Research Reports of the National Instituteof Industrial Safety, NIIS-RR-2005 (2006)
UDC 537.24:544.778.4

着火性放電を抑制したノズル型除電器の除電特性*崔

光石**, 山隈瑞樹**，児玉勉***，鈴木輝夫****， 最上智史****Static Eliminating Properties of Incendiary Discharge-Free,
Nozzle-Type Electrostatic Neutralizer*
by K.S. CHOI**, M. YAMAGUMA**, T. KODAMA***, T. SUZUKI****, and T. MOGAMI****

Abstract; Most industrial operations with powders, such as transport, filling, and emptying, involve collision andseparation between the particles themselves and between particles and the walls of the installations. Thus,
electrostatic charges are often generated on powdery substances. Such a charging phenomenon may cause
electrostatic discharges (ESD) inside the silo and result in the ignition of flammable dusts. As one method toprotect a dust explosion due to ESD, we have developed a new nozzle-type electrostatic neutralizer with acorona discharge. In this study, it was investigated experimentally whether the developed neutralizer can be usedin an explosive atmosphere. The ignition testing apparatus, which consists of a testing needle electrode and an
ignition vessel (2500 cm3) filled with an ethylene-air mixture (6.5 Vol. %), was used for evaluating theignitability of the high-voltage electrode system. The performance of the neutralizer (flange-type) was also
evaluated with a full-size pneumatic powder transport system. The results obtained from the experiments are as
follows:(1) The needle electrode system with a 100 MΩ-coupling for the developed neutralizer can be used in the
explosive atmosphere with a MIE (minimum ignition energy) of more than 0.1 mJ up to the applied
voltage of AC or DC 7 kV.
(2) The nozzle-type neutralizer can be used for producing air ions to reduce effectively the static charges on
polymer materials in a pneumatic powder conveying system by controlling the applied high-voltage.
Keywords; electrostatic neutralizer, incendiary discharge, ignitability, powder* IFAC Symposium on Power Plant & Power Systems Control, Vol.3, pp.1161-1164 (2003)及び JJAP (Japanese
Journal of Applied Physics, Vol. 44, No. 3, 2005, pp.L.1026-1029, 2005)で一部誌上発表** 物理工学安全研究グループ Physical Engineering Safety Research Group*** フェロー研究員 Fellow Researcher**** 春日電機（株）Kasuga Denki INC− 45 −

産業安全研究所研究報告

1. INTRODUCTION
As powder technologies have been rapidly
progressing, a variety of new flammable powders
have been produced and used in various industrial
processes. Most industrial operations with powders,such as transport, filling, emptying, blending, andmilling, involve friction between the particles
themselves and between particles and the walls of theinstallations. Thus, static charges often occur on solidand powdery substances in powder-handling
production plants.
Especially, when fine powders and dusts aretransferred, loaded, or stored within a specific section
and in cases of powder coating using compressed air
flow, fine powders are strongly tribo-charged by thefriction and collision of pipe-particles and interparticles [1]. Such a charging phenomenon may block
the flow of fine powder and generate electrostatic
discharges (ESD), causing the ignition of flammabledusts.
As recently reported, some fine powders are so
sensitive that even a spark with very low energy canignite them. Technologies for the prevention of such
accidents are essential to avoid discharge sparks.
For this reason, a nozzle-type of high-voltage
electrostatic neutralizer has been previously proposedby Kodama [2]. However, whether a nozzle-type
neutralizer can be used in an explosive atmosphereremains to be seen. Therefore, in the present study,
we evaluated the practical version of the neutralizer
through several tests.

NIIS-RR-2005（2006）

tube 3 mm in diameter for air supply. Compressed air
with a pressure of 0.1 to 0.3 MPa was supplied to thenozzle to protect the needle electrode from thedeposition of powder as well as to blow ionized air
toward the charged powder within the pipe.
It should be noted here that the DC-type neutralizer
in the present paper was used with a positive polarity,
as the charged powder pellets in our pneumatic
powder-transport facility were negatively charged.

Fig. 1. Structure of a nozzle-type
electrostatic neutralizer.

