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Isolation transformer and connection to ground
A question on isolation transformers and their ground path. I have
seen 2 different wiring diagrams. One has the ground from the copper screen inside the transformer going to shore ground and the other has it going to ship earth. My instinct tells me that the former is correct - if there is a problem inside the transformer the current path is back to shore. If the shore supply has a RCD/GFCI then it should be detected and the shore side supply breaker should trip. If there is no shore RCD/GFCI, then at least the hull potential is not being raised. With the latter the current path is to hull - which I think raises the potential of the hull - not a good idea. If there is no GFCI/RCD on the shore supply breaker, the leakage may not be detected. The ship's RCD/GFCI will not detect it either as it is installed downstream of the transformer. Former at http://www.victronenergy.com/upload/...ransformer.pdf Latter at http://www.sterling-power.com - products - isolation transformer Comments/Thoughts??? |
Isolation transformer and connection to ground
Should be
Former at http://www.sterling-power.com - products - isolation transformer Latter at http://www.victronenergy.com/upload/...ransformer.pdf |
Isolation transformer and connection to ground
Very few people use isolation transformers. Everybody should, but ignorance
prevents them from doing so. If you use one, never connect the safety earth from shore or you might as well not have an isolation transformer. This isolates your vessel earth. Afterall, the reason to use these is to protect your hull from leakage current from Granny's decrepid 30 year old vacuum cleaner on the other end of the marina. Steve wrote in message oups.com... A question on isolation transformers and their ground path. I have seen 2 different wiring diagrams. One has the ground from the copper screen inside the transformer going to shore ground and the other has it going to ship earth. My instinct tells me that the former is correct - if there is a problem inside the transformer the current path is back to shore. If the shore supply has a RCD/GFCI then it should be detected and the shore side supply breaker should trip. If there is no shore RCD/GFCI, then at least the hull potential is not being raised. With the latter the current path is to hull - which I think raises the potential of the hull - not a good idea. If there is no GFCI/RCD on the shore supply breaker, the leakage may not be detected. The ship's RCD/GFCI will not detect it either as it is installed downstream of the transformer. Former at http://www.victronenergy.com/upload/...ransformer.pdf Latter at http://www.sterling-power.com - products - isolation transformer Comments/Thoughts??? |
Isolation transformer and connection to ground
I have one with the earths in accordance with the sterling power
diagram. It was not very expensive - £175 for a 3.6kVA. When I was installing the shore power its use was instantly demonstrated whilst still on dry land in the builders shed. The building's neutral was showing +12 compared to earth, once onboard after the ship side neutral had been bonded to earth, the ship side neutral was of course at Ov. The other major advantage of an IT is that you no longer have to worry whether the shore supply itself is wired correctly - Live is Neutral and vice versa. |
Isolation transformer and connection to ground
The only function of the shore power ground wire when you have an
isolation transformer is to return short circuit current back to the source so it will blow the dock circuit breaker. Although the probability of an electrical breakdown on a transformer is minimal, the metal frame and the copper shield should be connected to the shore power ground to provide this return circuit. The transformer frame should not be connected to anything on the boat and it should be mounted on an insulated support in a metal boat. When purchasing an isolation transformer, getting one with adjustable voltage taps on the input is worth the extra expense if you are traveling to foreign ports. Our transformer can accept input from about 80 volts to about 450 volts with different tap settings and in addition to foreign adaptability, it has helped out on docks suffering from low voltage. wrote: I have one with the earths in accordance with the sterling power diagram. It was not very expensive - £175 for a 3.6kVA. When I was installing the shore power its use was instantly demonstrated whilst still on dry land in the builders shed. The building's neutral was showing +12 compared to earth, once onboard after the ship side neutral had been bonded to earth, the ship side neutral was of course at Ov. The other major advantage of an IT is that you no longer have to worry whether the shore supply itself is wired correctly - Live is Neutral and vice versa. |
Isolation transformer and connection to ground
Andina Marie wrote:
