Fuse: fast blown vs slow blown

[Winston]

bud-- wrote:

(...)

There was a building downtown, maybe 8 stories, that burned down. Just
the 1st floor and basement were left, and they were used as a parking
lot. The existing service remained - a mere 208/120V, but fed by 4 or
6 parallel conductors. They cleverly stored ice melting salt on top of
the service. It burned down. The utility guys said some of the
conductors burned back into the supply conduits and remained hot. Some
others burned back and welded to the conduit. (They were fed at the
utility end with "cable limiters", which are lugs combined with fuses.
Limiters on the welded cables opened.)

'Cable limiters'. That's a new one on me.
If it weren't for the guys that insist on 'belt, suspenders *and* rope,
we'd be caught with out pants down much more often.


(...)

Plasma is at the arc, and you should get blown away from it, an
advantage of the explosion.

"It's A Feature!" we explained, as the electrician (slumped against the
wall on the other side of the garage) finally opened his eyes.

This'll be my entry! http://www.bulwer-lytton.com/

The emphasis is to work on dead circuits. I am not sure how you do
useful work in an arc flash suit. Can't remember - the protection may
be so you 'only' get 2nd degree burns, but a lot better than
"nothing". One of the things you don't want to wear is polyester - it
melts.

Which wouldnt be too bad, except that it *sticks* too. Ouch.

(...)

It is a commonly used garden variety 2" x 0.5"d "time delay" cartridge
fuse. It is safe on 200kA available by clearing long before the
current through it reaches 200kA.

I use my arc welder at ~120A (~30 V) for my hobby projects.
I'm trying unsuccessfully to get a visceral understanding of 200kA
at 220 V.

--Winston <-- Nope. Still don't get it.

 

[bud--]

On Jun 2, 1:15 pm, Winston <Wins...@BigBrother.net> wrote:

bud-- wrote:
On Jun 1, 1:30 pm, Winston<Wins...@BigBrother.net>  wrote:

(...)

I learned about 'flashover' only very recently.
Once you know about it, it's obvious.

Called arc-flash in US.

Yes.

 > The hazard isn't always obvious.

I meant 'conceptually speaking'. At higher voltage levels,
a lot more objects look like conductors.  At higher current
levels, a lot more objects *continue* to be conductors.
When we combine higher voltage with higher current, it is
amazing that more electricians aren't injured.

I spent my career working on the really low-power
end of the spectrum and never thought about high-power
hazards.  I recently helped troubleshoot a 208 V
3 phase system and thought I was in the 'big leagues'!

Um. Turns out I wasn't.  

There was a building downtown, maybe 8 stories, that burned down. Just
the 1st floor and basement were left, and they were used as a parking
lot. The existing service remained - a mere 208/120V, but fed by 4 or
6 parallel conductors. They cleverly stored ice melting salt on top of
the service. It burned down. The utility guys said some of the
conductors burned back into the supply conduits and remained hot. Some
others burned back and welded to the conduit. (They were fed at the
utility end with "cable limiters", which are lugs combined with fuses.
Limiters on the welded cables opened.)

 > One of the

smartest electricians I have run across wanted to measure the motor
current in a food plant. The motor starters were in a motor control
center, which is a frame with many motor starter modules - busbar feed
in back. He defeated the door interlock, opened the module door and
put a clamp-on ammeter on one of the contactor motor wires - all
absolutely routine. No one knows what happened - the guess was there
was a loose screw. In any case there was an arc-flash. He had bad
burns, some from vaporized copper condensing on his skin. He spent a
lot of time in the hospital with multiple plastic surgeries. But he
survived. You can be killed from burns, concussion from the explosion,
shrapnel.

That's one of those situations where living can be worse
than dying.

These days to be OSHA compliant and make the same measurement you
might have to wear an arc-flash suit. OSHA interest in arc-flash is
relatively recent. Some related equipment issues are now in the US-
NEC.

http://www.texsoinstruments.com/arc-flash-suits

I see that this company makes several suits that apparently
comply to NFPA 70E but only their most expensive is said to
protect against shrapnel and none are said to protect
against plasma (reasonably enough!).
I guess 'something' is better than nothing.

Far as I know NFPA 70E is not adopted as an enforceable standard
anywhere, but is the default standard for working on electrical
equipment (arc-flash is only part of it). Protection required for arcs
depends on voltage, available fault current, clearing time of the
overcurrent protection, and distance. "Current limiting" fuses, with
fast clearing times, can greatly limit the damage to equipment and
hazard to workers. New equipment is supposed to get a label giving the
hazard (calories per square cm.). One label I saw said there was there
was no safe protection.

Plasma is at the arc, and you should get blown away from it, an
advantage of the explosion.

The emphasis is to work on dead circuits. I am not sure how you do
useful work in an arc flash suit. Can't remember - the protection may
be so you 'only' get 2nd degree burns, but a lot better than
"nothing". One of the things you don't want to wear is polyester - it
melts.

