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#1
posted to rec.boats.electronics
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Isolation transformer and connection to ground
Andina & Chuck,
This is not rocket science. There is NO safety issue by not using shore safety earth. Shore safety earth ultimately connects with the distribution transformer nuetral and tied to a ground stake at that transformer. That ground stake is the reason for having an isolation transformer in the first place. It will never be a closer reference to earth than your boat. Any leakage current ANYWHERE in that distribution net will seek your hull to earth, as it will offer the least resistive path to earth. (It is in the water!) The plan is to magnetically couple energy from the primary to the insolated secondary with the secondary referenced by end tap (110 V) or center tap (230 V) to the boats ground plate. There must not be any electrical connection between your boat and shore power. If a fault would occur on either side of the transformer, fault current will run quite nicely to the ground plate tripping the feed circuit breaker no matter where it is. The only reason to use the shore safety earth on the transformer case is if the transformer is physically mounted on the dock, not in your boat. Steve "Andina Marie" wrote in message oups.com... Chuck, I don't agree with your grounding advise. If you ground the frame of the transformer to the boat ground and there is an internal short from the shore power primary winding to the frame, there is no return path for the current so you will not trip the supply breaker. In addition, you now have the boat ground, including underwater metal, live at 110 or 220 volts which can electrocute persons in the water or boarding from a metal dock. Andina Marie |
#2
posted to rec.boats.electronics
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Isolation transformer and connection to ground
I can't agree, Steve.
The question involves safety not electrolysis. It is well understood that there must be no connection between the grounds to eliminate the electrolysis currents and that is the primary reason for installing a transformer. However considering the safety question, the concern is the (remote) possibility of a short from the primary winding to the frame. I disagree with your statement that a fault on either side of the transformer will trip the circuit breaker - that is incorrect. If, as has been suggested, the transformer frame is connected to the boat ground and you develop a fault from the primary winding to the frame - and hence the boat ground - and hence the underwater metal, you will NOT draw enough current to trip a 30 or 50 amp circuit breaker. The underwater metal will be alive at a high AC voltage and considerable current will be flowing to the water generating all sorts of nasty gasses but there is no way the water is going to carry enough current to trip the breaker. A swimmer in the vicinity is then at risk of a lethal shock. As Chuck pointed out, a GFI will disconnect as soon as it detects leakage current to the water but GFI outlets on the dock are very rare and GFI protection on the boat input side of an isolation transformer is virtually non-existent. A short from the secondary side to frame, if the frame is connected to either shore or boat ground is a far less critical situation since the secondary side is floating so no lethal voltage would be present on the underwater metal in either case. Regards, Andina Marie Foster, |
#3
posted to rec.boats.electronics
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Isolation transformer and connection to ground
Andina Marie wrote: I can't agree, Steve. The question involves safety not electrolysis. It is well understood that there must be no connection between the grounds to eliminate the electrolysis currents and that is the primary reason for installing a transformer. However considering the safety question, the concern is the (remote) possibility of a short from the primary winding to the frame. I disagree with your statement that a fault on either side of the transformer will trip the circuit breaker - that is incorrect. A short on either primary or secondary would trip the breaker, of course, provided it has been sized properly. If, as has been suggested, the transformer frame is connected to the boat ground and you develop a fault from the primary winding to the frame - and hence the boat ground - and hence the underwater metal, you will NOT draw enough current to trip a 30 or 50 amp circuit breaker. The underwater metal will be alive at a high AC voltage and considerable current will be flowing to the water generating all sorts of nasty gasses but there is no way the water is going to carry enough current to trip the breaker. A swimmer in the vicinity is then at risk of a lethal shock. I agree with your analysis. As Chuck pointed out, a GFI will disconnect as soon as it detects leakage current to the water but GFI outlets on the dock are very rare and GFI protection on the boat input side of an isolation transformer is virtually non-existent. It should be kept in mind that GFI protection onboard is both inexpensive and relatively simple to install. I believe a strong case can be made for using GFI protection even when the transformer frame