Galvanic Isolator

[Slim]

I've just ordered a Safeshore galvanic isolator with a remote monitoring facility to replace my existing one. Whilst there's nothing wrong with this one it's fitted in an awkward position to monitor regularly. My question is, Is there any technical reason why the new one cannot be wired in series with the old one. Installation would be easier and as I see it provide a degree of redundancy.

Besides, it was my birthday today and I hadn't bought myself a present.  ? 

[matty40s]

8 minutes ago, Slim said:

Is there any technical reason why the new one cannot be wired in series with the old one. Installation would be easier and as I see it provide a degree of redundancy.

 

They would need to be at least 2m apart, and should only be wired outdoors.

 

Sorry, am I mixing up isolation and social distancing....

[Jen-in-Wellies]

15 minutes ago, Slim said:

I've just ordered a Safeshore galvanic isolator with a remote monitoring facility to replace my existing one. Whilst there's nothing wrong with this one it's fitted in an awkward position to monitor regularly. My question is, Is there any technical reason why the new one cannot be wired in series with the old one. Installation would be easier and as I see it provide a degree of redundancy.

Besides, it was my birthday today and I hadn't bought myself a present.  ? 

You would still need to regularly test the original, as a failed diode would still need to be found, making fitting the second, easy to test one pointless.

Sell the original on to someone else.

Jen

Edited January 4, 2021 by Jen-in-Wellies

[Sea Dog]

10 hours ago, Slim said:

My question is, Is there any technical reason why the new one cannot be wired in series with the old one.

Technically, doing something suspect because it's more convenient rather than correct, is known as a bodge. That would be technical reason enough for me.

 

[Alan de Enfield]

If there is 'nothing wrong with the old one', why not just move it a few inches to where it can be more easily monitored ?

[Jen-in-Wellies]

Thinking about it some more. It is even less of a good idea. With two galvanic isolators in series, you have doubled the potential difference needed for the boat earth to connect to the shore earth. Around 2.8V, instead of 1.4V. Each isolator has two silicon diodes in series, giving a 1.4V voltage difference before they conduct.

Jen

[Keeping Up]

3 minutes ago, Jen-in-Wellies said:

Thinking about it some more. It is even less of a good idea. With two galvanic isolators in series, you have doubled the potential difference needed for the boat earth to connect to the shore earth. Around 2.8V, instead of 1.4V. Each isolator has two silicon diodes in series, giving a 1.4V voltage difference before they conduct.

Jen

I can't see any problem there, nothing wrong with 2.8v potential difference. But I do totally support your earlier point that it is a very bad idea because not only is there twice the risk of failure but also the original isolator would still need to be tested as regularly as before despite the temptation not to bother; a successful test on the new one would give you a false sense of security at the same time as a failed diode on the old one was presenting a major hazard.

[Sea Dog]

7 minutes ago, Jen-in-Wellies said:

Thinking about it some more. It is even less of a good idea. With two galvanic isolators in series, you have doubled the potential difference needed for the boat earth to connect to the shore earth. Around 2.8V, instead of 1.4V. Each isolator has two silicon diodes in series, giving a 1.4V voltage difference before they conduct.

Jen

That might make it better Jen as any true earth fault is still going to be above the GI trigger voltage, but it's still a bodge. If it fails open circuit, the OP may have no earth and never know. 

[Slim]

2 hours ago, Sea Dog said:

Technically, doing something suspect because it's more convenient rather than correct, is known as a bodge. That would be technical reason enough for me.

 

Physically sometimes a bodge is a necessity. My excuse for considering such a 'bodge' is a combination of dodgy knees and being blind in one eye(from birth). These combine to make accessing the existing GI very difficult. However in my own defence the purpose of asking the question was to clarify the safety of such action. I rather think I will be replacing not duplicating. It looks like I've got 2 or 3 months to think about it so it won't be a rushed (or unsafe) decision. 

 

Forgot to say thanks to everyone.

Edited January 5, 2021 by Slim

[dor]

Fit the new isolator somewhere easy to get to, and just connect a bit of wire across the old isolator which is probably easier than removing it or trying to fit the new one in its place.

 

 

[Sea Dog]

2 hours ago, Slim said:

Physically sometimes a bodge is a necessity. My excuse for considering such a 'bodge' is a combination of dodgy knees and being blind in one eye(from birth). These combine to make accessing the existing GI very difficult. However in my own defence the purpose of asking the question was to clarify the safety of such action. I rather think I will be replacing not duplicating. It looks like I've got 2 or 3 months to think about it so it won't be a rushed (or unsafe) decision. 

 

Forgot to say thanks to everyone.

The other way to look at it is to disconnect and ignore the existing GI and run a new earth cable from your shore connection point to the site of your new GI - this might be easier depending on whether you can access the back of your shore supply connection.  

 

This is obviously an easier issue to sort for those without those additional challenges you refer to though, Slim.  If you're anywhere near the upper reaches of the GU once we're allowed back to our boats, I'll happily pop round and help you or do it for you as you prefer.  I fancy there may be other members willing to lend a hand in many areas. 

[Slim]

4 minutes ago, Sea Dog said:

The other way to look at it is to disconnect and ignore the existing GI and run a new earth cable from your shore connection point to the site of your new GI - this might be easier depending on whether you can access the back of your shore supply connection.  

 

This is obviously an easier issue to sort for those without those additional challenges you refer to though, Slim.  If you're anywhere near the upper reaches of the GU once we're allowed back to our boats, I'll happily pop round and help you or do it for you as you prefer.  I fancy there may be other members willing to lend a hand in many areas. 

That's very kind of you but I wouldn't want to make too much of things. As you say I can pick up the earth before the GI and take it from there. Best wishes