shore power polarity

[BlacknBlue]

Hi all, is anybody able to explain to me the mechanics behind the polarity of shore power. I can't see how AC can be the wrong polarity, but my boat has a warning about connecting to the wrong polarity and to use the test box before plugging into shore power. I'm specifically talking about connecting to marina power. The boat has mechanical safety switching in place to prevent more than one AC supply source (genset, inverter or shore power) being connected at a time. I fully understand the reasoning there (phase), but the reverse AC shore power polarity has got me stumped.

cheers, thanks in advance

[TheRealAndy]

Shore polarity exists because earth and neutral are tied together. I cant remember what the regs are for boats (gensets) but its possible that you genset has a floating neutral. I know the genset I have floats, so there is no polarity as such. What you need to make sure is that all the neutral connections on the boat are wired the same way, and connect to the correct neutral on you shore connection. I would be surprised if this is not the case, but it all depends on the origin on your boat.

[finga]

Yep, all to do with binding and the multiple earth neutral system we have in Australia.
Also the Marina or your boat could have isolation transformers in their relative systems as well.
The book would have been written to include other parts of the world as well....and who knows what goes on in some places and it would be the easiest thing to just to tell people to use the test box.

If you want a PDF file of the Australian Standard (AS3004:1:2008 Electrical Installations-Marinas and Recreational Boats) just PM me your email address and I'll send a copy.
There is some informative information in there as well as the technical.

Cheers then
Scott

[seastorm]

My understanding of polarity when connecting a boat/motorhome ect to shore power is, that all power point and light switches in the boat should be double pole ( the switch disconnects active and neutral cables) this is to ensure that when you turn the power point off and there is an appliance plugged into the power point there is no power running through the appliance. What you have to look out for is if you do not have double pole switches only the active cable gets switched, now if your extension lead has reversed polarity and you plug it into your boat all will work fine but lets say you have a toaster on board and the toast gets stuck so you turn the toaster off at the power point and put your metal knife in to free the toast, if you hit the element you could get a shock because the extension lead has reverse polarity and the single pole switch has only disconnected the neutral cable so the active cable in now putting power through the toaster.
I hope this round about explanation helps with reverse polarity

[BlacknBlue]

thanks all, I get it. A floating neutral would be a great idea- the vessel is fitted with a 3 yo Paguro 8000 genset http://www.cowesonline.com/zonexml/s...y_id=7123;cp=0- will chase that up. my last boat (BW26) had shore power, but no test box or warning and it had me stumped as to why this one needed one....
cheers

[testlab]

Going from memory, the wiring diagram for my searay shows the European export model has RCDs fitted and therefore the boat side neutral and earth are bonded but the generator neutral floats (it gets bonded through the double pole isolation switching when selected). So it would be necessary to get the incoming polarity correct.

The best way to think about AC power circuits is tha the active swings above and below neutral, and not that they alternate polarity.