NZS3004 Changeover Switch

[Nite_Shift]

Query for the gurus out there;

I'm trying to come to terms with the intent of NZS 3004.2:2014 3.5.3.2 under "changeover switch for boat with its own ac power system"

I understand the concept of a changeover switch iac with this standard and 3000, What I dont understand is why this clause mandates a specific device to protect against shore supply polarity reversal, but only in the case where the vessel has its own ac power system, an appliance inlet from a shore supply and a changeover switch to select between the two. Surely if no on board power generation or changeover switch existed the hazard presented by a shore supply polarity reversal would be no greater?

Can anyone explain this to me? A schematic in the standard would have been very useful at this point.

Thanks in advance.

[AlecK]

I agree the hazards from incorrectly polarised shore power are identical regardless of whether or not there's an on-board source.
I suspect the condition for the requirement was adopted, along with the requirement itself, from US practice;
as that's where a lot of the detail added to 3004 in 2008 edition came from, along with quite a bit from IEC.

[Nite_Shift]

Cheers, Thanks.
After having a good think through the problems, a super dangerous condition only really exists with a P/E transposition. In this case the hull is at 230V above earth and other correctly wired ship hulls on shore supply. Appliances will not work, but unless the fault trips the supply pillar RCD (if installed) no other protective devices will operate. This must be the problem the standard tries to negate.

[AlecK]

One difference with boats as compared to land-based connectable installations is that the boat's hull is immersed in water that is at local earth potential. Even for non-conductive hulls, there are conductive parts in direct contact with the water, and these are usually connected electrically to the "earth" of the electrical system(s).
So where an A-E reversal on a vehicle (with rubber tyres) results in chassis being livened above local ground,but no current flowing (until someone bridges the gap; boats are different .

With the boat being in effect an earth electrode, any difference in potential between the water and the shore-based installation's earthing system will cause current to flow.
Even very small currents, especially d.c., flowing via this path can cause massive amounts of corrosion, and the source may be some other boat.
Hence the use of isolating transformers or galvanic isolators to block d.c.


Another difference is that boats may travel to other places where supply is not MEN.

Many ex-UK caravans have polarity reversal indication (generally just a neon between N & E); which can cause problems when doing the mandatory IR test (A+N - E) as part of WoEF checks.

The draft new "3001" includes a requirement for reverse polarity detection for transportable structure that have supply via transformer / converter with polarised output. This is the same requirement, and again not universal, but triggered by a different condition. Very strange.
Will be interesting to see what emerges in the final document.