# Shorepower and Reverse Polarity



## oft

Perhaps someone can enlighten us about reverse polarity. Don Casey's tip (copied below) is clear enough, but still fails to explain our 'current' situation.

Earlier today we went for the first sail of the season. When we arrived at the dock all was well with the shorepower that we had been connected to over the winter. Upon our return from sailing I plugged us back in and the AC panel indicated reverse polarity so I disconnected shorepower. The reverse polarity indicator lit up once last year when one of the batteries died, but it has since been replaced and this time both batteries measured 13V. The only thing that changed was the boat that has the other outlet/meter on our electrical post had plugged in and was charging. A few questions:

1) Any ideas why after 1 year a reverse polarity problem would appear? Could it involve the old boat charging next door or are marine meters/outlets usually adequately isolated from one another?

2) What implications does reverse polarity have for charging batteries?

3) How can one adequately protect themselves from bad shorepower? Is an older Protech 4 enough or???

Thanks

*Don Casey Tip #40*

Alternating Current/Reverse Polarity

Since alternating current, by definition, flows in one direction then the other, what is meant by polarity when applied to an AC shorepower connection and why is polarity so important on a boat? Even though the current flow reverses, the "hot" wire is connected to the generator at the power plant and the "neutral" wire is connected to ground there. That means the electricity flows to us through the hot wire. All switches and circuit breakers must be in this side of the circuit to disconnect the load from the power.

Now suppose connections to the dockside receptacle are reversed. That puts all the AC breakers on the boat in the neutral side of the circuit. An overload might still trip the breaker, but since the breaker is in the neutral side, the circuit is unprotected from a short. Current will continue to flow until the circuit burns open. A fire aboard is the likely consequence.

Reversed polarity also presents a serious shock risk. Turning off a breaker appears to remove power from the circuit because it turns off all appliances connected to that circuit. But with reversed polarity you have disconnected the appliance from ground, not from power. The circuit is stll live!

If your AC switch panel does not have a polarity tester, buy a plug-in tester and use it. Most also detect an open grounding wire and other dangerous conditions.


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## sailingdog

> 1) Any ideas why after 1 year a reverse polarity problem would appear? Could it involve the old boat charging next door or are marine meters/outlets usually adequately isolated from one another?


They may have re-wired something over the winter. That could have reversed the polarity of the wiring at the dockside, even though nothing appears to have changed.



> 2) What implications does reverse polarity have for charging batteries?


Maybe none/maybe a lot-the batteries will still charge normally in most cases. It depends on the AC-based charger you have. Some will not work if the polarity is reversed.



> 3) How can one adequately protect themselves from bad shorepower? Is an older Protech 4 enough or???


The real danger with reversed polarity is that the "neutral" wire is now *"HOT"* and on the side without the breakers. *So if you decide to shut off the shore-power, but leave the cable connected, the "neutral" terminals at all of the outlets are still going to be live and electrocution is a good possibility. *


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## Valiente

So _that's_ what that little red light on the AC panel means...


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## donradclife

The little red light is hooked up between the neutral and the ground wire, and as the other posts indicate, it means somewhere in the shore power system the neutral and load wires have been switched. Stick your mutimeter (carefully) in the marina socket--Assuming you are using 110V, if you measure 110V between the neutral and the ground, talk to them about their wiring.

On a 15 amp North American circuit, the neutral is the wider blade and the ground is the round plug.

On a 30 amp circuit, the ground is the blade with the extra bend. Looking at the socket, if the ground is at 6 o'clock, the neutral should be at 10 and the load should be at 2.


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## TSOJOURNER

If that other boat is not wired correctly, your panel sensor may be detecting it. I'd start by unplugging the other boat momentarily and see if your red light goes out. If that doesn't change anything, get a portable polarity tester and test the marina's outlet. If it shows it's reversed, then it's a problem they will need to correct.


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## Valiente

Guys, I was joking. I am new to the boat, but not new to the topic. My little red light is currently off (no pun intended) and I hope it stays that way.


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## sailaway21

And all it takes is one outlet with the hot wired to the neutral terminal to cause a problem. This condition can "backfeed" the entire neutral bar, energizing it, and in essence negating the breakers. Bears investigation soonest.


