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Bad shore power
Chris wrote:
If youb boat is not connected to shore power or ground, I don't see how the marina's or other boater's faulty electrical installation could eat your zincs. Chris Well, if your boat is in a strong DC field between two other boats, that could speed up zinc depletion. Imagine the boat on your right dropping his battery cable into the water and current flowing through the water to the boat on your left, and returning to the battery via the shore power grounding conductor. Your boat is in the middle and current enters a through-hull at the bow, travels through the DC wiring to the prop and out again. The current seeks out a low resistance path and your boat is it. Now you have a strong DC voltage at the prop that cancels the effect of your zinc and eats the prop. Impressed current systems on docks or larger vessels can create such fields also. Chuck |
Bad shore power
Chuck, I don't think you understand the problem. If there is a ground fault detecting breaker some where in the distribution net, it clearly should function whether or not we elect to use the green equipment line or not at the service point. I agree that the green wire should provide a low impedance path to earth. In fact , as I stated, that connection must be made at the distribution transformer. The problem is that the low impedence path provided at the transformer will NEVER be as low an impedance path to earth as your hull is. Therefore energy leakage to earth anywhere within the net, we are now talking about the whole marina, will route itself to your hull simply because it is a better ground than the one provided at the transformer. Your statement that fault current does not flow through the green wire cuircuit is flat incorrect. That is its sole purpose. The inverse is also true, if there is no fault, no current will flow there. If we take the case of the in-line AC line filter, you know, the capacitor type with the metal case that bolts to equipment frames? Or granny's 25 year old vacuum cleaner with worn out brushes and carbon dust all around the commutator? All that is leakage current. Now, I concede that the earth point at the transformer is also the neutral tranformer leg tie point and the leakage also returns to the transformer at that point, but your hull has also created a huge ground loop in the net and by default routes all earth reference current through your hull. Please also remember that your boat, if wired correctly, is a complete power system AC and DC all properly tied together at your hull ground plate and when hooking directly to shore power without an isolation transformer, you are paralleling the shore based grid and the ground connection you make is effectively an equalization bus and every code book I have ever read clearly states that an equalization bus should never be longer than one meter. So the correct advice is, if you don't want to waste your metal bits below the waterline due to electrolosis and galvanic protection is your goal, never direct wire to shore power. Instead, you should couple your energy magnetically through the use of an isolation transformer and leave other's faults to themselves. Steve Hello Steve, I would caution strongly against using the water-earth path in lieu of the green equipment grounding conductor. Ever. The purpose of that conductor is to provide a low impedance path for the purpose of clearing a ground fault. In proper wiring, the impedance is no more than a few ohms and the fault is cleared at a breaker. Fault current does NOT flow through the earth. By contrast, the water-earth path may have a resistance of 25 ohms or more for salt water, and tens of thousands of ohms or more for fresh water. This path will NOT clear a ground fault. It will not trip a 20 A breaker. Witness the 120 volt power cords dropped off a dock without tripping a breaker! If one has a problem with wiring impedances in the marina, deal with it by getting the wiring inspected. Ask the marina operator when it was last checked. Yell. Scream. Buy your own tester and tell the marina what you measured. Tell the other boaters. Take names and publish on the Internet. But don't throw out the safety net because even with corroded connections, the wiring may still allow a ground fault to be cleared. If there is a problem with a nearby source (boat or otherwise) then an isolation transformer would be a good idea. But even with an isolation transformer, always use approved wiring practices. There is probably no more thoroughly reviewed standard than the NEC. The ABYC standards for AC wiring are similarly solid. And install GFCIs for the smaller ground fault currents that won't trip a regular breaker but can electrocute. Good luck. Chuck ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
Bad shore power
Steve Lusardi wrote:
Chuck, I don't think you understand the problem. If there is a ground fault detecting breaker some where in the distribution net, it clearly should function whether or not we elect to use the green equipment line or not at the service point. OK. You are referring to a GFCI or ground fault breaker. You are correct, provided that only 125 volt circuits are involved. Such a device in a 250/125 volt circuit will not detect ground faults. Many boats utilize 250/125 volt systems. Not all marinas and not all boats utilize GFCI equipment. In any case, your original post, to which I responded, made no mention of GFCI protection. I agree that the green wire should provide a low impedance path to earth. In fact , as I stated, that connection must be made at the distribution transformer. The problem is that the low impedence path provided at the transformer will NEVER be as low an impedance path to earth as your hull is. Therefore energy leakage to earth anywhere within the net, we are now talking about the whole marina, will route itself to your hull simply because it is a better ground than the one provided at the transformer. What you are saying here suggests you are thinking about charge distribution in the earth, as is important in lightning protection but I don't think it applies to this discussion. The green wire is connected to the earth (and water) to maintain people in contact with the earth and equipment cases at the same potential. This is somewhat different from the ground fault clearing function of the green wire. It is an important function because even small leakage currents flowing from a hot wire-to-equipment case connection (not a direct short) through a person to earth (or water) can be dangerous. So even though the current is too small to clear the fault, it may nonetheless be dangerous and we can minimize its danger by keeping the equipment case and the person at the same potential. Hence: the green wire is connected to ground. Whether you are on water or earth, there is no guarantee you will ever be at exactly the same potential as someone else, even a few feet away. GFCIs for 125 volts can provide even better protection when leakage currents are detected. Your statement that fault current does not flow through the green wire cuircuit is flat incorrect. That is its sole purpose. Of course. My exact words: The purpose of that conductor