I am at the point that I feel I really need a galvanic isolator. Problem is that I can't get a handle on what I really need in design type, and I'm trying to save boat bucks as much as possible and still get the job done. I am fully aware of the ABYC change for remote monitoring. For the sake of this discussion, let's eliminate this part of the topic.
For this discussion, I am trying to decide between a Newmar GI-30:
http://www.defender.com/product.jsp?path=-1|328|303336&id=52807
and the Yandina DI-50:
http://www.defender.com/product.jsp?path=-1|328|303336&id=605562
From the Newmar description on the first link above:
"the units feature a large capacitor, providing a secondary low impedance path for stray AC current and improved galvanic protection.
Note: galvanic isolators are available from other manufacturers without the secondary path because they eliminated the capacitor to reduce the cost. It is worth the few extra dollars to purchase a isolator that contains the very important capacitor"
Now, I go over to the Yandina site and find this general page:
http://www.yandina.com/galvanicIsolator.htm
"Since some electrolytic voltages are higher than 0.6 volts, a good galvanic isolator should have two diodes in series in each direction to give a 1.2 volt isolation. Some also add a capacitor to increase the ability to conduct AC current, however I personally think this is a mistake as it does allow low level AC currents to flow and cause electrolytic type activity, even if not true electrolysis. This activity can remove paint from the fitting and generate chlorine bubbles that damage surrounding antifouling paint."
and more detail on the capacitor side here:
http://www.yandina.com/GIsolCap.html
"NO. The galvanic isolator is designed to carry 130% of the maximum shore power AC current rating you are using. However very small AC leakage currents can reduce the level of DC protection available from a galvanic isolator.
There is no easy answer to the capacitor question. The use of a capacitor is sufficiently controversial that the ABYC continue to list it as optional in their specifications.
The theory is that if AC is leaking to ground through your galvanic isolator, then the isolating diodes are going into conduction for a percentage of each half cycle and while they are in conduction, they are no longer suppressing any superimposed DC current. The DC potential would be added to one half cycle and subtracted from the other. If the total voltage when they are added exceeds about 1.2 volts then there is a path for just a portion of the DC voltage you are trying to block.
So by adding a capacitor you are trying to provide a path for the AC current while still blocking the DC. The size of the capacitor boils down to what you consider acceptable leakage current from the AC equipment on your boat and you have to size the capacitor so the voltage across it at this current is typically small compared to the DC voltages you are trying to avoid. If you are protecting from DC voltages typically in the 0.9 volts range maximum, then you have to keep the AC voltage across the capacitor to less than 0.3 volts.
[snip]
Therein lies the dilemma and the original question. What is acceptable leakage? My philophosy is you shouldn't be trying to cover up a problem by conducting it to ground. I maintain you should check every item on your AC supply and if any show any leakage at all they need to be repaired. That is a safer alternative to just allowing the leakage to flow to ground."
So, with all of the above, which seems to reflect a lot of opinion, can any of our electrical gurus address this issue of whether a capacitor is or is not a good idea?
What else should I be looking at/am missing as I look at these units?
Thanks!
For this discussion, I am trying to decide between a Newmar GI-30:
http://www.defender.com/product.jsp?path=-1|328|303336&id=52807
and the Yandina DI-50:
http://www.defender.com/product.jsp?path=-1|328|303336&id=605562
From the Newmar description on the first link above:
"the units feature a large capacitor, providing a secondary low impedance path for stray AC current and improved galvanic protection.
Note: galvanic isolators are available from other manufacturers without the secondary path because they eliminated the capacitor to reduce the cost. It is worth the few extra dollars to purchase a isolator that contains the very important capacitor"
Now, I go over to the Yandina site and find this general page:
http://www.yandina.com/galvanicIsolator.htm
"Since some electrolytic voltages are higher than 0.6 volts, a good galvanic isolator should have two diodes in series in each direction to give a 1.2 volt isolation. Some also add a capacitor to increase the ability to conduct AC current, however I personally think this is a mistake as it does allow low level AC currents to flow and cause electrolytic type activity, even if not true electrolysis. This activity can remove paint from the fitting and generate chlorine bubbles that damage surrounding antifouling paint."
and more detail on the capacitor side here:
http://www.yandina.com/GIsolCap.html
"NO. The galvanic isolator is designed to carry 130% of the maximum shore power AC current rating you are using. However very small AC leakage currents can reduce the level of DC protection available from a galvanic isolator.
There is no easy answer to the capacitor question. The use of a capacitor is sufficiently controversial that the ABYC continue to list it as optional in their specifications.
The theory is that if AC is leaking to ground through your galvanic isolator, then the isolating diodes are going into conduction for a percentage of each half cycle and while they are in conduction, they are no longer suppressing any superimposed DC current. The DC potential would be added to one half cycle and subtracted from the other. If the total voltage when they are added exceeds about 1.2 volts then there is a path for just a portion of the DC voltage you are trying to block.
So by adding a capacitor you are trying to provide a path for the AC current while still blocking the DC. The size of the capacitor boils down to what you consider acceptable leakage current from the AC equipment on your boat and you have to size the capacitor so the voltage across it at this current is typically small compared to the DC voltages you are trying to avoid. If you are protecting from DC voltages typically in the 0.9 volts range maximum, then you have to keep the AC voltage across the capacitor to less than 0.3 volts.
[snip]
Therein lies the dilemma and the original question. What is acceptable leakage? My philophosy is you shouldn't be trying to cover up a problem by conducting it to ground. I maintain you should check every item on your AC supply and if any show any leakage at all they need to be repaired. That is a safer alternative to just allowing the leakage to flow to ground."
So, with all of the above, which seems to reflect a lot of opinion, can any of our electrical gurus address this issue of whether a capacitor is or is not a good idea?
What else should I be looking at/am missing as I look at these units?
Thanks!