Galvanic isolator

[ditchcrawler]

It will vary tremendously according to the purity of the water - pure water being a pretty good insulator.

Agree, but would still be nice to have some idea, if its a good insulator you wouldn't get electrocuted in the bath, so we are really talking relative.

 

So is that a good insulator compared with a conductor to a mat. or 250 sq inches surface area of earth rod against 240 sq feet of steel in water. which if on the broads would probably be acidic, I don't know about canals, or in the sea, saline.

 

Food for thought

The proper way to lay an earth is not a stake but an Earth Mat, they don't come cheap

As I said I have not worked in the field for many years, but you are not talking fault current but 30ma for a RCD to work

[GUMPY]

Agree, but would still be nice to have some idea, if its a good insulator you wouldn't get electrocuted in the bath, so we are really talking relative.

 

So is that a good insulator compared with a conductor to a mat. or 250 sq inches surface area of earth rod against 240 sq feet of steel in water. which if on the broads would probably be acidic, I don't know about canals, or in the sea, saline.

 

Food for thought

 

As I said I have not worked in the field for many years, but you are not talking fault current but 30ma for a RCD to work

Yes but every PE should be able to carry the full fault current...

[cuthound]

The proper way to lay an earth is not a stake but an Earth Mat, they don't come cheap

Depends on the local geography, you can't lay a mat and get it to work in a rock substrate.

 

I have used mats, rods (including dosing them with salts to get the resistivity down) and a combination of the two in the past, particularly with earths for HV substations.

[smileypete]

hmm.. now this is confusing ..especially as my degree is in the social field not electrics.

 

I am on a river mooring and used to be moored away from my lovely fellow moorers. My choice as I do like to dance on my decking and sing !!

 

Recently two other boats have moved nearer to me and frequently use their landlines, which is not an issue, so thinking that I was being prudent, I bought a GI from a well known and published company. The model I bought is below, but after reading this thread I am now unsure which was the best option.. a GI or the T thingybob

 

GI with 2 Status Monitors,

Fault Indicator

1 LED Light = DC Leakage

2 LED Light = AC Leakage.

 

Hi tree

 

Sounds like it's one of the Safeshore ones so will be fine, the LEDs will warn if there's a bit too much voltage between the boat hull and shore earth that the GI can deal with.

 

To check the LEDs work for some reassurance, you could try the test described in post #51:

 

http://www.canalworld.net/forums/index.php?showtopic=76834&page=3#entry1588866

 

A side effect of these topics is that the expert wibbling can be worrisome...

 

cheers, Pete.

~smpt~

Edited June 19, 2015 by smileypete

[jonathanA]

hmm.. now this is confusing ..especially as my degree is in the social field not electrics.

 

I am on a river mooring and used to be moored away from my lovely fellow moorers. My choice as I do like to dance on my decking and sing !!

 

Recently two other boats have moved nearer to me and frequently use their landlines, which is not an issue, so thinking that I was being prudent, I bought a GI from a well known and published company. The model I bought is below, but after reading this thread I am now unsure which was the best option.. a GI or the T thingybob

 

GI with 2 Status Monitors,

Fault Indicator

1 LED Light = DC Leakage

2 LED Light = AC Leakage.

Don't worry you have bought one of the better GIs and if nothing else this thread should have shown you that at the end of the day there are pros and cons with both the GI and IT.

 

I wouldn't attach much credence to the claims of high failure rates for GIs quoted by others without some evidence, especially when pushed it seems many of these "failures" where in fact poor installations.

[jonathanA]

You're right of course, but on the other hand it might be easier to unplug the shore lead and then measure the voltage between the shore earth pin and the hull, that way you will know whether it is a boat fault or a shore fault. That would be easier than spending hours checking out the boat only to find it was a shore fault!

Yes agreed I wouldn't decide the fault was with the marina or boat before I got there and did some testing/investigation.

[smileypete]

This has been a know fact for more than 5 years that as more switch mode power supplies are used

 

Some GI will start to conduct because of a small voltage which is on the earth wire at the power bollard

 

This voltage is because of switch mode power supplies.

 

Good information on the safe-sure site

 

Keith

 

Thing is, a certain well known site lengthily asserts it as fact, without offering any shred of evidence otherwise.

 

And forum members who have tried to find evidence have concluded the complete opposite for canal boats.

 

Last time I was at WB, admitedly 15 years ago the L&N on the bollard I had were reversed!

Somewhere like WB I would not use anything but an IT as it removes all possible yard wiring problems.

unless its labeled.

 

An inexpensive socket tester costing a few pounds would detect that no problem.