is to provide a low impedance path for the purpose of clearing a ground fault. My point was that the path through the water and earth has too high a resistance to clear a ground fault. A connection from an appliance case to ground is NOT a substitute for a low impedance equipment grounding wire back to the distribution panel. The inverse is also true, if there is no fault, no current will flow there. If we take the case of the in-line AC line filter, you know, the capacitor type with the metal case that bolts to equipment frames? Or granny's 25 year old vacuum cleaner with worn out brushes and carbon dust all around the commutator? All that is leakage current. Now, I concede that the earth point at the transformer is also the neutral tranformer leg tie point and the leakage also returns to the transformer at that point, but your hull has also created a huge ground loop in the net and by default routes all earth reference current through your hull. Please also remember that your boat, if wired correctly, is a complete power system AC and DC all properly tied together at your hull ground plate and when hooking directly to shore power without an isolation transformer, you are paralleling the shore based grid and the ground connection you make is effectively an equalization bus and every code book I have ever read clearly states that an equalization bus should never be longer than one meter. Perhaps I'm misunderstanding, Steve, but every one of the millions of residences in the US subject to the NEC parallels the shore based grid and ground connection. While not a perfect system design, I don't believe it bears on our discussion. So the correct advice is, if you don't want to waste your metal bits below the waterline due to electrolosis and galvanic protection is your goal, never direct wire to shore power. Instead, you should couple your energy magnetically through the use of an isolation transformer and leave other's faults to themselves. Again, I fully agree that a properly installed isolation transformer is one of several ways that galvanic currents through the shore power grounding conductor can be mitigated. I continue to maintain that the grounding conductor should be utilized from the dock pedestal to the isolation transformer connection at the primary. At the secondary, you should create and use your own grounding conductor by connecting to the secondary's neutral for that purpose. Chuck Steve Hello Steve, I would caution strongly against using the water-earth path in lieu of the green equipment grounding conductor. Ever. The purpose of that conductor is to provide a low impedance path for the purpose of clearing a ground fault. In proper wiring, the impedance is no more than a few ohms and the fault is cleared at a breaker. Fault current does NOT flow through the earth. By contrast, the water-earth path may have a resistance of 25 ohms or more for salt water, and tens of thousands of ohms or more for fresh water. This path will NOT clear a ground fault. It will not trip a 20 A breaker. Witness the 120 volt power cords dropped off a dock without tripping a breaker! If one has a problem with wiring impedances in the marina, deal with it by getting the wiring inspected. Ask the marina operator when it was last checked. Yell. Scream. Buy your own tester and tell the marina what you measured. Tell the other boaters. Take names and publish on the Internet. But don't throw out the safety net because even with corroded connections, the wiring may still allow a ground fault to be cleared. If there is a problem with a nearby source (boat or otherwise) then an isolation transformer would be a good idea. But even with an isolation transformer, always use approved wiring practices. There is probably no more thoroughly reviewed standard than the NEC. The ABYC standards for AC wiring are similarly solid. And install GFCIs for the smaller ground fault currents that won't trip a regular breaker but can electrocute. Good luck. Chuck ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
Bad shore power
Chuck,
You are not misunderstanding. In fact you have an excellent understanding of the problem. Our discussion has led us to your statement below and while you are correct that the same code is used for millions of residences, they don't float and are not subject to galvanic damage and because of that, it does bear on our discussion. My point is that code is not appropriate for marina use. So it comes down to personal choice. I use a custom made isolation transformer, they are cheap insurance. Steve Perhaps I'm misunderstanding, Steve, but every one of the millions of residences in the US subject to the NEC parallels the shore based grid and ground connection. While not a perfect system design, I don't believe it bears on our discussion. So the correct advice is, if you don't want to waste your metal bits below the waterline due to electrolosis and galvanic protection is your goal, never direct wire to shore power. Instead, you should couple your energy magnetically through the use of an isolation transformer and leave other's faults to themselves. Again, I fully agree that a properly installed isolation transformer is one of several ways that galvanic currents through the shore power grounding conductor can be mitigated. I continue to maintain that the grounding conductor should be utilized from the dock pedestal to the isolation transformer connection at the primary. At the secondary, you should create and use your own grounding conductor by connecting to the secondary's neutral for that purpose. Chuck Steve Hello Steve, I would caution strongly against using the water-earth path in lieu of the green equipment grounding conductor. Ever. The purpose of that conductor is to provide a low impedance path for the purpose of clearing a ground fault. In proper wiring, the impedance is no more than a few ohms and the fault is cleared at a breaker. Fault current does NOT flow through the earth. By contrast, the water-earth path may have a resistance of 25 ohms or more for salt water, and tens of thousands of ohms or more for fresh water. This path will NOT clear a ground fault. It will not trip a 20 A breaker. Witness the 120 volt power cords dropped off a dock without tripping a breaker! If one has a problem with wiring impedances in the marina, deal with it by getting the wiring inspected. Ask the marina operator when it was last checked. Yell. Scream. Buy your own tester and tell the marina what you measured. Tell the other boaters. Take names and publish on the Internet. But don't throw out the safety net because even with corroded connections, the wiring may still allow a ground fault to be cleared. If there is a problem with a nearby source (boat or otherwise) then an isolation transformer would be a good idea. But even with an isolation transformer, always use approved wiring practices. There is probably no more thoroughly reviewed standard than the NEC. The ABYC standards for AC wiring are similarly solid. And install GFCIs for the smaller ground fault currents that won't trip a regular breaker but can electrocute. Good luck. Chuck ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
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