Dumb question on polarity. .

comsnark

comsnark

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Apr 10, 2007
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NJ Shore
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SeaRay SunSport
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. . .ok, sparked by a previous thread, I have a dumb question regarding reversed polarity when hooked to shore power. When I first got my boat, and first hooked up shore power (120V source), I got the "reverse polarity light".

Can some explain the hazard (either to life or equipment) that this represents? I never really thought about AC power having "polarity".

The problem I had was caused by reversed leads on the power outlet I was using. I confirmed the polarity problem with a $5 gizmo from Home Despot. And then rewired the outlet -> no problems since (except when I left the #*$& charger turned off for three weeks. D'oh!)
 
The problem comes because of the assumption that the neutral wire should be at earth potential and that the line wire is hot and therefore the dangerous one. When the polarity is reversed the "safe" wire becomes dangerous. Any device constructed under the assumption that the neutral wire is safe becomes dangerous. The term "double insulated" refers to devices so built that both line and neutral are isolated.

Now as a protection, a device can be constructed such that the green ground wire is attached to the frame of the device. Any current leakage between neutral and ground will be harmlessly channeled away. Any leakage between the hot wire (line) and the chassis will also be channeled away and probably cause a breaker-trip. Also, if a GFI is in the circuit, the GFCI will sense that current on the line and neutral wires is not balanced, causing a trip.

At home, most electrical panel boxes bus the white and ground wires together at some point. This terminal is connect to the lead-in wire with the two live wires back to the pole transformer and usually to a grounding stake on the pole. Also, at your home there should be at least one 8' grounding stake connected to the ground bus with heavy gauge (#6 I think?) wire. Back in the olden days, they would sometimes connect the ground bus to the copper water pipe leading into the house. Any way, lots of good grounding at home.

On the boat grounding is a bit different. The green ground wire and the while neutral wires are supposed to remain isolated from each other. The green ground wire is typically bused to all of the underwater hardware in the boat. The green wire is also connected to the grounds on shore via the shore power connector. It's possible that low level voltage differences between ground on your boat and ground on shore could exist. Isolators are installed in the ground wire to break this low voltage current flow. But the isolator will conduct when the voltage rises because of a hazardous fault.

That's the short form overview.

A few problem scenarios follow.

Lets say that a three prong device is connected to a socket aboard. This device could also be the built-in battery charger, water heater, air conditioning, etc. Let's postulate that in the device, ground and neutral are connected either though design or a fault. Now the ground circuit is at neutral potential. You may think that this isn't a problem. But remember that electricity seeks the easiest path. So the easiest path from the device to ground might be from your underwater fixtures to the water by way of the green bonding wire. This will cause accelerated erosion of zinc anodes, though hulls, shafts, struts, propellers, rudders, stern drives, or what ever you have immersed in water. Or it could be from the swim ladder, which often is bonded, though your body, to the water. There are many more possibilities, but this should illustrate the problem.

Best regards,
Frank
 
Last edited:
What a fantastic reply. That is what this forum is all about. Thanks Frank I eagerly read every word you post.
 
And so the question of reverse polarity has been answered by an Electrical Engineer on THIS forum!!! Someone just needed to ask.
 
Yup. Putting my question in the other thread would have been off topic.

When I "fixed" the fixture, is was also upgraded to a GFI with an appropriate weather resistant cover for the power cord that is now in there all the time (boat in summer. .deicer in winter)
 
osd9 said:
And so the question of reverse polarity has been answered by an Electrical Engineer on THIS forum!!! Someone just needed to ask.
Click to expand...

Uh no. I'm mathematics / computer science. There are some real EEs on the forum, but I am not. Of course this does not require an EE and I need to go back and add some stuff which I should have but I turn into a pumpkin after 10pm.

Best regards,
Frank
 
The problem comes because of the assumption that the neutral wire should be at earth potential and that the line wire is hot and therefore the dangerous one. When the polarity is reversed the "safe" wire becomes dangerous. Any device constructed under the assumption that the neutral wire is safe becomes dangerous. The term "double insulated" refers to devices so built that both line and neutral are isolated.

Now as a protection, a device can be constructed such that the green ground wire is attached to the frame of the device. Any current leakage between neutral and ground will be harmlessly channeled away. Any leakage between the hot wire (line) and the chassis will also be channeled away and probably cause a breaker-trip. Also, if a GFI is in the circuit, the GFCI will sense that current on the line and neutral wires is not balanced, causing a trip.

At home, most electrical panel boxes bus the white and ground wires together at some point. This terminal is connect to the lead-in wire with the two live wires back to the pole transformer and usually to a grounding stake on the pole. Also, at your home there should be at least one 8' grounding stake connected to the ground bus with heavy gauge (#6 I think?) wire. Back in the olden days, they would sometimes connect the ground bus to the copper water pipe leading into the house. Any way, lots of good grounding at home.

On the boat grounding is a bit different. The green ground wire and the while neutral wires are supposed to remain isolated from each other. The green ground wire is typically bused to all of the underwater hardware in the boat. The green wire is also connected to the grounds on shore via the shore power connector. It's possible that low level voltage differences between ground on your boat and ground on shore could exist. Isolators are installed in the ground wire to break this low voltage current flow. But the isolator will conduct when the voltage rises because of a hazardous fault.

That's the short form overview.

A few problem scenarios follow.

Lets say that a three prong device is connected to a socket aboard. This device could also be the built-in battery charger, water heater, air conditioning, etc. Let's postulate that in the device, ground and neutral are connected either though design or a fault. Now the ground circuit is at neutral potential. You may think that this isn't a problem. But remember that electricity seeks the easiest path. So the easiest path from the device to ground might be from your underwater fixtures to the water by way of the green bonding wire. This will cause accelerated erosion of zinc anodes, though hulls, shafts, struts, propellers, rudders, stern drives, or what ever you have immersed in water. Or it could be from the swim ladder, which often is bonded, though your body, to the water. There are many more possibilities, but this should illustrate the problem.
Click to expand...

Beavis_and_Butthead_horror.png
 
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