is connected to the shore power grounding conductor. A short from the secondary side to frame, if the frame is connected to either shore or boat ground is a far less critical situation since the secondary side is floating so no lethal voltage would be present on the underwater metal in either case. Is that true? If the secondary wire that is connected to the boat's ground shorts to the frame, which in turn is connected to the shore power grounding conductor, then I would agree with your statement. However, if the "hot" secondary wire shorts to the frame, the full secondary voltage will be applied between the boat's underwater metal and the shore power ground! Just what we are trying to prevent. If the frame is connected to the boat's ground, then a short from the secondary hot wire to the frame would simply trip a breaker. Chuck Regards, Andina Marie Foster, |
#4
posted to rec.boats.electronics
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Isolation transformer and connection to ground
chuck wrote:
....snip A short on either primary or secondary would trip the breaker, of course, provided it has been sized properly. ....snip Chuck I am following your discussion with much interest as I am planning to install very soon an isolation transformer in my boat. I add a further question: I have just learnt that a firm is going to commercialize a Switch-Mode Isolation Transformer (they claim is the first in the world!) and, if you are interested, just make a Google search. My question is: is it indeed equivalent to the ol' faithful and massive isolation transformers? Thanks for your help Daniel |
#5
posted to rec.boats.electronics
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Isolation transformer and connection to ground
The real problem, that no one has yet mentioned, is inrush charge current.
If the transformer is sized correctly for maximum shore load, the initial inrush will most certainly pop the breaker at most marinas. The solution I use works very well, if anyone is interested. Tie in series with the primary coil of the transformer a standard light bulb socket. Insert a 100 watt light bulb of the appropriate voltage and connect a switch capable of handling the maximum current of the transformer in parallel across the lamp socket. In use, open the switch and connect the transformer to shore power, then close the switch before applying secondary load. Initially the bulb will light and then fade out. Anytime after that bright phase, it is safe to throw the switch shorting out the lamp. Steve " |
#6
posted to rec.boats.electronics
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Isolation transformer and connection to ground
Good point, Steve. Has a lot to do with the trip curve of the pedestal-mounted circuit breaker, but that's usually beyond the reach of boaters to change. A step up from your manual (and probably bullet-proof) solution might be to use a simple 120 VAC, SPST relay with coil directly across the transformer primary and a 100 watt light bulb in series with the transformer hot wire. The relay's contacts would short the bulb when the inrush current dissipates and the relay coil is energized. A shortcoming is that the relay is energized whenever the transformer is in use. Another step up would be to insert a manual SPDT switch so as to select either the light/bulb relay circuit or a direct connection. That way the light bulb/relay circuit could be reserved for those cases where inrush is tripping shore power breakers. Probably $15 worth of parts and 20 minutes to do the job. It should be simple enough to design a better circuit that would automatically drop the relay out after initial inrush. Caution: these are lethal voltages and the wiring is best left to professionals for those uncertain of their skills and understanding. Chuck Steve Lusardi wrote: The real problem, that no one has yet mentioned, is inrush charge current. If the transformer is sized correctly for maximum shore load, the initial inrush will most certainly pop the breaker at most marinas. The solution I use works very well, if anyone is interested. Tie in series with the primary coil of the transformer a standard light bulb socket. Insert a 100 watt light bulb of the appropriate voltage and connect a switch capable of handling the maximum current of the transformer in parallel across the lamp socket. In use, open the switch and connect the transformer to shore power, then close the switch before applying secondary load. Initially the bulb will light and then fade out. Anytime after that bright phase, it is safe to throw the switch shorting out the lamp. Steve " |
#7
posted to rec.boats.electronics
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Isolation transformer and connection to ground
Thanks for the info. I was not aware of the product and as you can
