Electrical equipment to be protected from overvoltage is connected with a possible source of overvoltage via an input conductor. A fuse is connected in series with the input conductor. A spark gap is connected between the input conductor and ground for conducting the overvoltage current to ground and for blowing the fuse to open the circuit to the electrical equipment. A pulse attenuator network is provided between the spark gap and the electrical equipment to be protected for attenuating the pulse of energy passing through the fuse and spark gap prior to blowing of the fuse. The pulse attenuator network includes additional shunt spark gaps, series inductance, and a series connection of a twisted shielded pair of conductors having low-voltage insulation

Cambra, J. M.

English

NASA Technical Reports Server

REPLY TO
ATTN OF: GP
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON. D.C. 20546
MAR 2 0 1974
TO: KSI/Scientific & Technical Information Division
Attn: Miss Winnie M. Morgan
FROM: GP/Office of Assistant General
Counsel for Patent Matters
SUBJECT: Announcement of NASA-Owned U.S. Patents in STAR
In accordance with the procedures agreed upon by Code GP
and Code KSI, the attached NASA-owned U.S. Patent is being
forwarded for abstracting and announcement in NASA STAR.
The following information is provided:
U.S. Patent No. :
Government or
Corporate Employee
Supplementary Corporate
Source (if applicable)
NASA Patent Case No.
NOTE - If this patent covers an invention made by a corporate
employee of a NASA Contractor, the following is applicable:
YES NO
Pursuant to Section 305(a) of the National Aeronautics and
Space Act, the name of the Administrator of NASA appears on
the first page of—the—pa-tent-;, however, the name of the actual
inventor (author) appears at the heading of column No. 1 of
the Specification, following the wordj^ v^with respect to
an invention of ..."
Bonnie L. Woerner
Enclosure
https://ntrs.nasa.gov/search.jsp?R=19740009816 2020-03-17T14:39:26+00:00Z
U-mted States Patent
Cambra
[ i i ] 3,795,840
[45] Mar. 5, 1974
[54] OVERVOLTAGE PROTECTION NETWORK
[75] Inventor: Joseph M. Cambra, Santa Clara,
Calif.
[73] Assignee: The United States of America as
represented by the Administrator of
the National Aeronautics and Space '
Administration, Washington, D.C.
[22] Filed: Nov. 29, 1972
[21] Appl. No.: 310,624
[52] U.S. Cl 317/16, 317/31
[51 ] Int. Cl H02h 3/20
[58] Field of Search 317/16,31
Primary Examiner—James D. Trammell
Attorney, Agent, or Firm—Darrell G. Brekke; Armand
G. Morin, Sr.; John R. Manning
[57] ABSTRACT
[56]
3,382,409
3,353,066
3,334,271
3,480,832
References Cited
UNITED STATES PATENTS
5/1968
11/1967
8/1967
11/1969
Assowetal ........................... 317/16
DeSouza ........................... 317/16 X
Bacon...: ............................... 317/16
Person .................................. 317/16
Electrical equipment to be protected from overvoltage
is connected with a possible source of overvoltage via
an input conductor. A fuse is connected in series with
the input conductor and a spark gap is connected be-
tween the input conductor and ground for conducting
the overvoltage current to ground and for blowing the
fuse to open the circuit to the electrical equipment. A
pulse attenuator network is provided between the
spark gap and the electrical equipment to be pro-
tected for attenuating the pulse of energy passing
through the fuse and spark gap prior to blowing of the
fuse. The pulse attenuator network includes additional
shunt spark gaps, series inductance, and a series con-
nection of a twisted shielded pair of conductors having
low-voltage insulation.
3 Claims, 3 Drawing Figures
I 100 FT. OF TWISTED
I SHIELDED PAIR
(LOW VOLTAGE
INSULATION)I SPARK| GAP
FIRING
THERMO- ,
COUPLE
(TYPICAL
TRANSDUCER)
X
VOLTS 90V[2pf | .