2.2 Experimental method
2.2.1 Test for an incendiary discharge
from the neutralizer

2. EXPERIMENTAL
2.1 General structure of the nozzle-type neutralizer
The high-voltage nozzle-type neutralizer is briefly
reviewed as follows [2]. The actual structure,including the dimensions of the nozzle-type ofneutralizer, is shown in Fig. 1. It consists of a needle
electrode (diameter: 2 mm) situated within agrounded shield (stainless steel; length, 32 mm; innerdiameter, 2.1 mm; opening diameter of the nozzle 3mm) for a corona discharge, a high-voltage powersource (AC or DC), electrode support, and a slender

The testing method for the ignition voltage andcurrent of the neutralizer is described as follows.
Figure 2 shows the overall structure of the ignition
testing apparatus, which consists of a testing needle
electrode and an ignition vessel (2500 cm3) [3]. The
needle electrode is connected to a power source via a
resistor to reduce damage to the minimum even if theneedle is accidentally grounded. A needle with acoupling resistor was selected for use as one of theneedles of the test neutralizer.
Inside the ignition vessel, an artificial spark was
generated between the needle electrode and a− 46 −

着火性放電を抑制したノズル型除電器の除電特性

grounded rotating plate electrode (20 mm × 33 mm)
when the rotating plate electrode approached theneedle electrode. The space between the center of theneedle tip and the metal plate was 0.5 mm.
The typical ignition testing procedures aredescribed as follows:(a) The ignition chamber was filled with an
inflammable gas-air mixture.
(b) The level of voltage of the power source wasset at the initial level.
(c) Generating spark by rotating the grounded
plate electrode with the use of a motor(60rpm).
(d) If no ignition occurred, the spark was repeatedevery second up to 100 times.
(e) The level of voltage was increased, if
necessary (no ignition within 100 times), andtesting continued until the least sparking
voltage level was obtained.
It is noteworthy that, rather than fine powders, theinflammable gas used in the experiment was ethylene(6.5 Vol. %), which is so sensitive that even a sparkwith very low energy, such as 0.1 mJ, can ignite it[4]. The test conditions were 20 ± 3 ℃ and 35 ± 5% RH.
In general, an incendiary discharge from aneutralizer depends upon the resistance, capacitance,
and inductance of the power circuit of the neutralizer.
However, the effect of the inductance can be ignored
since the circuit line is very short. Accordingly, inthis paper, the effect of the power circuit on the
incendiary discharge from the neutralizer wasinvestigated using the resistance as parameters.

2.2.2 Evaluation of the performance of theneutralizer
A full-size pneumatic powder transporting facility,
as shown in Fig. 3, was used to evaluate theperformance of the nozzle-type neutralizer. The fullsize pneumatic powder transport facility consisted ofa cylindrical silo (stainless steel; diameter, 1.5 m;
straight body length, 2 m, and capacity 4.8 m3), a
pipeline (stainless steel; diameter, 0.1 m; total length,
20 m), an air blower (10 m 3 /min) driven by aninverter motor, and an air conditioning unit
controlling the temperature (30 ℃) and humidity (30% RH) of the blowing air.
The silo provided a rotary valve driven by aninverter motor to release powder (pellet flow rate,
about 32 kg/min) from its bottom. For observing theelectrostatic discharge inside the silo, a camera or avideo camera with an image intensifier (Hamamatsu
Photonics Ltd., Night Viewer C3100, Gain 60,000)
was set on the window glass on the silo roof.
As powder, polypropylene (PP) pellets with mean
particle sizes of 3 mm were used in this experiment.
The test conditions were 26 ± 2 ℃ and 35 ± 5 %RH.

Fig. 3. Pneumatic powder transportation facility
used to evaluate the performance of theneutralizer.

3. RESULTS AND DISCUSSION
Fig. 2 Schematic diagram of the ignition testing
apparatus used to evaluate the incendiarydischarge occurring from the neutralizer.

