Table 11.—Safety factors and ignition voltages

for capacitor circuits

Disk material ry B ~ Ve Safety
Vide factor
A uF:
Cadmium . 7.481 2877x10" 162.6 = 1.00
Aluminum . 4446 4.977K10% = 207.7 1.28
Brass 12665 3.556x10TM% 2536 1.56
Lead 18.249 1.067x10 = 291.3 1.79
Tin. 7.239 1.175x10% 299.8 1.84
Copper 3.902 1.43910 333.0 2.05
Stainless steel... 13.202 1.205x 10% 423.7 2.61
Cold-rolied steel... 6.639 2793x107 = 440.6 2.71
At 10.3 uF:
Cadmium . 9.917 1.334 10% 39.9 1.00
Aluminum . 90.484 © 1.028x 10% = 40.51.01
Brass .. 23101 2811x10 = 46,6 1.17
Copper 24,744 6,902 x 10% 4701.18
Lead .. 5.153 -1.371x10% = 5251.32
Tin... 9.265 4.406 x 10 583 1.46
Cold-olled steel... 5.221 9.750x 10% 827 2.07
Stainless steel... 12.352 5.821x10% 920 2.31
‘AL 107 uF:
Aluminum ... 31.532 9.441x10% = 17.383
Copper .. 48.744 5,957 x 10% 20.8 1.00
Cadmium . 14.206 1.303x10% = 208 1.00
Brass 28.858 3.44310 = 21.01.01
Tin 15.563 1.892x10% 21.5 1.08
Load + 23.081 1.578x10" = 217 1.08
Cold-rolied steel... 14.209  3.963x10% = 27.31.31
Stainless steel... 24.384 4.634 x 10” 28.1 1.35
At 4,310 uF:
Aluminum . 2.582 x 1075 92 70
6516x107 13.2 1.00
6.483x10% 13.7 1.05.
4.093x10% 139 1.05
1814x10% = 15.11.14
Brass . 2218x10% = 154 1.17
Stainless steel 17.721 3990x10% = 16.11.22
Cold-rolled stool... 27.649 5.675x10% = 17.31.31

VA 15

In breakflash testing, as the electrodes separate, an
arc forms at the contact point. If this arc has sufficient
energy, then an ignition will occur. In the case where
inductance is low, i.e., <100 mH, formation of the arc is
almost always caused by vaporization of the contact point.
This metallic vapor serves to conduct current as long as
the circuit voltage can maintain it. Eventually, the arc gap
becomes so great that the spark is quenched and current
ceases to flow. At higher inductances, i.c., above 100 mH,
formation of the arc may also be caused by vaporization of
the contact material (5, p. 83).

Because the disk contact material vaporizes, it may be
hypothesized that the boiling point of the material may in-
fluence how easily an arc is maintained in some circuits
(S, pp. 86-89). For resistor and inductor circuits, a crude
relationship exists between ignition current and boiling
point for the elemental disk materials. This relationship
is presented graphically in figures 20 and 21. These graphs
have the form

IC = 10°F) +D, ©
where IC = ignition current, mA,
and BP = boiling point, °C.

The values for C and D for each of the particular circuits
are given in table 12, From this, a rough estimate of the
ignition current is possible if the boiling point is known.
No such relationship was found to exist for capacitor
circuits.

Table 12.—Values of C and D for Ignition
current versus bolling point equation

c D Cc D
B0Vde,, 284x 10% 271 |] 10mH.., 145x104 2.39
40Vde.. 314x104 241 |] 100mH., 931x105 2.00
50Vde., 3.29x10* 2.24 || 600mH.. 218x104 1.51
mH... 1.27x10% 2.75



‘Table 4.—Voltage-current versus probability of Ignition for cold-rolled steel disks

Direct current, mA Probability of Ignition Voltage, V de Probability of ignition
ALBOVde .. 3,330 89% 101 ALL2uF ... 327.0 9.1% 10%
450 13x 1 x r
320 box tot srs 27x tot
3,780 1.8 x 107 400.0 29x 104
At4oVde .. 1,785 8.6 x 105 425.0 1.2 103
1,800 1.8 x 104 A103 uF ... 78.0 72x 104
1,877 1.8 x 104 85.0 1.2x 10%
1,900 27x 104 90.0 1.8 x 10?
1,966 5.0 x 104 AL 107 uF... 25.0 5.1 x 104
2,000 1.4 x 109 27.0 3.6 x 104
AtsOVde .. 1,200 90% 107 285 20% 10
1,300 1.01 320 12x11
1,400 22x 10% AL1,310 uF .. 15.8 1.4.x 104
1,500 8.4.x 107 16.5 9.1 x 104
Ati mH.... 1,410 7.3 x 104 17.0 8.9 x 104
1,436 5.6 x 10° 18.5 68x 107
1,450 3.5 x 107
1,474 4.0.x 107
At1OmH... 540 1.8x 104
545 41x 104
550 95x 104
600 3.8 x 107
At 100 mH .. 164 71x 104
169 1.5 103
174.5 1.4.x 109
180.5 23x 10°
At 600 mH .. 108 3.2% 104
110.5 3.2.x 104
N25 27x10?
115 63x 107
-
10 T T T T T T
KEY
Cadmium
$ —— Brass y
- 10% -—ALead a / 4
S --—4# Aluminum f f
1} I—|—O Tin / fo p
L ---—@ Copper i / q fl}
-+++-+-0 Stainless steel int
fo} - Stainless si 4 ‘ig
> 13+ --1—x Cold-rolled rey . ei 4
a steel | /$ fli
Z yi}
a / fi}
a a ‘ip
g oy 4 74
a
107 1 1 1 1 L 1
| 2 3 4 5 10 15 20

DIRECT CURRENT, 10% mA

Figure 11.—Probability of ignition versus current results for 50-V de resistor circult.


19

CONCLUSIONS

This project was designed to generate ignition currents
(voltages) producing a probability of ignition of 10°+
5x 10+. The margin of safety attained when using mate-
tials other than cadmium is dependent upon disk electrode
material in resistor or inductor circuits. Capacitor circuit
results show a smaller margin of safety; it appears that
other materials, aluminum in particular, require ignition

voltages comparable with those of cadmium. At several
capacitances, aluminum exhibited lower ignition voltages
than cadmium. Further research is needed to determine
the cause of the wide range safety factors among the
circuit types. Based on these results, the approval tests
used by MSHA should not be modified to account for the
safety factor provided by the material involved.

