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FAST FRED (65.154.177.181)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, November 25, 2003 - 6:11 am: | |
The marine industry is getting very concerned with another source of Electrical fire that could bite any of us.
In the US most dock or campground wiring , and perhaps the wiring inside the coach is 2 legs of 120V with 240V supply .
The loose neutral or open neutral can cause a big problem , a FIRE.
EG, there is a heavy draw 120V appliance on one leg (air cond?) wired to the coach neutral and a low draw 120v item (TV) on the other leg,with both on , and there is a poor conection to the coach or shore power neutral, that creates a high restance .
Because of its heavy draw the air cond has low internal resistance.
The net result will be that instead of the neutral line from the air cond being at ground (0) volts., it will be at somthing closer to line voltage(120V)all the way to the recepticle , and only at ground potental on the ground side of the resistance.
The TV which is wired across the other 120V leg, and the same neutral will now efectivly be wired across 2 X 120V legs to something near 240V.!!
This can cause a fire depending on what it is that is now being run on almost 240V.
A fault like this could be caused ANYPLACE in the campground supply line , if the 120V is taken , as usual from a 240V source.
The only "solution " ( for 240V input ,besides inspection and care and maint) is an isolation transformer, as the supply neutral is not conected into the coach, its bolted to the frame of the transformer.
There is nothing that can be done for just the 120V power user, if the fault is in the supply line.
Sugestion was to use a Panamax automatic overvoltagw & surge protection device.
www.panamax.com
IF we chose to use propane for heat , fridge , cooling ect , is not the question as all have some 120 or 240 aboard .
Information from Professional Boatbuilder #86 Dec/Jan 2004
FAST FRED |
DonTX/KS (66.82.9.32)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, November 25, 2003 - 6:52 am: | |
Now you are talking something to be really concerned about, campground wiring. I have seen it all in those places. I am not an electrician by trade, but got elected resident electrical repairman for a local RV Park, and have had to "fix" sites on the road to be acceptable.
Common practice is to totally ignore the "extra" pole on the four wire 50 amp, just saw an entire addition, not even one wire hooked to even one ground lug. Another I have seen several times, is to wire the RV 30a receptacle "just like any other range receptacle" and put two 120v legs to it. I would recommend ALWAYS checking what is going on at where you are going to plug your bus into. I always did, except for one time, I arrived late, was tired, and had stayed in that campground before-big mistake. It is not nice to see your bus putting on a fourth of July display, no fire, but lots of fireworks display! I do another check too, put one lead of the voltmeter against the bus, the other stuck into some moist dirt on the ground. |
Doug Dickinson (Dougd470) (65.161.188.11)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, November 25, 2003 - 9:29 am: | |
I had this same thing occur to me in my house! I lived in Pennsylvania and my house was fed from the pedestal with what is called a concentric grounded cable, meaning the two insulated hot lines are encircled by an aluminum wrap of the neutral wire. It has NO jacket so the aluminum wire is in contact with the usual Pennsylvania acidic clay soil.
After 10 years of living there, the neutral gave way and if it were not for the ground at the electric meter (where the bond is between neutral and ground) I would have exactly the situation FF describes. In my case, the voltage swing was about 30 volts - enough to see a light bulb get a good bit BRIGHTER when the AC kicked on.
I called the power company and the service guy told me they encounter this a lot in 1970s and 80s vintage homes in that area. No AL wire in the house but the feed to the house STILL IS.
FWIW
Doug
St Louis MC9 |
Phil Dumpster (12.230.214.167)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, November 27, 2003 - 8:44 am: | |
Concentric ground cable was a crazy idea that some engineer with no real world experience came up with.
The vast majority of high voltage drop service wire is aluminum. If it were copper, it would weigh so much that it would stretch under its own weight.
The hazards of a loose neutral problem in an RV can easily be prevented with a pair of circuit breakers or fuses and some large metal oxide varistors, with a 130 volt clamping voltage. These are commonly used in power strip surge supressors, but for an RV service you'd need some that are big enough to conduct a current at least as large as your circuit breakers are rated. These devices have a very high resistance across the terminals, but the resistance drops quickly when the voltage across the terminals approaches the clamping voltage, until the device appears as close to a dead short above the clamping voltage.
