http://www.consumerist.com/consumer...-comcast-plugs-in-wrong-cable-222665.php

I would like to know what really happened here. I don’t see how the cable tech could cause this much damage by connecting the “wrong cable”.

Curt

"It seems likely that the young technician connected the rooftop end of the cable to a similar-appearing, but now obviously incorrect electrical cable"

I smell a rat, anyone else think this a wee bit improbable?

I woud say that to the totalaly untrained - an unused SE drop would 'look' simular, and be quite capable to cause this damage. And the decsribtion sound as if just that happened.

I tried to post on that forum, and asked if we could get pic's of 'what' this cable guy connected to.

I'm trying to imagine someone putting an RG-6Q connector on a service drop.

Elviscat, I'll bet you a beer that it was a pre-MCOP short. If you have ever had one - not suggesting you do, but it sounds, and even smells exactly as decribed there. Full brunt of POCO will go off like a grenade - trust me...

$10 (Mark's beer money ) says that it's at least partly Apple's Fault. And this is coming from someone with an iBook on their lap right now.

Ian A.

It iooks like that the coax got connected to the mains 120 or 240 Volts or perhaps even a higher voltage to that kind of damage.

Sorry kid, this is why one never plays with downed power lines!!!! The AIC for a main circuit breaker connection is ~10,000 amps plus. The connections before it are without any protection for the most part, there are usualy no fuses and a short will recieve all that the neighborhood transformer can deliver - instantaniously and contiuously thoughout the duration of the short!

I relay a story as an example..... I was closing a gutter about ten years ago while on a 20' ladder. The gutter was on the POCO side of the meters and mains, and a set of the 250MCM conductors had some spring to them when pushed in the box. Notheless, it shifted as I was putting the cover on, and the split-bolt slapped against the cover, and pierced the tape covering it creating a short to the cover I was holding at face level. It blasted a 3" hole in the cover, sounded like a grenade, and splattered molten metal specs into my face, and shirt! I then rolled down the ladder, and luckily got caught 1/4 of the way down by one leg, the ladder rolled over and I then rode it to the ground upside down. Not hurt by the fall - but the flash tepmerarily blinded a portion of my vision - a big white spot. Not to mention my nerves were pretty shot.... I had to go out to replace the cover the next day, and it took several hours for me to get the nerve up to even climb the ladder without shaking. And since then I have learned quite a lot about proper PPE and the reasons for it... Did I mention that I am one of the luckiest persons I know????

120 - ground will do it... The damage will be proportional to the size of the transformer and relivant distance to it. Look up Arch Flash Calculations.


[This message has been edited by e57 (edited 12-18-2006).]

I would think that the circuit board traces in the modem would burn up long before anything in the Powerbook ever did. (I am well aware of the effects of 120V on circuit board traces. I have never seen one continue to arc over after the trace burns up).

[This message has been edited by brianl703 (edited 12-18-2006).]

I would say that all of that damage was instantanious...


[This message has been edited by e57 (edited 12-18-2006).]

You'd think, but I've seen circuit boards where 120V was applied directly to the 5v/12v power bus and not every chip on the board was (physically) damaged--the traces gave up before the chips did.

[This message has been edited by brianl703 (edited 12-18-2006).]

This looks possible if the braid from the coax hit a hot leg off the service drop. The grounds typically get carried all the way from incoming equipment to a ground plane on the pcb in the cable modem which can be tied to the negative side of the data, or even more likely going thru the shield on a USB cable if that was the method of connection.

That's possible--the ground traces tend to be nice and big.

The Ethernet port is supposed to be isolated to 1500V. USB has no such isolation requirements.

Incidentally, it was the fear of something like this happening that made me put that nice screened (shielded) Ethernet cable my old cable modem came with in the trash and use a non-screened one.

And perhaps the Ethernet port WAS used and IS a screened Ethernet cable. I have this exact modem, let me go take a measurement..

Well, this modem does not have an Ethernet port that is screened (has the metal piece on it which makes contact with the metal shield on the RJ45 plug). The old one I had did.

However, the USB port's ground is connected directly to the incoming coax cable's ground. I checked it with a multimeter.

Conclusion? Using the USB port on this cable modem is a DUMB idea. (Especially if the condition of the ground block connection on your cable drop is questionable, like most of them).


[This message has been edited by brianl703 (edited 12-18-2006).]

brianl703, IMO this would have happened reguardless of what was connected to what as far as the normal computer connections - The connection to an electrical conductor is the cause here. The resulting energy from that transformer outside your house can be mind boggling...

Some information.... http://www.alliantenergy.com/docs/groups/public/documents/pub/p015092.hcsp http://www.mikeholt.com/mojonewsarchive/NEC-HTML/HTML/What-is-Arc-Flash~20040512.php

Mark, I don’t see how having a large amount of fault current available at the transformer comes into play here. 1 amp or 100,000 amps at 120 volts would result in the same damage to the modem. The resistance of the coax and electronic components in the modem greatly limits the amount of current that can flow. I have a hard time believing that even if this coax had 120 volts on it that this much damage would have been done. I would guess that the tuner in the cable modem would be destroyed but that’s it. This seems more like a lightening strike or medium voltage fault.