REFERENCES

1, US. Code of Federal Regulations. Title 30—Mineral Resources;
Chapter 1—Mine Safety and Health Administration, Department of La-
bor; Subchapter D—Electrical Equipment, Lamps, Methane Detectors;
Tests for Permissibility, Fees; Part 18—Electric Motor-Driven Mine
Equipment and Accessories. July 1, 1984, pp. 111-113.

2. American National Standards Institute. Standard ANSI/UL 913.
Tntrinsically Safe Apparatus for Use in Class I, II, IIl, Division 1. Haz-
ardous (Classified) Locations. Underwriters Laboratories, Northbrook,
IL, 4th ed., July 29, 1988, 41 pp.

3. Cawley, J.C, Probability of Resistive Spark Ignition Caused by
Very Low Currents. BuMines RI 9114, 1987, 11 pp.

4, ___.. Probability of Spark Ignition in Intrinsically Safe Circuits.
BuMines RI 9183, 1988, 15 pp.

S. Magison, E. Electrical Instruments in Hazardous Locations.
Instrum. Soc. Am., Research Triangle Park, NC, 34 ed., 1978, pp. 83,
86-89,

INT.BU.OF MINES,PGH.,PA 29552


UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

°c degree Celsius min minute
cm?/min cubic centimeter mm millimeter
per minute
ms millisecond
in inch
rev revolution
kohm kilohm
rpm revolution per
L/min liter per minute minute
mA milliampere s second
BF microfarad volt
mH millihenry Vde volt, direct current
OTHER ABBREVIATIONS USED IN THIS REPORT
A,C,D,E constant terms NDIR _nondispersive infrared
B slope of graph N, number of ignitions
BP boiling point N, number of sparks
d acceptable probability of PB probability of ignition
ignition error
q probability of no ignition
Ic ignition current
s spark sample size
I, current yielding
Pi = 1/1,000 v. voltage yielding p,
= 1/1,000
TEC International Electro-
technical Commission Zz Z score



16

VOLTAGE, V de
Figure 17.—Probability of ignition versus voltage results for 10.3-uF capacitor circult.

10 : 1 : ———
KEY °
— Cadmium ----v Stainless steel
—-—* Brass ----@ Cold-rolled 4
3 —--4 Lead steel
E 3) 7-4 Aluminum as /
Z 10° —-Tin - de 4
L —--e Copper 7 e
3 4 Yr
- es
a Yr,
2 v a ;
Pa A | 4
fe)
«
a
10° f Nl fi ! fo
100 200 300 400 500
VOLTAGE, V de
Figure 16.—Probabliity of Ignition versus voltage results for 1.2-uF capacitor circult.
-1
10 T T TM—T TT re
KEY
— Cadmium “
z= —-—# Brass
°Q lo2L —-~4 Lead
E --« Aluminum
S —/-0 Tin
= —-—e Copper
6 os -—7 Stainless steel
> 10°F ----@Cold-rolled steel
(=
a
z
41
g lor
a
10° [er ee ee
20 30 40 50 60 70 80 90 100

CLADE, Ve


18

30 T T T T T T T TST ¥
4 . Pia Toppan)
E20} weer } 4
‘ aes ° 2
2 I5 F wed . 4
E uec7
z -
ir ae ww
© OF wee a 2. 4 I
3 757 “ 8 2 3 Aluminum _|
z ” 7
° 3
E 5h a & Lead 4
z °. a
2 4
= 47 Cadmium

3 on \ f j j : , j ;

6 8 0 (2 4 16 (8 20 22 24 2
BOILING POINT, 102 °c

iva

“ Figure 20.—Elemental disk material boiling point versus resistor circuit ignition current.

) [=}
Aluminum ¢ Copper,

Tin wn

IGNITION CURRENT, 10? mA
N
T
1

IF Lead: ty
et - KEY
Cadmium ueeo7 seeQeee mH
\ a ——o-- 10 mH
id oat —o— 100 mH
--<-- 600 mH
3 fn \ a ! 14 !

1
6 8 10 [2 14 16 18 20 22 24 26 28
BOILING POINT, 10% °c

Figure 21.—Elemental disk material bolling point versus Inductor clrcult ignition ‘current.

28


' B
Table 8.—Voltage-current versus probability of Ignition for tin disks
Direct current, mA Probability of ignition Voltage, Vde Probability of Ignition
AtgOVde .. 1,500 84x 10 ALL2uF .... 220.0 22x 104
1,912 45x 104 260.0 2.3 x 104
2,110 1.8 x 10% 290.0 1.8 x 104
2,230 3.6 x 104 300.0 1.1.x 109
2,450 5.7 x 104 310.0 1.2 10
2,660 7.3 x 104 330.0 66 x 10"
2,860 1.3x 10° A103 uF... 48.5 27x 104
At40Vde .. 1,400 1.9.x 104 52.5 45x 104
1,620 47104 56.5 4.1.x 104
1,700 1.1.x 10% 60.5 88x 104
1,792 24x 10° 655 3.4x 107
AL5OVde .. 1,006 3.2 104 705 86x 107
1,050 7.0 x 104 ALO7UF.... 21.6 7.4x 104
1,160 3.1.x 107 218 1.1.x 108
1,200 9.0.x 10° 22.4 7.7 x 104
ve 1,120 1.8 x 104 23.1 1.2% 10?
1,180 1.3.x 104 23.8 57x 107
1,233 3.2.x 104 AL1310 nF .. 13.35 1.8 x 104
1,290 1.1 10% 13.5 3.6 x 104
1,386 64x 10° 13.65 1.8% 10%
At1OmH ... 582 2.5 x 104 140 28x 10
600 7.5 x 104
607 9.3 x 104
627 73x 10%
‘At 100 mH .. 170 4.0.x 104
176 47x 104
183.5 4.0 x 10°
190 1.8 x 10?
‘At 600 mH .. 111 3.0 x 10%
112.5 9.9 x 10%
115 1.0 10%
130 6.4 x 107
10° T T T ——— —
P
10"! |
3 iV P
E } /
Zz oe / 4 {4
- p 1B
we A cath
S Ios KEY ff |
> Cadmiun d IH}
EF — —*Brass / i
=)
= —--—t Lead ve /
2 04+ —--—4 Aluminum may
oO ——I—0 Tin
2 ——-—0 Copper
os —"—"-¥ Stainless steel J
40 © Cold-rolled steel
106. 1 A 1 rr ee 1
OS 04 05 06 #07 08 09 | I 121314

DIRECT CURRENT, 10? mA

Figure 15.—Probability of ignition versus current results for 600-mH inductor circult.



sparks was chosen as the basis upon which to compare
materials. This probability of ignition was arbitrarily
chosen because (1) it is easy to generate enough data for
a reasonable confidence level, (2) past research has shown
that the current (voltage) versus probability data graphs
are linear on log-log coordinates, and (3) results may be
extrapolated to probabilities of ignition as low as 107 (3).
To calculate the spark sample size, equation 1 was used.

