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wnjj

Cornelius, Oregon

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Posted: 04/26/21 12:32am Link  |  Quote  |  Print  |  Notify Moderator

Bobbo wrote:

DrewE wrote:

Bobbo wrote:

wnjj wrote:

The other thing that can fry stuff in a RV is a standard 4-pin 50A connection with an open neutral. That can cause half of the devices in your RV to see much higher than 120V and why many suggest installing some kind of protection system.

Yes, an open neutral on a 50 amp rig will fry stuff. However, it doesn't fry just half of the stuff, it fries everything. Since there is no NEUTRAL, everything sees the full 240v. Fortunately, an open NEUTRAL is rare.


Everything doesn't see 240V, not at the same time. The 240V is split between the two legs; the one with the higher load and hence lower impedance sees proportionally more of the voltage, and the one with the lower load sees less voltage. In theory, if the legs were perfectly balanced, there would be no change in their voltage and no problems at all; indeed, in that situation, there is no neutral current, so it's effectively an open circuit anyhow. The legs are not perfectly balanced in normal cases, though, and how far out of balance they are can change rather rapidly and spectacularly if devices are damaged and become open circuits (or short circuits, followed by their circuit breaker tripping, making an open circuit).

A broken neutral is indeed very bad news. It does not, however, lead to 240V at all the outlets and loads.

Agreed that all items don't see 240v AT THE SAME TIME. However, when leg 1 sees 240v and blows out its 120v items, they stop pulling power and leg 2 now sees the 240v. So, saying they don't see 240v AT THE SAME TIME is accurate. It is milliseconds between the two.

When the fried items “blow” and stop pulling power, the circuit is now open and nothing sees any or voltage. The L1 and L2 loads are in series so when one side loses all of its devices and there’s no neutral, there is isn’t a complete circuit.

The more heavily loaded side pulls down below 120v and the other side goes up. As items blow open, the mismatch is greater until the lightest loaded side goes open circuit. The heavily loaded side will never see more than 120v.

Bobbo

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Posted: 04/26/21 05:36am Link  |  Quote  |  Print  |  Notify Moderator

wnjj wrote:

Bobbo wrote:

DrewE wrote:

Bobbo wrote:

wnjj wrote:

The other thing that can fry stuff in a RV is a standard 4-pin 50A connection with an open neutral. That can cause half of the devices in your RV to see much higher than 120V and why many suggest installing some kind of protection system.

Yes, an open neutral on a 50 amp rig will fry stuff. However, it doesn't fry just half of the stuff, it fries everything. Since there is no NEUTRAL, everything sees the full 240v. Fortunately, an open NEUTRAL is rare.


Everything doesn't see 240V, not at the same time. The 240V is split between the two legs; the one with the higher load and hence lower impedance sees proportionally more of the voltage, and the one with the lower load sees less voltage. In theory, if the legs were perfectly balanced, there would be no change in their voltage and no problems at all; indeed, in that situation, there is no neutral current, so it's effectively an open circuit anyhow. The legs are not perfectly balanced in normal cases, though, and how far out of balance they are can change rather rapidly and spectacularly if devices are damaged and become open circuits (or short circuits, followed by their circuit breaker tripping, making an open circuit).

A broken neutral is indeed very bad news. It does not, however, lead to 240V at all the outlets and loads.

Agreed that all items don't see 240v AT THE SAME TIME. However, when leg 1 sees 240v and blows out its 120v items, they stop pulling power and leg 2 now sees the 240v. So, saying they don't see 240v AT THE SAME TIME is accurate. It is milliseconds between the two.

When the fried items “blow” and stop pulling power, the circuit is now open and nothing sees any or voltage. The L1 and L2 loads are in series so when one side loses all of its devices and there’s no neutral, there is isn’t a complete circuit.

The more heavily loaded side pulls down below 120v and the other side goes up. As items blow open, the mismatch is greater until the lightest loaded side goes open circuit. The heavily loaded side will never see more than 120v.

