Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE: IMPROVED LEAKAGE RESISTANCE
DETECTOR AND ALARM CIRCUIT
BACKGROUND OF THE INVENTION
This invention relates to a detector and alarm
circuit for use with electric apparatus which is connected
to power lines, and where the body of the apparatus is
insulated from ground. More particularly, this invention
relates to a detector and alarm circuit for electrically
powered mobile apparatus such as trolley buses and the like
for determining incipient leakage resistance between the
body of the apparatus and the power lines such that when the
body is insulated from ground and one of the power lines is
substantially at ground potential, a voltage occurs on the
body of the apparatus which might be hazardous to any human
lS who might come into contact with it.
One such leakage resistance detector and alarm
circuit is shown in our earlier United States Patent
3,987,425. According to this patent, the circuit provides a
warning alarm after experiencing a voltage above a
particular level for certain duration and once so sensed, a
second delay circuit is activated and if the voltage remains
for a second duration, an alarm is activated.
In trolley cars, for example, the overhead lines
are such that one is normally at ground potential and the
other is at about 600 volts. Leakage will occur between the
electrical connections of the lines to the trolley car and
the apparatus body will be raised from its normal ground
potential. This potential remains on the body as the body
is isolated from ground. A certain minimum voltage is
normal, however, as this voltage continues to climb, the
danger to passengers leaving or entering the car increases
as they may provide an electrical connection between the
ground and the vehicle body. The danger to the passengers
dramatically increases as the voltage of the apparatus body
exceeds a predetermined minimum, normally about 30 volts.
In the case of a complete failure where the apparatus body
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would be at several hundred volts, the passenger is in
extreme danger.
This prior art leakage resistance detector and
alarm circuit does not take into account the magnitude of
the voltage sensed, and the time duration between the first
occurance of the sensed voltage and the time of initiation
of a warning is not variable in accordance with the
magnitude of the voltage.
SUMMARY OF THE INVENTION
According to the present invention, in a leakage
resistance detector and alarm circuit of the type generally
referred to above, an improvement has been made in that
means are provided for automatically varying the time period
or duration that the sensed voltage has to be present before
initiating the warning alarm, with this variation decreasing
the time period as a function of the magnitude of the sensed
voltage.
According to a preferred aspect of the invention,
this time duration is decreased to approximately zero when a
maximum voltage is sensed.
In the improved leakage resistance detector and
alarm circuit of the present invention, the magnitude of the
sensed voltage is taken into account and the time duration
to actuation of a warning alarm is decreased in accordance
with the magnitude of the sensed voltage. In this way, the
driver of the vehicle will have an immediate warning in the
case of an extreme breakdown and will realize there is a
potential danger to passengers immediately as opposed to
only after the duration of the fixed time period of the
prior art devices.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are shown in
the drawings, wherein:
FIG. l is a general block diagram showing the
relationship of this invention by polar circuit;
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FIG, 2 is a detailed circuit showing various
aspects of the components of the detector and alarm
circuitry; and
FIG. 3 is a circuit layout showing additional
components of the alarm circuit of FIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Apparatus according to this invention is shown
diagrammatically in FIG. 1, and somewhat more specifically
in FIG. 2 with respect to the major components of the
detector and alarm circuits. Thus, there is shown in
FIG. 1, a detector and alarm indicated generally at lû in
series with a resistor 12 which is connected to a pair of
lines 14, 16 by a pair of back-to-back diodes 18, 20
respectively. It will be seen from more detailed discussion
hereafter that resistor 12 is, at any instant in time,
connected only to the most negative one of the pair of
electric power lines 14, 16 when there is a voltage
difference between them.
The detector and alarm circuitry 10, in accordance
with this invention, is further connected in series with the
chassis, body or other portion of the apparatus which might
be contacted by a human who is at ground potential, and the
apparatus body or other portion is shown generally at 22.
It, in turn, is connected in series with the resistor shown
at 24, which is connected to an electric power line 15. It
will be noted in FIGS. 1 and 2 that resistors 12 and 24 are
designated RT and RL respectively; and those resistors
are respectively a known test resistance which is interposed
between the apparatus body 22 by way of the detector and
alarm circuitry 10 to the most negative one of the electric
power lines 14, 16, and the incipient leakage resistance
which may occur in the apparatus between the apparatus body
22 and the most positive of the electric power lines, shown
at 15 in the drawings. A battery 26 is shown being
connected across the detector and alarm circuit 10, and one
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terminal of the battery 26 is also connected to the
apparatus body 22.