S = (Zpaq)/d’, )
where S = spark sample size,
Z = Zscore, 2.326,
P, = target probability of ignition, 10°,

q = probability of not getting an ignition, or
1-p,

and d = acceptable probability of ignition error,
5x 107,

Thus, with a 98%-confidence level, the probability of
ignition would fall between 5 x 10* and 1.5 x 10° (fig. 6).
Equation 1 yields a sample size of approximately 22,000
sparks. A typical data set would then produce 22 ignitions.
Whenever 22 ignitions or 22,000 sparks were accumulated,
a data set was considered complete and testing was halted.
For each material, at least three data points were taken.

Figure 7 illustrates the gas mixing system used in the
experiment. The test gas was delivered to the breakflash
apparatus explosion chamber at a rate of 500 cm3/min and
consisted of 8.3% +0.3% methane, 20.0% oxygen, and the
balance nitrogen. Each gas has its own mass flowmeter
and accompanying flow controller. Each flowmeter is
calibrated for the particular gas and has an accuracy of
+3% full scale. So that the gas concentration would
remain consistent, the methane and oxygen flow rates
could be set to track the nitrogen or, if necessary, be
controlled manually. The methane concentration was
monitored by a nondispersive infrared analyzer, which had
flow rates limited to 1.0+0.5 L/min. The oxygen monitor
was installed in parallel with the breakflash explosion
chamber, and its flow rate was automatically limited to 100
to 150 cm*/min. The spark test apparatus verification
specified in section 8.4 of ANSI/UL 913 was performed
daily to verify the integrity of the test gas. Installed on the
inlet of the breakflash explosion chamber was a flame
arrestor to prevent propagation of an ignition back into
the gas mixing system (fig. 8). Also installed on the
explosion chamber was a pressure switch. In the event of
an ignition, a delay circuit triggered by the switch stopped
the electrode shafts, allowing the system to purge the
products of combustion for 3-1/2 min before restarting the
electrode shafts.

SZ score is a statistical parameter whose value is dependent upon the
level of confidence chosen; in this case, 98%.

1x 10%

15x10"

1104}

5x10

PROBABILITY OF IGNITION

<4
1x 104

1
7000

DIRECT CURRENT, mA
DIRECT VOLTAGE, V

70,000

Figure 6.—Acceptable probability of ignition range of 5 x 104

to 1.5 x 10

Ne

3

FM
FC
M

Flowmeter
Flow controller

Motorized valve

Figure 7.—Diagram of gas mixing system.

Na

Gos | Blended

KEY

=> Gas circuit

Blended
gas

—— Electrical circuit

cHa—

mixing

system | 998

Flame arrestor

To exhaust
NOIR
gas
taikel [analyzer]
Breoktiosh|

Figure 8.—Diagram of gas flow.


INTRODUCTION

Intrinsic safety (IS) is concerned with the ignition of
explosive gases by electrical or thermal means. In regard
to mining, the primary objective of IS is the prevention of
methane explosions caused by accidental energy discharge
from low-energy electrical/electronic devices. Two ele-
ments must be present to create a potentially hazardous
condition: an ignitable fuel-air mixture and an ignition
source. In poorly ventilated areas, methane released
during the mining process may accumulate in explosive
concentrations. The methane may then be ignited by a
malfunctioning electrical device. Because IS utilizes
energy-limiting techniques, it is effectively limited in use to
low-power applications, IS is only one of several tech-
niques used to prevent explosions in such hazardous areas.
Other techniques, such as explosion-proof enclosures, re-
quire a physical barrier between the electrical circuit and
the hazardous environment. If this physical barrier is
breached, a margin of safety cannot be maintained; a sit-
uation that does not exist if IS techniques are used.

The US. Mine Safety and Health Administration
(MSHA) must approve all devices intended for use in or
inby the last open crosscut or in return air underground as
per 30 CFR 18 (1).? Because certain areas of these speci-
fications are somewhat vague, American National Stand-
ards Institute/Underwriters Laboratories (ANSI/UL)
Standard 913 (2), with some modifications, is presently
used as the de facto standard for electrical/electronic de-
vice approval. The ignition current/voltage curves avail-
able in ANSI/UL 913 are frequently used in the approval
process. Initially, the energy discharge of the device is
compared with the curves. Those devices with energy dis-
charges well below the curves may be approved by elec-
trical inspection only. Other devices, whose energy dis-
charge falls near the curve, must be ignition tested using
a device similar to the breakflash apparatus shown in
figure 1 (2, p. 38).

The breakflash apparatus simulates the making and
breaking of a circuit using four tungsten wire electrodes
and a single cadmium disk electrode. The electrode ma-
terials used in the breakflash apparatus were not chosen
arbitrarily. Tungsten was chosen as the wire material for
its toughness, resulting in increased electrode life. Cad-
mium was chosen as the disk material for its ability to
produce ignitions at the lowest currents or voltages known
for any material. The electrodes counterrotate—the wire
electrode shaft at 80 rpm and the disk electrode shaft at
19 rpm—to create sparks in an energized circuit. This

breakflash apparatus. MSHA specifications state that two
worst case circuit faults should be added to the CUT and
15 times the normal energy discharge of the device should
be applied at the point of test. Two ignition tests, each
consisting of 1,000 rev of the breakflash apparatus, are
conducted. If there are no ignitions of the test gas, the
device is approved as intrinsically safe.