3.1 Incendiary discharge occurring fromneutralizer− 47 −

産業安全研究所研究報告

The minimum ignition voltage (MIV) of the test gas(ethylene) was determined by repeated ignition tests
with high-voltage AC (50Hz) and DC (in a positive
polarity). It should be noted here that the lower the
MIV, the more easily the gas can be ignited. The
possibility of the suppression of ignition by the circuit
impedance with a resistor was examined. Resistors
from 10 MΩ to 100 MΩ were used in this study.
As a result, when the AC power source was used, theMIV showed a slight decrease with increasing thecoupling resistor; 10.8 kV at 50 MΩ, 10.6 kV at 60
MΩ, and 10.5 kV at 100 MΩ.
On the other hand, the MIV increased with anincrease in the coupling resistor with the DC powersource: 7.5 kV at 20 MΩ, 17 kV at 50 MΩ, 30 kV at
75 MΩ, and 37 kV at 100 MΩ.
In additions, concerning the characteristics of theion current, it is well known that a neutralizer with acoupling resistor is superior to a neutralizer with acoupling capacitor [5].
These facts suggest that a resistive-coupledelectrode safely produces ion pairs for reducing static
charges and that incendiary discharges are suppressedby the resistor and voltage supplied to an AC-type(50HZ) and DC-type (positive polarity) electrode of a
maximum of 7 kV.

NIIS-RR-2005（2006）

3.2 Performance of the nozzle-type
neutralizer
Taking the experimental results reported above into
account, a 100 MΩ resistor was connected to an
ionizing electrode, and high voltage was applied to
the resistor to generate a corona discharge andevaluate the performance of the nozzle-type
neutralizer.
For that purpose, first, the ion current, Ic, comingfrom the neutralizer was measured using a metal plate
impressed with a high voltage, Vp (see Fig. 4). The
high voltage applied, Va, to the needle electrode andthe corona current, Ic, were controlled and recorded
automatically by a computer through a GP-IB
interface. The space d between the nozzle opening of
the neutralizer and the metal plate (0.1 m × 0.1m)
was 0.05 m.
The I c was measured with an electrometer
connected between a metal plate impressed with Vp
and the ground. The Ic as a function of the supply
voltage of the neutralizer and the supply air pressure,Pa, are shown in Figs. 5 and 6, respectively. The
experimental value represents an average of ten
measurements performed under the same conditions;the scatter limits for the ten measurements are givenin each figure.

Fig. 5 Relationship between Ic and Va.

Fig. 4 Schematic drawing of ion current measurements for the nozzle type neutralizer.

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着火性放電を抑制したノズル型除電器の除電特性

Fig. 6 Relationship between Ic and supply airpressure Pa.

When a high-voltage DC (in a positive polarity) is
applied to the needle electrodes for corona discharges,
the neutralizer produces a larger I c than theneutralizer to which a high-voltage AC (50Hz) has
been applied.
The Ic increased clearly with an increase in both the
Va and the Pa for both types of neutralizers, DC (in apositive polarity) and AC. The Pa (or airflow) has
been reported to have an effect on the suppressiongas, such as O 3 and NO X , generated from thedischarge area [6]. To maintain the corona discharge,
seed electrons for discharge must be continuously
produced. Any suppression gas that remains in the
discharge area disturbs the maintenance of the corona
discharge, resulting in a decrease of Ic at the same Va.
The change in Ic as a function of time when a highDC voltage of 5 kV was applied to the needle
electrode was discussed. As a result, the Ic exhibited a
rapid drop in 60 s of elapsed time in the case withouta Pa. However, when a Pa of 0.3 MPa was provided
to the corona electrode, the Ic changed very little overtime.
These facts agree with those reported in the
hypothesis, namely, that the effect of the Pa is todrive out any discharge suppression gas from thedischarge area near the electrode.
Secondly, the performance of the nozzle-type
neutralizer was evaluated with the full-size pneumatic
powder transport system. The neutralizer, provided
with sixteen ionizing needle electrodes (flange-type),

