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Re: Powerbook Explodes After Comcast Plugs in Wrong Cable #73153 12/18/06 11:02 PM
Joined: Dec 2002
Posts: 228
J
JCooper Offline
Member
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.

Work Gear for Electricians and the Trades
Re: Powerbook Explodes After Comcast Plugs in Wrong Cable #73154 12/18/06 11:06 PM
Joined: May 2005
Posts: 166
B
brianl703 Offline
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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).]

Re: Powerbook Explodes After Comcast Plugs in Wrong Cable #73155 12/19/06 12:18 AM
Joined: May 2003
Posts: 2,876
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e57 Offline
Member
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 Heller
"Well - I oughta....." -Jackie Gleason
Re: Powerbook Explodes After Comcast Plugs in Wrong Cable #73156 12/19/06 01:25 AM
Joined: Apr 2002
Posts: 558
C
caselec Offline OP
Member
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 Swartz
Re: Powerbook Explodes After Comcast Plugs in Wrong Cable #73157 12/19/06 03:29 AM
Joined: May 2003
Posts: 2,876
E
e57 Offline
Member
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. [Linked Image] And, medium or high voltage would have done in more than this laptop.

Quote
It seems likely that the young technician connected the rooftop end of the cable to a similar-appearing, but now obviously incorrect electrical cable. In any case, both technicians stated that the company-installed "system" of cables on the roof were "a real mess" and were unsafely stretched over and near an electrical box and associated cables.


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

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.


Mark Heller
"Well - I oughta....." -Jackie Gleason
Re: Powerbook Explodes After Comcast Plugs in Wrong Cable #73158 12/19/06 09:34 AM
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Posts: 166
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brianl703 Offline
Member
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).]

Re: Powerbook Explodes After Comcast Plugs in Wrong Cable #73159 12/19/06 12:09 PM
Joined: Jul 2004
Posts: 625
S
SolarPowered Offline
Member
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).]

Re: Powerbook Explodes After Comcast Plugs in Wrong Cable #73160 12/19/06 12:19 PM
Joined: May 2005
Posts: 166
B
brianl703 Offline
Member
The powerbook could have had a TV tuner card, but I doubt it.

Re: Powerbook Explodes After Comcast Plugs in Wrong Cable #73161 12/19/06 02:21 PM
Joined: Jul 2001
Posts: 599
J
JBD Offline
Member
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.

Re: Powerbook Explodes After Comcast Plugs in Wrong Cable #73162 12/19/06 02:31 PM
Joined: May 2005
Posts: 166
B
brianl703 Offline
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"Power cable hub"? Is that like a "power strip"?

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