The approval process is quite safe as there have been
no explosions traced to intrinsically safe devices. MSHA
employs a “safety factor" in its approval testing. By in-
creasing the energy at the point of test by 50% over the
conditions of actual use, MSHA testing allows for varia-
tions in supply currents (voltages), circuit tolerances, gas
concentrations, environmental conditions, and other fac-
tors that may influence the probability of igniting the test
gas. In addition, the cadmium disk electrode in the break-
flash apparatus causes ignition at lower current (voltage)
levels than disk electrodes of copper or steel. This in
effect, creates a hidden safety factor in addition to the

sparking serves to ignite the methane-air mix in the 3 rane 3 Bowe Lol a Bearing plate
. . aa Ql wmber
surrounding explosion chamber. The circuit under test 3 Upper plate 9 Shaft adapter /5 Whisker holder assembly
(CUT) is connected in series or parallel with the 4 Brush bracket /0 Drive gear 76 Chamber top
5 Motor 11 Siip ring 17 Chamber knob
6 Post 12 Center plate -

{talic numbers in parentheses refer to items in the list of references
at the end of this report,

Figure 1.—Breakflash apparatus used to collect spark ignition
data,



INFLUENCE OF ELECTRODE MATERIAL
ON SPARK IGNITION PROBABILITY

By Jeffrey Shawn Peterson’

ABSTRACT

The testing procedures of the U.S. Mine Safety and Health Administration specify that intrinsic safety
acceptance tests be conducted using a standard tungsten-cadmium electrode configuration in the break-
flash apparatus. However, in realistic mining environments, other materials may be more likely sources
of sparking. Information defining the probability of spark ignition between common materials such as
aluminum, brass, copper, lead, tin, cold-rolled steel, and stainless steel is of more practical value in
determining ignition hazards, The U.S. Bureau of Mines has completed an investigation of the influence
of material on the ignition probability using the breakflash apparatus. By comparing ignition currents
or ignition voltages corresponding to a probability of one ignition per thousand sparks to those found
previously for cadmium, a margin of safety may be estimated for each material. This report presents
the results of an investigation into the influence of disk electrode material on the probability of ignition.

‘Blectrical engineer, Pittsburgh Research Center, U.S, Bureau of Mines, Pittsburgh, PA.


14

Table 9.—Safety factors and ignition currents

Table 10.—Safety factors and ignition currents

for resistor circuits for Inductor circults
Disk material A B l,mA Safety Disk material A B mA Safety
factor factor
At 30 V de: ALT mH:
Cadmium . 9.063 3.159x10% 8391.00 6.189 2410x107 7021.00
Brass . 11.705 1.640x 10 1,388 1.65 10.702 1.845x 10° 8721.24
Lead 12.321 6.237x10% 1,520 1.81 9.038 1.211x10% 9531.36
Auminum ...... 9.497 4.375x10% 1,999 2.38 9714 3.419x10% 1,079 1.54
Tin..seceeeeees 8.788 7.812 1077 2,828 3.37 9.084 1.497x 10% 1,157 1.65
Copper . 30.503 1.857x 10% 2,926 3.49 + 20136 © 3.129x10% 1,271 1.81
7712 1116x110" 3,125 3.73 Cold-olled stee!.. 68.627 1.170x 107 1,401 2.00
25.282 7.114x10TM 3,679 4.39 Stainless steel... 38.007 7.318x10"% 1,426 = 2.03
At 10 mH:
7.268 4997x107 4831.00 Cadmium . 18.082 6.687x10 3151.00
7.306 6.256 x 105 798 1,76 21.753 4.146 x 10° 434 1.38
11.818 1.322 10% = 8941.97 59.951 3.34210" = 4391.39
14873 1.54510 1,189 2.63 24.760 2.420x 107 5381.71
10.071 3.287 10% 1,680 3.71 48.034 1.468x10% 5551.76
9.028 4.852x10% 1,767 3.90 21.886 -7.820x10% 558 1.77
18.429 1.169x10% 2,024 4.47 22.439 1466x105 = 5751.83
Stainless steel... 11.562 2.936x10 2150 4.75 45.153 2.880x10" 6031.91
At 50 V de:
6217 2.969x10% 314 1.00 19.310 9.253x10% 118 1.00
14.388 1.07910 5091.91 95.404 2493x107 1371.16
8897 7.854x107 6832.11 15.652 1.593x 10° 1441.22
8435 4,281x10% 1,017 3.24 47911 1,041x 10% = 1631.38
19.465 1.226x10 1,063 3.39 10.603 2676x107 167 1.42
13.033 3.627x10“ 1,267 4.04 29.077 1.243x10 = 1711.45
19.416 2.051x10% 1,336 4.25 36.298 2766x110 177 1.50
15.786 1.007x10 1,470 4.68 30.138 9.366x10% 1811.53
‘At 600 mH: 6
Capacitor circuit results are included in tables 2 through Cadmium wee toe 6 te
8 and presented graphically in figures 16 through 19. The — Brass | 28730 2692x10% 101 230
safety factors listed in table 11 show that typically the Aluminum . 27.495 = 1.978 x 10” 106 2.44
margin of safety attained by using materials other than _Cold-olled stool... 82.849 7.975% 10%" 1112.52
cadmium decreases as the circuit capacitance increases. Cope “pcat cree oa jou ing 3a
Surprisingly, aluminum disks produced lower ignition volt- Tiheeeeccee 32.120 7.965x10TM 114 259

ages than cadmium at 107 and 1,310 uF. The surprising
results at 1,310 uF prompted testing using a cadmium disk
electrode. Test voltages of 9.0, 9.4, and 11.0 V de used in
aluminum testing were repeated with the cadmium disk.
‘Using 22,000-spark data sets, no ignitions were observed
in any of the tests. A retest of aluminum disks at 9.0, 9.4,
and 11.0 V de resulted in ignitions in sufficient quantities
to verify the previous aluminum results. Testing at 107 pF
illustrates only negligible differences between copper, cad-
mium brass, tin, and lead ignition voltages. Their ignition
voltages were clustered around 21.0 V de.

Some of these unusual results may be attributed to ex-
perimental error inherent in the testing. As an example,
for a data set of 22,000 sparks, if the probability of ignition
is 10°25 x 10°, the data set will produce 22+11 ignitions.
The large number of tests required to complete the project
effectively prohibited testing at lower probability levels
or narrower error ranges because of the large amounts of
time involved. If the acceptable error range is still

considered to be 50% of the target probability of ignition,
then decreasing the target probability of ignition by an
order of magnitude increases the spark sample size by a
factor of 10, or from 22,000 sparks to 220,000 sparks.
Cutting the acceptable error range in half would increase
the sample size by a factor of 4 from 22,000 sparks to
88,000 sparks.