You put one circuit breaker on each leg of the AC coming in (not on the neutral wire) and wire the MOVs between each hot leg and the neutral, after the circuit breakers.
If the neutral connection is lost and the loads on each leg aren't balanced, the voltage across one load will increase. When it gets to 130 volts, the MOV will go to a low resistance, which does two things - it prevents the load from seeing a voltage much larger than 130 volts, and it causes the voltage across the load on the other leg to climb. When that happens, the MOV across that load will go to a low resistance. In a minor imbalance, the voltages will see-saw back and forth as one MOV clamps, then the other, then both open and the cycle repeats, and this will be visible in any incandescent lamp, or even a television set, which will alert you to the loose neutral problem. Extreme imbalances will cause a surge which will trip one of the circuit breakers or blow the fuse, also alerting you to the problem. If done correctly, no load on either leg will see a voltage greater than the clamping voltage of the MOV across that leg.
You can use multiple small MOVs wired in parallel, or you can use the really big and chunky ones. They are designed to absorb some pretty large surges without burning out, although I have a surge supressor that was hit with 25K volts when a car took out a utility pole down the street. The MOVs exploded and the insides of the supressor case was covered with soot, but the unit remained clamped as a dead short across the line even when power was restored. All this did was trip the breaker until the supressor was removed from the line. |
gary Stadler (68.7.217.217)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, November 27, 2003 - 9:24 am: | |
Woah a minute Phil... what you're describing is:
(a) no excuse (or fix) for faulty neutral wiring
(b) "might work" but if so, it would only work on 240 volt 4 wire systems (H-N-H-G) that are rarely found in RV's
If someone's going to take the time to mess with MOV's, that time and money would be better spent on doing the basic wiring job better in the first place. MOV's were never created as a "fix" for a broken neutral. They are for surge supression only...while they "may" temporarily balance a load that has a broken neutral on a 240 volt system, that's NOT what they are designed to do by any stretch of the imagination. Even if they did, the result would be a masking of the fact that there was a basic and serious problem, and may as Clarke Echols used to so strangely put it, someone may get "critically killed" (whatever that really meant!!)
Phil, this idea "might" work under some circumstances, although I personally would never rely on it for safety. It's too "iffy".
Also remember that a LOT of the folks here are NOT electrical or electronic engineers and may have a little understanding of electricity but not much. Presenting things like this to folks who don't understand it can get them in a lot of trouble....
My suggestion if you don't trust your neutral you'd better redo it correctly. Mount a little 120 volt neon indicator on your panel and hook it between ground and neutral, and any problems will instantly and glowingly make themselves known...
Cheers
gary |
Darryl (68.184.122.103)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, November 27, 2003 - 10:05 am: | |
Gary, I think the scenerio to which Phil is referring would be a neutral problem with campground wiring, not the wiring in your own coach. In the case of the former, it would not be practical to fix the cause of the problem, because you don't own it. You see my point. Phil's scenero was an interesting perspective.
I've been thinking lately about an issue that apparently isn't a problem, but appears to be related. When you have a 4 wire service (240V) you have two out of phase hots, one neutral and a ground. If you are pulling two different loads (one off each phase), you still have only one neutral for both. Why doesn't that overload the wiring for the one neutral?
Thanks for any replys.
Darryl Howell
Smyrna GA |
Abajaba (12.217.194.92)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, November 27, 2003 - 10:49 am: | |
The reason for the single neutral is that some of the return current is actually flowing through the other hot line.
I hope this ASCII art works.
120 VAC a o------------------------
Neutral o------------------------
120 VAC b o------------------------
If you have a 7 amp load on '120VAC a' and a 10 amp load on '120VAC b' then there will be only 3 amps flowing through the neutral. If the loads on both legs of the supply were equal then there would, in an ideal world, be no current flowing in the neutral wire. That is one of the reasons that the books all say to balance the loads as good as possible on a 240 volt source.
Al Butler remove any 'x' from email add to use.
ka0ies |
Richard Bowyer (Drivingmisslazy) (24.196.191.70)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, November 27, 2003 - 3:04 pm: | |
What Al states is correct for linear loads.
