Shore power polarity

[halcyon]

Re: worst case scenario

Theres no earth connection on a RCD, only live and neutral. ELCB had live, neutral and earth, and for that matter one make of RCD's back in the early 90's did. The RCD measures inbalance, ELCB measures the current in the earth wire. Thus the RCD protects no matter were the stray current goes, or how it reaches 'earth', ELCB requires a known path, i.e. earth wire, and died out in the early 90's.

Brian

<hr width=100% size=1>

 

[MainlySteam]

Thanks Rick for the response, I see where you are coming from.

I can only add that when the shore power is connected to the boat then it is a requirement of all codes I am aware of that the neutral to earth bonding on the boat be automatically broken (usually by the switch making shore power or automatically in some invertors). If that is done then I believe the risk of a ship to shore current path through the sea does not exist with reverse polarity unless there is a neutral to ground fault on board AND the ground conductor in the shore cable is faulty.

Clearly other faults can also cause a current path back through the sea endangering others, such as a live to ground fault on the boat and a faulty earth conductor in the shore cable.

I strongly empathise with the message that many do not realise the danger they may be putting others at through incorrect marine ac wiring practices.

John

<hr width=100% size=1>

 

[Rick]

John,

I suspect we are violently in agreement! This thread started with building a cable to "reverse polarity" in the event that the onboard "polarity detection thingie" implied "wrong way round". It scares the shit out of me!

There are all sorts of assumptions being made here, including blind faith in rcd/earth leakage units working as expected. The reality is that there are lots of situations in which reversed polarity / seawater bonding etc will not cause an issue, until a certain type of fault is encountered. At that time, IF everything works as expected, no-one is any the wiser, apart from a fuse/circuit breaker or an RCD trips - BUT (and a big BUT but a worthy one) if someone has played with things, we can have a situation where live AC is masquerading at what we expect earth to be - and when it does, people can die.

At essence, if your on-shore neighbour (next door or five doors up) "plays" with his/her AC wiring, you won't die, unless you go onto their property - nothing they do can upset your AC supply (apart from cease it as result of major short!). When we're on the water in a marina, ANY neighbour who plays with their AC supply can cause you a life threatening situation in the event a fault occurs in their AC supply, and cause your AC supply to be faulty - your earth can be live, and you can die.

Solution - go the extension cord solution, or better, get an isolating transformer - they're heavy, they get hot, they're a pain in the butt, but you (or those precious to you) aren't going to die.




<hr width=100% size=1>

 

[andyball]

When we're on the water in a marina, ANY neighbour who plays with their AC supply can cause you a life threatening situation in the event a fault occurs in their AC supply, and cause your AC supply to be faulty - your earth can be live, and you can die.




As far as I can see....any playing with the marina supply onboard/i] so as to connect "live" to "earth"


in a correctly wired marina , would cause...

1: a pontoon fuse/circuitbreaker to trip due to the high current, disconnecting the power

2: the pontoon rcd to trip, disconnecting the power






<hr width=100% size=1>

 

[Rick]

Andy wrote:
>As far as I can see....any playing with the marina supply
> onboard/i] so as to connect "live" to "earth" in a correctly
>wired marina , would cause...
>1: a pontoon fuse/circuitbreaker to trip due to the high
>current, disconnecting the power

>2: the pontoon rcd to trip, disconnecting the power

Remember this thread started with having a "polarity reversing" cable made up? If someone in a marina you visit uses one of these, and if they have AC earth bonded to the water via the DC earth, the potential exists for electrocution.

It's all ifs and buts, however you wouldn't leave bare wires on an appliance cord at home because the RCD would trip is anyone touched them would you?

On the subject of pontoon fuses / RCD's, everything Andy points out is true in a "correctly wired marina", but how sure are you that every marina is correctly wired? Certain nobody used oversize wire to fix a fuse blowing problem, or "bridged" an RCD to solve a fault?


<hr width=100% size=1>

 

[claudio]

In the event of a live wire being shorted to earth with the pontoon fuses bridged and the marina RCD non functional, there are two likely scenarios

The live wire burns out, therefore removing the danger to other vessels.
The earth wire burns out, therefore now making all vessels connected to earth LIVE.

I would imagine that the latter scenario is highly unlikely, however not impossible.



<hr width=100% size=1><A target="_blank" HREF=http://shop.yachtronics.co.uk>http://shop.yachtronics.co.uk</A>

 

[pandroid]

Like others, I use a little Mains polarity checker/detector built into a plastic plug and bought from Homebase, but I'd like to build the checking into the boat itself.

Can any of you engineers out there provide a circuit of whats going on inside the checker? I assume its a few diodes and a couple of Neons, but short of taking the plug apart I'm not sure how its done


<hr width=100% size=1>

 

[MainlySteam]

A green one from line to ground shows polarity is correct when glows, and a red one from neutral to ground glows red if polarity is reversed. To minimise the current in the shore supply earth conductor the resistance of the lamps should be greater than 25- 30 kohm (by adding resistors if necessary).

My view would be that if you needed to ask how it was done, then you are safest getting a marine electrician to do it for you.

John

<hr width=100% size=1>

 

[Eam0885]

Re: worst case scenario

I was wondering what causes reverse polarity We have a 46ft carver that the reverse polarity light is on and will not go off do you have any suggestions on how to fix the problem
'
Thanks

<hr width=100% size=1>