Galvanic Isolator

[southchinasea]

I was told, last night, I should get a galvanic isolator on the new boat. Hands up,
I don't really know what it is/does! I'd appreciate an idiots guide.
Thanks.

 

[HaraldS]

Hi Jonathan,

it's nothing too complicated. In practise a galvanic isolator consists of just 4 diodes, two sets of two diodes in series and the two sets connected antiparallel.

The effect of that whole arrangement is that it's two ens become conductive if the voltage between the two is above 1.4V, either polarity. For lower voltages the thing does not conduct current.

The idea is that all galvanically generated voltages, between dissimilar metals will always be below the 1.4V so that not galvanic current camn flow.

On the other hand, should a fault occur and the protective ground be needed the AC level will be higher and thus you should still have your protective ground, with no more than 1.4V added to ground potential.

There is some risk that the diodes can burn out on a serious ground fault and the ground connection become open as a consequence. Even with powerfull diodes, a ground fault switch (RCB) would be very advicable when using such an isolator. But a RCB is always a good investment.

The parts are very cheap, yet good marine isolators are not particularly cheap, but still less expensiv than an isolation transformer.

Since your RCB will trip very quickly on a ground fault, the peak current capability of the diodes is important, but you don't need big heat sinks, since the loading in case of a fult will be very short.

What is not well known is that galvanic isolators fail to isolate if there is a small residual AC current going through the ground wire. Some RF filters in some devices may be enough to cause a small leakage of say 1 mA, way below an RCB tripping level. Since this current is associated with AC line voltage, it will happily go through the galvanic isolator.

While the AC current in itself is no problem, it effectively disables the galvanic isolator from doing it's job. Any galvanic current that would flow without isolator will then start flowing, piggy packed on top of the AC.

So you see, whether a galvanic isolator works, depends a bit on the AC appliances you have on board.

I think galvanic isolators are only the second best choice and it is better to have one rather than none, but I would personally always go with an isolation transformer. More expensive, but worth its money, especially if you plan to visit odd places, maybe with differing shore voltages and systems. A good transformer allows you to switch the voltage.

Another good alternative that is becomeming increasingly more practical, is to use shore AC only to drive a big DC charger, and then generate your own AC from powerfull inverters. The charger is isolating and could be self selecting to the input voltage. Today still more expensive than other solutions, especially if you want to drive your calorifier that way, but there is big future potential in this approach.

 

[softop]

Hi,
Harlods has said it all. I got one from www. power-store.com and paid about £75.

 

[AndrewB]

Another question.

Thanks for a really comprehensive reply, HaraldS, which is very helpful.

What I'm not clear about is whether it is strictly necessary to earth AC aboard, anyway. And if the earth is omitted, the isolator is unnecessary. Earthing is mainly a protection against the casing of an appliance becoming live as a result of a short. But as all our on-board appliances are plastic cased, most are two-wire anyway, the risk seems small, and the RCB ought to catch shorts anyway.

Is this wishful thinking?

 

[ccscott49]

Re: Another question.

Andrew I tend to agree with you, none of my earths to any mains equipment is connected to anodes, negatives on batteries or anything else, apart from the earth back toi the shore supply, and I have an RCD, nobody has yet been able to convince me I need to have thenm connected to my anodes or anywhere else. I don't have a corrosion problem, never have, theres no circuit!

 

[HaraldS]

Re: Another question.

Hi Andrew,

that is a rather good question and experts are still not in agreement on this. Let me start by saying that the current standards I know, CE and US, both require shore AC ground and ship AC ground to be connected. So current a yard would not get the CE certification for a boat if they would leave the two unconnected unless an isolation transformer is used.

Ther is a new ISO standard (ISO13297) being worked on, that may in the end allow for a couple of choices:

(1) Just an isolated charger. Charger chassis connected to shore AC ground, but not to ship ground. This is staright forward and o.k. today.
Variants of this are to create your own on board AC net, via an inverter. In this case the option is to create a neutral by tieing one inverter output to ship ground. In this case single pole breakers are allowed. In the floating case, two pole breakers would be required. In all cases a ship side AC ground would need to be created, coming from ship earth. And a RCb would be a requirement.

