DIY Galvanic isolator.

[Vara]

This months (Nov) PBO has an article by David Berry about making a DIY galvanic isolator, it looks to be well within my limited capabilities.

He is a bit optimistic about price as I reckon it will be about £20 not the "under £10" quoted.

Is there any compelling reason why I shouldn't proceed, or do the ones from chandlers contain a mystery ingredient worth the £50 difference.

[markdj]

I thought the article was a bit over complicated as it was using underpowered diodes in parallel to give the same protection as one larger one and this is bad practice just for the sake of saving money.

There was no mention of the rigorous testing that commercial galvanic isolators go through before they are approved as safe for use and I thought this was the most glaring omission.

Making one yourself with a poor heatsink as shown gives a false sense of security as it could be a fire hazard if supplied with current in an earth leakage situation, albeit for less than a second.

The standards of galvanic isolators is as follows:

"Galvanic isolators shall be designed to withstand the application of power from a short-circuit test from a source capable of delivering 5,000 A r.m.s. symmetrically to its output test terminals for the time required for the circuit-breaker in the test circuit to trip. After
three applications of the short-circuit test, the electrical and mechanical characteristics of the isolator shall be unchanged."


Just my 2p

Mark
www.boatdoctorni.com

[oldvarnish]

Where is the sense in saving fifty quid to produce an inferior product which has been through no testing and has to conform to no specification, as the previous poster correctly points out?

Properly built, tested and installed, these are important safety items.

To make your own to save a few quid is to lose sight of the priorities.

[garvellachs]

I haven't read the article, but there is a similar idea here:
http://quinco.890m.com/Library/Galva...torDIYAssy.pdf
There the diodes are not really in parallel - they are used to provide the necessary voltage drop in both directions of current? I have made one up using 50 amp diodes from China.

[Cantata]

Hear hear. This thing is there to save your life. Nuts to mess about making one to save a few quid.

[VicS]

My antivirus software stopped me opening the above link

But the idea of using two bridge rectifiers is well known

See http://www.yandina.com/acrobats/GIDIYAssy.pdf

and also http://www.yandina.com/galvanicIsolator.htm

This design and the one in PBO both exploit the bridge rectifier to use to diodes in series to obtain the required voltage drop and the two arms of the bridge in parallel to obtain the required current rating.

Bridge rectifiers can be obtained quite cheaply from Maplin Electronics


The danger of an improvised unit is that the diodes could fail when subject to a fault condition which will then leave the safety earth connection to the shore power installation open circuit. For this reason a GI should be tested after any fault that causes the mcb to trip

[misterg]

"Galvanic isolators shall be designed to withstand the application of power from a short-circuit test from a source capable of delivering 5,000 A r.m.s. symmetrically to its output test terminals for the time required for the circuit-breaker in the test circuit to trip. After
three applications of the short-circuit test, the electrical and mechanical characteristics of the isolator shall be unchanged."

Looked at making one a while back, and came across something similar (I think it was the ABYC?? standard). I couldn't find any bridge rectifiers that got close to that for less than £100 each (150 amp + rating). The 'normal' 10 - 50 amp bridges will pop like fireworks if they get a full phase-earth fault across them, and probably do so in the first 10 or 20ms, probably before your RCD has had a chance to trip (40 - 200ms) and certainly before your MCB has tripped (400ms+). Upshot is that power stays on, and everything that should be 'earthed' is now sitting there at mains voltage.

I opted to unplug the shore power lead instead.

I thought that the PBO article was shoddy in not pointing out what these things *should* be built to withstand.

Andy

[wotayottie]

This months (Nov) PBO has an article by David Berry about making a DIY galvanic isolator, it looks to be well within my limited capabilities.

He is a bit optimistic about price as I reckon it will be about £20 not the "under £10" quoted.

Is there any compelling reason why I shouldn't proceed, or do the ones from chandlers contain a mystery ingredient worth the £50 difference.

Its your decision but I have gone ahead. I've no doubt that the end result is not quite as safe as an expensive commercial product but then I see the risk of failure as being pretty small since most mains electrical equipment is well insulated these days and indeed I use little of it aboard the boat. Certainly I see no sense in going out in rough weather in a small boat , itself a risky occupation, and then being totally risk averse about everything else.

Given that few of us have a limitless amount of money to spend on boating, the safety bit needs to be spent where there is the highest risk. How many people have proper modern gas lockers and up to date pipe for example? How many have decent quick acting fuses in their 12v systems? Liferafts? etc etc.

[seadog30]

The real point to consider is if you buy an expensive swindlery supplied unit if it kills you you have someone to sue

[markdj]

Thank goodness I'm not the only one who thinks some of these articles are just not up to scratch. There have been previous electrical pieces too which I personally could not contemplate to do to any person's boat.

I think if you want mains power plugged in all the time then a proper tested isolator is a small price to pay for protecting your anodes against stray currents. Having a unit on the boat that WILL burn or explode upon an earth leak is just simply careless at best and dangerous at worst.

For those who have such items installed, make sure they are mounted above the waterline and away from any flammable material, gas lockers, petrol cans etc.

I can't believe that some people are asking things like having in date gas pipe, fuses, liferafts !!!! this is basic, basic stuff.