was attached to the end of the inlet pipe of the silo.
The pellets were negatively charged because thepipeline was made of stainless steel, which has atendency to be positively charged. The electric fieldstrength in the silo was measured with an electrostatic
air-blow field sensor (KSF-0201, Kasuga Denki
INC), which was installed on the side wall of the silo
at a distance of 40 mm from the heap surface. The
distance for testing was always constant. The fieldsensor, which is equipped with 100kPa air blower toprevent dusts from coming into the detector, wasoriginally developed to measure the field due to acharged dust cloud in a silo [7].
The switch of the neutralizer was turned on (DC 3
kV in a positive polarity), and the change in theelectric field strength in the silo was observed. The
results are given in Fig. 7. The electric field strength
above the pellet heap decreased significantly by theoperation of the neutralizer. And then, the fieldstrength increased to near the original value when theneutralizer was turned off.

Fig. 7 Field strength in the silo as a function of time.

A large number of electrostatic discharges along thepellet heap in the silo were observed using a camera
with an image intensifier set on the window glass ofthe silo roof, as shown in Fig. 8(a).
However, in the case with the neutralizer (DC 3kV),
the electrostatic discharges completely died out in thesilo (Fig. 8(b)). It was confirmed that the nozzle-type
neutralizer is effective for reducing electrostatic
charges from polymer materials.
Figure 9 shows the performance of the AC (50Hz)
and DC-type (in a positive polarity) neutralizer as a

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産業安全研究所研究報告

function of the voltage applied to the neutralizer. The
electrostatic field strength in the silo decreased withan increase in the applied voltage to the AC-type
neutralizer. The AC 7 kV used in our experiment was
not sufficient for the elimination of the charges of thepowder.
On the other hand, the performance of the DC-type
neutralizer was superior to that of the AC-type one.
However, a DC (in a positive polarity) voltage ofabove 5 kV brought about significant reverse
charging.
The application of a grid electrode was found to beeffective to improve the performance of the AC-type
neutralizer [8]. A feedback control-type neutralizer
may also be effective for the prevention of reverse
charging in the DC-type neutralizer. This type will be
described in our next report.

NIIS-RR-2005（2006）

4. CONCLUSIONSThe safe performance of a nozzle-type electrostatic
neutralizer was examined experiment-ally in an
explosive atmosphere. The electrostatic elimination
performance of the nozzle-type neutralizer was also
evaluated with a full-size pneumatic powder transportsystem. The results are summarized as follows:(1) The resistivity of a 100 MΩ-coupled electrodeproduced ion pairs safely for reducing static
charges, and incendiary discharges were
suppressed by the resistor and voltage supplied
to the electrode.
(2) The nozzle-type electrostatic neutralizer
performed well in general, but there were also
several problems, i.e., the nozzle-type neutralizer
with AC-applied voltage had insufficientelimination, while the DC-applied voltage
brought about reverse charging unless theapplied voltage was properly controlled.

REFERENCES
Fig. 8 Photos of the electrostatic
discharge inside the silo.

Fig. 9 Effect of AC- and DC-type neutralizers as afunction of the voltage applied to theneutralizer.

[1] K.S. Choi, M. Yamaguma, T. Kodama, J.H.
Joung, D.S. Shin, N. Iwai, T. Kashiwazaki, and
M. Takeuchi: KIEE Int. Trans. on EA, 11C(2001),1.
[2] T. Kodama, T. Suzuki, K. Nishimura, S. Yagi,
and S. Watano: Proc. 2000 IEEE/IAS Annual
Meeting (2000), p.652.
[3] Research institute of industrial safety, Japan(1991): Tech. recommendation of the researchInst. of industrial safety (RIIS-TR-91-1), p.11.
[4] Y. Tabata, T. Kodama, S. Kamachi, T. Suzuki, S.
Yagi, N. Nomura, and F. Yamada: The lecture
paper presented of IEJ(1991), 161[5] T. Suzuki, S. Yagi, K. Kamihashi, and Y.
Tabata: Proc. Inst. of Electrostatics, Japan(1989), 245.
[6] K.S. Choi, T. Fujiki, and Y. Murata: Japanese J.
Applied Physics, 43 (2004) 11A, 7693.
[7] Y. Tabata and T. Kodama: Research Reports of
the Research Institute of Industrial Safety, RIISRR-93 (1994), 143.
[8] T. Kodama, T. Suzuki, K. Nishimura, S. Yagi,
and S. Watano: J. Electrostatics, 55 (2002), 289.
（平成18年1月23日受理）− 50 −

