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"
<m...@california.com> To: "Dan Kwok"
<dk...@intetron.com>, <emc-p...@majordomo.ieee.org>
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 m...@california.com
408 286 3985 fx 408 297 9121
AJM International Electronics Consultants
619 North First St, San Jose, CA 95112
-----Original Message-----
From: Dan Kwok <dk...@intetron.com>
To: Robert Macy <m...@california.com>; emc-p...@majordomo.ieee.org
<emc-p...@majordomo.ieee.org>
List-Post: emc-pstc@listserv.ieee.org
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 dk...@intetron.com
>Internet http://www.intetron.com
>
>----- Original Message -----
>From: "Robert Macy" <m...@california.com>
>To: "Roman, Dan" <dan.ro...@intel.com>; <emc-p...@majordomo.ieee.org>
>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 m...@california.com
>> 408 286 3985 fx 408 297 9121
>> AJM International Electronics Consultants
>> 619 North First St, San Jose, CA 95112
>>
>> -----Original Message-----
>> From: Roman, Dan <dan.ro...@intel.com>
>> To: emc-p...@majordomo.ieee.org <emc-p...@majordomo.ieee.org>
>> 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:sco...@world.std.com]
>> >Sent: Wednesday, October 24, 2001 7:43 PM
>> >To: Gary McInturff
>> >Cc: emc-p...@majordomo.ieee.org
>> >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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