(2) Classic: Shore AC-ground on board connected to ship ground and lead to all receptacles and appliances. A 30mA RCB at the input. Hot and Neutral coded. Single pole circuit breakers/fuses for the individual circuits.

(3) Classic with isolation transformer: Shore AC-ground connected to core of isolation transformer. A RCB at the input side of the transformer is optional. On the output side, one end connected to ship ground creates the new neutral, the other end becomes the hot. The ship AC ground is connected at the same point where the neutral is created. An RCB on the output side is required. Breakers and fuses can be single pole.

(4) Floating AC with isolation transformer: Shore AC connected as above to the transformer. Input RCB still optional. Noe of the output ends is grounded, there is no neutral and no hot. Ships AC ground as above, provided by the ship ground. This mode requires 2-pole breakers/fuses. It is debated whether an insulation monitor is needed to indicate a loss of ground independance of any one of the AC leads.

(5) No shore AC connection: This is what you suggested and is controverse. This is what it looks like: From shore to the boat come just neutral and hot, no AC ground wire. It goes trhough an RCB, if it can be insured that neutral and hot cannot be accidetially switched from shore, then single pole breakers are ok. (in the hot wire). The boat will have it's own AC ground systen, connected to ship ground.


So let's look at Nr.5 in more detail: There are no intentions to allow an AC net without a ground system, the main reason is that Ac devices are today built with the assumtion that there is one.
It start with RF filters that wouldn't work, but goes to some dangerous parts like for example the calorifier. If the calorifier wasn't grounded, a short to the casing of the heat element could energize your fresh water. With all the tubing being plastic, and fresh water with some cloride in it not particularly conductive, not much would happen until you stand on ground and shower with electric water. Depending on the current level the RCB may trip or not. If it is below 30mA it would'nt, but I don't think showering with 20mA is fun.
So lets agree that for all these reasons, you need an AC ground on the boat. With this coming from the ships earth and an RCB you should be well safe on your boat.
The main argument against this setup is that it can be lethal to swimmers or divers in the vicinity of such a boat. It takes less than 30mA going through the water, to completely paralize a swimmer or diver. A potentially faulty RCB increase this risk dramatically.

there is another variant not in the discussion for good reasons. It would be to bring in the shore AC-ground to the boat, and use it as boat AC-ground wiithout connecting it to the ship ground. The problem with this is that due to faults on shore a huge potential difference between ship ground and this shore based ground could arise, which would put the chassis of grounde AC deviced onto dangeroous voltage, even with the breakers of, and of course no RCB will catch this either. -- So don't choose that variant.

I think many people seem to create their own version of 5 by disconnecting the shore ground these days, so it is around whether we like it or not. So when swimming in a marina, you might not be sure exactly what will kill you....

 

[AndrewB]

Re: Another question.

Many thanks for this, another very lucid explanation. In fact the set-up we have is the final (unnumbered) one you list and regard as dangerous, no on-board earth but reliance on the shore earth.

I've tested from time to time at the marinas where I've been resident, and never found more than a tiny potential difference between shore earth and my yacht. Of course, the chances are I'm earthing through a neighbouring yacht's un-isolated earth, rather than a distant earthing point .

However, the shore earth might be unreliable if another similarly unearthed boat nearby had a short. Equally, if I understand you right, I might present a risk to a neighbour were there to be a short on my boat that was not trapped by the RCB, which raised the potential on the earth line. I think you've convinced me to fit a yacht earth and isolator.

 

[southchinasea]

Thanks Harald for both postings. Very informative. My electrical knowledge is sadly not a strong point and I've just finsished Miner Brotherton's 12 Volt bible to try and remedy this deficiency!
Cheers
Jonathan
PS Re your last paragraph. Clearly??? this solution only works if the AC appliance
you are running is rated less than your charger/inverter. e.g. no way I'm going to run an aircon via a charger/inverter?