AC electrics

[gauntlet]

I have got a bit confused reading the various posts about this. Perhaps someone can help me with a simple, clear explanation? I am a layperson when it comes to boat electrics, but trying hard to understand 1 word in a hundred of Nigel Calders book...

I am in the process of installing an ac connection on our wooden boat. I have fitted the socket and a double-pole rcd from index marine and the proper arctic cable. I am replacing the previous owner's dodgy wiring (domestic junction boxes in the bilges!) with a clear run in B&Q conduit. We intend to occasionally use shore power for tools; and to add a portable generator for longer cruises...we have no invertor or anything else more comlicated. before I finish, I have 2 questions which I remain puzzled about, having read some conflicting posts...

1. Isolation & earthing: I was planning to connect the ac earth to our common grounding point (hull anode) that my husband is busily connecting all our dc negs. to...I understand that this creates the possibility of making one big circuit in the marina so was thinking of fitting an isolator. However a post from someone - John maybe? - seemed to suggest that I might not need to ground the earth at all. Please can someone explain that idea? If I do need to fir an isolator, any recommendations as to the best buy?
2. The long thread about reversed polarity got me really confused! Given the set up above, can someone please explain to me what the thinking is about best practice regarding fitting any kind of polarity indicator, reversing cable etc.? I got a bit lost in the argument about New Zealand etc..!!
Thanks,

Yacht Ame's proud new owners!

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[bruce]

if you think of the shore power as a big long extension cord with its own breaker at the end in the boat, you have it. if you put breakers in the boat of a lower value that the dockside box, if you 'blow a fuse' you just trip the breaker in the boat rather than having to go outside to the dock box. the shore power is grounded thru the dock box, the third wire does that.

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[john_morris_uk]

Polarity:

Put a neon warning light between neutral and ground. It shouln't light. If it does, it means that live and neutral have got mixed up. I keep a short lead with a plug and inline socket on each end to insert into the supply with the wires crossed over to sort this out. Hope that this is clear...

Earthing:

Best practice is a Galvanic isolator in the earth line where it comes on board. Most people suggest connecting boat negative with mains earth. No doubt someone with argue the opposite, but with the isolator in place you should be ok.

Best of luck.

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[MainlySteam]

<<<1. Isolation & earthing>>>

The ac system earth conductor must always be bonded to the boat's common ground - usually to the anode or engine in contact with the sea.

The ac system neutral must not be bonded to ground on board the boat while the boat is connected to shorepower, but the ac neutral must be bonded to ground on board the boat while the boat is not connected to shore power and there is an onboard ac supply eg from an onboard generator or invertor (small portable generators present a special problem as they often have "floating neutral" systems). This must be done by means of a switch which selects either shore power or ships power and in doing that automatically switches the neutral bonding to/from ground.

The DC negative need not be grounded to the boat's ground (ie with the ac ground). However, in most older boats one has no easy choice as their engines normally have all their sensors, alternator, starter with negative connected to the engine block and that is in turn normally connected to the ac ground by default (newer engines are often made with all those things isolated). The American Boat and Yacht Council (ABYC) standard requires the DC negative to be bonded in common to the ac ground but I do not think there is any modern good reason for this (Calder gives one reason I think, from memory, but the shipyard electrical engineers I have spoken to about this are of the same view as me and do not normally do it on large yachts). By having the DC negative bonded one opens up the possibilty of stray DC currents through the boat, which is alleviated if the DC system is completely isolated from the boat - of most importance for metal boats but, in my view is so for other constructions too. If you have the DC isolated you should fit double pole breakers on your DC panel instead of single pole (in my opinion, double pole are a good idea in any event but they do make the DC panel considerably larger).

2. Reversed polarity.

This makes no difference to your installation and should be forgotten about except to the extent that a polarity indicator is desirable on the boat. This is installed as described by another post but one should ensure that the resistance of the lamps is at least 25-30 kohms to avoid leakage to the ac system earth. Incorrect positioning and configuration of the polarity indicator will upset your RCD. As you do not appear very confident I would advise getting a good marine electrician to help you with that and also for the correct positioning of a galvanic isolator (as another post recommends).

If you run into reversed polarity and you boat is correctly wired, then it is the marina's fault. From what I understand (I am not in the UK so stand to be corrected) you are unlikely to run into the problem in UK marinas and if you do the marina would be required by regulation to fix it.

Sorry about the length of this but it is impossible to describe how to wire a boat in a couple of paragraphs. You do not seem very confident and so my main advice would be to seek some professional help from a respected marine electrician. To minimise costs, perhaps you could get him on board, have him say what he needs, where it is to be fitted and where he needs access to, then get those things, install them if just a screw on job, and open the boat up so he has access everywhere or put mousing lines through areas which cannot be dismantled so cables can be pulled, and then get him to do it. A boat can be wired very quickly if all the bits and pieces are on hand, any panels made, bulkhead outlets fitted, etc. A great part of the time is usually getting access to the various areas for cable pulling, etc - you can reduce that time by opening up what you can yourself.

John

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[bruce]

PBO in last month or so, had an extra section, and it has an instruction diagram to do what you want. it shows connecting the ground to battery neg thru a 'galvanic isolator valve' or 'isolation transformer' article in pbo 421.

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