In any two wire trolley line installation, one of
the electric power lines is nominally at ground potential or
zero voltage; and the other of the electric power lines is
at the nominal working voltage which is a high voltage
relative to ground. The electric power lines are not,
however, polarized so that one or the other of the lines is
always the OV line or HV line, because of the manner in
which such overhead lines are connected, particularly at
road or line intersections and between separate line
sections, etc. Thus, a pair of back-to-back diodes 18, 20
is provided; they are connected respectively to lines 14 and
16. It will be noted that lines 14 and 16 are designated
HV/OV and OV/HV respectively; that is to say, when line 14
is the HV line, line 16 is the OV line and vice versa.
Thus, the lower end of resistor 12 is connected either
through diode 18 or diode 20 to whichever of lines 14 and 16
is the most negative line, at any instant in time. Because
of the circuit arrangement within the apparatus in which the
detector and alarm circuitry according to this invention is
installed, the leakage resistance 24 is found between the
chassis or body of the apparatus 22 and whichever of lines
14 and 16 is the most positive electric power line -- which,
for purposes of this discussion and ease of illustration, is
shown as line 15 in the drawings.
A resistance network exists between the most
positive and most negative of the electric power lines to
which the electric apparatus in which the detector and alarm
circuitry according to this invention is installed; and that
resistor network includes the leakage resistance from the
apparatus body to the high voltage line which may be lumped
and designated RL, and is shown at resistor 24, and a
known test resistor RT shown as resistor 12.
The apparatus body 22 is, for purposes of this
discussion, considered to be a short circuit -- that is to
say, a zero resistance element in the resistor string; and
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the resistance string connection within the detector and
alarm circuitry 10 from the leakage resistance 24 to the
known test resistance 12 is also a zero resistance
component. Therefore, there is installed between the most
negative electric power line and the most positive electric
power line a known resistance and an unknown incipient
leakage resistance, each of which is also connected to the
body, chassis or other portion of the apparatus which a
human might contact.
It has been noted above that it is desirable that
the apparatus body 22 be permitted to rise in voltage
relative to the most negative electric power line (which is
substantially at ground potential) to only about 30 volts;
and that no condition be permitted where a short circuit
current from the apparatus body to ground and connected
through the leakage resistance 24 to the most positive of
the electric power lines 15, may exceed 2 ma. That is to
say, V~ as shown in FIG. 1 may not exceed 30 volts
relative to the most negative of electric power lines 14,
2û 16; and the leakage current through the resistor string 24,
12 may not exceed 2 ma. Given those conditions, it can be
seen that when the value of the known resistance 12 is l5K
ohms, a current of 2 ma. through it causes a 30 volt drop
across the known resistance. In a conventional trolley
system, when the voltage between the electric power lines is
600 VDC, it will be seen that a total resistance which can
- be accommodated between the electric power lines is 300K,
and any resistance lower than that would cause a current of
greater than 2 ma. to flow in the resistance string. Thus,
because the known resistance 12 is 15K, a leakage resistance
24 of 285K can be tolerated in that circumstance.
When the leakage resistance, in the circumstances
described above where the voltage between the electric power
lines 14, 16 is 600 VDC, exceeds 285K, the current through
resistance string 24, 12 does not exceed 2ma. and the
voltage drop across the known test resistance 12 does not
exceed 30 volts. If, however, the leakage resistance 24
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drops below 285K in the given circumstances, the current
through the resistance string 24, 12 exceeds 2 ma., anû the
voltage drop across the known resistance 12 excee~s 30
volts. A potentially hazardous condition then exists,
because a person who stands at ground potential and solidly
connects to the apparatus body 22 would then be in series
with the leakage resistance 24 to the HV line, and when the
leakage resistance 24 has reduced to the extent that a
current of approximately 2 ma. might flow through the
person's body, the detector and alarm circuitry according to
this invention becomes operative to warn of the incipient
hazardous condition. This might be especially necessary in
any condition when a sudden drop in leakage resistance 24 --
even, perhaps, tending towards a short-circuit from the
apparatus body 22 to the HV line 15 -- would cause a
dramatic rise in voltage VB of the apparatus body 22 with
respect to the most negative of the electric power lines,
i.e. substantially ground potential.
The power for the logic circuits of the detector is
supplied by the 12-24 volt battery 100 in the trolley. lt
comes into the detector through the 150 ~H choke 102 to roll
off high frequency transients. Additional filtering is
provided by the 5.1 ohm resistor lû4, the 33 volt metal
oxide varistor 106, the diode 108 (also providing protection
against incorrect wiring polarity), and the capacitors
2200,uF 110 and O.l~F 112. The voltage is then regulated to
5 volts by the regulator IC 114. The 220 and 680 ohm
resistors 116 and 118 establish the output voltage of the
regulator. Capacitor 120 (O.l,uF) and capacitor 122 (10/35)
provide for instantaneous current demands of the switching
circuits and smooth the output of the regulator.
