Steel Narrowboat

[boathunter]

It is a bridge of diodes, usually 4, two connected in series one way, and two in series the other way.

 

This allows it to block low voltages, such as those resulting from galvanic interaction in both directions, because the voltage is not high enough to switch the diodes on.

 

When you get an ac fault, the diodes switch on and direct the fault current to earth.

 

Often they have a capacitor across them, to shunt high frequency earth currents which result from switch mode power supplies, such as phone chargers etc.

 

Edited to add a missing "y".

That last bit is interesting. I have odd fresh bright pits here and there, very shallow. Perhaps my isolator doesn't have a capacitor.

I use and charge my phone a lot. The last guy didn't.

[matty40s]

It is a bridge of diodes, usually 4, two connected in series one way, and two in series the other way.

 

This allows it to block low voltages, such as those resulting from galvanic interaction in both directions, because the voltage is not high enough to switch the diodes on.

 

When you get an ac fault, the diodes switch on and direct the fault current to earth.

 

Often they have a capacitor across them, to shunt high frequency earth currents which result from switch mode power supplies, such as phone chargers etc.

 

 

 

I am also interested, I have noticed more and more boats having bright orange galvanic corrosion visible, do you think that mobile chargers(and wireless mifi etc) would be leading to this increased corrosion??

 

I rarely see severe galvanic corrosion in CC boats - although there can be some, it is almost always marina based boats which suffer, however, two boats from the same marina may have different levels of corrosion, even if both fitted with GI.

[cuthound]

That last bit is interesting. I have odd fresh bright pits here and there, very shallow. Perhaps my isolator doesn't have a capacitor.I use and charge my phone a lot. The last guy didn't.

Possibly, SMPSs work by rapidly switching the mains on and off to regulate the output voltage. This causes high frequency harmonics to appear in the earth conductor, and the capacitor is to provide a path for them.

 

My practical experience is with power supplies for critical applications, such as data centres, telephone exchanges and hospitals, rather than boats although the theory applies to both (although I'm beginning to climb the learning curve on boats).

 

Have you tested your GI? Providing you can get to it and have access to a multimeter with a diode test function, it is easy.

 

Basically you put a wire across the GI's earth connections to discharge the capacitor. Remove the wire, set your meter to diode test, and put the meter across the earth termination, and note the voltsge.

 

Then repeat the above, but this time swap the meter leads over on the GI's earth connections.

 

The meter readings should be around 0.9 volts or so. Open circuit or short circuit ratings indicate faulty diodes, so the GI must be replaced.

 

Hopefully someone with practical experience of GI's and the manifestation of galvanic corrosion will be along to provide more advise.

[boathunter]

If I set my multimeter to the diode symbol, the reading defaults to 1. When I touch the probes together it squeels and reads 0.00. Does that sound right?

[matty40s]

it means you aren't testing the galvanic isolator yet....

[cuthound]

If I set my multimeter to the diode symbol, the reading defaults to 1. When I touch the probes together it squeels and reads 0.00. Does that sound right?

Yes, as Matty says it proves the meter is working.

 

Next you need to follow the steps in post #28

[Rendelf]

As our boat doesn't have shoreline, would it be prudent to get a GI on the negative feed to the engine block? We use DC-DC buck converters to power laptops and phones - a lot, because I work from home. We also use an inverter for washing machine and hoover.

 

 

There are some shiny sacrifices in the water on the other side of the hull from the engine... so would the GI just be preserving them, in this case?

 

Also, does anyone have an opinion on these?

 

http://www.ebay.co.uk/itm/60-Amp-Galvanic-isolator-D-I-Y-Kit-for-Canal-boats-leisure-craft-Marina-use-/272297386040?hash=item3f66306038:g:j9cAAOSweWVXe8QA

 

I could just buy the diodes myself, but in principle...seems good for non-mains use (no capacitor)

 

 

 

Also, to try and get back to what the OP was saying....

 

There's always epoxying the hull...which should in theory protect it longer than bitumen? That is, if you drive really carefully.

 

I want a stainless boat now. I'm going to call it "Nobel Norman's an Island"

[cuthound]

As our boat doesn't have shoreline, would it be prudent to get a GI on the negative feed to the engine block? We use DC-DC buck converters to power laptops and phones - a lot, because I work from home. We also use an inverter for washing machine and hoover.

 

 

There are some shiny sacrifices in the water on the other side of the hull from the engine... so would the GI just be preserving them, in this case?

 

Also, does anyone have an opinion on these?

 

http://www.ebay.co.uk/itm/60-Amp-Galvanic-isolator-D-I-Y-Kit-for-Canal-boats-leisure-craft-Marina-use-/272297386040?hash=item3f66306038:g:j9cAAOSweWVXe8QA

 

I could just buy the diodes myself, but in principle...seems good for non-mains use (no capacitor)

 

 

 

Also, to try and get back to what the OP was saying....

 

There's always epoxying the hull...which should in theory protect it longer than bitumen? That is, if you drive really carefully.

 

I want a stainless boat now. I'm going to call it "Nobel Norman's an Island"

If you don't have a shoreline, then there is no point in having a GI. The ex-working g bozts have managed without them fine for 70 years plus!

 

With respect to your link. There is no heatsink, so one wonders how it will perform with potentially several thousand amps of ac fault currentflowing through it for a few milliseconds. Looks homemade.

