Michael,
The arcing was between hot and neutral. GFCI outlets would have made no
difference.
- Robert -
Robert A. Macy, PE [email protected]
408 286 3985 fx 408 297 9121
AJM International Electronics Consultants
619 North First St, San Jose, CA 95112
-----Original Message-----
From: [email protected] <[email protected]>
To: [email protected] <[email protected]>
Cc: Robert Macy <[email protected]>
List-Post: [email protected]
Date: Thursday, October 25, 2001 5:52 PM
Subject: 120V Ground Faults
>
>Robert,
>
>>From my experience, I think you will find that arcing ground faults are
>inherently high-resistance in nature and, while dramatic, do not
>necessarily pull significant amounts of current. Most 15 amp breakers will
>likely require several seconds/minutes to blow at 60/30 amps, which is what
>you'd get with a 2/4 ohm arc. In reality, I wouldn't be surprised to see
>something more like a 10-20 ohm figure being used for this type of
>phenomenon, which would allow a 15 amp breaker to arc virtually
>continuously (i.e. the home arc-welder). The fact that arcs are drawn -
>and sustained - at 120 volts is, I believe, relatively rare.
>
>Higher voltage systems (and GFCI outlets) have ground fault systems that
>rely on the detection of zero sequence (neutral leakage) currents. My
>understanding is that the decision to require this type of protection on
>480 volt systems over 1200 amps was largely due to the increased likelihood
>they'd be able to draw and sustain an arc, as well as the damage that can
>be caused at these higher power levels (balanced with the concerns of
>cost-effectiveness of installing them more broadly). I feel they drew the
>line in an appropriate place.
>
>In my experience, while problems, such as this, do arise, the frequency and
>relative damage caused by them is relatively small. I think you would have
>seen a change (like the addition of GFCI about 25 years ago) if the case
>were otherwise. You should be able to add zero sequence current sensing to
>your household panel, should you care to do so, for ~$500, but where
>out-of-the-box systems exist for 480 volt systems, this would need to be a
>custom design amploying the combination of a sensor relay and a shunt-trip
>breaker. Of course any nuisance trips you may experience similar to your
>GFCI would take down your main.... You can buy a LOT of GFCI breakers for
>these dollars. Caveat Emptor/Engineer.
>
>Regards,
>
>Michael Garretson
>Compliance Engineering Manager
>RadiSys Corporation
>+1 503 615-1227
>
>
>
> "Robert Macy"
> <[email protected]> To: "Dan Kwok"
<[email protected]>, <[email protected]>
> Sent by: cc:
> owner-emc-pstc@majordom Subject: Re: skinny
power cords.
> o.ieee.org
>
>
> 10/25/01 02:36 PM
> Please respond to
> "Robert Macy"
>
>
>
>
>
>
>
>It definitely was not supplied by the heater company. It was a high
>quality
>UL approved cord. It's just that this cord carbonized and burst into flame
>as the arc was existing. The flames did immediately extinguish when the
>arc
>was stopped by unplugging the cord which is good.
>
>But again, it was disturbing that the 15A breaker provided no protection.
>
>Anyway, it was a good lesson for this "sleeping" guy. Now I take
>electrical
>distribution inside my home much more seriously.
>
> - Robert -
>
> Robert A. Macy, PE [email protected]
> 408 286 3985 fx 408 297 9121
> AJM International Electronics Consultants
> 619 North First St, San Jose, CA 95112
>
>-----Original Message-----
>From: Dan Kwok <[email protected]>
>To: Robert Macy <[email protected]>; [email protected]
><[email protected]>
>Date: Thursday, October 25, 2001 1:42 PM
>Subject: Re: skinny power cords.
>
>
>>
>>Hi Robert,
>>
>>Recently, I bought several similar heaters for my home. I recall reading
>in
>>the operation instructions, explicit safety warnings against using
>extension
>>cords with the heater. Was the extension cord supplied with the heater?
>>
>>
>>---------------------------------------------------------------------
>>Dan Kwok, P.Eng.
>>Principal Engineer
>>Electromagnetic Compatibility
>>Intetron Consulting, Inc.
