Quote:
Originally Posted by RaymondR
The arbitrary designation of one side of the output of one side of the secondary winding of an isolation transformer does not make sense to me. Also connecting the case earth to the arbitrarily chosen "neutral", thereby making it live also appears contra-intuitive.
The use of double pole breakers appear to be in response to the realization that both sides of the output from the secondary windings are live?
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What is “live”? When you have a
power source presented on two live conductors, calling one of them “neutral” and safe to touch is remarkable. Next step on that path is connecting that “neutral” to earth and to a DC “negative” (remember that the metal
hull of
boats was used for DC negative just like the chassis of a car still is today… and our
engine blocks are still used as DC negative conductor!
Still, for shore based
power distribution I have read reasoning that kind of makes sense, but remember that this was all devised a long time ago before breakers and RCD safeties. To replicate this setup aboard our sailboats doesn’t make sense, because we can afford to build the same levels of
reliability and
safety for our power as an operating room in a hospital or a clean room in a factory.
Looking at an isolation transformer aboard, you have to view this as a power source that is fueled by
shore power but not galvanically connected to it, similar to a genset or a
battery with
inverter.
Out of any of those you get two conductors presenting the power source. You can safely touch one of the conductors as long as you don’t touch the other. When you connect one of them to ground, you can still touch that one safely, but not the other one anymore because we make a connection to ground just by standing on it, floating in
water that connects to ground or touching anything else that connects to ground. By connecting one of the conductors to ground, we have made the other one more dangerous and we call it “live” or “hot”.
Luckily, we can use an isolation transformer and start over. Take the two grounding jumpers that come with a
Victron isolation transformer and hide them so they won’t be found again. If someone tells you that you can’t use your
shore power that way when hauled out, then use an extension cord instead. I always like to have a separate
battery charger that can connect to every shore power in use in the world regardless of voltage or frequency so I can connect it to that extension cord and then simply use the
inverter when hauled out.
Double pole breakers: when one pole triggers the breaker, the other pole is interrupted as well, i.e. they are mechanically linked. Even though it is safe to touch one conductor in a floating power system, it’s still safer not to touch any. When you have connected one of them to ground even before getting to the breakers, there is little use for double pole breakers because you created a path around the breakers that is almost unavoidable to touch, namely everything connected to ground and even the ground itself.
But for our isolated and floating system we have eliminated that alternate return path and we can and we do install double pole breakers.
Note that in the US a 240V circuit is floating too (both conductors are “hot” but unlike aboard a
boat with isolation transformer, each of those conductors also has a 120V potential to ground and are much more dangerous than the power behind an isolation transformer) and you will find double pole breakers for those branch circuits. I think many new
boats use double pole breakers even when they use a grounded neutral. Makes it easier to convert when installing an isolation transformer.
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