My recommendations are made to help other MH owner's out there who hopefully will never find themselves in a similar situation.

1) Use the shortest length of shore cord possible to the power pedestal.

2) Keep the 30-50A adapter outside of your coach. If it does catch on fire, it won't burn your coach.

3) Watch all 30A male connections. They melt and fail. Get a new 30-50A adapter every several years.

Safe Motorhoming.
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Old 08-02-2013, 07:49 AM   #112
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I guess that all of the theory, formulas and capacitive/inductive impedance arguments each of us is responsible for our own riggs. At least we have a chance of discovering the threat in our shore connections. We are at the mercy of the refrigerator manufacturers when it comes to a fire there. One of the Sowers lost their 5th wheel because of a frig fire. I hope none of us ever face ANY rig fires!!!

This has been a great discussion. Great input all!
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