Making Electrical Fuses out of Copper Wire
W. Curtiss Priest
Dr. W. Curtiss Priest
Director, CITS
February 13, 2002
Subject: Making Electrical Fuses out of Copper Wire
The electrical chemistry and composition of commercial fuses is
not widely available.
The ideal fuse works up to its rated current, then, "magically"
opens the circuit just above that current. Some fuses are
also designed to accomodate the peak currents of inductive
loads, referred to as "SLO-BLO" by Littlefuse.
Once the fuse opens, it should remain open and present no
fire hazard. One problem with using a wire as the fuse
element is it turns into vaporized atoms providing the
opportunity to form a plasma discharge. The ability for
these atoms to sustain a plasma discharge varies directly
with the voltage, thus the voltage rating on fuses which
relate to the "clearing voltage" -- the ability for the atoms
to clear without beginning a plasma discharge.
To quench the plasma discharge at higher currents, the wire
element is emersed in either fine sand or fine glass beeds.
This is the reason fuses starting at about 30 amperes are
contained in a cardboard tube with brass ends. The interior
contains the quenching material. Such fuses are often
contained in a "fuse block."
For a fuse to "blow" the wire element must have a distinctly
higher resistance than the wires connecting it. If it is
the case of a copper wire element connected to copper wire,
this means the diameter or guage of the wire must be
considerably finer than the connecting wires.
Other ways to increase the resistance without going to such
finer guages is to use copper alloyed with materials such
as lead or other metals such as aluminum, iron (steel), or
"fuse wire." The literature is unclear about the precise
meaning of "fuse wire." A commonly available fuse wire
in Europe is tinned copper wire. The wire is tinned to
reduce oxidation. Fuse wire is also referred to in the 1959
edition of the Handbook of Chemistry and Physics and is shown
to have a resistance of about ten times that of copper (or
tinned copper). The composition of this particular wire, which
has fusing currents shown in a table labeled "Fusing Currents
for Wires" is not stated. The Handbook does contain another
table labeled the "Resistance of Wires" and the resistances
of 24 different metals from Brass to Zinc are presented.
These resistances will relate to how these metals might
behave as a fuse, but the relationship is not simple.
It should be noted, for safety reasons, that most fuses are
contained in ceramic, glass or other non-combustible materials,
if they are NOT surrounded by sand. Even brief periods of
arcing or plasma discharge represent a fire threat if not
properly contained. Thus, the use of copper wire as a fuse, whether
tinned or not, should be approached cautiously. Generally, one
should not even attempt to repair a fuse that has blown with
copper wire of the corresponding guage as the containment for
the original element may not correspond to what is needed for
a copper wire.
That said. There are situations where it is useful to know
and use a copper wire fuse. What is ideal about such a fuse
is its cost is essentially zero.
For example, one way to limit the current to a spot weld, from a
fixed current source, is to put such a fuse in series with the
apparatus. While this technique is not as reliable a source of
a known amount of energy (such as derived from a charged
capacitor), it can suffice in certain cases.
While the amount of energy to perform a spot weld relates
to the cross-sectional area of the area to be welded, it is
still useful to have a sense of the diameters or gauges of
wires that might be employed.
The following chart has been created with this purpose in mind.
Notice that there are two columns of "fusing currents for wires."
The first column is derived by an empirical, mathematical
expression shown in the footnotes. The second column is
from the Handbook of Chemistry and Physics (above).
Notice that the two sources of empirical data agree well at
around 43 guage but diverge as one goes to larger guage wires.
That the Handbook Advisory Panel decided to drop the table
of fuse currents in the 1964 edition and have not replaced
it suggests the variables affecting the fusing current are
complex. For example, one would expect different results
based on the length of the wire and based on its orientation
(vertical or horizontal).
The preface to the 1959 table probably states it well when
they say:
Owing to the influence of various factors which control
the rate of loss of heat energy the following values
can be considered only as approximations
That the table was not returned to the Handbook, even 35
years later is an egregious loss. Certainly rough data
is better than no data at all.
The "computed" column is based upon a formula (below). Should
anyone wish to augment this formula with better knowledge,
you are invited to contact this author for the Basic program
that creates the following table.