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## wlcoxe

Just had a similar problem. I installed a galvanic isolator during the spring prep period and it worked great, including at the dock after launch. Went for a short sail, came back, and the RP sounded every time I lit off the inverter. Dock-side sages said that blowing the isolator would do that. Took the isolator out of the circuit, no more alarm, will monitor zincs closely. Heard tell that there was a surge at the dock at some point, but it would have been over unless it occurred in the short time between plug-in and turn on. FWIW.
Bill Coxe, O40 Kukulcán, New London, CT


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## sailingdog

USCGRET has a good point. It very well could be a wiring problem on the other boat causing problems with yours. Unplug it and see what your boat reports.  If no change, ask the marina what they changed over the winter??


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## oft

Thank you for all of the input. 

We managed to get back down to the boat and alas the problem has had the nerve not to self-correct (and since the boat next door was no longer plugged in we'll have a hard time continuing to blame it on them). When we briefly tried plugging our boat in again (she was left not plugged in), we found that not only did the reverse polarity indicator light up but also that we had no power at any of our 120V outlets. Attempting to throw and reset the circuit breaker and the GFI outlets had no effect. I'll explore further with the marina any changes they have made, but any answers to a couple more questions would be greatly appreciated:

1) What impact could it have on a boat if the marina wiring (30A) was changed so that when the boat was plugged in the ground, neutral, and hot wires got switched (e.g., if hot went to the neutral or ground wire on the boat)? Could these switched wires have taken out my 120V system (from what I could find there were no obvious burn marks, triggered breakers, etc.) and how? 

2) Given all that can happen, is it the best policy to always leave one's boat connected to shorepower?

Thanks again.


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## ebs001

Your GFI probably won't reset because of the reverse polarity and therefore none of your other 120v receptacles will work. You need to check the polarity of the marinas 30 amp receptacle. You could also try plugging into the other 30 amp receptacle on the same pedestal to see if that corrects the situation. Then you'll know if it's the pedistal or just your receptacle.


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## sailingdog

Did you check with the marina to see if they had done any work on the wiring over the winter? If so, they may have accidentally reversed the polarity on the system.


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## phaiakian

credit goes to Art Richard IP32 #34 "Lagniappe" (St. Petersburg, FL) for the following dialogue:

"LOW GLOW OF REVERSE POLARITY LIGHT:
A low-level reverse polarity light on an IP when there is heavy current draw (like a heater or air conditioner) is not unusual. The reason is as follows:

1. The reverse polarity light is connected between the AC neutral (white) and AC ground (green) wires inside the distribution panel. If there is a voltage between these two lines, the light lights in proportion to the voltage.

2. When using shore power, the neutral and ground wires are connected together somewhere on the shore at a power distribution box. Between that point and your boat load (heater) are many feet of wire. Normally the AC current flows through the hot (black) wire to the load (heater) and back through the neutral (white) wire to the aforementioned distribution box (of course the current also flows through the black and white wire back to the power generator, but that is not pertinent to this discussion).

3. When current flows through a wire, the small resistance of the wire causes a voltage drop which is proportional to the length of the wire and the resistance of the wire. You see this on your AC voltage meter when you turn on a high current load (heater). Depending on the load and the length of wire, this voltage drop is sometimes 10 or more volts.

4. The reverse polarity lamp is effectively measuring the voltage between the neutral and ground wires at the boat. Since the ground wire is not (normally) carrying any current, there is no voltage drop in this wire. The neutral wire however is carrying current and does have a voltage drop. This voltage drop in the neutral wire appears across the reverse polarity lamp, causing it to glow, the brightness depending on the actual voltage drop in the neutral wire. This glow is not an indication of reverse polarity, and will not normally cause a problem.

BRIGHT GLOW OF REVERSE POLARITY LIGHT:
When you have a bright glow in the reverse polarity here is what happens:

1. In order for the bright glow to happen, there must be a high (about 100+ volts) voltage difference between the neutral and ground wires in your boat.

2. When the polarity is reversed, the power source's hot (black) wire is connected to the boat's neutral distribution, and the white (neutral) wire is connected to the boat's black wire. This can cause electrolysis and lots of other problems as well as being very unsafe. NEVER IGNORE A BRIGHT REVERSE POLARITY LIGHT! "

( Art continues with the following - not pertinent to your problem, but included anyway )

"YOUR GENERATOR PROBLEM:
Just because the generator does not light the reverse polarity light on another (non IP boat) does not prove that you do not have reverse polarity. It is hard to determine your problem without measuring voltages, but here are some possible causes:

1. Your generator has a reversed internal polarity. You need to measure the AC voltage between the neutral and ground pins on the generator output connector to verify this. On a "standard" (wall type) socket) the neutral is the wider flat slot and the ground is the round hole. If you get over 100 volts, the generator has reverse polarity. If you get a fluctuation in voltage, you probably have a floating ground.