>>Ph (604) 432-9874
>>E-mail [email protected]
>>Internet http://www.intetron.com
>>
>>----- Original Message -----
>>From: "Robert Macy" <[email protected]>
>>To: "Roman, Dan" <[email protected]>; <[email protected]>
>>Sent: Thursday, October 25, 2001 8:49 AM
>>Subject: Re: skinny power cords.
>>
>>
>>>
>>> Just have to jump in here with personal experience:
>>>
>>> In our bedroom we have a deLonghi radiator heater which uses an
>extension
>>> cord (high cost UL approved) heavy guage #12 wire to power it - when
>it's
>>> used. This extension cord plugs into a "multi outlet" adapter, also
>heavy
>>> duty UL approved. At the time of the incident there was no power being
>>used
>>> from this outlet.
>>>
>>> I was in another room, my wife was sitting on the edge of the bed
>watching
>>a
>>> news blurb on TV when she heard a funny sound, a scritch, scritch. She
>>> called to me to come listen. Scritch, scritch, scritch got louder. As
>I
>>> arrived, flames started lapping up the wall from the outlet while still
>>> making arcing sounds. The flames were less than 6 inches from curtains.
>I
>>> reached into all this and unplugged the extension cord which luckily
>>stopped
>>> the fireworks display. Imagine, if we had not been there.
>>>
>>> Upon examination, it appeared that an arc had formed between the blades
>of
>>> the extension cord (remember no power at the time). That arc was not
>>> sufficient to drop the 15A breaker to the outlet, yet was sufficient to
>>> carbonize the UL approved material which further sustained the arc.
>>>
>>> I posted this to the newsgroup alt.home.repair where a fireman jumped in
>>> describing how this exact mechanism is what starts most home fires!
>Isn't
>>> that an encouraging thought!
>>>
>>> Anyway, a little damn fuse in the plug would not have helped in this
>>> circumstance, complete waste of time, much like the main breaker was.
>>>
>>> - Robert -
>>>
>>> Robert A. Macy, PE [email protected]
>>> 408 286 3985 fx 408 297 9121
>>> AJM International Electronics Consultants
>>> 619 North First St, San Jose, CA 95112
>>>
>>> -----Original Message-----
>>> From: Roman, Dan <[email protected]>
>>> To: [email protected] <[email protected]>
>>> Date: Thursday, October 25, 2001 7:41 AM
>>> Subject: RE: skinny power cords.
>>>
>>>
>>> >
>>> >I agreed completely with Scott. A 6 to 9 foot 18AWG cord will handle
>well
>>> >in excess of 20A for a short period of time without starting to smoke
>>> (heck,
>>> >it'll handle close to in excess of 60A for a very very short time
>without
>>> >bursting into flames--not that it was a good experience finding this
>out).
>>> >Point is, the cordage will handle a fault either indefinitely or long
>>> enough
>>> >for the branch circuit breaker to trip provided you are connected to a
>15A
>>> >or 20A branch circuit.
>>> >
>>> >Another data point, you routinely pass more current through the cord
>when
>>> >doing the earthing test and that uses more current than the cord is
>rated.
>>> >Leave the tester on for awhile and the cord does not really heat up
>>either.
>>> >
>>> >What this list needs is a power cord manufacturer or agency safety
>>engineer
>>> >that does power cords to settle this once and for all!
>>> >
>>> >Dan
>>> >
>>> >-----Original Message-----
>>> >From: Scott Lacey [mailto:[email protected]]
>>> >Sent: Wednesday, October 24, 2001 7:43 PM
>>> >To: Gary McInturff
>>> >Cc: [email protected]
>>> >Subject: RE: skinny power cords.
>>> >
>>> >
>>> >
>>> >Gary,
>>> >I believe the answer is that the power cord rating of 6 or 10 amps is
>the
>>> >operating current, at which it will have minimum temperature rise.
>Under
>>> >fault conditions it will experience a rather dramatic temperature rise
>>that
>>> >is still well below the melting temperature of the insulation. The
>breaker
>>> >or fuse should clear well before the cord is "cooked" to the point of
>>> >failure.
>>> >
>>> >Scott Lacey
>>> >
>>>
>
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