屋根工事における軒先からの墜落防止に関する研究
日野泰道
本報は，低層住宅工事における屋根からの墜落災
害に焦点を絞り，災害発生状況とその防止対策について検討を行ったものである。検討の結果，①足場
先行工法のガイドラインの基準を満たす足場が設置された現場でも，墜落死亡災害が発生している場合があることが分かった。また実験的検討により，②墜落防護を完全に行うためには，限られた本数の手摺・中桟を使用する方法では難しいことを明らかにした。これらの結果を踏まえ，墜落防護方法としてネットを用いた方法を取り上げ，その適切な設置方法について実験的に検討を行った。その結果，③ネットに衝突後の跳ね返りにより，被災者が頭部を屋根面に強打することで傷害の発生可能性があることが明らかとなった。そのため，④屋根からの墜落防止用として安全ネットを設置する場合には，屋根面への跳ね返りを防止するための措置が必要であることが明らかとなった。 （図14，表3，参考文献16）

RFIDを用いた広大空間における再起動防止に関する研究深谷 潔
危険空間が広大になると，操作装置の位置から全体を見通すことができず，作業者がその空間内にいても分からず誤って機械を起動し，事故となることがある。このような事故を防止するため，RFIDを用いて危険空間内部の作業者の位置を常時モニターするシステムを開発し，評価した。
格子状に設置したタグを作業者が所持するリーダ
ーでその位置を読み取るシステムであるが，周りの物の配置により電波の受信距離が異なり遠くのタグを受信することもあり，位置計測の誤差となることがあった。実用化のためには，装置を小型化すると共に誤差を考慮したシステム設計が必要である。
（図5，表2，写真3，文献7）

着火性放電を抑制したノズル型除電器の除電特性
崔光石，山隈瑞樹，児玉勉，鈴木輝夫，最上智史
粉体輸送プロセスにおける静電気トラブルの発生を防止する方法として，粉体の帯電量を静電気トラ
ブル発生レベル以下に制御できるノズル型除電器（コロナ式）を開発した。しかし，コロナ式除電器
は，高電圧を利用しているために何等かの原因で異常作動すると，まれに着火性放電を起こし，爆発，火災を誘発する危険性がある。そこで著者らは開発したノズル型除電器の安全性を着火試験によって評価した。その結果，放電針と高電圧源の間に100M
Ωの高抵抗（結合抵抗）を有する放電電極は，印加電圧が7kV以下（交流又は直流）であれば，放電針からの放電火花が最小着火エネルギー0.1mJ以上の可燃性雰囲気の着火源にならないことが判明した。
また，このノズル型除電器を内蔵したフランジ型除電器を実規模大の粉体空気輸送帯電実験装置のサイロ内に設置して，実際に粉体を除電してその効果を調べた。その結果，上記の電圧範囲内で印加電圧を制御することによって粉体の帯電を効果的に除電できることが明らかになった。したがって， 本除電器
は，改良を加えることにより可燃性雰囲気を伴う粉体プロセスにおける静電気障災害の防止に十分寄与できると考えられる。
（図9，参考文献8）

高強度アルミニウム合金重ね継手の疲労き裂モニタ
リングとその疲労破壊特性
佐々木哲也，本田 尚
高強度アルミニウム合金重ね継手の疲労破壊を防止するために，ボルト内に埋め込んだひずみゲージの出力で疲労き裂をモニタリングすることを試みた。
実験の結果，ひずみゲージ出力の平均値よりも変動幅の方が疲労き裂進展に伴う変化の割合が大きく，
疲労き裂モニタリングに適していることが明らかになった。なお，ボルト穴に初期切欠きのない重ね継
手の疲労破壊位置は，大きく3箇所に分類できたが，いずれの場合も疲労寿命に大きな差はなかった。
（図6，写真1，表2，参考文献12）− 76 −