[boathunter]

Yes, as Matty says it proves the meter is working.

 

Next you need to follow the steps in post #28

Test done. Numbers are...

1768 one way round

1728 other way round

No decimal points.

It has a monitor box attached with two LEDs, one lit is DC fault, both lit is AC fault, neither is lit. Disconnected this, repeated test and got the same result.

I need help making sense of that please.

Edit to add the multimeter didn't squeak.

Edited July 15, 2016 by boathunter

[Rendelf]

With respect to your link. There is no heatsink, so one wonders how it will perform with potentially several thousand amps of ac fault currentflowing through it for a few milliseconds. Looks homemade.

Yep, the point was that I don't use AC for much of anything...so I could make one like this for 12v. Somewhere earlier I'm sure someone said the GIs were useful even for preventing non-shoreline related corrosion. . . but I may well be bowing to the wrong dog

 

In the event of a fault with the inverter, there's an inline fuse and a circuit breaker....so I'd have thought the cheapy would survive?

 

Have just seen the other thread on GIs ...

[Iain_S]

As our boat doesn't have shoreline, would it be prudent to get a GI on the negative feed to the engine block? We use DC-DC buck converters to power laptops and phones - a lot, because I work from home. We also use an inverter for washing machine and hoover

 

 

A galvanic isolator will do nothing to protect the steel from corrosion due to the presence of more noble metals; that's the job of the anodes and the blacking. The galvanic isolator is connected into the shore line earth, and isolates "boat earth" from "mains earth", unless there is a fault and the diodes go into conduction.

 

It would be a really bad idea to fit one in the negative feed to the engine block. It wouldn't provide any protection, would probably be destroyed the first time you started the engine, and, if not, would reduce the charging voltage on the batteries by about 1.2V (probably more), assuming a machine sensed alternator.

[cuthound]

Yep, the point was that I don't use AC for much of anything...so I could make one like this for 12v. Somewhere earlier I'm sure someone said the GIs were useful even for preventing non-shoreline related corrosion. . . but I may well be bowing to the wrong dog

In the event of a fault with the inverter, there's an inline fuse and a circuit breaker....so I'd have thought the cheapy would survive?

Have just seen the other thread on GIs ...

You don't need one for dc. All that will happen is that one set of diodes will conduct all the time, as the 11 volts will be higher than the switch on voltage.

 

As Iain_S says, it is to prevent currents flowing due to the small voltage difference between different earths.

Test done. Numbers are...

1768 one way round

1728 other way round

No decimal points.

It has a monitor box attached with two LEDs, one lit is DC fault, both lit is AC fault, neither is lit. Disconnected this, repeated test and got the same result.

I need help making sense of that please.

Edit to add the multimeter didn't squeak.

What was your meter set to, it should be "diode test"? When I do this test I get a maximum reading of around 0.9 volts each way. (the reading gradually climbs up from zero as the capacitor charges).

 

The fact you have a GI with ac and dc fault indicators that are not lighting up shows that it is ok.

[boathunter]

Thanks for your reply. I've lost the manual for the multimeter but it'set to the only diode symbol. I've never used this function before. No sign of a decimal point, could it be measuring resistance?It's quite a nice multimeter Uni-T UT105 if that means anything to you.

ETA: Found the manual on line, it's measuring resistance.

[cuthound]

Thanks for your reply. I've lost the manual for the multimeter but it'set to the only diode symbol. I've never used this function before. No sign of a decimal point, could it be measuring resistance?It's quite a nice multimeter Uni-T UT105 if that means anything to you.ETA: Found the manual on line, it's measuring resistance.

Not familiar with your multimeter, but this shows how to use the diode test facility. The reading should be in volts or millivolts. It is the same for every multi ever I have ever seen so should be the same for yours.

 

http://en-us.fluke.com/training/training-library/test-tools/digital-multimeters/how-to-test-diodes-using-a-digital-multimeter.html

[boathunter]

Not familiar with your multimeter, but this shows how to use the diode test facility. The reading should be in volts or millivolts. It is the same for every multi ever I have ever seen so should be the same for yours.

 

http://en-us.fluke.com/training/training-library/test-tools/digital-multimeters/how-to-test-diodes-using-a-digital-multimeter.html

Well, mine measures resistance, here's a link to the page of the manual.http://www.manualslib.com/manual/538657/Uni-T-Ut105.html?page=21#manual

Edit:ahhh was reading the wrong bit, but I stand by my crazy readings earlier.

Edited July 15, 2016 by boathunter

[Rendelf]

A galvanic isolator will do nothing to protect the steel from corrosion due to the presence of more noble metals; that's the job of the anodes and the blacking. The galvanic isolator is connected into the shore line earth, and isolates "boat earth" from "mains earth", unless there is a fault and the diodes go into conduction.

 

It would be a really bad idea to fit one in the negative feed to the engine block. It wouldn't provide any protection, would probably be destroyed the first time you started the engine, and, if not, would reduce the charging voltage on the batteries by about 1.2V (probably more), assuming a machine sensed alternator.

 

 

You don't need one for dc. All that will happen is that one set of diodes will conduct all the time, as the 11 volts will be higher than the switch on voltage.

 

As Iain_S says, it is to prevent currents flowing due to the small voltage difference between different earths.

 

 

 

 

 

Thanks both, for clearing up my misunderstanding there.

 

I think I should do a bit more reading before posting anything else!