Wednesday, October 20, 2010

Shore power issues

If In Doubt, Change It Out

If your shore power inlet looks like this, you know you need to change it.  That pin got hot because of a poor connection, which could have been in one of three places:

  1. Where the wire is connected to the pin, at the back of the inlet fitting.
  2. Where the plug contact makes contact with the pin
  3. A poor connection between the wire and the contact in the mating plug

    In any case, now you must change out the shore power inlet fitting.  But you should also change out the plug which mates with it.  Here's why:  In order to ensure a good connection, the contacts inside the plug spring apart as the pin from the shore power inlet is inserted.  This spring action results from the shape of the contact, the material, and its temper.  If the pin has been overheated, its mating contact likely has too, and probably has lost its temper.  Therefore, if you only change out the shore power inlet, the connectors in the mating plug, now having lost their springiness, will make a poor connection with the pins in the shore power inlet (see #2, above).  The next time the boat draws a big load, it is likely that the plug will heat up, destroying both itself and the newly installed shore power inlet, and possibly even the entire boat.

    Be a Loner - Get Isolated
    There are three wires in that connector.  Traditionally, these are called the hot, the neutral (these two carry the power) and the ground.  Strictly speaking, the ground is unnecessary - it is there as a safety precaution.  If, for example, one of your appliances should develop an  internal short (say, the casing of the heating element in your water heater cracks, and water seeps in, connecting the actual heating element and the casing, and in turn the water, and the water heater tank, etc.), this ground lead, which is connected to the external parts of everything electrical on your boat, ensures that you are at the same potential as the cases, knobs, etc. and therefore do not receive a shock.  If substantial current flows thru the short, a breaker will trip - which is the desired action.

    On shore, "ground" is established by driving a metal rod literally into the ground (your house ground will also be connected to your water supply, which is even more metal in the ground).  On the water, what gets used as "ground"?  This is not as easy to explain.  The obvious answer is: the water.  But on a boat, there is something else going on.  Much of the submerged external metal on your boat will be protected from galvanic corrosion by attached zincs which, being higher in the electromotive series, will corrode to protect that submerged metal.  But if we connect the boat AC ground to the water, then we are at great risk of electrolytically dissolving that external metal!  More on this in a moment.  Selfishly, look at it this way.  If your zinc-protected external metal is attached to your neighbor's external metal, then your zinc will be protecting both your submerged metal and his.  If both boats have their green ground wire connected directly to the boat ground, then your zinc will protect his metal.  But it is much worse than this...  your zinc will also be protecting all the marina's submerged metal as well, starting with your dock.  So how to provide the safety protection that a ground delivers to someone onboard, without truly sacrificing your zinc?

    There are two ways.  First is the galvanic isolator.  This device gets inserted into your ground wire right after it comes aboard.  You just cut the wire, and attach the two ends to the two terminals you see there.  Thru electronic magic involving diodes, ground currents are carried but electrolytic leakage is barred.  This is the lower cost solution.

    Unfortunately, the galvanic isolator is not a perfect solution.  Chief among its flaws is that it can silently fail.  That is, it can go open circuit, in which case your first notification would be when you got that shock we were talking about earlier.   A better but more costly answer is an isolation transformer.  This device literally isolates shore power from the boat, with the only connection being via a magnetic field.  Perfect.  But they are heavy, and expensive.  Like the man at the carnival says, "You pays your money, and you takes your chances."  But the only true losers at this game are those who do not play.  You need one of these devices.

    Belt And Suspenders
    Adding a second layer of protection, Ground Fault Interrupters are now, like in houses, required equipment on all circuits on a boat near water:  the galley and the heads.  But realistically speaking, any circuit on a boat is near water.  They should all be GFI protected.  This one is easy to do.  How many AC outlet circuits do you have on board?  One?  Two?  Three?  Well then you just need to go to Home Depot and buy one/two/three GFI outlets.  Locate the first outlet on each breaker (it'll probably be the closest one...), and replace the standard outlet with the GFI outlet.  All the downstream outlets on that circuit are now protected (they even include little stickers in the GFI packaging that you can affix to the downstream outlet covers to attest to this).  This would certainly qualify as a small boat project.

    In Should Equal Out
    When a large Krogan here on G Dock was hauled for a sales survey, an expensive problem became immediately apparent: the 3" thick full length stainless steel keel shoe was completely eaten away by electrolysis. Lawsuits flew. In the end, the marina staff checked each boat on the dock for leakage, by simply applying a clamp-on ammeter over the shore power cord. If the amount of current being supplied to the boat was the same as that returning down the neutral wire, the ammeter would read zero. Surprisingly several boats were found to be leaking tens of amps into the water. Remember that cracked water heater element casing from above? As long as the total current drawn did not exceed the breaker's trip rating, that extra current was leaking out from the boat, energizing everything in the vicinity.

    Clamp on ammeters do not need to be expensive.  So here's another small boat project:  get one, and check your boat's electrical system from time to time.  Check your neighbor's too!  Heating elements in water heaters and coffee pots crack long before they fail completely, motor windings short to their casings thru conductive dust from the brushes, insulation on wires chafes.  An annual check is a good thing.

    Final Words
    Shore power is a wonderful boon to the boater, but it is not as simple as just plugging in the cord.  But then, nothing on a boat is.

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