Since the negative side of the trolley battery 100
is connected to the body 22 of the trolley, and one of the
overhead rails is earth grounded at each sub station, as the
body of the trolley rises above earth ground a current path
is established through the input OP amps 150 and 152, one
high voltage resistor 154 (lMeg), one high voltage
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diode 158, and the associated high voltage lead. Two high
voltage dio~es are provided on each high voltage lea~ to
provide for a failsafe condition should one dioae fail to
open. Two resistor/diode paths are provi~ec, since we must
always find the most negative overhead rail (allowing the
detector to operate regardless of the direction the trolley
runs down the tracks). A high voltage connector 162 is also
supplied which when removed, disconnects the connection
between the two high voltage diodes to allow testing of each
lû diode in~ependently shoul~ the unit be suspecte~ faulty.
OP amp 150 is set up as a linear DC amplifier with
the gain established by the 150K and 200K resistors 1~4 ana
166. Disregarding the 'Hysteresis' feedback loop (on pin
12), the output of this amplifier will swing from 0 to 5
volts as the trolley body swings from 0 to +60 volts
relative to ground (-ve overhead rail). OP amp 152 acts as
a comparator and toggles the output when it detects a
voltage in excess of the preset level on pin 6. The voltage
divider on pin 6 allows the trip threshold level adjustment
to be between 20 and 6û volts. When the threshold is
exceeded the output pin 7 goes positive and 'shmitts' OP amp
150 through the lMeg and lOK ohm resistors on pin 12 thus
preventing an unstable condition about the thresholo level.
The test push button anû remote test circuitry
(FIG. 3) pull OP amp 150 pin 12 positive to force a 'detect'
condition regardless of the voltage on pin 13. For the
remote test circuit, the transistor 2ûO acts as a switch
inverting the polarity of the input allowing the remote test
switch to return to a common negative (-ve battery). The
lOK resistor 202 to positive ensures that the transistor is
normally off. The other lOK resistor 209 on the base
provides base drive to the transistor when the remote switch
is operated. The lO~uF capacitor 206 and the two 27K
resistors 208 and 210 provide a charge and aischarge time
constant delaying the action of the remote button by about
20 milliseconds to roll off contact bounce an~ any voltage
spikes which may be present due to the length of wiring to
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the trolley's console. The diode 212 ensures that the
circuit only recognizes a negative or grouna potenti~l.
OP amp 250 is again set up as a linear DC amplifier
with OP amp 252 as a voltage following buffer. The gain of
250 is set by the 12K and 200K resistors 254 and 256 so that
the output swings between 0 and 3 volts as the trolley bo~y
swings between 0 and +600 volts relative to ground. This
pegs the negative end of the lOO,uF capacitor 258 that
establishes the amount of delay time to warning output. The
diodes 160 on the inputs prevent the OP amps 250 and 252
from latching up if an extremely negative voltage appears on
their inputs. The two O.Ol,uF disc capacitors 260 on the
input provide transient roll off on the signal that feeds
the OP amps.
OP amp 270 is set up as a comparator with
hysteresis to detect AC voltage. A time constant is formed
by the O.l,uF capacitor 272 and l~eg ohm resistor 274 pumped
by the diode 276 to yield a DC voltage on pin 3 that excee~s
the level set on pin 2 when an AC voltage peak in excess of
the detect threshold appears on pin 7 of OP amp 152. This
again 'shmitts' OP amp 270 (through the O.ljuF) and causes
the output pin 1 to go positive. The net result is that an
AC voltage (typically 6û hz) at the input to the detector
who's peak voltage exceeds the trip level will also cause an
alarm. The output of this circuit goes directly to the
~ drive transistor 280 that feeds the 100 ohm current limiting
; resistor 282 and red 'Detect' led 284. This output also
prevents the constant current source from charging the lOO~uF
timing capacitor 258, when an alarm is not present, by
pulling a low on the capacitor through the lK resistor 286
and the diode 288.
The constant current source forme~ by the 'Warning
Alarm Delay' potentiometer 290, the transistor 292 and the
three other resistors 294, 296 and 298 in that area charge
the lOO~uF capacitor 258 with a linear ramp. If the
capacitor's negative end is pegged at 0 volts then, when an
alarm is generated, the maximum time (depending on the
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setting of the current limiter) will result. If the
negative end of the capacitor is not at 0 volts then the
charge time for this capacitor will be reduced accordingly.