For non-linear loads such as solid state switching devices, SCR's ect. the neutral current can sometimes be double that of the primary current. In many computer installations, we doubled the size of the neutral as compared to the hot legs.
Richard |
Darryl Howell (Darryl) (68.184.122.103)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, November 27, 2003 - 8:32 pm: | |
That's really interesting Richard. For some reason, I've never noticed this subject being broached on either of the two boards. When I hooked up my Heart 3000, I brought in 50 amp 240 volt service to it. This particular model has two inputs (inputs can be either in-phase or out-of-phase) as well as two outputs. The second leg simply passes through --- the first actually runs the charger and does the power sharing if needed. Anyway, they require two neutrals be run with the hot legs. That got me thinking about this subject. Thanks for your info. |
Abajaba (12.217.194.92)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, November 28, 2003 - 1:03 am: | |
Hi Richard,
Yeah I should have put the statement that in a real world application the current could be higher than the calculations would show. Inductive or capacitive loads can really throw a curve ball at the calculations.
If every load in the system were pure resistance even then, the calculations could be wrong due to power factor on the supply side.
All one can really do is to try your best at balancing the loads on the lines and make it as even as possible.
I have noticed that some of the power supplies for personal computers these days claim to have power factor correction built into them. This would make it appear as a purely resistive load.
I have worked on some power supplies that did this and it really isn't too overly complicated. It just adds a few more parts to the power supply. I think in the future I will be getting power supplies for my computers that have this feature.
For clarification if there is a capacitive or inductive load on the AC power line what happens is that the maximum current draw is not at the same point as the maximum voltage. With an oscilloscope on the line looking at both voltage and current it would appear as if both sine waves were on top of each other with no power factor.
If there is a power factor in the system then the two sine waves would not be on top of each other. The current would either lead the voltage or lag behind it depending on if the load was capacitive or inductive.
Al Butler remove any 'x' from my e-mail to use it
ka0ies |
Richard Bowyer (Drivingmisslazy) (24.196.191.70)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, November 28, 2003 - 4:40 am: | |
Good explanation Al, on capacitive or inductive loads and the resulting change in current due to power factor. This is one reason everyone should talk in VA or KVA instead of Watts or KW. In most capacitive or inductive loads, the power factor never exceeds 0.8, so the overall current would be approximately 20% more than if it were a pure resistive load.
The other type of load, as you mentioned in computers, are non-linear loads. This is typically caused by switching power supplies in equipment such as computers, electronic furnaces or other equipment that uses large amounts of power and the amount of power used is controlled electronically. They take a great big gob of power for a portion of the waveform and create a large distortion of the wave form. These types of loads can cause the neutral current to actually double or more, as opposed to the 20% increase caused by the inductive or capacitive load.
Probably more than most need to know for wiring a bus, but I would strongly recommend that the neutral conductor always be at least as large as the main power conductors.
Richard |
David Anderson (168.215.176.194)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, November 29, 2003 - 6:41 pm: | |
I always knew about the neutral carrying only the unbalanced load from L1 and L2, but never knew about the non linear switching that Richard talked about. I learn something every time I come here.
Just for clarification, if those of us who have 240 volt generator applications, (me), how much do we need to worry about over loading the neutral (considering my loads are in close balance on each phase of L1 and L2?)
PS. neutral is #6THHN, same size as L1 and L2.
L1 and L2 on a 50 amp Double pole breaker.
David Anderson |
Abajaba (12.217.194.92)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, November 29, 2003 - 10:01 pm: | |
Hi David:
Tell us what you have in the way of loads on your system and we can give you a better answer. Most likely though you won't have a problem with it. The problem only comes into play when there are some BIG non-linear loads on the system.
remove any 'x' from e-mail to send me something.
Al Butler
ka0ies |
Richard Bowyer (Drivingmisslazy) (24.196.191.70)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, November 29, 2003 - 10:50 pm: | |
David, Al is correct and I seriously doubt if you will have a problem in your bus. I should have mentioned that when I talk about computers, I am talking about rooms full of P/Cs or the big main frame computers.
Richard |
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