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Old 04-10-2017, 09:17 PM   #21
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I did more digging and it turns out that the result of an open neutral are even more complicated than so far described......

So, I am now a believer. For the record, I've used a monitor/protector for a long time, and this year added a Hughes Autoformer.......

Here's more science...

Quote:
Normally the neutral makes each side a 120v leg but if the neutral is gone, the center reference is basically gone as well.
So it's floating to whatever the two leg's devices allow it to float at.
Let me give you an example.
Suppose you have a 1200 watt @ 120 VAC heater plugged into one 120 VAC side, and a 600 watt @120 VAC heater plugged into the other.
a 1200 watt heater @ 120 V == 10 amps (pure resistive load), which means the resistance is about 12 ohms. A 600 watt heater will be about 24 ohms.
Together, those are in effect now a resistor dividing network, and if you measure the voltage on the floating neutral line from each hot, you'll not get 120v, but more like 60 and 180

So, one 120v leg is now at 60 VAC, and one is at 180 VAC. It would be complicated to describe how this would affect things -- in the heater world, chances are the element would burn out as it was not intended for this use, but different things like motors or power supplies would work badly when they get arbitrary voltages.

Also, if one leg had only one resistive device, like that heater, the voltage change would likely cause too much current to flow, and it would burn out (at best)

The math is harder when you consider things like power supplies, motors, etc.
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Old 04-10-2017, 09:54 PM   #22
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X2 Oscar - Progressive EMS & a Hughs Autoformer for the win. Don't leave home without them!
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Old 04-11-2017, 11:44 AM   #23
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Oscar,
I'm glad that you found another reference that confirms my explanation.

But, to be frank, it doesn't say any more than what I said. I just provided the formulas for the calculations that the other reference left out.

–Gordon
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Old 04-12-2017, 01:18 PM   #24
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Trip the post breaker before connecting and disconnecting 50A shore power

After thinking about the open neutral problems, and realizing that one of the most likely causes of an open neutral is a bad connection between our power cords and the posts, I realized that it is possible to momentarily have an open neutral while connecting or disconnecting our power cords to a 50A post. Thus, it is advisable to turn off the power post circuit breaker before connecting or disconnecting from shore power. This is only necessary for 50A power connections and not 30A or 15A power connections, because the latter don't have a dangerous situation if the neutral is open.

I've been lackadaisical about this, and will be more careful in the future.

Here is how the problem can arise: A 50A power plug has one long round pin for ground and 3 equal-length blades for power. (The ground pin connects first and disconnects last, which is good.) The neutral blade is in the middle. If the outside (hot) pins connect before the neutral, you have an open neutral until the neutral blade connects as well. This could happen if you connect by pushing on the power cord at the same time as the plug, because the neutral blade would be angled away from the receptacle.

Turning off the circuit breaker alleviates the problem.

I realize that the shore power transfer switch on our coaches has a delay that might mitigate the problem while connecting, but there can be nothing more exciting than racing against the transfer switch every time we power up. And, the delay in the transfer switch won't protect us when we disconnect.

–Gordon
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Old 04-12-2017, 02:38 PM   #25
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Transfer Switches that protect against open neutrals and surges

Noticing that people are installing hardwired surge protectors, and that the power transfer switches could be used to test for an open neutral before transferring to shore power, I looked up a few transfer switches that claim to sense open neutrals and protect against surges. They can be used to replace our existing transfer switches if they don't have this capability. As a replacement, they won't take up valuable space in the area where we store our power cord.

SurgeGuard
Surge Guard RVC Compatible ATS with Full RV Power Protection | Technology Research, LLC
Surge Guard 50A Automatic Transfer Switch with Limited Protection | Technology Research, LLC

Progressive Dynamics
Automatic Surge Protected Transfer Switch

I'll be looking into my transfer switch to see what features it has when I next get my RV out of storage.

–Gordon
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Old 04-22-2017, 09:39 AM   #26
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I replaced mine with the Progressive Dynamic product. It was. 30$ more than the original. It's been in for about a year and works good.
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