The only function of the shore power ground wire when you have an isolation transformer is to return short circuit current back to the source so it will blow the dock circuit breaker. Although the probability of an electrical breakdown on a transformer is minimal, the metal frame and the copper shield should be connected to the shore power ground to provide this return circuit. The transformer frame should not be connected to anything on the boat and it should be mounted on an insulated support in a metal boat. To clarify, a short in the transformer primary (or secondary, for that matter) should trip the boat's breaker. If it instead trips the dock breaker there is a design or equipment problem. Although the transformer's metal case could be connected to the shore power ground wire, insulation of the transformer case is then required. Because wiring in this way provides dubious benefits, I believe grounding the metal case to the boat's ground system is a better approach. In terms of safety, it seems preferable that the transformer case be at the same potential as the metal cabinets of all other AC devices on the boat. When purchasing an isolation transformer, getting one with adjustable voltage taps on the input is worth the extra expense if you are traveling to foreign ports. Our transformer can accept input from about 80 volts to about 450 volts with different tap settings and in addition to foreign adaptability, it has helped out on docks suffering from low voltage. While a tapped primary is generally a good idea, isolation transformers with 450 volt primary taps and internal shields are not common in the US. An alternative approach would be to use a "standard" isolation transformer (untapped primary) and a separate autotransformer. The latter does NOT provide isolation, but can be used on either side of the isolation transformer to step voltages up or down as desired, and is fairly inexpensive. Even a Variac (a continuously adjustable type of autotransformer) can provide an inexpensive means of dealing with a range of input voltages. Unfortunately, if used on the shore side of the isolation transformer, the metal case of a (non-adjustable) autotransformer would have to be insulated. On the secondary side of the isolation transformer, the case could be grounded as I recommended for the isolation transformer case. Although it may be ok to apply 450 volts AC to the primary of an isolation transformer designed for 120 volts, the manufacturer's input voltage ratings should always be strictly observed. As a practical matter, the autotransformer used at the isolation transformer secondary will safely and conveniently correct under- and overvoltage conditions, but it may not provide a universal solution. The autotransformer (or tapped isolation transformer) can be a double-edged sword. If it is set to raise an incoming line voltage from 80 to 120 volts, and in the middle of the night the line voltage rises to 120 volts, there is a good chance that damage will occur to connected appliances. Chuck wrote: I have one with the earths in accordance with the sterling power diagram. It was not very expensive - £175 for a 3.6kVA. When I was installing the shore power its use was instantly demonstrated whilst still on dry land in the builders shed. The building's neutral was showing +12 compared to earth, once onboard after the ship side neutral had been bonded to earth, the ship side neutral was of course at Ov. The other major advantage of an IT is that you no longer have to worry whether the shore supply itself is wired correctly - Live is Neutral and vice versa. |
Isolation transformer and connection to ground
As I recall from training many years ago, the shield is a noise remedy.
It seems it should connect to the vessel's signal ground / counterpoise through a radio frequency capacitor, to cancel and block noise from the radio circuits, via the ship's neutral connector. The transformer case and frame should be connected to shoreside "Earth", and neutral at the power pole / entrance. This presumes that the shield is not connected to the frame of the transformer. If it were, I would try it both sides and disconnected, to satisfy noise and galvanic requirements while at the dock. |
Isolation transformer and connection to ground
Yes and no. The type of shield you are thinking about was indeed
intended to reduce noise, and it typically was grounded on the side most likely to be noisy. The Faraday shields in those "non-marine" isolation transformers are designed to handle signal-level currents. Marine-type isolation transformers (as contemplated by the ABYC) utilize shields designed to handle the full current rating of the transformer. Presumably the presence of that shield would cause primary leakage currents to flow through the shield to the shore grounding wire (thus tripping a GFCI breaker) rather than through the transformer core to the secondary. The shield is intended to provide belt and suspenders safety, rather than noise reduction, although it undoubtedly offers up some of that as well. Chuck Terry K wrote: As I recall from training many years ago, the shield is a noise remedy. It seems it should connect to the vessel's signal ground / counterpoise through a radio frequency capacitor, to cancel and block noise from the radio circuits, via the ship's neutral connector. The transformer case and frame should be connected to shoreside "Earth", and neutral at the power pole / entrance. This presumes that the shield is not connected to the frame of the transformer. If it were, I would try it both sides and disconnected, to satisfy noise and galvanic requirements while at the dock. |
Isolation transformer and connection to ground
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 |
Isolation transformer and connection to ground
Thanks for your comments.
