Lighting strike taking out Virgin cable in E11?

[watercooled]

So, can anyone recommend a coax cable isolator/lightning arrestor?

BTW don't forget your PC is grounded, I don't think the modem is apart from at the ISP's end - lightning hit the cable, found it's way to your PC then through the case to the ground connection. I'd advise isolating the PC from ground and from the modem.

It would be interesting to find out what the lightning hit though - the coax cables are buried and, while the ground around a strike is charged I doubt the strike would continue back above ground again...

[TAKTAK]

BTW don't forget your PC is grounded, I don't think the modem is apart from at the ISP's end - lightning hit the cable, found it's way to your PC then through the case to the ground connection. I'd advise isolating the PC from ground and from the modem.

The power supply is grounded... So you can't really isolate it... The case is grounded to the PSU so that if it becomes live, you're safe and the PSU trips...

[watercooled]

By isolate it I mean pull the plug.

[TAKTAK]

By isolate it I mean pull the plug.

aaaah, only during a storm, i thought you meant permanently ...

[aidanjt]

So, can anyone recommend a coax cable isolator/lightning arrestor?

BTW don't forget your PC is grounded, I don't think the modem is apart from at the ISP's end - lightning hit the cable, found it's way to your PC then through the case to the ground connection. I'd advise isolating the PC from ground and from the modem.

It would be interesting to find out what the lightning hit though - the coax cables are buried and, while the ground around a strike is charged I doubt the strike would continue back above ground again...

Well, the modem would be grounded, and a large portion of the strike will go into the modem's ground before it completely burns out, forcing the rest into your router and computer.

[Blastuk]

that's impossible! virgin uses FIBER OPTIC ... oh.... wait.

[TheAnimus]

It will always go like that, electrons will always follow the shortest path to ground (your PC, probably the one connected to the first ethernet port of a router), it's always a good idea to isolate the first connection coming into the house, preferably the phone line, in the case of cable internet, get a surge protector that can filter ethernet. It'll save you a large chunk of money and headaches.

Yes but isn't resistance a factor, the path, might be more than one, if the z is to high to allow the discharge?

So the idea that it would use the first socket on the router, is erm, yes anyway.

Most routers you can see the metal 'shield' this is called the magnetics. This is ground.

But yet it still goes out to the PCs on the LAN!

[peterb]

This is why i have a UPS with ethernet isolation on my WHS!

If you get a direct lightening strike, there is nothing that is going to protect your equipment, but then, if you do get a direct hit, you will have more to worry about than your computer!

The power supply is grounded... So you can't really isolate it... The case is grounded to the PSU so that if it becomes live, you're safe and the PSU trips...

Well, the modem would be grounded, and a large portion of the strike will go into the modem's ground before it completely burns out, forcing the rest into your router and computer.

As I said above, a direct strike is pretty much impossible to protect against - the initail voltage will be megavolts, and once the strike has commenced, the current flow will be mega amps. Fortunately direct strikes are rare, (but if you look at photographs of trees that have had a direct strike, they are blasted apart by the water in them instantly vapourising) and many lightening strikes are cloud to cloud and don't hit the ground anyway.

The damage is caused by electromagnetic induction. A current flowing in a conductor generates a magnetic field, and a changing magnetic field induces a vooltage in another conductor. The size of that voltage depends on the intensity of the field and the rate of change of that field. So a high current flowing in the ionised air produces a very large transient induced voltage in both powerlines and communications cables, and it is this that causes the damage. A UPS will help prevent damage to connected mains equipment from mains induced spikes. A surge protector (a good one) will help protect communications equipment. these are fitted at the exchange and are expensive) but since the transient is of short duration, the wavelength of the spike will be short and so may propogate to consumer equipment. A domestic type (Belkin - whatever) MAY provide some protection, but if it has been subjected to a surge, it should be changed - they can only absorb so much energy. (The exception are gas filled devices - expensive!)

Yes but isn't resistance a factor, the path, might be more than one, if the z is to high to allow the discharge?

So the idea that it would use the first socket on the router, is erm, yes anyway.

