first, been in the CATV business for over 8 years now. seen and done it all.

2nd, any “voltage charge” that would cause this would not cause things to burst into flames, sparks yes, smoke yes, flames no. any flames would have been from a longer exposure to the voltage, first causing melting. if the guy had an exposed portion of the drop outside touching a power line with also an exposed portion, you would get the arced power and some damage, but the modem would die out and the ethernet port on the computer would be fried most likely, and possibly even the motherboard. that, i have seen happen. no flames. this would also have been along with a tv or more not liking life either.
now, as for cable signal itself, there is voltage on the main lines, from pole to pole, anywhere between 60 and 90 volts, to power the line extenders and amplifiers on the feeder line which feeds service to the whole street/neighborhood, THERE IS NO VOLTAGE COMING OUT OF ANY TAP TO YOUR HOUSE. the signal on the cable to your house is measured in dBmv (decibels in millivolts) in that signal it is RF and digital frequencies. if there were any voltage on there, all of your tvs would look like crap, and nothing would work. the way the phone service works from cable companies is also through another modem called an MTA. this is plugged into the house electrical outlet to provide the ring voltage for phones. you do not need 70 volts to power ring voltage like someone posted above. 5 volts will do. most of the electronics in your house would be fried at 70 volts.

now that being said, there could be a major problem with the customer’s power-ground. that means, the power to her house is unstable and the grounding out of the meter box is in a “floating status” that means, if the cable drop is grounded, and has a better connection to the ground than her power, guess what gets used as the ground? the cable does, and then things go boom, pop, spark, sizzle. been fried by that myself, it isnt fun. if that is the case, guess who should pay? part of my job has also been to assess the damage a customer has in claim against us, and things can go either way. most of it has to do with drilled holes or something a contracted installer did. i have also had to deal with claims of fried computers and a lot of people TRY to blame us for their own misfortune. want a tell tale way of being sure? look at the coax cable where the drop hits the house, there will be a ground block, grounded to the power ground or the ground rod itself. check that fitting, see if it has any blackened or sparked spots on it. then check the cable leading to the modem. if there is no damage out there….no blame.
last one i had like that, the guy was all over me bitching about how we were gonna pay for all of his stuff that was fried: microwave, washer, computer, tv’s, clocks, and phones. see anything yet? yeah, a lot of it wasnt hooked to cable. i showed him the cable lines outside. yeah, the drop TO THE HOUSE WAS FRIED, but at the groundblock, the line going INTO THE HOUSE WAS GOOD. that shows the groundblock did its job and blew out….WHEN LIGHTNING HIT 30 FEET FROM HIS HOUSE. and he tried to blame us……?
sorry buddy, it rode in on your power lines.

nice try….class dismissed.

We don’t know the exact nature of the accident, but even if the installer applied 120VAC to the coax shield it’s still quite possible for only the cable modem and computer equipment to be destroyed while sparing her TV.

For damaging current to flow, a complete path to ground is necessary. If the cable modem were plugged into the laptop with USB, then the current could flow from the coax shield through the cable modem and on to the laptop via the USB ground. (As others point out, unlike Ethernet, USB provides no isolation). From there, the current could reach ground through either the laptop’s power supply or through one of the attached hard drives and their power supplies.

Most TVs, on the other hand, do not have 3-pin grounded power plugs. So even if you apply 120V to the coax shield going to the set, there would not necessarily be an onward path to ground to allow damaging current to flow through the TV.

I’ve always been a little surprised that the electrical code allows
cable TV even with these grounds. Look at the cable TV line running
along a typical power/phone pole. It’s usually uninsulated. Now look
up at the top of the same pole. Those are the primary (high voltage)
power lines, and they’re also usually uninsulated. If a storm or an
accident causes a primary wire to fall on the TV cable, you can guess
what might happen in the houses in that neighborhood. Even proper
grounds can divert only so much fault current away from people and
property.

Telephone lines also share poles with power lines, but they’re always
insulated, the phone company tends to be more attentive to protection,
and line isolation transformers in modems and phones are standard
practice.

If fiber-to-the-home ever replaces phone and cable TV, events like
these can be a thing of the past.