Perhaps I wasn't clear enough in my message about the ungrounded neutral system.
I am NOT proposing the elimination of the grounding (green wire) system that connects all non-current carrying metal parts of electrical equipment (cabinets, motor housings, etc) to ground. These wires connecting touchable parts of electrical equipment to the hull (central grounding point at earth potential) are a simple and effective way to drain off stray electricity to protect personnel in the case of electrical defects (notice "defects" is plural - more on that later). Let us first consider a grounded neutral system. When electricity does go astray, say when a hot line touches the cabinet in an electric clothes dryer, the cabinet becomes hot compared to ground. Just how hot is it? If the defect makes a good connection then no more than 30 amps can flow or the breaker will trip. (Jan's dryer in her condo has a 30 amp breaker.) So lets say that in the case of our defective dryer, 120 vac is on the cabinet (defect #1) and almost 30 amps is flowing through the grounding (green) wire, to the grounding terminal block, thence through the neutral/grounding jumper and back to the power source. There is also a connection between the grounding terminal block and the bilge (and the earth) so that between the bilge and all touchable bits of metal the boat there will be more-or-less the same electrical voltage. Just how much voltage (the more-or-less voltage) will be between the cabinet and the bilge, or that is to say, how much voltage will be developed across the grounding (green) wire? According to my "Ugly's Electrical References" handbook, an ampacity of 30 amps is carried by a number ten wire and number ten wire has a resistance of 1.25 ohms per thousand feet. Grounding wires are the same size as conducting wires and say this number ten dryer grounding wire is fifty feet long. Fifty feet is 1/20th of a thousand so it's resistance will be 1/20th of 1.25 or .065 ohms. .065 ohms times 30 amps is 1.95 volts, hardly life threatening, UNLESS the grounding circuit has a defect. If the grounding line is defective (defect #2) and the resistance in this line goes up, so does the voltage developed across it, up to a maximum of 120 volts which can be lethal. (Bear in mind that in the 240 system that while the black and the red (the two hot wires in a 240 volt feed) are 240 volts apart , each line is only 120 volts higher than neutral so you would only measure 120 volts to ground even in a 240 volt feed. ) So in a neutral-connected-to-the-grounding system if there was a stray connection in the dryer from hot to cabinet, and there was a defect in the grounding circuit, a potentially lethal voltage could develop between cabinet and bilge. So in the grounded neutral system there must be TWO defects for the dryer cabinet to someone standing in the bilge. Now let's take the case of the ungrounded neutral system. Since there is no connection between the bilge and any current carrying conductor, then when the dryer cabinet became hot (defect #1) a person standing in the bilge would feel nothing when touching the dryer because the electricity could not find its way back to its source. If there was a defect of leakage from neutral to grounding or bilge (defect #2) then the person will be exposed to 1.9 vac, too low to even be aware of. Only if there was an additional defect (defect #3) of excessive resistance in the grounding (green) conductor would the dryer cabinet become dangerous to a person standing in the bilge. So there you have it. The grounded neutral system requires two defects to become dangerous. The un-grounded neutral system requires three defects to become dangerous. Note also that a method of indicating there is an electrical leak between current carrying conductors and ground would be a Good Thing in both systems. This could be a simple light bulb, as I have seen used on "the big ships", or meters/switches etc. The offending equipment can then be found by disconnecting the various suspects one at a time. I have done this myself finding a leak in an air compressor motor cooling fan. As things are with such an indicator, either system could have leakage and one would never be aware of it until a second or third defect (such as a failure of the grounding (green wire)) lets the cabinet become hot. Since there are very few antique tube type radios or TVs on our vessels a normally hot radio chassis is a rare and special case that should be dealt with accordingly. Norm S/V Bandersnatch Lying Julington Creek FL N30 07.68 W081 38.47 See Annotations Below 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. Norm - Agreed. 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. Norm - I most often see the neutral terminal strip and the grounding terminal strip in the breaker box. The neutral-to-grounding jumper is not mentioned. I don't know what a '"Marina" inverter' is. 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 Norm - Yes, keep electrical currents off things that should not have electrical currents flowing through them. Futher, they should require devices to indicate when a defect causes current to flow through "grounding and bonding paths" so folks will be aware of otherwise hidden defects and take corrective action. 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. 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. Norm - If the neutral was not connected to the earth, there would be zero volts max to earth so there is no benefit of the "bonded neutral". 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. Norm - True, if the neutral was not connected to earth in this case then the breaker would not trip, but the Ground Fault Indicator would be screaming! 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. Norm - If the dryer had its hot touching the cabinet and the washer had its neutral touching the cabinet and we put a hand on each one we would feel nothing because the grounding (green) lines connect the two cabinets together. If the "touching" were good contacts then the breaker would trip. There would have to be the additional defect of a bad grounding (green) conductor for the cabinets to become dangerous. Again - three defects needed for danger. 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. Norm - Having an antique TV on board with a hot chassis would be a special case requiring special considerations. 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) Norm - This is true. I have had surge suppressers smoke due to the afore mentioned leaky air compressor cooling fan. I do have a "whole house" style surge supperssor (also called a lightening arrestor" at the breaker box. 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! Norm - That depends where the short is. If it betwee the two hots then the breaker will pop. If it is between a hot and neutral (driers use 120 vac for the controls and the motor) then the breaker will pop. If it is between a hot and cabinet that situation is discussed above. I repeat - ALL vessels should have ground fault indicators whether they have grounded or ungrounded neutrals to show that a potentially dangerous condition exists. This is a failure of the safety standards writers. This last point is because all the rest of the electrical systems are designed 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. Norm - I have trouble understanding this statement. Yes, if "a neutral to safety ground bond" is "not present, then perhaps some of the design approaches are now invalidated." I agree. Changing "design approaches" from two barriers to death to three barriers to death would be a Good Thing. 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. Norm - This is an interesting point. However, in my case thirty years of living with a ungrounded neutral has shown it to be moot. 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. Norm - Agreed. I have an 2-winding isolation transformer. I think everyone should. A 7.5 KVA unit with taps, I got it for a reasonable price at Groban Supply. 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 - The big ships have a lot of 440 vac three phase to run blowers, compressors, winches, etc. This voltage can be instantly deadly so the ungrounded system, along with fault indicators and a professional electrician on staff, bring a degree of needed safety. I prefer to formulate my own "Thou Shalls" and "Shall Nots". (see previous essays on factory installed defects in diesel fuel systems) In the case of the ungrounded neutral system, I prefer three barriers between me and electrocution over two. PS - This essay also had another lasting benefit; I have finally learned to spell "neutral" correctly! Norm S/V Bandersnatch Lying Julington Creek FL N30 07.68 W081 38.47
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