You are correct, if you assume that everything, literally EVERYTHING, on the dead leg blows. If anything, anything at all, keeps pulling current, the other leg does see 240v. A lot of stuff that plugs in, in this day and age, are rated for 100v to 240v and 50hz to 60hz. Those items will keep pulling current. I don't think it is safe to assume that the first leg will completely blow out and the second leg will be saved. But, I don't usually deal in absolutes. It seems much safer to me to say everything in the RV is at risk than to say that half of the RV will be safe. It may, or it may not be.


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joebedford

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Posted: 04/26/21 02:21pm Link  |  Quote  |  Print  |  Notify Moderator

This is the RV receptacle I bought at Home Depot. They should know what it is because it's on the shelves in the store as are others.

RV 50A receptacle at Home Depot

wnjj

Cornelius, Oregon

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Posted: 04/26/21 02:33pm Link  |  Quote  |  Print  |  Notify Moderator

Bobbo wrote:

wnjj wrote:


The more heavily loaded side pulls down below 120v and the other side goes up. As items blow open, the mismatch is greater until the lightest loaded side goes open circuit. The heavily loaded side will never see more than 120v.

You are correct, if you assume that everything, literally EVERYTHING, on the dead leg blows. If anything, anything at all, keeps pulling current, the other leg does see 240v. A lot of stuff that plugs in, in this day and age, are rated for 100v to 240v and 50hz to 60hz. Those items will keep pulling current. I don't think it is safe to assume that the first leg will completely blow out and the second leg will be saved. But, I don't usually deal in absolutes. It seems much safer to me to say everything in the RV is at risk than to say that half of the RV will be safe. It may, or it may not be.

That's not quite right. Based upon past discussions I know DrewE knows his stuff, but got it reversed in this case. [emoticon] Lower impedance means lower voltage across that impedance and higher across the high impedance ones. If some of the loads on the high voltage leg blow, the impedance and therefore voltage across those remaining loads increases which continues to lower the voltage across the other leg. So one side will only see decreasing voltage unless all loads go away on the high side after which the low side sees nothing. Nevertheless, this can still be damaging the some devices that do not like low voltage (e.g. A/C).

Dutch_12078

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Posted: 04/26/21 05:47pm Link  |  Quote  |  Print  |  Notify Moderator

joebedford wrote:

This is the RV receptacle I bought at Home Depot. They should know what it is because it's on the shelves in the store as are others.

RV 50A receptacle at Home Depot

Yes, the housing is typically used for RV power, but the outlet inside is a standard NEMA 14-50, the same outlet used for many ranges and other 50 amp appliances. A friend of ours installed this exact box and outlet for the range in his summer kitchen at his farm in Vermont. He installed a second one on the corn crib/storage shed at our RV site there.


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grizz272

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Posted: 04/28/21 05:24am Link  |  Quote  |  Print  |  Notify Moderator

Well yesterday was interesting. After opening ad closing the outlet box a 1/2 dozen times the rivet on the left side of the top came off. This left the top hanging by only 1 rivet. Of course this was just after I had mounted the box to my garage wall. Looking closely at the rivet it was not set right and fell out. Today I need to buy a 5/32 screw, washers and a nut to replace the rivet.
I will probably buy 2 of everything so when the other side fails I can fix it right away.

Bobbo

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Posted: 04/28/21 05:28am Link  |  Quote  |  Print  |  Notify Moderator

Or drill out the other rivet and replace it now to be done with it.

Cummins12V98

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Posted: 04/28/21 09:05am Link  |  Quote  |  Print  |  Notify Moderator

Bobbo wrote:

Or drill out the other rivet and replace it now to be done with it.


That's how I roll.


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Cummins12V98

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Posted: 04/28/21 09:08am Link  |  Quote  |  Print  |  Notify Moderator

grizz272 wrote:

Well yesterday was interesting. After opening ad closing the outlet box a 1/2 dozen times the rivet on the left side of the top came off. This left the top hanging by only 1 rivet. Of course this was just after I had mounted the box to my garage wall. Looking closely at the rivet it was not set right and fell out. Today I need to buy a 5/32 screw, washers and a nut to replace the rivet.
I will probably buy 2 of everything so when the other side fails I can fix it right away.


I have installed several of that exact model with zero issues. The one i installed at our RV pad has been there for 15 years. That in my opinion is one of the higher quality items like that. Shows anything can have a defect.

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