2. Your generator has a floating ground (This can cause the reverse polarity light). If this is the case, you need to connect the neutral line to the ground (case) of the generator."

now back to sailnet: 

for the problem you have of the indicator light, a question would be : is it totally off, low, or bright? 

Clues to your situation are: 
1) you're always plugged in at the dock - so with your batteries constantly topped, there would not even be a low glow (very faint perhaps) of the light. 

2) when you replaced a dead battery and plugged in again, the system started working harder to get the bank up to specs. A low glow would be normal and actually shows everything is working OK. But the low glow would be a change from no light before and begs an explanation. Hopefully this is it.

3)before sailing, you have no light (batteries are topped, there's little draw), then after sailing, and perhaps using more amps than you made while underway, you plug in again - if a low glow is seen this is normal because you need to draw on you system to get back to full banks.

good luck. all the advice in the world doesn't mean you shouldn't get a reverse polarity tester and plug it in. cost is about 10 dollars at a hardware store.


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## bhcva

Thanks for the post by Art Richards....I've had exactly that problem for 12 years and no one has been able to explain it except to say "don't worry"...the light glows very brightly when the water heater is on and very faintly at other times.

Bruce3


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## bestfriend

*Oh Crap!*

I just happened to push the "test polarity" button today on the boat and it lit up like a christmas tree. I didn't think much of it, everything was working fine and has been since we moved to the new slip a month ago(yikes!). Not being much of an electrician, I figured I would check out sailnet for the answer. Not the answer I was hoping to hear. Now I am across the Bay for the night, 40 minutes away with a couple (read half a bottle) of glasses of red wine in me. So, stay tuned for tomorrow's local fire report! Wish me luck.


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## bestfriend

Well, she survived through the night. The only way I could get the polarity light to go off was to unplug the shore power. I didn't have time to check the dock receptacle with a meter, that will be tomorrow. Watch the news for "sailor electrocuted". If the dock receptacle is not reversed, could the problem be on board?


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## sailingdog

Yes, but I doubt it. Get one of the dongles that allows you to plug a regular extension cord in to your shorepower outlet and then get a regular household polarity checker... keep them both aboard the boat, and use whenever you're at a different marina—to check the wiring status before you plug your boat in.


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## TSOJOURNER

Oft Junior member! I think,that this system your boat is connected to,is a 240 120 V system.Which means that you have two or three wires in to the distrybution box on the dock +groundwire.If you measure between one of them two or three wires and the ground you get 120 Volts.The ground and the neutral wire are the same thing.Red light on your switchpanel means leak to ground on your outlet.Since here in America you don´t have groundleak breakerswitch,this light is in stead.
 Capthor


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## bestfriend

Please forgive my naivete with electrical systems on the water. There is a button marked "test polarity" and then a light next to it that says "reversed". I think I misunderstood that as "push this button to test your polarity". I am guessing that it means "push this button to test the polarity light/alarm system". So, you push the button and the light goes on telling you that your check system is operative. My light is not always on. Thinking on it more now, I would think that the light would go on automatically if there was a problem.
Update: I got the confirmation that I needed. The button is a check system. Thanks all.


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## oft

UPDATE: As it turned out, our neighbor, while unplugging from shorepower, had bumped our cable causing it to pull out somewhat from the connection and this, in turn, resulted in the reverse polarity situation. So, checking connections and terminations is not a bad place to start.


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## sailingdog

Layer 1 is always important to check...


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## captain jack

phaiakian said:


> credit goes to Art Richard IP32 #34 "Lagniappe" (St. Petersburg, FL) for the following dialogue:
> 
> "LOW GLOW OF REVERSE POLARITY LIGHT:
> A low-level reverse polarity light on an IP when there is heavy current draw (like a heater or air conditioner) is not unusual. The reason is as follows:
> 
> 1. The reverse polarity light is connected between the AC neutral (white) and AC ground (green) wires inside the distribution panel. If there is a voltage between these two lines, the light lights in proportion to the voltage.
> 
> 2. When using shore power, the neutral and ground wires are connected together somewhere on the shore at a power distribution box. Between that point and your boat load (heater) are many feet of wire. Normally the AC current flows through the hot (black) wire to the load (heater) and back through the neutral (white) wire to the aforementioned distribution box (of course the current also flows through the black and white wire back to the power generator, but that is not pertinent to this discussion).
> 
> 3. When current flows through a wire, the small resistance of the wire causes a voltage drop which is proportional to the length of the wire and the resistance of the wire. You see this on your AC voltage meter when you turn on a high current load (heater). Depending on the load and the length of wire, this voltage drop is sometimes 10 or more volts.
> 
> 4. The reverse polarity lamp is effectively measuring the voltage between the neutral and ground wires at the boat. Since the ground wire is not (normally) carrying any current, there is no voltage drop in this wire. The neutral wire however is carrying current and does have a voltage drop. This voltage drop in the neutral wire appears across the reverse polarity lamp, causing it to glow, the brightness depending on the actual voltage drop in the neutral wire. This glow is not an indication of reverse polarity, and will not normally cause a problem.


