Electrical issues - outdoor cable & isolator switch

[GUMPY]

The reason ( apart from choice) boats ended up with the isolator in the -ve is that in the early days of the BSS (25+ years ago) it was stipulated there should be one isolator for both start and domestics and no one made a DP isolation switch so as the -ve was usually common  it got put in the -ve.

 

[WotEver]

1 hour ago, Rick Savery said:

Good point Wotever - I suppose land-based electricians won't have the 12V experience if nothing else.

They generally know nothing about boat electrics, plus most boat 'standards' are completely contrary to their learning. We don't use twin & Earth for mains, for instance.  We don't use the hull for a negative return as they might have got used to in cars, low voltage cables are selected for voltage drop, not current carrying capacity... etc.

[Paul Gwilliams]

53 minutes ago, WotEver said:

They generally know nothing about boat electrics, plus most boat 'standards' are completely contrary to their learning. We don't use twin & Earth for mains, for instance.  We don't use the hull for a negative return as they might have got used to in cars, low voltage cables are selected for voltage drop, not current carrying capacity... etc.

12v is alien to me more than happy with 240 & 415v. My apprenticeship was commercial based so never got to house bashing.

But it's a good learning curve with the expert help on here. 

[ditchcrawler]

3 hours ago, Keeping Up said:

Assuming that it is correctly connected (ie so that it disconnects the battery terminal from absolutely everything) then having the isolator switch in the negative lead CANNOT cause a spurious return path via the hull, even if various things that shouldn't connect to the hull do in fact do so (such as a car radio aerial outer). Actually I prefer it being in the negative because having switched it off results in the safe situation that when working on the battery connections it doesn't matter if any one cable should happen to drop and land on the hull; if the switch is in the positive and you should happen to accidentally drop the positive cable on to the hull there will be sparks. In fact the only convincing argument I've seen for it being in the positive is because that is where you would expect to find it! And yes I am a qualified electrical engineer (or was until I retired).

I point was people adding bits not taking a negative switch into account mabe connecting negatives back to the battery.

3 hours ago, Loddon said:

The reason ( apart from choice) boats ended up with the isolator in the -ve is that in the early days of the BSS (25+ years ago) it was stipulated there should be one isolator for both start and domestics and no one made a DP isolation switch so as the -ve was usually common  it got put in the -ve.

 

Didn't know that, but as you say an easy way to overcome it, that is how our share boat was wired

[RickS]

Thankyou all for your comments and advice - I can't pretend to understand all of it but read them all with interest and I think some of it has stuck - at least I have a better understanding of what the problem may well be, and know what to ask the electrician. Thanks again