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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 |