Some of the test results produced curves with unusually
low slopes, such as tin at 30-V de resistor circuit testing.
This also may be attributed to the large acceptable error
range. Adjusting the probabilities of ignition at the test
currents (voltages) by 50% could drastically alter the slope
of the graph. Continuing research includes extrapolating
data that produced unusually low slopes, such as tin,
30 V de, to find a current (voltage) yielding a probability
of 105 and then testing it to verify that it falls reasonably
close to the extrapolated curve.


vary current in 24-V de circuits using test inductances of 1,
10, 100, and 600 mH.

Aluminum electrolytic capacitors were used to conduct
the capacitor circuit testing because they are the most
common large-value capacitors used in electronic construc-
tion, Test capacitor characteristics are listed in table 13
The charging resistor was calculated to provide a charging
time constant of approximately 100 ms for the capacitor.
Tf this resistance was too small, the power supply would
cause current to flow into the arc as the capacitor dis-
charged. It was imperative that the energy contribution
from the power supply be minimal compared with that of
the capacitor discharge at contact. Checks of the dis-
charge time for each test capacitor confirmed that the
power supply contribution was minimal. If the value of
the charging resistance was too large, then the time con-
stant would be too long, causing the capacitor to discharge
at less than the full test voltage. One tungsten wire elec-
trode was used in capacitor circuit testing, and the speed
of the breakflash was slowed so that the capacitor could
charge for approximately five time constants. As a result,
capacitor circuit tests progressed much more slowly than
resistor or inductor circuit testing. The charge-discharge
cycle of the capacitor was also monitored with an
oscilloscope.

Table 1.—Capacitor circult specifications

Mallory Sprague
TC56 C5010 “Te1407__‘Tet2i1
Capacitance... uF. 1.2 ‘1,310 103 107
Charging
tesistances.kohm.. 92.9 0.075 967 0.935
Charging time
constant ..... 8. 0.111 0.098 0.100 0.100

Using the specifications in ANSI/UL 913 detailing
the cadmium disk, disks of materials commonly found in
mining environments were machined (fig. 5) (2, p. 39).
These materials included aluminum, lead, copper, tin,

Reference in text or tables to specific products does not imply
endorsement by the U.S. Bureau of Mines.

Charging
resistor
WW
dc power 1EC
supply Te breakflash
‘test machine

Figure 4.—Test circult used to obtain spark ignition data for
capacitor testing.

brass, cold-rolled steel, and stainless steel. Because
ignitions are less likely to occur with new disks, each disk
required a 20-min break in period in an unenergized cir-
cuit With only two exceptions, one disk of each material
was used in each type of circuit. During resistor circuit
testing, the softness of tin and lead resulted in wearing
inconsistent with that of the other materials, facilitating the
need of two disks of each of these materials to complete
the work. The tungsten wire electrodes had a diameter of
0.020 mm and an unsupported length of 11mm. To
prolong their usable life, the tungsten wire electrodes were
annealed prior to use. Unfortunately, these electrodes
were still susceptible to splintering, breaking, and wearing
down. They were replaced when visibly worn, or after
4,500 sparks for resistor and inductor circuit testing, or
1,000 sparks for capacitor circuit tests. If any two-wire
electrodes failed during resistor or inductor circuit testing,
the experiment was halted and all four electrodes were
replaced. The electrodes counterrotated to create the
sparks, which were then electronically counted. The
resulting ignitions were counted in a similar manner.
Essential to the experiment was the determination of
a manner in which to compare the disk materials. A cur-
rent (resistor and inductor circuits) or voltage (capacitor
circuits) yielding a probability of one ignition per thousand

‘Work done by Denver Research Institute under Bureau of Mines
contract H111585.

TOP VIEW | ee

t
‘vo ri SIDE VIEW

Figure 5.—Disk electrode, constructed of various materials.
(All dimensions are in millimeters.)



RI| 9416 REPORT OF INVESTIGATIONS /1992

Influence of Electrode Material on Spark
Ignition Probability

By Jeffrey Shawn Peterson

UNITED STATES DEPARTMENT OF THE INTERIOR

BUREAU OF MINES



‘Table 2.—Voltage-current versus probability of Ignition for aluminum disks

Direct current, mA Probability of ignition Voltage, Vdo Probability of ignition
AL30Vde .. 1,803 3.2.x 104 AUL2uF .... 190.0 46 x 104
1,873 9.1.x 104 205.0 1.9x 10
1,973 8.6 x 104 220.0 1.1.x 103
2,100 1.0 x 103 235.0 1.8 109
2,520 9.8 x 107 AL10.3 uF... 38.4 3.6 x 104
At4oVde .. 950 45x 10% 40.4 68 x 104
1,080 1.4x 104 42.4 14x 10%
1,200 24x 10% 444 4.1107
1,960 5.9 x 10° AL1O7 uF... 163 23x 104
AL5OVde .. 847 1.4x 10% 17.3 9.4 x 104
g92 46x 104 183 23x 10?
980 1.1.x 107 19.3 6.5 x 107
1,108 1.5 10° AL 1310 uF .. 9.0 1.3 x 104
AtimH.... 900 1.3 104 9.25 1.3 x 10
1,000 1.4.x 10% 94 3.6 x 10°
1,100 4.0 x 104 10.0 44x 10%
1,140 23x 107 11.0 1.1.x 10?
1,224 4.1.x 10°
1,300 1.2 103
At1OmH ... 520 22x 104
535 27x 104
‘550 1.3 x 107
575 1.6 x 107
At 100 mH .. 160 2.5 x 104
168 1.4.x 104
171 1.3 x 107
175 5.3 x 10
200 9.1 x 107
‘At 600 mH . 101 26x 104
105 6.9 x 104
110 28x 10%
115 8.9 x 107
lo ' T T T T T T T T
2 4
é /*
é 10% a/ > 5
2
8 /
w
fo)
lo3b
Fated KEY ah
a Cadmium L
o ——# Brass
s -—4 Lead wow °
© jo4h —--- Aluminum on
& i—'—0 Tin
--—=@ Copper
- Stainless steel
-1—* Cold- rolled steel
107-5| 14 L 1 i 1 1
7 8 9°10 15 20 25° 30-35 «40

DIRECT CURRENT, 107 mA

Figure 9.—Probability of ignition veraus current results for 30-V de resistor circult.



