Lightning melts ground wires

[Kajunbee]

I was talking to a friend of mine that owns a commercial shrimp boat about lightning strikes on his vhf antennas. He said in his experience that wires only on the ground side of battery connections would be melted. I was wondering if someone could give and explanation of why this would be so. Is this typical of most lightning strikes of antennas to affect ground side and not positive. The antennas are usually fastened with u-bolts to the steel rigging and in splinters after strike.

 

[betwixt]

.... because the negative side probably has better connection to Earth. Lightning takes the shortest route to the water which is usually via the engine and chassis.

Brian.

 

[Kajunbee]

Thanks, so would it be safe to say that if the radio is connected to say a 8-d battery alone with no path to earth ground it would be completely safe from a lightning strike. Can you think of any scenario where the radio could be damaged.

 

[FvM]

Are we talking about direct lightning strike to the antenna? If antenna and receiver have no ground connection at all, lightning will jump over from receiver to metal parts or water. It's also creating additional risk to persons.

Big vessels have lightning proof antennas on earthed pylons and overvoltage arrestors in antenna line.

 

[D.A.(Tony)Stewart]

The tops of clouds being positive cause bigger negative discharges than positive

Thanks, so would it be safe to say that if the radio is connected to say a 8-d battery alone with no path to earth ground it would be completely safe from a lightning strike. Can you think of any scenario where the radio could be damaged.

Yes. Many. and the user too. If lightning voltage was high enough to bridge the cloud to earth, what makes you think a small gap of floating batteries to the hull will be protected?

With an antenna exposed to lightning, a fringe strike could be life-ending if you are tuning the radio during the strike as a body becomes a conduit to the nearest conductor to the hull. An arc flash can also ignite a fire.

There are several layers off protection to divert lightning current.

 

[G4BCH]

would it be safe to say that if the radio is connected to say a 8-d battery alone with no path to earth ground it would be completely safe from a lightning strike.

Probably not. If it was a direct hit to the antenna the lightening would probably just jump from the radio to the nearest ground (earth). The spark will have travelled several hundred metres from a storm cloud to the antenna, a few more metres of air are unlikely to stop it. Even if it did not flash to ground the front end would be likely fried; the radio would act like a capacitor to ground and the charging current of that capacitor would do the damage.

Can you think of any scenario where the radio could be damaged.

A nearby strike could induce enough current into the antenna to destroy the frontend of the radio,

Lightning protection is all about diverting the destructive current away from the equipment you want to protect before it gets there; simple in theory more difficult to put into practice, it take s a lot of care the voltages and currents involved are very large.,

 

[D.A.(Tony)Stewart]

As far as the -ve side always getting fried, understand that the path of highest resistance creates the most heat in series and lowest resistance when in parallel. Considering the ground -ve side is probably hull connected to a salty body of water for shrimp, which mechanism do you think causes the burnt negative side.

Then is it common mode or differential mode?

 

[Kajunbee]

Thanks to everyone. I had a few more questions but these were answered in each member’s reply.
I spoke to my buddy again and he said that he was electrocuted also. At the moment of the strike he was rolling his cursor ball on his chart plotter and the shock caused his arm and wrist to curl up and lock.
What’s crazy is the vhf ground wire burnt in half close to the radio but a 2 amp fuse on positive wire was still good. Also the solder on center pin of the coax connector melted. This seems to indicate that a large current was flowing on the center conductor. How that chinky little 2 amp fuse survived is beyond me.

 

[D.A.(Tony)Stewart]

Thanks to everyone. I had a few more questions but these were answered in each member’s reply.
I spoke to my buddy again and he said that he was electrocuted also. At the moment of the strike he was rolling his cursor ball on his chart plotter and the shock caused his arm and wrist to curl up and lock.
What’s crazy is the vhf ground wire burnt in half close to the radio but a 2 amp fuse on positive wire was still good. Also the solder on center pin of the coax connector melted. This seems to indicate that a large current was flowing on the center conductor. How that chinky little 2 amp fuse survived is beyond me.

If you re-read my last comment, you will figure it out. He was lucky as the lightning protection was insufficient.

 

[D.A.(Tony)Stewart]

If you re-read my last comment, you will figure it out. He was lucky as the lightning protection was insufficient.

No?

"lowest resistance when in parallel" via ground, common mode

Inject HV to antenna to ground then resonate while conducting, both ground and V+ side potential is raised while being shunted to hull and salty? water is the lower impedance side, thus regardless of polarity of strike, this side is more likely to burn. Note the fuse side.

Hence common mode.

 

[D.A.(Tony)Stewart]

Not the fuse side

 

[Kajunbee]

I relayed the information everyone gave to my buddy. He said that after the incident another fisherman told him if he disconnected the antenna he should place the end in a glass jar to further insulate it. It seemed like sound advice at first but I’m not sure. Wouldn’t this be much like a Leyden jar but without the water?

--- Updated Jan 10, 2024 ---

I relayed the information everyone gave to my buddy. He said that after the incident another fisherman told him if he disconnected the antenna coax he should place the end in a glass jar to further insulate it. It seemed like sound advice at first but I’m not sure. Wouldn’t this be much like a Leyden jar but without the water?

 

[D.A.(Tony)Stewart]

Sounds a lot safer than exposed coax but only about 1 pF