ECN Forum Forums General Topics of the Trade General Discussion Area Powerbook Explodes After Comcast Plugs in Wrong Cable

"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.