METHOD OF COMPUTER ANALYSIS

A commercially available software package, RS/1, re-
lease 3, was used to analyze the test data using multiple
regression techniques. Each data set contained a test cur-
rent (voltage) and a corresponding probability of ignition,
defined as the number of ignitions (N,) divided by the
number of sparks (N,).

BENIN, Q

Initially, the log of the current (voltage) was plotted versus
the log of the probability of ignition. A straight-line curve
fit of this plot yielded an equation of the form

Log p= B(logIorlogV)+E, 3)

where B = slope of graph,

and E = constant term.

From this, the probability of ignition can be estimated at
each test current or voltage and plotted with the test re-
sults. Graphs of these data would be the logarithmically
transformed current (voltage) and probability of ignition
data and would have the form

p= BIS ®
or
p= BVA, )
where P, = probability of ignition,
I = current, mA,
V = voltage, V de,

B = slope of graph, same as in equation 3,
and A = antilog of E, as in equation 3,

Further analysis using equation 3 would generate the igni-
tion current or ignition voltage corresponding to the target
probability of one ignition per thousand sparks, By com-
paring these to previous Bureau research involving cad-
mium disks, safety factors may be assigned for each
material,

TEST RESULTS AND ANALYSIS

Resistor circuit results are included in tables 2 through
8. These results are shown graphically in figures 9 through
11, The cadmium curves shown are those of previous Bu-
reau research (4), The graphs illustrate that the ignition
currents are highly voltage dependent. Table 9 summa-
izes the test results, Listed in this table are exponent A
and coefficient B of equation 4; I., the current yielding one
ignition per thousand sparks, and the safety factor for each
material. As shown, the safety factors vary widely among
the test materials, with cadmium having the lowest ignition
current at all test voltages, In general, the order of the
materials from lowest to highest safety factor was consist-
ent throughout the resistor circuit testing. This fact and
the magnitudes of the safety factors illustrate that ignition
current in resistor circuits is highly material dependent.

Inductor circuit test results are included in tables
2 through 8 and in figures 12 through 15, Here, the igni-
tion currents were highly dependent upon inductance.
Table 10 summarizes the results. The safety factors for
inductor circuits were somewhat less than those for resis-
tor circuit work; i.e., the influence of the material on the
probability of ignition was less. Cadmium still had the
lowest ignition current. The results also illustrate that as
circuit inductance increased, the goodness of fit increased.
Or, as the test circuit became more like a simple resistor
circuit (ie,, as the circuit resistance-circuit inductance ratio
increased), test results became more random or
unpredictable,


50% energy increase. The magnitude of this additional
material-based safety factor is unknown,

‘When sparking occurs in actual use, the materials acting
as electrodes will likely be the materials more commonly
found in the mining environment, Although cadmium is
commonly used in mining environments for plating of
cases, nuts, bolts, etc., other materials may be more
common. As examples, steel is frequently used in the con-
struction of equipment enclosures and copper is used ex-
tensively as an electrical conductor. Spark test data gen-
erated using disks of steel, copper, and other materials
commonly found in mining environments are essential in
determining their effect upon the probability of ignition.
Computer analysis of these data yields ignition currents

(voltages) corresponding to specific probability of ignition
when sparking occurs between steel, copper or other elec-
trodes. By comparing these currents (voltages) to previous
US. Bureau of Mines research involving cadmium disks,
a safety factor may be assigned for each material. This
safety factor is more precisely defined as the ratio of the
ignition currents or ignition voltages that yields an equal
probability of ignition when testing two materials under
similar test conditions, Cadmium was used as a reference
material and assigned a safety factor of 1.00.

As part of its program to enhance safety in the mines,
the Bureau has conducted an investigation of the influence
of disk electrode material on the probability of ignition.

ACKNOWLEDGMENTS

The author wishes to thank the following Pittsburgh
Research Center employees: Robert Kemp, physical sci-
ence technician, for his efforts in compiling the test data
for this project, Arthur Hudson, electronics technician, for

his instrumentation of the project, and Anne Oyler, com-
puter programmer, for her computer analysis of the test
results.

EXPERIMENTAL DESIGN

Test circuits investigated included simple resistor, in-
ductor, and capacitor circuits (figs. 2-4). For the resistor
circuit, decade boxes consisting of noninductive resistors
were utilized to vary current over the range of test volt-
ages. Voltages below 50 V dc represent the most likely
range commonly found in low-power devices in the mine.
For disk materials other than cadmium, testing at or below
20 V de became impractical. The increased currents nec-
essary to conduct testing at <20 V dc could raise the wire
electrode temperature to the point where a hot-wire
ignition is possible. Spark ignition and hot-wire ignition

Rtest

de 1EC
power breakflash
supply machine

mechanisms are different. Spark ignitions require the
transfer of electrical energy from an arc to the gas mix.
With hot-wire ignitions, thermal energy is imparted to the
test gas by the electrically heated wire. Eventually,
the gas will become so hot that it will ignite spontan-
cously. Because hot-wire ignitions could render low-
voltage ($20 V dc) resistor circuit test results invalid, test
voltages were limited to 30, 40, and 50 V de.

Air-core inductors were used in the inductor circuit
testing. Noninductive decade boxes were again used to

Current- L
limiting resistor test
TH
IEC
a pone breakflash
machine

Figure 2.—Test circuit used to obtain spark Ignition data for
resistor testing.

Figure 3.—Test olrcult used to obtain spark ignition data for
inductor testing.



17

10° 1 :

VOLTAGE, V de

T
KEY
— Cadmium
Zz yo'L ——® Brass 4
° 10 —--4 Lead a i
E —---4 Aluminum . 74 a
z —/-0Tin / 2l/
= 102 —--© Copper ' by 4
i a
5 —--7 Stainless steel / : KK
> ----@Cold-rolled steel /, |/ kh
F -3 / if 4
a10°h “3 4
oO :
< / of
o 4
a 10 4
10° n 1 1
10 15 20 25 35
VOLTAGE, V de
Figure 18—Probability of ignition versus voltage results for 107-yF eapacttor circult.
10" ; —;
KEY
— Cadmium Yo
<2] ——s Brass
z a 4
2 10 —--4 Lead / ?
EF —---« Aluminum a4 y
S | ~"OTin “ $j
~ 10° —-*Copper $ 4
ro} —+--7 Stainless z
> steel ® ¢
- -q| 777-@ Cold-rolled Oo og
Ss 107h steel o 4
oO
<
8
& 10h 4
10° ! !
5 ie} 15 20

Figure 19.—Probability of Ignition versus voltage results for 1,310-uF capacitor circult. .