PATENTED HAR 51974
SHEET 1 Of 2
3,795,840
THERMO- ,
COUPLE
(TYPICAL
TRANSDUCER)
X
^ I /3^20K'^AMp]25a lOOnh i
O ^ A ~ T. ' «*^ _ yiwnr
21
I SPARK
I GAP
[FIRING
I VOLTS 9OV
14 ' (/? I Y5 ^"^
/6J 2kI 25H 'OO^h i| ZO.ll. •»"'/•••
1 ^ 1 /7 J_ /9 ^"^ L
/fl. /5s
I 100FT. OFTWISTED
, SHIELDED PAIR
(LOW VOLTAGE
INSULATION)
lOKfl
Fig.±
PATENTEDMR 5(974
SHEET 2 Of 2
3,795,840
t
H - h
I I
I I
ICT7SEC
TIME
t
2 X IO*6SEC TIME
.• 3,795,840
1 2
OVERVOLTAGE PROTECTION NETWORK electrical equipment to be protected via .an additional
nonlinear conductive device selected to become con-
The invention described herein was made by an em- ductive when a certain threshold voltage appears there-
ployee of the United States Government and may be across, for further shunting leakage pulse energy from
manufactured and used by or for the Government for 5 the electrical equipment to be protected,
governmental purposes without the payment of any Other features and advantages of the present inven-
royalties thereon or therefor. tion will become apparent upon a perusal of the follow-
DESCRIPTION OF THE PRIOR ART ing sPeci5'cati°n 'aken in connection with the accom-
panying drawings wherein:
Heretofore, protective networks for protecting elec- 10
trical equipment from overvoltages have employed BRIEF DESCRIPTION OF THE DRAWINGS
spark gaps and gas filled tubes to shunt overvoltage and FIG. 1 is a schematic circuit diagram, partly in block
excessive currents from the input conductor to ground. diagram form, depicting an electrical circuit incorpo-
In addition, such protective networks have employed a
 rating an overvoltage protection network of the present
series connected fuse for disconnecting or open circuit- 15 jnvention
ing the source of overvoltage or current from the elec-
 FIG 2 'is a waveform diagram depicting the wave-
trical equipment to be protected. Such a protective net-
 form of a ieakage overvoltage pulse at the output of the
work is described in U. S. Pat. No. 3,448,341, and
 fuse in the drcujt of FIG t> and
NASA Tech Brief No. 69-10490, October, 1969.
 F,G 3 is a waveform diagram of ^ ,eakage pulse en.
While such a prior art overvoltage protective net- 20
 efgy M fina,ly Hed tQ ^ jnput Qf the e|ectrica,
work is useful for protecting electrical equipment from equipment to be protected in the circuit of FIG. 1.
sources of overvoltage it is desirable to provide addi-
tional protection. More particularly, such a prior art DESCRIPTION OF THE PREFERRED
overvoltage protective network serves to convert a step EMBODIMENTS
function of overvoltage to a pulse of overvoltage which 25
 Referri nQW to F,G , there js shown an electrica|
can travel into the circuit to be protected. The pulse of
 measuri circuit j incorporating an overvoltage pro-
energy comes about due to the finite time required for ^^
 netwofk 2 of the nt invention ,„ the mea.
the fuse to blow and disconnect the electrical circuit to
 sufi circuj a thermocouple 3 or other suitable trans.
be protected from the source of overcurrent or voltage.
 ducer ^  ,ocated .^ environment for sensi a .
In certain applications where very high voltage and 30
 eter to be measured For , he ^^J ,e 3
power is encountered, such as in a Mach 50 wind tun- , . , . ., .
 f , ,. , .
, ... . ., . . . .. , . may be located in the region of a plasma discharge of
net utilizing a 15-megawatt electrical discharge in a / .. , . ., , fn . . ^ . . ...
, , , j u - u i - j a shock tube in a Mach-50 wind tunnel where i t is ex-
shock tube to produce a high velocity and temperature , ... ,. . . , .
.. • i. I i i posed to a possible source of overvoltage 4, such as thegas stream, the overvoltage leakage pulse energy can T- . , • •
 t u i j- • . • _ • _ if , • i i • j • ifi . -j i. is high-voltage terminal of the plasma discharge by break-be substantial. It is desirable to provide an overvoltage •", , f . . ... ,. j , .
protective network which greatly reduces the leakage down ,of th? *as '" * e VICI"!ty °,f the **Tv 1\*
pulse energy flowing to the electrical equipment to be typ,cal_wind tunnel, the anode voltage is +9 kV and the
protected power in the discharge is approximately 15 megawatts.