Curt

Curt, while it is true that the relatively small conductors of the coax, and the rest of the circuitry involved could of and DID lessen the current value here. This is why Breakers have AIC ratings - so they don't catastrophically explode like this. But I think it is pretty clear. Lightning is a shot in the dark - pun intended. And, medium or high voltage would have done in more than this laptop.


RG-6Q and #2AL service drop conductors look very similar - until you connect them apparently.

Anyway.... We disagree on that for now. I looked at the forum this is posted on earlier, and looking to see if they might be able to elaborate on 'What' he connected to.

Then there is the problem that the tech would have had to have stripped back the #2AL service conductor and attempted to install a Snap 'n' Seal "F" connector on it.

How the heck did he do this? The results of using a coax stripper on live #2AL service should have resulted in something that looked very wrong and was quite shocking...

Ethernet does provide isolation which is missing from USB. I would expect that the effects of applying 120V (of ANY amount of available fault current) directly to an ethernet port would be a blown isolation transformer..the damage to that laptop was much more extensive than I would expect from that...



[This message has been edited by brianl703 (edited 12-19-2006).]

Despite the title, from the photos it doesn't look like the Powerbook actually exploded. Rather, it looks like a cable exploded, spraying soot all over the place.

I'd guess that house power was somewhere shorted to the cable system, and made its way to ground when the cable was connected to the laptop.

The picture of the cable that exploded is clearly of a shielded cable of some sort. It looks like coax to me, but it's so badly damaged I can't tell for sure. If it is coax, what was it connected to?--I don't think a Powerbook has any coax connections.

In the same photo, there are some other parts that also appear to have been involved in the event. I can't tell what they are, or where they came from. They don't appear to be part of the Powerbook.


[This message has been edited by SolarPowered (edited 12-19-2006).]

The powerbook could have had a TV tuner card, but I doubt it.

I fyou read the article closely you will see the phrase "Metal bits and electronic debris from the power cable hub". It is clear that the hub exploded not the computer.

Later in the article it mentions that the coax cables "were unsafely stretched over and near an electrical box and associated cables"

The probable explanation is that damaged insulation allowed the coax shield to come into contact with the POCO conductors or connections. The resulting gound fault caused the hub/modem to fail violently.

"Power cable hub"? Is that like a "power strip"?

"Power cable hub"?

OK, here's a hypothesis: The "Power cable hub" might be a "Power Over Ethernet" inserter or hub that they had for, say, powering an IP phone. The PoE components would NOT be transformer isolated, so some sort of fault on the ground of the cable system could have made its way to that point, causing the overdramatic failure.

My hypothesis is that whomever wrote that story had no idea what they were talking about when they used the term "power cable hub".

Here's a repeat of wwhat I posted at Mike Holt's on this subject.

<<I'd bet this is another urban myth being started. There is nothing I can think of that would be a "similar-appearing, but now obviously incorrect electrical cable" to which anyone in their right mind could find a way to connect to a coax cable. And at best, they coupld connect to what, 120V? Not likely to be that explosive. This hoax was concocted by someone who has little or no knowledge of electrical installations or equipment.>>

Some of the discussion here may have swayed me though. I still doubt anyone "hooked up" anything, but the possibility of his accidentally dropping an exposed shield onto a live conductor somewhere does make some sense.

[This message has been edited by jraef (edited 12-19-2006).]

Mark, I was being sarcastic.

But it wouldn't surprise me if a Comcast Service Man/Contractor hooked up the wrong lines.

Ian A.

[This message has been edited by Theelectrikid (edited 02-04-2007).]

I don't doubt for one minute that despite some errors in describing the equipment, that this happened pretty much as described and shown.

We can all argue that the impedance of the coax, length, etc. should make this event unlikely or even impossible, but realize this:

Electricity is one of the most unpredictable forces in physics. It can and will behave in manners contrary to what the "rules" say.

And I have personally seen faults on the POCO side of an "ordinary" 120/240 volt service drop, and yes the fault current far exceeds what one would get on the load side of most residential circuit breakers.

One example that comes to mind is when a triplex secondary lateral fell last summer. It arced a 3" diameter hole into DRY concrete!! (No rain or water runoff from sprinklers.)

Never underestimate the destructive power of electricity!!

And for those who doubt this event, anyone care to donate the same gear for a forensic re-enactment? I'll do the testing....


edited for spellling

[This message has been edited by mxslick (edited 12-19-2006).]

At least I'm not totally alone here...