CONTENTS

Abstract... cece cc ce essen eee cece eeee eee n eee eeeeeeenaes
Introduction . .
Acknowledgments
Experimental design . .

Method of computer analysis
Test results and analysis .
Conclusions ......
References .,

ILLUSTRATIONS

1, Breakflash apparatus used to collect spark ignition data

2. Test circuit used to obtain spark ignition data for resistor testing
3. Test circuit used to obtain spark ignition data for inductor testing .
4, Test circuit used to obtain spark ignition data for capacitor testing
5. Disk electrode 6... 0... 6c cece cceeeee eee e cece eeeeanee
6.
7.
8.
9.

. Acceptable probability of ignition range of 5 x 104 to 15 x 10°.

. Diagram of gas mixing system

. Diagram of gas flow .

. Probability of ignition versus current results for 30-V de resistor circuit
10. Probability of ignition versus current results for 40-V de resistor circuit
11. Probability of ignition versus current results for 50-V de resistor circuit .
12. Probability of ignition versus current results for 1-mH inductor circuit
13. Probability of ignition versus current results for 10-mH inductor circuit .
14. Probability of ignition versus current results for 100-mH inductor circuit
15. Probability of ignition versus current results for 600-mH inductor circuit
16. Probability of ignition versus voltage results for 1.2-4F capacitor ‘circuit
17. Probability of ignition versus voltage results for 10.3-yF capacitor circuit
18. Probability of ignition versus voltage results for 107-uF capacitor circuit
19. Probability of ignition versus voltage results for 1,310-uF capacitor circuit .
20. Elemental disk material boiling point versus resistor circuit ignition current .
21, Elemental disk material boiling point versus inductor circuit ignition current

TABLES

Capacitor circuit specifications 6.0.0.0... ccc ccee cece eee eeeneeeeeeeeeeeeeeeneens
Voltage-current versus probability of ignition for aluminum disks .
Voltage-current versus probability of ignition for brass disks . . .

Voltage-current versus probability of ignition for cold-rolled steel disks .
Voltage-current versus probability of ignition for copper disks ........
Voltage-current versus probability of ignition for lead disks ..........
Voltage-current versus probability of ignition for stainless steel disks .
Voltage-current versus probability of ignition for tin disks .....
Safety factors and ignition currents for resistor circuits ...........
Safety factors and ignition currents for inductor circuits ....
Safety factors and ignition voltages for capacitor circuits... .
Values of C and D for ignition current versus boiling point equation . . .

BE Spar anayne



10

Table 5.—Voltage-current versus probability of ignition for copper diske

Direct current, mA Probability of ignition Voltage, Vde Probability of ignition
ALQOVdo .. 2,770 9.1 x 105 AUL2uF .... 220.0 3.2x 104
2,830 7.7 x 104 226.0 9.1 x 105
2,900 1.2 10° 233.0 5.4.x 104
3,000 1.4.x 10% 250.0 27x 104
At4OVde .. 1,580 1.4x 104 278.0 27x 104
1,640 6.4x 104 310.0 1.1% 10%
1,709 23x 10% AL10.3 uF... 43.0 9.1.x 105
2,000 24x 107 45.0 5.9.x 104
2,200 6.5 x 10° 475 9.3 x 104
AL5OVde .. 1,110 1.8.x 10% 50.0 47x 10°
1,191 5.5 x 104 ALIO7 UF... 20.3 3.1 x 104
1,305 9.7 x 104 20.5 9.0 x 104
1,462 7.8 x 10° 21.0 6.3 x 104
AtimH.... 1,000 5.0 x 104 215 83x 10"
1,025 5.4.x 104 22.0 1.7% 10?
1,100 1.4 108 ALY 310uF .. 13.2 21x 10
1,195 26x 10% 13.4 5.0.x 104
1,200 27x 10° 13.8 1.0x 10?
At1OmH ... 500 1.8 x 10% 143 1.7.x 10
525 7.2x 104 148 3.3 x 103
550 9.2 x 104 15.3 1.3 x 107
600 1.9 x 10?
A100 mH . 155 1.3.x 104
189 1.8.x 104
163 1.5 10°
166 3.8 x 10°
170 5.8 x 10°
‘At 600 mH .. 102.5 5.9 x 104
105 77x 104
110 53x 10°
130 1.6 x 107
“1
10 T T T T TTT TT 7
KEY
— Cadmium —---¥ Stainless steel 4
3 _p| —~7BBrass — ---@ Cold-rolled steg ZA iv :
Ee 10 + —-~Lead , 1,4
z -~4 Aluminum y ° $3
2 -—>Tin au
a7 Wan I /
u —--© Copper aw MT, :
Mats Ly FY I
> 10h MY LOY Ps
Lad ae e- vA vi
2 4 xo kh
oO “
sa ”~ a ON
J 10 4
a
10% ! \ \ [en ee ee ee |
5 6 7 8 9 10 It 12 13 14 15 16

DIRECT CURRENT, 10% mA

Figure 12,—Probability of ignition versus current resutts for 1-mH inductor circuit.


12
Table 7.—Voltage-current versus probability of ignition for stainless steel diske
Direct current, mA Probability of ignition Voltage, Vide Probability of ignition
AtaoVde .. 2,640 27x 10% ALL2uF .... 355.0 9.9 x 105
2,820 5.1 x 10% 366.0 1.4.x 104
2,950 5.5 x 10% 445.0 1.9.x 10
3,410 20x 10 AL10.3 uF ... 70.0 9.1 x 105
3,700 3.7 x 10% 80.0 45x 105
3,860 8.1.x 10? 845 1.8 x 104
At40Vde .. 1,696 6.6 x 10° 94.0 3.7 x 109
1,800 17x 104 At 107 uF .... 26.0 1.8 x 10%
1,900 27x 104 27.0 3.6 x 104
2,050 4.1.x 104% 28.0 6.1 x 104%
2,200 7.2 x 10% 30.0 6.0 x 10?
2,500 9.2 x 10 AL1310uF .. 180 1.7.x 104
AtSOVdo .. 1,250 1.1% 104 15.5 5.0 x 104
1,350 1.3 104 16.0 1.9.x 10%
1,480 1.4.x 107 18.0 5.3 x 10%
1,598 3.5 x 10°
AtimH.... 1,300 8.7 x 104
1,400 3.5 x 10%
1,450 8.6 x 104
1,500 6.3 x 10°
1,516 1.8 x 10?
At1OmH ... 565 9.1.x 104
580 7.7.x 104
600 25x 103
620 6.1.x 107
At 100 mH .. 175 5.5 x 104
180 6.3 x 10%
185 1.2 10%
190 7.0.x 10°
At 600 mH .. 110 7.2 105
112.5 9.6 x 10%
115 44x 10°
120 3.0 x 107
10° T r T
to! a 4
3 7
E
2 4
Fg
a
S fs
> los 7
= i/
= de
oO -
a 104
oOo
2 KEY
* 108 Cadmium =—i——0 Tin
— —* Brass ———* Copper
—--* Lead —-—-¥ Stainless steel
‘6 —--—« Aluminum iad Cold-rolled steel!
lo" 1
oo 1.25 15 1752225