imagine, I have no experience with it. My comments are therefore necessarily abstract. The two primary advantages of a switch-mode device are lower weight (less iron) and a built-in capability to regulate output voltage (and perhaps adjust for different input voltages) automatically. Potential disadvantages, however, are numerous. The standard isolation transformer is perhaps the most reliable electrical component we have. Switch-mode circuits tend to involve many electronic components, some of which operate under life-shortening stresses. While an isolation transformer could easily outlast a boat, I doubt the same could be said of the vastly more complex switch-mode devices. Without examining the actual circuit, it is impossible to discuss the degree of isolation achieved or the potential isolation fault modes of the device. No mention is made in the specifications about the purity of the sine wave output. For some appliances, this may not be critical; for others, it is very important. Possible electronic noise from the switching circuits could also be troublesome. While modern switch-mode circuits can be designed to deal acceptably with these issues, it is difficult to evaluate the product to which you refer since no mention of them is made on the website. Other considerations are susceptibility to noise and voltage transients (e.g., lightning) on the shore power line and radio frequency interference from onboard radios, battery chargers, fluorescent lamps, and radar. It is unclear how the ABYC and various EU standards will view such a device. Sorry I can't offer anything more specific. Perhaps others have had some experience with switch-mode isolation devices. Good luck. Chuck Daniele Fua wrote: chuck wrote: ...snip A short on either primary or secondary would trip the breaker, of course, provided it has been sized properly. ...snip Chuck I am following your discussion with much interest as I am planning to install very soon an isolation transformer in my boat. I add a further question: I have just learnt that a firm is going to commercialize a Switch-Mode Isolation Transformer (they claim is the first in the world!) and, if you are interested, just make a Google search. My question is: is it indeed equivalent to the ol' faithful and massive isolation transformers? Thanks for your help Daniel |
#8
posted to rec.boats.electronics
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Isolation transformer and connection to ground
Chuck,
I have been down the route of automatic inrush control. The circuit I designed was keyed off of secondary voltage in that when secondary voltage was developed, would the relay short the lamp. This worked very well, but I like the manual switch better. It is cheap clean and bulletproof. On the switched transformers the problem is efficiency, reliability and noise. I do use however a modified UPS for minor AC use and it works a treat. It is a 1400 KVA Smart UPS by APC, but instead of using it off internal batteries, I run it off ship's batteries (24V). Normally it will not fire up without an external AC source, but if you use the undocumented cold start procedure, it will. It has an additional utility as well. When pluged into shore power and is powered up, it will act as an intelligent battery charger, keeping the ships batteries fresh. Steve "chuck" wrote in message hlink.net... Thanks for the info. I was not aware of the product and as you can imagine, I have no experience with it. My comments are therefore necessarily abstract. The two primary advantages of a switch-mode device are lower weight (less iron) and a built-in capability to regulate output voltage (and perhaps adjust for different input voltages) automatically. Potential disadvantages, however, are numerous. The standard isolation transformer is perhaps the most reliable electrical component we have. Switch-mode circuits tend to involve many electronic components, some of which operate under life-shortening stresses. While an isolation transformer could easily outlast a boat, I doubt the same could be said of the vastly more complex switch-mode devices. Without examining the actual circuit, it is impossible to discuss the degree of isolation achieved or the potential isolation fault modes of the device. No mention is made in the specifications about the purity of the sine wave output. For some appliances, this may not be critical; for others, it is very important. Possible electronic noise from the switching circuits could also be troublesome. While modern switch-mode circuits can be designed to deal acceptably with these issues, it is difficult to evaluate the product to which you refer since no mention of them is made on the website. Other considerations are susceptibility to noise and voltage transients (e.g., lightning) on the shore power line and radio frequency interference from onboard radios, battery chargers, fluorescent lamps, and radar. It is unclear how the ABYC and various EU standards will view such a device. Sorry I can't offer anything more specific. Perhaps others have had some experience with switch-mode isolation devices. Good luck. Chuck Daniele Fua wrote: chuck wrote: ...snip A short on either primary or secondary would trip the breaker, of course, provided it has been sized properly. ...snip Chuck I am following your discussion with much interest as I am planning to install very soon an isolation transformer in my boat. I add a further question: I have just learnt that a firm is going to commercialize a Switch-Mode Isolation Transformer (they claim is the first in the world!) and, if you are interested, just make a Google search. My question is: is it indeed equivalent to the ol' faithful and massive isolation transformers? Thanks for your help Daniel |
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