 

Also, an IT may not fully protect you from stray voltages in the water from faulty wiring in other boats. It could be useful to monitor the voltage between hull and shore supply, as some GIs do.

 

I think for a typical canal boater, the choice of an IT over decent GI is for 'peace of mind' and personal preference really, not necessarily for increased safety or corrosion protection!

 

It will vary tremendously according to the purity of the water - pure water being a pretty good insulator.

 

Yeah but remember water is a great dielectric too.

 

My own findings indicate that current leakage from a fairly small area of exposed metal in the water (a few square inches) will cause an RCD to trip immediately.

 

Would be good to get some earth loop fault impedance readings that include a narrowboat hull in the water, I expect they'd be surprisingly low.

 

cheers, Pete.

~smpt~

 

Edited June 21, 2015 by smileypete

[GUMPY]

 

An inexpensive socket tester costing a few pounds would detect that no problem.

 

Also, an IT may not fully protect you from stray voltages in the water from faulty wiring in other boats. It could be useful to monitor the voltage between hull and shore supply, as some GIs do.

 

 

 

As would a neon between N and E or even a meter which is what I prefer to use as I can read more information from it.

 

Please explain how a boat that is totally isolated from shore earth, as a boat with an IT is, can sucumb to stray voltages from other boats.

Edited June 21, 2015 by Loddon

[nicknorman]

As would a neon between N and E or even a meter which is what I prefer to use as I can read more information from it.

Please explain how a boat that is totally isolated from shore earth, as a boat with an IT is, can sucumb to stray voltages from other boats.

Because the current from the other boats creates a voltage gradient in the water. If the hull is spanning the voltage gradient then current will prefer to "short circuit" that gradient by flowing along the hull, thus causing corrosion.

[GUMPY]

Because the current from the other boats creates a voltage gradient in the water. If the hull is spanning the voltage gradient then current will prefer to "short circuit" that gradient by flowing along the hull, thus causing corrosion.

Much the same as a human in the water...........except that paralises them and they drown.

[Traveller]

Does that mean that a GI is not THE answer then? It is not (I hope) a problem for me as where I moor there is no shore power or can these "stray currents" originate from a source other than shore hook-ups? Apologies if this has been covered before but I just cannot get my head around the solution to this problem :-(

[ditchcrawler]

Because the current from the other boats creates a voltage gradient in the water. If the hull is spanning the voltage gradient then current will prefer to "short circuit" that gradient by flowing along the hull, thus causing corrosion.

So how many mili volts will that be over 60 ft?

[nicknorman]

So how many mili volts will that be over 60 ft?

Depends on the steepness of the voltage gradient.

[tree]

Many thanks smileypete for easy to understand advice as always ..huggles and thankyou as well jonathan.

[cuthound]

This has been a know fact for more than 5 years that as more switch mode power supplies are used

 

 

Keith

For the benefit of the non-electrical forum members, switch mode power supplies regulate voltage by rapidly switching the manins on and off, typically 20,000 times per second or more. This causes harmonics (distortion of the mains sine wave), which flow back down to earth resulting in a voltage between neutral a earth.

 

The more and larger the SMPS's the greater the earth leakage and the voltage developed.

 

In a boat the SMPS's will be relatively small (phone chargers, laptop power supplies etc) compared to the overall load, mainly the fridge (if its mains operated).

 

When functioning both work equally well at low differential voltages between shore and boat earths, but IT's continue to work at higher voltages, where GI's "switch on" and connect the two together.

 

Whether to choose a GI or an IT depends on budget, space availability and the users willingness to check the functionality of a GI. As NN says, IT's also consume a small amount of power and are not completely silent in operation.

 

In my case I had no choice, the boat I bought secondhand came with a GI and there is insufficient space for an easy IT retrofit.

[nicknorman]

For the benefit of the non-electrical forum members, switch mode power supplies regulate voltage by rapidly switching the manins on and off, typically 20,000 times per second or more. This causes harmonics (distortion of the mains sine wave), which flow back down to earth resulting in a voltage between neutral a earth.

The more and larger the SMPS's the greater the earth leakage and the voltage developed.

In a boat the SMPS's will be relatively small (phone chargers, laptop power supplies etc) compared to the overall load, mainly the fridge (if its mains operated).

 

Not sure that description is quite right? Isn't it due to having smoothing capacitors between the switching elements and earth? There has to be some connection to earth anyway. However many small chargers don't even have an earth connection. Some have a plastic earth pin. I noticed that when someone kindly trod on my old-style iPhone charger at the gliding club, and broke it in half, that internally the earth pin is not connected to anything.

 

I'm pretty sure that capacitors to earth are frowned upon these days so I am unconvinced that this is a problem in the real world.