This should show the need for a linear charging ramp since a
300 volt fault should take 1/2 the time and a 600 volt fault
should go ostensibly instantly. The 470 ohm resistor 294 on
the current limiting transistor limits the range of the
delay adjustment and the lK and 56K resistors 296 and 29
establish the transistors bias.
OP amp 300 is again set up as a comparator with a
47K resistor 302 and lOK resistor 304 (FIG. 3) on pin 5
setting the threshold. This OP amp senses the voltage on
the lOO~F timing capacitor 258 and the output sits normally
high. This then feeds OP amp 310, which, due to the
positive feedback on pin 3 through the 33K resistor 312,
forms 1/2 of a 'latch'. OP amp 310 pin 3 normally sits at
1.2 volts with the lOK resistor 314 pulling low from the
output of OP amp 320 and the 33K resistor 312 pulling high.
When an alarm is detected, OP amp 310 pin 2 goes below the
threshold on pin 3 forcing pin 1 positive. This releases
the pull down (through the 270 diode 324 and lK resistor
326) previously held on the second current limiter for the
latching alarm delay and allows the second lOO,uF capacitor
330 to charge. The current limiter here operates exactly
the same way as for the 'Warning Alarm Delay'. Amp 32û
forms the second 1/2 of the 'latch'. When the lOO~uF
capacitor reaches the threshold level set on pin 13 of amp
320 (same as that set on amp 320 pin 5), then pin 14 of
amp 320 goes positive and causes OP amp 310 pin 3 to be
; 30 raised from 1.2 volts to the positive supply rail thereby
latching the alarm since a high or a low on amp 310 pin 2
has no effect. The latch reset circuitry 340 is i~entical
to the test and remote test circuitry. This operates by
; first - forcing a positive on pin 13 of OP amp 320 thereby
causing pin 14 to be driven back down low, removing the
positive feedback from OP amp 310 pin 3, and secon~ly -
forcing pin 5 of OP amp 300 positive to make the output
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return high so that ûP amp 310 pin 1 must now return low ana
discharge the lOOJuF through the dioae 324 ana lK resistor
326 to get back to the unlatched condition.
Since the 'latched condition' can only occur a~ter
the 'latching delay time' has expired, an output for
'Warning Alarm' can be taken from pin 1 of OP amp 310 an~ an
output for 'Latching Alarm' can be taken from pin 14 of OP
amp 320. Two identical output drive circuits 360 and 370
are provided on these locations. They consist of a voltage
divider 372 to provide base drive to the transistor 274
who's collector drives the led through a 100 ohm resistor
276 (limiting the current) and a diode 278. The collector
also provides the base drive to the level shifting
transistor 280. When the transistor 274 is turned off, the
collector rises to +24 volts (trolley battery). The ~iode
278 is required to protect the led against reverse
polarity. The transistor 280 output drive transistors are
normally held off by the lOK ohm resistor 282, ana are
turned on by the lK resistor 284. Their collectors are
normally pulled low by resistor 286 to overcome any
emitter/collector leakage that may be present in the
transistor. The collector then supplies current to the
console device through the 27 ohm 5 watt resistor 288 (to
limit the output current) and another diode 290. This diode
protects the transistor from a positive going transient in
excess of the 24 volt battery and allows 'wire OR'
capability of the outputs i.e., two outputs can be tied
directly together without having one that is off, preventing
the other from turning on.
The sensitivity of the detector can be a~justea to
sense a voltage on the trolley body from 2û to 60 volts DC
relative to the grounded overhead rail. The actual voltage
sensitivity required varies from city to city and is
specified by the transit authority. The adjustment is made
with the detector installed in the trolley with the trolley
disconnected from the overhead rails and instea~ connected
to earth ground (i.e., a cold water pipe). To make the
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adjustment, a current limited power supply, set to the
required voltage, must be placed on from trolley body to
ground (+ve to body). The voltage sensitivity potentiometer
is then adjusted to the point where the ~Detect' led just
lights. This completes the sensitivity adjustrnent.
The time delays (again specified by the transit
authority) can be adjusted by creating a 'Detect' condition
with the use of the 'Test' button and timing with a stop
watch the delay that occurs before the 'Warning' and/or
'Latching' leds light.
Although various preferred embodiments of the
present invention have been described herein in detail, it
will be appreciated by those skilled in the art, that
variations may be made thereto without departing from the
spirit of the invention or the scope of the appended claims.
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