I think it is quite possible as mentioned for an "Ordinary" 120 drop on the POCO side of a main breaker to do just this type of damage. No 'freak of nature' or high voltage... Just 120 at several thousand amps vaporizing a very narrow path to ground in a microsecond... (A ball of super-heated metalic gas) Admittedly the scenario is strange, but not so far out there, IMO. There a lot we don't know about the incident, but having seen a POCO side short at 120 up close remove roughly 9 square inches of 14g steel in a simular explosive detonation, I'm gonna pull a 'mythbuster - plausable' on it. And not that I would suggest anyone try this at home.... (Without proper PPE and notification/participation of your local POCO) I don't think the RG-6 impedance would make any noticable at all at with that amount of energy.

e57
I will post the same response as I did on the Mike Holt site.
Have you ever looked at the short cicuit calculations to see how much the current is reduced by the impedance of the conductor? Using the short circuit calculator on the Bussmann site, if you start with 10,000 amps available short circuit current and run 50' of #14 in a non magnetic raceway, you only have 452 amps available line to neutral at the end of the run. Even if I take the available current up to 50,000 amps at the start of the run, the line to neutral available current at the end is only 464 amps. The impedance of the coax will be much much higher than than of #14 copper and reduce the available current even move. Note I used single conductors in non-magnetic conduit because that is the closest match for NM that they have on the calculator.
Don

OK, lets be conservative and say 400A ran at 120v till the conductive circuit, [ ie metal, plasma arc, etc. ] disrupted itself, and ignoring any voltage drop. Say for 100 milliseconds?

Energy = 400Ax120V= 48,000W. @ 100m/s = 0.0013kWh power.

That’s enough to practically instantaneously make a cubic inch of superheated steam at over 2000 psi gauge.

Alan

Brianl703, I've done hipot and lightning transient testing on random NIC cards and products in the past. I've seen good and bad ethernet isolation. In the IEC/UL 60950 world, and as confirmed by TR 62102, ethernet is SELV. It is usually only being isolated from other SELV circuits. The insulation is required to meet requirements of Functional Insulation. Per clause 5.3.4 of xx60950 (or 60950-1) this means the insulation must either meet creepage and clearance, pass a 500 VAC hipot, or pass a short circuit test under normal conditions. I think the reason there are some hi-isolation ethernet device out there is for use in isolating ethernet/SELV from POTS/TNV3 circuits.

This is what IEEE 802.3 requires(for 100BaseT--the requirements for 10BaseT are identical and covered in 14.3.1.1).

Who knows whether the equipment involved here actually meets these specifications?

23.5.1.1 Isolation requirement
The PHY shall provide electrical isolation between the DTE, or repeater circuits including frame ground,
and all MDI leads. This electrical separation shall withstand at least one of the following electrical strength
tests:
a) 1500 V rms at 50 Hz to 60 Hz for 60 s, applied as specified in subclause 5.3.2 of IEC 60950: 1991.
b) 2250 Vdc for 60 s, applied as specified in subclause 5.3.2 of IEC 60950: 1991.
c) A sequence of ten 2400 V impulses of alternating polarity, applied at intervals of not less than 1 s.
The shape of the impulses shall be 1.2/50 ìs (1.2 ìs virtual front time, 50 ìs virtual time or half
value), as defined in IEC 60060.
There shall be no insulation breakdown, as defined in subclause 5.3.2 of IEC 60950: 1991, during the test.
The resistance after the test shall be at least 2 MÙ, measured at 500 Vdc.

Thanks Brian. That explains a lot. I thought it was somewhat odd that most ethernet magnetics provide much greater isolation than required by the safety standard. My thinking was it was for use in a greater range of applications and maybe even for use where the Norway/Sweden deviations apply, but that didn't make since because the parts only had a hipot rating and not supplementary insulation with a stated creepage/clearance figure.

So, based on the info you've given, it seems there are a certain number of ethernet products out there that may not conform to all of the IEEE specs. (I'll keep make/model notes on testing I do in the future)

Funny thing here is what's missing. What did the installer have to say about this? Certainly he can provide the details on what happend on his end- if it happened at all. What about a statement from the cable company?

This smells like day old fish.

This is so catastropic, one-sided and written by someone who apparently has no technical knowledge that I have a hard time believing that this actually happened.

Until there are some answers I would chalk this up as another hoax and not waste any time on it.

-Hal

For the sake of multiple conversations at multple sites... My responce to Don...


I'll add a correction - something equivilent to #4-3...

seems like comcast cable employees are around allot of explosions lol.

http://www.forbes.com/feeds/ap/2006/11/20/ap3192492.html

maybe this time he was trying to hook up the lines to the gas. lol

The history on this is that the prototype 3 megabit Ethernet at Xerox PARC encountered around 90 volts differential in ground potential from one part of the building to another. That was problem. So when we wrote the spec for the 10 MB Ethernet, which could have coax segments spanning distances of as much as 500 meters, we made sure that we were adequately covered for differences in ground potential.

It would appear that subsequent revisions of the standard have retained this philosophy.

[This message has been edited by SolarPowered (edited 12-21-2006).]