So *that's* the '40,000 A' number in the 11Afuseratings.
I always wondered about those stunningly huge numbers.

A commonly available Bussmann FRN 20Afuseis "current limiting" and
rated for 200,000A available fault current. But much bigger.

Whoa.

It is a commonly used garden variety 2" x 0.5"d "time delay" cartridge
fuse. It is safe on 200kA available by clearing long before the
current through it reaches 200kA.

--
bud--

 

[Winston]

Robert Macy wrote:

(...)

It turned out the BEST fuse for this test equipment
was a small 3AG 1A FB, went off like a flash bulb, and created the 'in-
spec' waveforms.

I wonder what those waveforms looked like?

I *guess* that there was a >> 1 A current spike as the surge power
ionized the fuse element, then down to zero.
Or did the current peak at just over 1.0 A?

While visiting an Alcoa plant (in Iowa?), I saw a large casting of
aluminum, approx 18 feet long, 14 to 18 inches tall and 10 inches
wide, of a very strange shape. I was told that that was ordered by a
utility company. It is a FUSE and they had just blown one on one of
their high tension distribution lines and this was the replacement!
Now you want shrapnel and melted metal. Blowing that kind of fuse
must be awesome to behold.

The installation manual for that fuse must be a work of art.
Imagine trying to keep the oxide off of the contact surfaces!

--Winston

 

[Robert Macy]

On Jun 2, 8:23 am, bud-- <budn...@isp.com> wrote:

On Jun 1, 1:30 pm, Winston <Wins...@BigBrother.net> wrote:





bud-- wrote:

(...)

Nice post a couple levels up.

Thanks!

Meters like the Flukes are "Category" rated - 1, 2, 3. When working on
high capacity services, like 1000A 480V, you really want a cat 1
meter. You won't wind up wearing it as in your previous post. The
problem is not just the meter. Arc in meter causes vaporized meter
leads that turns into an arc between busbars - very dangerous.

I learned about 'flashover' only very recently.
Once you know about it, it's obvious.

Called arc-flash in US. The hazard isn't always obvious. One of the
smartest electricians I have run across wanted to measure the motor
current in a food plant. The motor starters were in a motor control
center, which is a frame with many motor starter modules - busbar feed
in back. He defeated the door interlock, opened the module door and
put a clamp-on ammeter on one of the contactor motor wires - all
absolutely routine. No one knows what happened - the guess was there
was a loose screw. In any case there was an arc-flash. He had bad
burns, some from vaporized copper condensing on his skin. He spent a
lot of time in the hospital with multiple plastic surgeries. But he
survived. You can be killed from burns, concussion from the explosion,
shrapnel.

These days to be OSHA compliant and make the same measurement you
might have to wear an arc-flash suit. OSHA interest in arc-flash is
relatively recent. Some related equipment issues are now in the US-
NEC.





In addition to the nominal voltage you have the hazard of transients
that could start an arc which will then sustain at the nominal
voltage. In the US, OSHA may also take strong exception to using a
meter without the right cat rating.

One of the hazards in high capacity services is available fault
current, which can be 200,000A. Fuses have a rating for available
fault current. The fix is to use "current limiting" fuses. For high
currents they have a clearing time of under 1/4 cycle. You handle a
200,000A available location by thefuseclearing before the current
increases to anything near that value. The earlier Flukefuseis
certainly current limiting. Thefuseopens before the meter leads turn
to plasma.

So *that's* the '40,000 A' number in the 11Afuseratings.
I always wondered about those stunningly huge numbers.

A commonly available Bussmann FRN 20A fuse is "current limiting" and
rated for 200,000A available fault current. But much bigger.

--
bud--

Two somewhat related incidences:

To pass a very stringent British Telecom specification that equipment
would survive adjacent equipment blowing a fuse, I desigened a
'transient generator' using 4 12Vdc 500A vehicle batteries in series
with a 50 ft 0000 Awg coil (to provide a bit of inductance), a starter
solenoid, and a fuse. The idea is the fuse shorts the 48 V, then
blows, and you get a 300+volt surge that wants to put more than 300A
someplace. If your adjacent equipment uses a 'shorting' protection
method, God save us all! The surge will strip off 1/4 inch wide traces
on a PCB. Anyway, I tested all kinds of fuses and discovered the fuse
designers very intelligently designed the fuse to 'gently' blow, just
die, go away. It turned out the BEST fuse for this test equipment
was a small 3AG 1A FB, went off like a flash bulb, and created the 'in-
spec' waveforms.

While visiting an Alcoa plant (in Iowa?), I saw a large casting of
aluminum, approx 18 feet long, 14 to 18 inches tall and 10 inches
wide, of a very strange shape. I was told that that was ordered by a
utility company. It is a FUSE and they had just blown one on one of
their high tension distribution lines and this was the replacement!
Now you want shrapnel and melted metal. Blowing that kind of fuse
must be awesome to behold.

