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Bankside TV ariel bypassing GI?


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Would a TV ariel mounted on a metal pole on the bank bypass the boat's GI and be a path for galvanic currents to flow between the boat and earth or other submerged metals? Several boats at my moorings have that arrangement but I can't figure it out. If the coax cable goes into the back of the TV and doesn't touch the hull but the TV is plugged into the boats AC system, is that a circuit?

 

I guess a multimeter between antenna pole and hull would confirm it one way or the other.

Edited by blackrose
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All depends on the design of the aerial and whether there is contact between the screen on the cable and the pole.

The same applies to the TV some have isolated cable screen and some don't.

The only way to tell is to check with a meter.

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4 minutes ago, Chewbacka said:

My tv is all plastic case with a 2 core mains lead - no Earth wire so the tv is only connected to ground by the antenna mounting, so a single earth point.  

 

Are you sure? Maybe if running off a shore line but if off an inverter the neutral in your twin core lead might be connected to form the earth at/in the inverter. Happy to be told  am wrong.

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11 minutes ago, Tony Brooks said:

 

Are you sure? Maybe if running off a shore line but if off an inverter the neutral in your twin core lead might be connected to form the earth at/in the inverter. Happy to be told  am wrong.

You are correct Tony.

 

Aerials are often responsible for earthing faults, car radios and TVs.

 

A 0.1uf 650v capacitor preferably a "Y" type between the braid and the aerial will isolate the voltage but let the signal through.

 

Edited by Tracy D'arth
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4 hours ago, Tony Brooks said:

 

Are you sure? Maybe if running off a shore line but if off an inverter the neutral in your twin core lead might be connected to form the earth at/in the inverter. Happy to be told  am wrong.

That is correct, but I doubt the neutral is directly connected to the aerial cable braid as if it was and someone replaced the plug and swapped the live and neutral wires the aerial would be live.  This would be a dangerous design error. 

  • Greenie 1
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There should be isolating capacitors on the aerial plug connection inside the set on the center and the outer braid.

I for one would not like to rely on them. They can get spiked by static in thunder storms and are possibly cheap poor components in modern cut price electronics from the Far East.

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I've measured the voltage from a bankside pole to hull with a multimeter and that's showing 0.558v but if I disconnect the coax cable it still reads 0.558v so I guess that's just the PV between the boat and earth. So how can I tell if the antenna cable is connecting the boat with the pole? I'm not getting any buzzing with a continuity test between boat and pole but the reading on continuity is 1.680. Is that the resistance?

 

I'm not really sure what I'm doing. 

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Posted (edited)
17 minutes ago, Ryeland said:

Just put some plastic sheet between the aerial clamp and the pole, and stop worrying!

 

Richard 

 

I wasn't worrying, I was trying to understand how to conduct a test in order to learn something. I assume you don't know either?

Edited by blackrose
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It's simply that there is a theoretical possibility of an issue as you described,  so why not just remove the possibility of the situation occurring? I do think that sometimes we over think things! 

 

Richard 

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Posted (edited)
11 hours ago, Ryeland said:

It's simply that there is a theoretical possibility of an issue as you described,  so why not just remove the possibility of the situation occurring? I do think that sometimes we over think things! 

Work

Richard 

 

I won't do that because it's not my boat/antenna. As I said in my original post some boats at my mooring have that arrangement and I was trying to check one of them with a meter. I'll give them your suggestion, but to be honest I'm not convinced it would work. Once you've tightened a metal clamp against a metal pole you might have easily gone through a bit of thin plastic sheet so it doesn't necessarily remove the possibility of the situation occurring. As Loddon said above, the only way to really know is to test it with a meter which is what I was trying to do...

 

Sometimes we also under-think things!

Edited by blackrose
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if you go back to basics the gi or it is there to stop electrical current flowing via the mains earth, causing (galvanic) corrosion. The metal pole and coax has the potential to connect the Hull to earth exactly the situation the gi/it is there to stop.  

 

So is the coax braid or inner connected to the pole ? Highly unlikely but if a Numpty has installed the antenna it could be or if the cable has chafed due to wind or other movement then the braid may be touching the boat or pole.

 

There may also be a possibility that other equipment connected to the TV could bridge to the boat Hull or boat 240v earth e.g something plugged into an audio or video socket on the TV might be connected to a chassis earth on say an amplifier or dvd player. 

 

I'm assuming the 1.68 was ohms not kilo or Meg ohms ! And that tells us there's a electrical path between pole and boat via the land and water so not unexpected. 

 

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The 1.68 reading shows there is a continuity with a resistance of 1.68 ohms this will be too high for the continuity beep to work. Because the the pole/ground/water/Hull isn't a great electrical conductor like a piece of copper wire would be.

 

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Nope you would need to check the boats 240v earth. Which is what the gi is protecting (effectively)

 

Just be careful if you start poking around in the 240v system and if in doubt etc.

 

 

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