Wires, Guages, and Fusing Currents of Copper
(best viewed in fixed point Courier)
Wire Computed Handbook
S.W.G. Number A.W.G./B&S A.W.G.Metric Fuse Current Fuse Current
(Inches) (Gauge) (Inches) (MM) (Amperes) (Amperes)
0.4000 0000 0.460000 11.6840 799.27
0.3720 000 0.409642 10.4040 691.36
0.3480 00 0.364796 9.2660 598.16
0.3240 0 0.324861 8.2520 517.49
0.3000 1 0.289297 7.3480 447.62
0.2760 2 0.257627 6.5430 387.19
0.2520 3 0.229423 5.8270 334.97
0.2320 4 0.204300 5.1890 289.77
0.2120 5 0.181900 4.6210 250.68
0.1920 6 0.162000 4.1150 216.85
0.1760 7 0.144300 3.6650 187.63
0.1600 8 0.128500 3.2640 162.33
0.1440 9 0.114400 2.9060 140.39
0.1280 10 0.101900 2.5880 121.45
0.1160 11 0.090700 2.3040 105.03 275.0
0.1040 12 0.080800 2.0520 90.87 225.0
0.0920 13 0.072000 1.8290 78.70 200.0
0.0800 14 0.064100 1.6280 68.04 160.0
0.0720 15 0.057100 1.4500 58.87 140.0
0.0640 16 0.050800 1.2910 50.92 120.0
0.0560 17 0.045300 1.1500 44.06 100.0
0.0480 18 0.040300 1.0240 38.11 80.0
0.0400 19 0.035900 0.9119 32.97 70.0
0.0360 20 0.032000 0.8128 28.55 60.0
0.0320 21 0.028500 0.7239 24.71 45.0
0.0280 22 0.025300 0.6426 21.29 40.0
0.0240 23 0.022600 0.5740 18.49 35.0
0.0220 24 0.020100 0.5106 15.97 30.0
0.0200 25 0.017900 0.4547 13.82 25.0
0.0180 26 0.015900 0.4038 11.91 20.0
0.0164 27 0.014200 0.3606 10.34 17.0
0.0148 28 0.012600 0.3200 8.91 15.0
0.0136 29 0.011300 0.2870 7.77 12.5
0.0124 30 0.010000 0.2540 6.67 10.0
0.0116 31 0.008900 0.2261 5.77 9.0
0.0108 32 0.008000 0.2032 5.05 7.5
0.0100 33 0.007100 0.1803 4.35 6.5
0.0092 34 0.006300 0.1601 3.75 5.0
0.0084 35 0.005600 0.1422 3.23 4.0
0.0076 36 0.005000 0.1270 2.81 3.0
0.0068 37 0.004500 0.1143 2.46 2.7
0.0060 38 0.004000 0.1016 2.12 2.3
0.0052 39 0.003500 0.0889 1.80 2.0
0.0048 40 0.003100 0.0787 1.54 1.8
0.0044 41 0.002800 0.0711 1.36 1.5
0.0040 42 0.002500 0.0635 1.18 1.3
0.0036 43 0.002200 0.0559 1.01 1.0
0.0032 44 0.002000 0.0508 0.89
0.0028 45 0.001800 0.0457 0.78
0.0024 46 0.001600 0.0406 0.67
0.0020 47 0.001400 0.0350 0.56
0.0016 48 0.001200 0.0305 0.47
0.0012 49 0.001100 0.0279 0.42
0.0010 50 0.001000 0.0254 0.38
Relationship between (tinned copper) fuse wire size and fuse rating is
d = 0.055*I^0.8 or I = 37*d^1.25
(where I = fuse rating in amps, d = wire dia. in mm)
Handbook values from the Handbook of Chemistry & Physics, 1959
Table: Fusing Currents for Wires, Copper
Note: Table discontinued in 1964 due to Advisory Board Decision
Gages Arranged In Columns As Follows:
AWG= American Wire Gauge
B&S= Brown & Sharpe
SWG= Imperial Standard Wire Gauge- (British legal
standard)
Comment:
Values are stated in approximate decimals of an inch
excluding the metric numbers. As a number of gauges are
in use for various shapes and metals, it is advisable
to state the thickness in thousands when specifying in
gauge number. Metric wire gauge is 10 times the
diameter in millimeters.
Fuse Conversion courtesy of:
ajw...@dial.pipex.com, News Group Posting
Gauge table courtesy of:
http://www.reade.com/Conversion/wire_gauge.html
--
W. Curtiss Priest, Director, CITS
Center for Information, Technology & Society
466 Pleasant St., Melrose, MA 02176
Voice: 781-662-4044 BMS...@MIT.EDU
Fax: 781-662-6882 WWW: http://Cybertrails.org