circuit.

DIRECT CURRENT, 102 mA
Figure 14.—Probabllity of ignition versus current results for 100-mH inductor



Library of Congress Cataloging in Publication Data:

Peterson, Jeffrey Shawn. ‘
Influence of electrode material on spark ignition probability / by Jeffrey Shawn
Peterson.
P. cm, — (Report of investigations; 9416)
Includes bibliographical references (p. 19),
Supt. of Docs. no.: I 28,23:9416.
1, Mine explosions~Prevention. 2. Methane. 3. Electric spark. 4. Electrodes,

Tungsten—Testing. 5, Electrodes, Cadmium—Testing. J. Title. II. Series: Report
of investigations (United States, Bureau of Mines); 9416.

N23.U43 [TN313] 622 s—de20 [622.82] 91-7375 CIP



Report of Investigations 9416

Influence of Electrode Material on Spark
Ignition Probability

By Jeffrey Shawn Peterson

UNITED STATES DEPARTMENT OF THE INTERIOR
Manuel Lujan, Jr., Secretary

BUREAU OF MINES
T S Ary, Director


‘Table 3.—Voltage-current versus probability of ignition for brass disks

Direct current, mA Probability of ignition Voltage, V.de Probability of ignition
ALgOVde .. 1,240 27 104 AUL2uF .... 327.0 9.1.x 105
1,285 4.0 104 345.0 45x 104%
1,473 20x 10% 373.0 27x 104
AL4oVde .. 2 45x 105 400.0 29x 104
820 6.8 x 104 425.0 1.2 10%
907 1.1 108 AL10.3 uF... 78.0 72x 104
1,110 1.1.x 103 85.0 1.2% 10%
AL5OVde .. 561 27x 104 90.0 1.5.x 10?
604 4.6 x 104 AL107 uF... 25.0 5.1.x 104
710 1.2 108 27.0 3.6 x 104
910 20x10? 28.5 20x 10
ALIimH.... 900 4.6 x 104 32.0 1.2% 10?
950 1.2% 10° AL1310 uF .. 15.8 1.4% 104
1,001 1.8.x 10° 16.5 9.1.x 104
1,306 1.8 x 10? 17.0 8.9 x 104
At1OmH ... 425 1.3 x 104 185 6.8 x 10
433 67x 104
440 1.3 x 103
450 12x 103
460 3.4x 107
At 100 mH . 137 1.8.x 10%
145 1.9x 107
150 3.2 x 10°
200 1.3x 107
‘At 600 mH . 95 9.1 x 105
100 1.4 x 10%
105 46x 10°
120 11x tot
yt
10"
T
TKEY T T T T T
Cadmium
2 ——a Brass
S -—4Lead rN
& 102%} --—a Aluminum 4
S —I—O Tin
oO -+-——@ Copper b/!!
uw -O Stainless steel // /
6 —x Cold-rolled / 4
> lob steel y / 4
E ff
2
a / /
<q
a / le
9 104 | 4
& /
.
1075] 1 i 1 1 1 i n
2 s 4 ~«*S 10 15-20 25 30

DIRECT CURRENT, 10% mA

Figure 10.—Probability of ignition versus current results for 40-V de resistor circuit.



1

‘Table 6.—Voltage-current versus probability of ignition for lead disks

Direct current, mA Probability of Ignition Voltage, Vde _ Probability of ignition
AL30Vde .. 1,317 27x 104 AVL2uF .... 250.0 14x 104
1,445 5.1.x 10% 276.0 85x 10°
1,710 55x 10% 283.0 4.1.x 104
2,000 20% 10° 290.0 7.0 x 104
AL40Vde .. 755 5.9 x 105 205.0 2.2 10°
814 1.0x 10° 300.0 35x 10°
906 3.7103 AL10.3uF ... 45.0 5.0x 104
1,200 1.7.x 10? 48.0 5.8 x 104
AL5OVde .. 496 9.1.x 105 51.0 8.2 104
862 23x 104 58.0 1.8 x 10°
506 1.4.x 10° AL 1O7 uF... 20.0 1.7.x 104
632 1.4. 10 21.0 62x 104
686 1.1% 107 22.0 1.1.x 109
AtimH.... 825 6.6 x 104 23.0 25x 10°
850 49x 104 25.0 3.7 x 10?
875 1.0 103 AL1310uF .. 140 91x 105
900 29x 10° 148 414x104
925 1.3x 10? 15.0 82x 104
At1OmH ... a2 27x 104 15.2 1.3 x 103
425 6.4.x 104 15.5 51x 10°
450 28x 10°
500 1.9.x 107
At 100 mH .. 190 1.6.x 104
133 4.1.x 104
137 1.2 10%
: 140 22% 10°
At 600 mH .. 80 1.4x 104
83 5.5 x 104
87 1.4x 108
90 3.8 x 10°
107 T T T a
2 y2
gS 10
Eb
&
~ 10°
we
° 9
/
> KEY { Ms
5 104 — Cadmium é ‘ 4
a — —aBrass
q —-—tLead
8 =--—a Aluminum
c 5 —— Tin
a 10 ——-* Copper 7
—--¥ Stainless steel
----@Cold-rolled steel
108 n n n rn 1.
2 25 3 35 4 #45 5 55 6 65

DIRECT CURRENT, 10% mA

Figure 13.—Probabllity of ignition versus current results for 10-mH Inductor circult.