A few questions re connecting shore power

[Murflynn]

Pretty certain it was in the old red book when I fitted our boat out. Not looked at the new BSC that much so not sure if it states similar in there or not. The more I think about it the more I am convinced it was "within 75mm but the ac and dc bonds couldn't be on the same stud (or words to that effect.)

I have checked the latest BSS checklist and there is no mention of grounding AFAIK.

I think the only regulation is the applicable BS/EN/ISO as referred by Chewbacka.

[Guest]

I have checked the latest BSS checklist and there is no mention of grounding AFAIK.

I think the only regulation is the applicable BS/EN/ISO as referred by Chewbacka.

Could be. My mind plays tricks these days

[nicknorman]

Your nice switch mode charger will be connected to the battery and the mains and any switch mode electrical noise will be dumped down the mains earth also connected to the battery and the hull by the negative earth bond.

Not really. Firstly SMPSs these days don't dump their noise to earth - in fact you average phone charger etc doesn't have an Earth connection (the Earth pin is usually plastic). I'm pretty sure that to be CE marked there is a very small limit as to what can be "dumped to earth". Your argument was one used in the early days of SMPS but now mostly obsolete.

 

But secondly even if there were some noise, does AC current cause electrolytic corrosion?

[WotEver]

... even if there were some noise, does AC current cause electrolytic corrosion?

Good question and I don't know. I've tried to research that in the past with little success. My gut tells me that yes it would on one half of he cycle and it's unlikely that the metal would be replated on the other half of the cycle. But that's only a guess.

[nicknorman]

Years ago I made a gadget to automatically water the greenhouse plants. It used a soil moisture sensor which consisted of passing a very small current through a couple of nails poked into the soil, the voltage being a guide to the moisture and if it was above a threshold, the water valve would be opened for a timed period,

 

But one of the nails disappeared in just a couple of weeks! Then I tried mk. II which used AC as the measuring medium. It can't now remember the exact details but I do remember that the nails no longer disappeared rapidly - I just can't remember if there was any electrolytic corrosion at all.

[Geo]

Just a few thoughts on DC and AC earth connections to a steel hull.

 

On a steel boat where and how the various DC and earth connections are connected to the hull has been the subject of conversations and arguments over the many years I have been involved in boats. In one way it is simple and in others can be complex.

 

The need to stop electrolytic corrosion means that the boats own DC system should only be connected to the hull in one place. This is to stop current flow through the hull causing electrolytic corrosion. That is the simple one.

 

Now comes the need to add AC supplies to the boat. If the there is absolutely no chance that there could be any interconnection between the AC supply and the boat’s other electrical supplies (the AC supply is fully isolated from all other electrical supplies) then the answer is simple it does not matter where on the hull the AC earth is connected to the hull but there must be only one connection. If there is more than one AC earth connection to the hull, there is the possibility of current flow around the earth system and thus through the hull. Now on the face of it being AC it cannot cause electrolytic corrosion because in one half cycle it takes off and in the next half cycle it puts back. I would suggest that the very action of removing ions from the steel and the putting it back leaves a steel with a different structure, ions that were within the steel have now been deposited on the surface of the steel. However that is not the main reason in my opinion for using only one connection. If corrosion occurs, say at the hull connections point or elsewhere, in the earth route there is a distinct possibility of the earth AC currents being rectified into DC and flow between various AC earth points on the hull, only a few volts worth but sufficient to cause electrolytic corrosion. Thus I believe there should only one AC earth connection to the hull.

 

If that AC supply can become interconnected to any part of the boat’s other electrical supply the two hull connections need to go very close together. Interconnection can happen when things like chargers and inverters etc are used and the negative and the AC earth are connected within the box. So it is important to make sure anything that is connected to both electrical systems does not allow an interconnection between the DC and AC hull connections if the connections to the hull are not close together. If they are close together, within a couple of centimetres, there is no problem. So personally I use the easy way, because I do not know what electrical stuff will be added in the future; I would always put the DC hull connection and the AC earth hull connection very close together and only one of each. It saves a lot of headaches further down the road.

[Neil2]

There's still something I'm not grasping here and that is why should there be "various" AC earth points on the hull? most AC appliances are likely to be double insulated, but those that are not are earthed via the green wire back to the consumer unit aren't they? I suspect this is something akin to domestic stainless steel sinks needing to be earthed but I still can't get my head round it.

[nicknorman]

There's still something I'm not grasping here and that is why should there be "various" AC earth points on the hull? most AC appliances are likely to be double insulated, but those that are not are earthed via the green wire back to the consumer unit aren't they? I suspect this is something akin to domestic stainless steel sinks needing to be earthed but I still can't get my head round it.

 

You do get some devices whose cases are earthed, for example inverters / combis. So if they are somehow attached to the hull as opposed to onto wood, then you have an extra Earth connection. Similarly a Travelpower has a braided lead going to the high voltage alternator, if this touches any hull metal there's another path. And I suspect, though haven't actually checked it, that the Travelpower alternator's case may be connected to Earth, and thus to the hull via the engine. Washing machine cases are normally earthed, although it seems unlikely that they would somehow be connected to hull. So you are right many devices are double insulated and therefore not earthed, but some are earthed.

[Geo]

You do get some devices whose cases are earthed, for example inverters / combis. So if they are somehow attached to the hull as opposed to onto wood, then you have an extra Earth connection. Similarly a Travelpower has a braided lead going to the high voltage alternator, if this touches any hull metal there's another path. And I suspect, though haven't actually checked it, that the Travelpower alternator's case may be connected to Earth, and thus to the hull via the engine. Washing machine cases are normally earthed, although it seems unlikely that they would somehow be connected to hull. So you are right many devices are double insulated and therefore not earthed, but some are earthed.

 

And thus connected, I would suspect, to the hull DC connection point