The output of a thermocouple 3, which includes a
SUMMARY OF THE PRESENT INVENTION 40 high-voltage conductor 5, a low-voltage conductor 6,
The principal object of the present invention is the and a shield conductor 7, is connected to respective
provision of an improved overvoltage protection net- input terminals 8,9 and 11 of a data recorder 12 via the
worjc intermediary of the over^voltage protection network 2.
In one feature of the present invention, a pulse atten- The overvoltage protection network serves to protect
uator network is provided in the circuit connecting the 45 the data recorder 12 from the possible source of over-
possible source of overvoltage to the electrical equip- voltage as applied to input leads 5, 6 and 7.
ment to be protected, such pulse attenuator means In the overvoltage protection network 2, each con-
being connected between a fuse which is blown and the ductor 5, 6 and 7 includes a series connection of a fuse
equipment to be protected for attenuating the leakage 13> sucn as a fuse rated for Vt A RMS, 20 kV and hav-
pulse energy otherwise flowing to the equipment to be 50 inS 12 ohm DC resistance. The output end of each fuse
protected. ' 13 's connected to ground via a spark gap 14 which has
In another feature of the present invention, the pulse a relatively low firing threshold voltage, as of 90 volts,
attenuator network includes a series inductor and a and which presents a relatively low capacitance, as of
shunt nonlinear conductive device, such as a spark gap, 2 picofarads, in the non-fired state,
gas filled tube, triac, SCR, or the like for shunting over- The spark gap 14 when the overvoltage appearing
voltage pulse energy from the electrical equipment to thereacross exceeds the firing or threshold potential,
be protected. fires and serves to shunt the source of high voltage 4
In another feature of the present invention, the pulse through the fuse 13. If excessive current is drawn by the
attenuator network includes a shielded twisted pair of fuse 13, the fuse blows and serves to disconnect the
low-voltage insulated wires one of which is series con- data recorder 12 from the source of overvoltage 4. In
nected between the fuse of the protective network and a typical example, the spark gap 14 is rated at 90 VDC
the electrical equipment to be protected for introduc- breakdown, SKA surge current, 5 A AC, with greater
ing a distributed nonlinear loss for heavily attenuating than 10'° ohms leakage resistance and less than 2 pf ca-
the pulse overvoltage energy as applied to the electrical
 65 pacity when not fired. In addition, the spark gap 14 is
equipment to be protected. preferably adequately heat sinked by mounting oppo-
In another feature of the present invention, the out- site terminals thereof to a pair of copper busbars, one
put of the pulse attenuator network is shunted from the of which is a copper ground bus 15.
3,795,840
15
Thus, in operation, when the overvoltage is applied,
.it builds up across the spark gap 14 until the firing
threshold is reached, at which time it conducts current
to ground, thereby, blowing the fuse 13 and disconnect-
ing the data recorder 12 from the source of overvoltage
4. This creates an overvoltage leakage pulse at the out-
put of the fuse 13 which is then attenuated by means
of a pulse attenuative portion 16 of the protective net-
work connected between the fuse and the data re-
corder 12.
More particularly, each of the pulse attenuative net-
works 16 includes a-resistor 17, as of 25 ohms, series
connected with each of the respective leads 5, 6 and 7.
A second spark gap 18, which is identical to the first
spark gap 14, is connected between the output of resis-
tor 17 and the ground bus 15 to form with resistor 17
a first nonlinear low-pass filter network. More particu-
larly, as the pulse of overvoltage leakage energy builds
up across the capacitance of the spark gap 18 and
reaches the firing threshold, the spark gap 18 fives and 20
shunts the respective conductor 5, 6 or 7 to ground.
That portion of the overvoltage leakage pulse which
passes the series resistor 17 and shunt spark gap 18 is
further attenuated by means of a series choke 19 hav-
ing an inductance as of 100 microhenries. In a typical 25
example, the choke 19 is an R.F. type, rated for 325
ma, 3.2 ohm, with a O greater than or equal to 42, and
a dielectric breakdown of 1,400 volts RMS. The output
of the inductor 19 is connected to a hundred-foot
length of twisted shielded pair cable 21 having conduc- 30
tors 8 and 9 and shield 11.