I understand your concern. Let's look in more detail at the 120 volt, single phase system. Case I. Transformer case connected to shore power grounding conductor. ABYC requires transformer case to be fully insulated with ventilation. ABYC requires no ground fault protection. Case II. Transformer case connected to boat AC grounding conductor (i.e., water). ABYC requires ground fault protection at the main shore power disconnect circuit breaker. No insulation or ventilation is required and the transformer case is at the same potential as all of the other grounded metal on the boat. Under case I, a direct short from the hot primary wire to the transformer case will trip the boat's breaker (hopefully) or the dock breaker. Under case I, a ground fault (leakage) of say 15 amps on a 20 amp circuit breaker can occur and continue indefinitely. This could be leakage from a hot primary wire to the case, to the grounded (neutral) primary wire, or to the secondary. Definitely not a desirable state of affairs, but one that grounding the transformer case to the shore grounding conductor does not and cannot prevent (unless the marina wiring has provided ground fault protection devices at their distribution box; quite rare). Under case II, a direct short from the hot primary wire to the transformer case (i.e., to the boat's grounding conductor) will trip the primary circuit's ground fault protection device once the current exceeds 5 milliamperes or so. Similarly for leakage currents in excess of 5 milliamperes but less than the circuit breaker trip current. [In either case I or II, leakage current between the hot primary wire and the neutral (grounded) primary wire will result in heat, lower output voltage, and without thermal overload protection, potentially serious consequences if the breaker doesn't trip. Has no real bearing on what we are discussing and ground fault protection will not help. Fortunately, transformers are among the most reliable electrical components.] I have omitted any discussion of the shield, which ABYC requires to carry the full current rating of the transformer. Depending on the manufacturing geometry, that shield may preclude a current path from primary to secondary or from the primary to the case, thus rendering the short and leakage scenarios very improbable. Ignoring the role of the shield, I believe Case II is a better solution, even if ground fault protection is added to Case I (which I strongly recommend). My current ABYC standards are not at hand and I am using an older edition, but I believe they are similar to standards now in effect. Does this provide better clarification? Chuck Andina Marie wrote: 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 |
Isolation transformer and connection to ground
In article . net, chuck wrote:
Yes and no. The type of shield you are thinking about was indeed intended to reduce noise, and it typically was grounded on the side most likely to be noisy. The Faraday shields in those "non-marine" isolation transformers are designed to handle signal-level currents. My understanding of shields in general, is that the shield should be connected to the ground with the least noise. greg Marine-type isolation transformers (as contemplated by the ABYC) utilize shields designed to handle the full current rating of the transformer. Presumably the presence of that shield would cause primary leakage currents to flow through the shield to the shore grounding wire (thus tripping a GFCI breaker) rather than through the transformer core to the secondary. The shield is intended to provide belt and suspenders safety, rather than noise reduction, although it undoubtedly offers up some of that as well. Chuck Terry K wrote: As I recall from training many years ago, the shield is a noise remedy. It seems it should connect to the vessel's signal ground / counterpoise through a radio frequency capacitor, to cancel and block noise from the radio circuits, via the ship's neutral connector. The transformer case and frame should be connected to shoreside "Earth", and neutral at the power pole / entrance. This presumes that the shield is not connected to the frame of the transformer. If it were, I would try it both sides and disconnected, to satisfy noise and galvanic requirements while at the dock. |
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 |
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, |
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, |
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 |
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 " |
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 |
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 " |
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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