Regards,

 

[William Sommerwerck]

"Winston" <Winston@BigBrother.net> wrote in message
news:isbc0i03i0@news5.newsguy.com...

The installation manual for that fuse must be a work of art.
Imagine trying to keep the oxide off of the contact surfaces!

All you need is a gallon jug of Cramolin!

 

[Winston]

William Sommerwerck wrote:

"Winston"<Winston@BigBrother.net> wrote in message
news:isbc0i03i0@news5.newsguy.com...

The installation manual for that fuse must be a work of art.
Imagine trying to keep the oxide off of the contact surfaces!

All you need is a gallon jug of Cramolin!

Interesting!

http://store.caig.com/s.nl/sc.2/.f

--Winston

 

[Rich Grise]

Winston wrote:

I use my arc welder at ~120A (~30 V) for my hobby projects.
I'm trying unsuccessfully to get a visceral understanding of 200kA
at 220 V.

The arc is pretty much the same, but 1,667 times as fat. ;-)

Cheers!
Rich

 

[Winston]

Robert Macy wrote:

(Snip installation details of stunningly huge over-current
protection device)

From Alcoa employee: the utility breaker was mounted on top the towers
and simply bolts in place with the surface the contact. Once bolted,
no oxide can form.

Sounds like the geometry and pressure used for the contact surfaces
was such that it broke through existing aluminum oxides and created
a gas - tight seal in the connector area. Sounds simple if you don't
have to actually design to achieve that effect.

(Snip effect of shorting huge batteries through an inductor)

Thanks! Very interesting!

--Winston

 

[Robert Macy]

On Jun 3, 12:17 pm, Winston <Wins...@BigBrother.net> wrote:

Robert Macy wrote:

(...)

It turned out the BEST fuse for this test equipment
was a small 3AG 1A FB, went off like a flash bulb, and created the 'in-
spec' waveforms.

I wonder what those waveforms looked like?

I *guess* that there was a >> 1 A current spike as the surge power
ionized the fuse element, then down to zero.
Or did the current peak at just over 1.0 A?

While visiting an Alcoa plant (in Iowa?), I saw a large casting of
aluminum, approx 18 feet long, 14 to 18 inches tall and 10 inches
wide, of a very strange shape.  I was told that that was ordered by a
utility company.  It is a FUSE and they had just blown one on one of
their high tension distribution lines and this was the replacement!
Now you want shrapnel and melted metal.  Blowing that kind of fuse
must be awesome to behold.

The installation manual for that fuse must be a work of art.
Imagine trying to keep the oxide off of the contact surfaces!

--Winston

From Alcoa employee: the utility breaker was mounted on top the towers
and simply bolts in place with the surface the contact. Once bolted,
no oxide can form.

From memory [can somebody find the reference number and post the
waveform], British Telecom Spec defined the 'boundaries of the voltage
waveform at the EUT's terminals and added that 350+A is available
during the waveform. During fuse blowing, the GND went down to -10
and the -48 came up to -20, then pops with a risetime [can't remember
the risetime] GND goes positive 30 or 40V and the -48 goes way down to
-300, or -500V and then decays back up to above -60 volts in 50mS.
That is a VERY long time for electronics.

The energy is approx 1/2 * 300V * 350A * 0.05 = 2600 joules When you
consider that the large protection MOV's inside your PC power strip
are rated for 5 - 10 j; you have some idea of the energy potentially
shoved into a telecom unit.

From memory and my understanding that the origin of the spec came
about after ....Telecom equipment are placed in racks in a room, the
batteries are usually in another room and 1 inch diameter rods go from
the batteries through the wall to the equipment room. At the equipment
room cables go down to the rods and simply clamp on the rods to get
power to an individual rack. A workman dropped his wrench across the
rods. The wrench was never found. The surge described in the spec
probably occurred, or worse, so the WHOLE room of equipment blew out,
destroyed. Since British Telecom had purchased all the equipment
without that power surge specced, British Telecom had to eat the loss,
thus the origin of the spec in future BT purchases.

The fuse and wiring and solenoid would pass up to 350 Amps, but not
for long, before the fuse disappeared. By the way, the coil of wire
would 'jump' up off the floor when the fuse went. When I first built
up the equipment, I insisted on placing everything inside a separate
room and activating remotely, just in case we lost a battery! We
never lost a battery. Turned out luckily to be an unnecessary
precaution.

Somewhere on the internet there must be pictures/videos of fuses
blowing under extreme current conditions. In such a picture, I
remember seeing a 'cone' of blasted metal progressing from some weak
point in the wire along the wire until most of it was gone. Plus
blowing a fuse in a controlled magnetic field will yield very
destructive EMP. Do a search for such weaponry. Perhaps, videos are
there.

Regards,
Robert