Ahoy!
Starting to reach the limit of my knowledge here - tried to comment on your points inline - hope the formatting comes through. Bottom line your question: if we are using an isolated source (e.g. Isolation transformer, or inverter/generator away from the dock) why can we not have a floating Natural line? To be honest, I do not know all the reasons. AND I know when working with old tube equipment, we used an isolation transformer that did not rebond the neutral line! But that was to protect us from hot chassis that seem to be common in tube equipment - we would not use an unbounded isolation transformer during normal operations - only while servicing the units. In short, I suspect there are some specific situations where a floating neutral has a benefit, and perhaps larger ships present some of those cases. Perhaps interference or steel vessels change the situation some. Myself - this is a bit beyond my knowledge, and most of the talk around the safety ground / neutral bonding centers on 'Thou Shall' - though read below for some cases where lacking this connection can cause concern. On my boat, I installed an isolating transformer and re-established the safety ground to neutral bonding point in my electrical panel. I cannot see a clear reason not to do this (though there might be some), but I can see some reasons why (read below) - and then there is the Thou Shall! Perhaps someone else has insight into larger ship system electrical systems, and more so their applicability to smaller boats such as ours. -al- From: liveaboard-boun...@liveaboardonline.com [mailto:liveaboard-boun...@liveaboardonline.com] On Behalf Of banders...@earthlink.net Sent: Wednesday, December 08, 2010 8:06 PM To: liveaboard@liveaboardonline.com Subject: Re: [Liveaboard] GALVANIC ISOLATOR Ahoy Al, I understand the wisdom of connecting all the touchable metal parts of electrical equipment to ship's earth so the bilge water and all touchable metal parts will be at the same voltage as the earth and I do this on my vessel. It is also a good idea to have a way to measure the ac voltage/current between the grounding (green or bare) wires coming from your system, and the grounding point to which you attach them, so you can be aware of any leakage. But would you please address the issue of connecting (or not connecting) the ac neutral to earth. On Boats, if we are plugged into Shore Power (w/o an onboard isolation transformer) we have no choice: the dock system will have this connection back at the master panel, and in this case we should not 'double connect' a 2nd bonding wire on the boat as that created ground loop issues. When we are not connected to dock and using an inverter, or generator (or an isolation transformer while at the dock) one is directed to re-establish a safety ground and neutral bonding point. This is typically done via the AC transfer switch (Shore / Generator) or internal to 'Marina' inverter. Why do we need to do this? According to the NEC 250.6: "To prevent a fire, electric shock, improper operation of circuit protection devices, as well as improper operation of sensitive equipment, the grounding of electrical systems, the bonding of equipment and circuit conductors must be done in a manner that prevents objectionable current (neutral return current) from flowing on conductive materials, electrical equipment, or on grounding and bonding paths". Thou Shall J Dock Power gives us no option (again, w/o an onboard isolation transformer), your question is what to do with isolated sources: Inverter, generator, isolation transformer. For example, my 240 vac loads (and those ashore) have no neutral. Why should 120 vac loads be any different? Wouldn't it make things safer to isolate *all* current-carrying conductors rather than connect neutral ( a current carrying conductor) to the ship's hull or other similar earth-like structure? Even if a 240v load is not using a neutral, it does enjoy the benefit of the bonded neutral by keeping the two 120v legs within 120v max to earth. More so if there was a fault in your dryer where one of the legs shorted to the case, then the breakers would pop due to the bonding point back at the house entrance. If we had a unbounded neutral - say away from dock running on a unbonded generator - then in this fault scenario a breaker would not pop. Given that the case of the dryer is connected to the safety ground, it is unlikely we would feel anything when working with the dryer. However we would now have the neutral at a potential of 120v above earth ground. Thinking this through, we might have these issues: 1. A 2nd fault somewhere else, say in this case where the Neutral was shorted to the case: In this situation someone coming into contact with that case would get a hard shock of 120v. 2. Or even if another device is not 'Faulted', but perhaps has a Hot Chassis by design. (I am remembering old Tube TVs where the Neutral line was connected to the metal chassis... Hence the polarized plug on them. 3. As I understand it, some electronics will not like to see more then 5v potential between safety ground and neutral - they may be damaged. (Surge Suppressers seems to be one likely candidates) 4. But perhaps most concerning to me at least: There is a SHORT in the 1st dryer, and none of the protection systems could detect it! This last point is because all the rest of the electrical systems are design with the idea that there IS a neutral to safety ground bond somewhere in the system. And if that is not present, then perhaps some of the design approaches are now invalidated. And of course the wild card is electronics: Why knows what if any damage can happen with a safety ground to floating neutral potential of say 120v with a hard fault. Or even 20 or 30v in the case of ever present leakage. I seem to have read that the European electrical system has ungrounded neutrals. Is this true or not? Not an expert here, but I have seen reference to this as well. However, this article implies is it more of an exception then the rule: http://en.wikipedia.org/wiki/Earthing_system I believe in the US power that comes into a house is from a center tapped secondary winding in the pole transformer which delivers 240 vac at the ends of the coil and the neutral comes from the center tap. The grounding of the neutral is only added in the house's breaker box by the neutral/grounding jumper wire. Correct me if I am mistaken. That is my understanding as well. And more so, only ONE such jumper is allowed. If you have a sub-panel (ala in a shed), then there is not a 2nd bonding point there. I have had an ungrounded neutral 120 vac system for 30 years on an electrically powerful and complex vessel and have never felt the slightest tingle. Do you have an isolation transformer, or never plug into shore power? Ref above, if you plug into shore power w/o an isolation transformer then the dock is providing this bonding point. I did this mainly because that was the way they did it on the many merchant marine ships I served on and because I believed the safety aspects of this scheme would work just as well on my own vessel. Have to tell you, here I am over my head. It is clear that an unbonded neutral has benefit in some situations - but I am not sure what those might be. I suspect larger ship systems have considerations beyond what we see on our level of boats - and that might drive different decisions. On my boat, I am not aware of any benefit of having a floating (unbonded) neutral, but I can see several concerns with it. And hence, I have re-established a safety ground to neutral connection in my system. Because I can see failure modes that cause safety issues without it, and then there is always the Thou Shall! Norm S/V Bandersnatch Lying Julington Creek FL N30 07.68 W081 38.47
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