Most routers you can see the metal 'shield' this is called the magnetics. This is ground.

But yet it still goes out to the PCs on the LAN!

A cable connecting two pieces of equipment that are isolated from mains can still pick up the spike and that can be sufficient to damage the equipment it is connected to, although that is unlikely to occur in a domestic environment.

Basically, if you are near a thunderstorm, powerdown and unplug everything, disconnect phone lines and TV antennas, and wait for the storm to pass. That won't protect against the direct hit, but as I said earlier, in that situation you will have more to worry about than the computer!

[watercooled]

If lightning is going to strike anyway there's not much that will stop it from arcing across any surge protectors etc but it doesn't just strike - it kind of finds a path first, and if you can make your computer a less tempting target (by unplugging it - and I don't just mean flick the switch as this doesn't isolate ground) it's far less likely to strike.

[peterb]

If lightning is going to strike anyway there's not much that will stop it from arcing across any surge protectors etc but it doesn't just strike - it kind of finds a path first, and if you can make your computer a less tempting target (by unplugging it - and I don't just mean flick the switch as this doesn't isolate ground) it's far less likely to strike.

No, it will make little difference to the liklihood of a strike - but unplugging will minimise the damage caused by surge caused by a close proximity strike.

But as you imply, a lightening strike is quite complex. The initila phase (when the generated vltage is high enough) is an electrostatic phenomemenon - the high intensity electic field local ionises the air which privides a path for electrons, which then progressivley ionises more air, so a sparc develops. that spark will follow the path of least resistance (air with more ions in it) until it reaces another point of lower potential. the charg built up then travels across that path ina discharge that is more akin to an arc. A spark is a high voltage, low current event, and arc is a low voltage high current event; in a lightening strike, the two fololow each other almost instantaneusly, and ceases when the charge is dissapated and there is insufficient energy to maintain the arc.

Before anyone mentions lightening conductors - they aren't. they would vapourise if they were hit. However they are charge conductors and serve to ensure that any charged air at the top of buildings (or typically chimnies - where warm, ionised/conductive combustion gases are discharged) is leaked away so that there isn't an easy path for the initial lightening fingers (the high voltage pahse) to trace through leading a path for the high current discharge. they are a preventative or protective measure, before the fact, not a palliative measure.

[watercooled]

It's like people say they are safe in a car during a thunderstorm because the rubber wheels are insulators - yes, it is safe but not for that reason. Lightning makes it from the cloud to the ground so it can easily make it from the base of the car to the road - the car acts as a Faraday cage, conducting the charge around the outside.
Lightning is pretty fascinating IMO - as well as what happens above the cloud...

[peterb]

It's like people say they are safe in a car during a thunderstorm because the rubber wheels are insulators - yes, it is safe but not for that reason. Lightning makes it from the cloud to the ground so it can easily make it from the base of the car to the road - the car acts as a Faraday cage, conducting the charge around the outside.
Lightning is pretty fascinating IMO - as well as what happens above the cloud...

Yes indeed - a fraday cage is an area of equipotential - although... ( ) with the currents flowing that might not be the case.

Thought expperiment. car, on its rubber tyres (we'll assume it is dry and they are a perfect insulator) is charged to say 1Mvolt. People in the car can touch any part of it because it it is at the same potentiol all over, and the same potential as them. Someone on the outside touching it and the ground will get a shock because they are the earth point.

Now if there is a current flowing through the car. assume the car (only an example) has a resistance of .01 ohms, but the voltage applied to the top of the car is 1MV then a current of 10,000 amps will flow throuhght the car. If someone inside touched the base of the car and a point half way up 9and we assume the resistance of the car is linear) they will experience a 500,000V shock - the fact that the car could act as a faraday cage is irrelevant.

It is actually this phenomena that kills people sheltering under a tree when it is struck by lightening (assuming the tree doesn't fall on them), or even on open ground. the current flowing away from the point of contact causes a potential difference across the ground, and someone straddling or lying across that PD will experience an electric shock that may cause burns and can kill. There have been recorded instances of several people in the vicinity of a lightening strike being killed, although no-on was directly hit.

However, we are going off topic now!