ok. i have this problem as of last night. i do have a heater plugged in. it has been plugged in for a few weeks and this is the first time this has happened. my battery is low and i tried to charge it. but, when i plugged the charger into the socket (i thought i'd try since the heater seems to be working fine) the needle on it's meter kept jumping back and forth like crazy and i imediately unplugged it. figuring it really was a reverse polarity issue, i called the marina to tell them they should check the wiring at the dock because my reverse polarity light was on.

it is very dim. not bright. if i turned off the 15 amp breaker (leaving the 30 amp main on) the light would go off. so, i am guessing that this is, indeed, my problem. however, the charger was acting strangely. that makes me wonder. any thoughts?

certainly not trying to burn my boat up.


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## pjs1554

Captain Jack,

Could you clarify where the 15 amp breaker is? On your boat panel? Is this the same circuit as the heater was/is connected to? The heater does it have a fan or just the wire that glows? 

Leakege to ground is not good. The two circuits ( ground and neutral ) should be isolated from each other all the way back to the main electrical service. With the corrosive environment involved it is not hard to understand why there are problems. Proper maintenance of the electrical system ( maria and yours ) should eliminate this problem.


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## captain jack

pjs1554 said:


> Captain Jack,
> 
> Could you clarify where the 15 amp breaker is? On your boat panel? Is this the same circuit as the heater was/is connected to? The heater does it have a fan or just the wire that glows?
> 
> Leakege to ground is not good. The two circuits ( ground and neutral ) should be isolated from each other all the way back to the main electrical service. With the corrosive environment involved it is not hard to understand why there are problems. Proper maintenance of the electrical system ( maria and yours ) should eliminate this problem.


it's an all new shore power system. the panel is a blue sea model with two breakers: a 30 amp main and a 15 amp branch breaker. the shore power plug is right in back of my breaker box so the distance between it and the panel is less than a foot worth of wire. all the wire gauges were chosen in compliance 
with what's recommended for the loads....erring on the big side where it was close.

i have been using it for heat battery charging, and power tool for a few months, now, with no issues.

the heater was plugged into the 15 amp circuit. i have two plugs on that line (a distance from each other chosen specifically to allow me to place the heater at different points).

when i got to the boat my battery was way down so i couldn't turn on the lights, as i usually would at night. i had to use the led lantern i used to use before i installed the lights and it wasn't as bright inside i think and that's how i noticed the reverse polarity light was lit. it was dim and i had to check to make sure it was actually on, at first.

when i flicked off the 15 amp breaker, the reverse polarity light went out. turned it back on and the reverse polarity light came back on.

i plugged my phone charger into the socket and it worked just fine. the heater was also working just fine.

i unplugged my phone charger but, not the heater, and plugged in the battery charger. instead of the needle tanking at maximum charge, which is what should have happened, it bounced up and down rapidly. so, i unplugged it. i wish, now, that i had tried it with the heat off. going to do that today.

the heater has no blower. it's an oil filled radiator type...and i had it on maximum. since it has no blower, the full wattage (which is the same draw as my previous ceramic heater) goes into heat. it won't heat instantly but it keeps the boat much warmer inside than the ceramic heater with a blower.

the green and white wires make no connections, except the reverse polarity light which blue sea wired up, on board my boat. if there is a connection made, it's at the dock. i know not to do that and there is no chance it happened do to shoddy work. i am quality minded bordering on the obsessive.

that's all the details i can think to tell. i know the marina hasn't done any work on the electricity. the one thing not the best about them is that they are slow to get stuff done :laugher.

i had thught that it might be something caused by the weather. we have had a bit of rain and freezing rain, lately.

now, i'm thinking it's nothing to worry about but, i want to be sure.


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## zeehag

always make sure you twist lock your shore power cord to make connection correct. not twist locking your cord will cause arcing and potential fire. 
ALWAYS double check your twist lock abilities each time.