The cable 21 is also terminated by connection to
ground via a parallel connection of a resistor 22 and a
third spark gap 23. Spark gap 23 is identical to the pre-
viously described spark gaps 14 and 18 and serves to 35
further attenuate pulse energy passing through the
lossy cable 21 to the data recorder 12.
In a typical example, resistor 22 comprises a one-half
the waveform of FIG. 2. This pulse waveform is heavily
attenuated by the pulse attenuator network 16 to pro-
.' dute a resultant pulse at the input of the data recorder
having a waveform of the type shown in FIG. 3.
5 The advantage of the overvoltage protective network
2 of the present invention is that it provides both per-
sonnel and equipment protection from overvoltage and
current which exceeds the fuse rating or the firing volt-
age rating of the spark gaps 14,18 and 23. Its simplicity
10 results in a large cost reduction over other methods. In
addition, the protection circuit 2 is passive except for
the condition of overload, therefore, it results in no
need for calibration.
What is claimed is:
1. A protection system for an a-c measurement sys-
tem operating in a high-voltage, high-energy environ-
ment comprising:
a transducer capable of generating an a-c signal;
said transducer having at least two conductors;
a ground bus;
each of said conductors having a fuse and a spark gap
connected in series between said conductor and
said ground bus, a high energy-handling, pulse-
attenuative, low-pass filter network having an input
and an output, said low-pass filter network having
a cut-off frequency above the frequency of said a-c
signal, said input of said network being coupled in
shunt with said spark gap;
means for reading out said a-c signal from said trans-
ducer;
said means for reading out said a-c signal being cou-
pled to said outputs of said low-pass filter network;
and
said filter network, in the case of a high-voltage, high-
energy fault, being able to reduce the energy reach-
ing said means for. reading out of a level that can
be safely tolerated.
2. A protection system as set forth in claim 1 wherein
said low-pass filter network includes a resistor havingwatt, 0.1 percent noninductive precision wire wound
resistor of 10 K fl resistance and cable 21 is 100 feet 40 first and second terminals, a second spark gap, said first
long consisting of two No. 24 tinned copper stranded terminal of said resistor connected to said node of said
wires in a 100 percent foil shield, having a nominal ca- fuse and said first spark gap, said second spark gap con-
pacitance of 55 pf per foot. Such a cable 21 has relative nected between said second terminal of said resistor
low voltage insulation and pulse energy is rapidly atten- and said ground bus, an inductance having first and
uated in the cable 21. The resistance of resistor 22 af- 45 second terminals, a third spark gap having first and sec-
fects the measuring characteristics and it may be al- ond terminals, said first terminal of said terminal of said
tered or omitted depending on the impedance of the inductance being connected to said node of said resis-
transducer and the impedance of the readout device. tor and said second spark gap, said first terminal of said
The output of the pulse attenuative network 16isap- third spark gap being connected to said ground bus,
plied to the respective inputs 8, 9 and 11 of the data re- so means for producing a long distributed capacitance
corder 12. with respect to said ground bus, said means being con-
In operation, with an overvoltage plasma discharge nected between said second terminal of said induc-
source 4 of 9.1 kV and a power of 15 megawatts ap- tance and said second terminal of said third spark gap,
plied directly to the input of the protective network 2, and said second terminal of said third spark gap com-
less than 6.0 X 10~8 wattTseconds (0.6 ergs) at a peak 55 prising an output terminal of said low-pass filter net-
voltage of 600 volts appeared at the input of the data work.
recorder 12. The fuse 13, in combination with the first 3. A protection system as set forth in claim 2 wherein
shunting spark gap 14, converted a high energy and said transducer has three conductors and said means
voltage step input function as produced by applying the for producing a long distributed capacitance comprises
aforecited potential to the input of the protective net- 60 a long twisted-pair shielded cable having a low-voltage
work 2 to a low energy overvoltage leakage pulse func- insulation,
tion of less than 200 nanoseconds wide, as shown by * * * * *
65


Overvoltage protection network

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