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## miatapaul

You are saying you plug in the charger, is this a regular car charger? The problem with most of those that I have read about is that the circuits do not have an isolated earth. So the car charger connects the 12 volt earth and the 120 volt and I think can cause issues like you are describing. If you have a marine isolated charger, never mind. But using an automotive charger on a boat is a big no no, as it can cause lots of issues with stray currents.


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## captain jack

miatapaul said:


> You are saying you plug in the charger, is this a regular car charger? The problem with most of those that I have read about is that the circuits do not have an isolated earth. So the car charger connects the 12 volt earth and the 120 volt and I think can cause issues like you are describing. If you have a marine isolated charger, never mind. But using an automotive charger on a boat is a big no no, as it can cause lots of issues with stray currents.


it is a car charger i am using temporarily til i get a permanent charger installed onboard. i hook it up when it needs a charge. i never had an issue with it before. the reverse polarity light was on before i tried using the charger. so, i don't think the charger was the issue.

to update.

i left the heater on and the shore power cord hooked up because there seemed to be no problem with the heater function. after reading up, i had wished i had turned it off and unplugged the cord. however, when i got to the boat today, the light was no longer on. (it was only dim before. not bright.) they never had a chance to check the dock electrics. (altough, they finally got the bubblers in place.) so, they did nothing to make the issue go away. but, away it went.

today, i had the heater on and i used the charger to recharge my battery. at first, the needle on the gauge on the charger acted squirrely again. i turned on the lights, as they should come on with the charger. they did but they flickered to match the needle. i turned the lights back off and continued with the charger. after a little, it steadied up like normal and everything went fine.

battery charged and i checked the lights. all is well.

the reverse polarity light never came back on; dimly or otherwise.

so, am i to assume it was just the draw of the heater?


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## OPossumTX

I do not know about your charger but the car charger I use with my trolling motor battery will cycle on and off like that if the battery is really low. I opened my charger up to replace the cord after the thing fell off the work bench while it was plugged in ripping the cord out. Inside mine there is one of those automatically resetting thermal breakers in series with the red battery cable. If the battery pulls more than 6 Ampres or the charger leads get shorted, the breaker starts doing its click-tick routine until the short is cleared or the battery charges enough for the current to drop below 6 A or so. Real MODERN battery chargers have electronics to limit the current and control the charge voltage. My 30+ year old "6A BATTERY CHARGER" has a selenium rectifier stack, a very roogie looking transformer, a neon pilot light, the amp gauge and that click-tick breaker I told you about. There is nothing else inside the thing. If I leave it on a battery too long it will boil that suckka DRY! 

I have learned to shut it off after a few hours at most. I should have pitched the junker years ago but the cracker box still works, so...

Have FUN!
O'


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## captain jack

OPossumTX said:


> I do not know about your charger but the car charger I use with my trolling motor battery will cycle on and off like that if the battery is really low. I opened my charger up to replace the cord after the thing fell off the work bench while it was plugged in ripping the cord out. Inside mine there is one of those automatically resetting thermal breakers in series with the red battery cable. If the battery pulls more than 6 Ampres or the charger leads get shorted, the breaker starts doing its click-tick routine until the short is cleared or the battery charges enough for the current to drop below 6 A or so. Real MODERN battery chargers have electronics to limit the current and control the charge voltage. My 30+ year old "6A BATTERY CHARGER" has a selenium rectifier stack, a very roogie looking transformer, a neon pilot light, the amp gauge and that click-tick breaker I told you about. There is nothing else inside the thing. If I leave it on a battery too long it will boil that suckka DRY!
> 
> I have learned to shut it off after a few hours at most. I should have pitched the junker years ago but the cracker box still works, so...
> 
> Have FUN!
> O'


mine isn't that old. it does self maintain. if it fills the battery, it shuts off. however, i was thinking that might be what's behind the way the charger was acting but, i wasn't sure. now, thanks to you, i am.

it still leaves me with that question about the dimly lit reverse polarity light, though.


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## bhcva

My reverse polarity light always comes on dimly when the hot water heater or other source draws a high current. I had it checked and was told it was not a problem. It has done it since the boat was new (94) and has never been a problem. I am at the end of a dock about 14 boats away from shore...long run.


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## babycop

i know this is old thread but have identical problem as OFT did ...would be extremely helpful to know how he fixed the problem


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## zeehag

did you remember to twist lock your plug???? that is major cause of sudden reverse polarity with dock power. always triple check. BOTH ends. if workers were on th e system, they willnot twist lock the plug when replacing. ask me how i know this ha ha h aha 
always check whenever you think workers may have changed anything,and before you leave and immediately upon return.

\this cannot be said enough different times


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