Note: Descriptions are shown in the official language in which they were submitted.
3~
- l - 9D-RG 11667
The present inyention xelates to p~otection
against leakage currents to ground~ par~icularly in an '!
electric range employing sheathed electrical resistance
heating elements.
In electrical load devices supplied through
conductors from a power source there is a particular
failure mode known as a ground faul-t where current flows
between one of the "hot" conductors and ground. To
alleviate the problems posed by such ground faults, various
lO devices known a5 ground fault interrupters (GFI's) have
been developed and are commercially a~ailable. A GFI
senses any minute leaXage current flowing within a load
device from a line "hot" conductor to ground. When such
leakage current is sensed, current to the load device is
15 immediately interrupted, thereby avoid`ing a shock hazardO
A power relay typically does the actual interrupting. The
specific way in which a ground fault condition is usually
sensed is by employing a differential current transformer
to detect a current imbalance in the power input lines.
20 If the current flowing into the load does not exactly
equal the current flowing out of the load, then it is
presumed that some of the current is diverting to ground.
Examples of such ground fault interrupters are disclosed
in U.S. Patent No. 3,633,070 - dated January 4, 1972 -
25 ~assos et al and U.SO Patent No. 3,899,717 - dated
August 12, 1975 - Legatti et al. The ground fault
interrupters disclosed in both of these patents also
include time delay means.
~.. ' ~ .
3~
9D-RG-11667
-- 2
It will be appreciated that most power sources
have some form of overcurrent protection. This
overcurrent protection may be provided by a ~use or
circuit breaker which serves to interrupt the power
source when a predetermined current threshold is exceeded.
In order to prevent false or nuisance tripping as a result
of momentary overloads, many fuses and eireuit breakers
have a time delay, with the amount of time delay before
actual circuit interruption usually inversely related to
the amount of o~ercurrent. Accordin~ly, an overcurrent
protection means associated with a power source must be
capable o~ reliably interrupting relatively lar~e fault
currents, substantially in excess of the predetermined
current threshold. It is with such an overcurrent-
protected power source that the present invention isintended to operate.
More particularly, the present invention is for
use with load devices which are supplied from such an
overcurrent-protected power source and which are subject
to two general elasses of failure. The first elass of
failure may conveniently be generally described as
encompassing relatiYely low current ground fault failures
more partieularly eharaeterized by exeessiv~ eurrent flow
bet~een at least one of the eonductors and ground, with
eircuit remaining at or below the predetermined current
threshold of -the power source overcurrent proteetion
means. The second class of failure may eonveniently be
g nerally described a~ encompassing relatively high current
failures, whieh includes high-current ground fault failures,
more particularly characterized by current through at
least one of the conductors being above the predetermined
eurrent threshold of the power source overcurrent
proteetion means. Eaeh of these classes of failure is
described in greater detail below in the particular
eontext of an eleetric range.
Specifically, a typical ~eating unit in an
~L~ 3~3~
9~-RG-11667
-- 3 --
electric range is a sheathed electrical resis-tance heating
unit comprising a heatiny element in the form of a
spiralled electrical resistance ~ire encased ~n an
elongated ceramic-filled metal outer sheath which is
electrically conductive and connected to the frame of the
range. The ceramic material, typically magnesium oxide,
transmits heat, but is an electrical insulator. Thus, the
outer sheath becomes hot, but normally remains electrically
insulated from the heating element.
One particular ~ailure mode possible in such a
sheathed electrical resistance heating unit is associated
with a breakdown in the insula-tion qualities of the
magnesium o~ide, permitting current to flow between the
heating element and the outer sheath. Typically, such a
failure begins gradually, drawing relatively little current
initially. This initial failure stage may be termed an
incipient ~round fault. However, if not immediately
interrupted, a destructive high current arc may ensue.
It should be menti~ned that, particularly in the
context of a sheathed electrical resistance heating unit,
it is possible to in effect directly sense leakage current
to ground and interrupt the sclme ~ithout employing a
differential cu~rent transformer. Such an approach is
implemented in the arrangement disclosed in commonly-
2s assigned U.S. Patent No. 4,044,224, - dated ~uyust 23,
1977 - Jenkins et al, whexein the direct electrical
connection between the outer conductive sheath and ground
of a dishwasher heating unit is interrupted by a fusible
link. Durin~ normal operation of the heating unit when
the ceramic insulation ~aterial is intact, substantially
no current flo~s through the ou-ter sheath ground
connection (with the exception of a small amount of AC
leakage current largely as a result of capacitive and
moisture effects). However, when a fault occurs,
significant current 10ws through the outer sheath ground
connection, causing the fusible link to open. Additionally,
.
~ 3~
9D-RG-11667
in the Jenkins and Herbs-t arrangement, a switch is
mechanieally arran~ed to open when the fusible link is
broken, and this switeh cuts off power to the heating
element.
Aeeordingly, it would appear desirable to
employ a ground fault interruptex t~pe deviee in eombination
with a range to sense a ground fault type failure or an
ineipient ground fault type failure of the sheathed
electrical resistance heating unit. In many cases sueh
a failure, and particularly an ineipient failure, is
aceompanied by only a slight increase ln the current drawn
rom the supply, and hence is a failure of the first
elass as deseribed above. In many instances, by
diseonneeting the power, the range is safe to operate
again beeause the failed element remains inoperati~e but
presents no safety hazard. Should a ground fault persist
in the failed element, the eireu;t opens again.
A deviee such as a range is also subjeet to the
second class of failure mentioned above characterized by a
mueh higher current than a typieal ineipient ground fault.
Such a failure of the second class may be a major areing
ground fault, a short eircuit between a conductor anywhere
in the ran~e and ~round where insulation may have
deteriorated, or other type of short circuit. Since a
household electric range is a relatively high current
device, typically operating from a 40 or 50 ampere cireuit,
at 240 volts, substantial fault current may oeeur. In
typical household range circuit protected by a 50 ampere
circuit breaker, a momentary fault current of 3,000
amperes may sometimes occur. As a result, in the absenee
of the present invention, the circuit interrupting portion
of any ground fault protection deviee for use in
combination with an eleetrie range would require eireuit
interrupting contacts suffieient to interrupt the maximum
fault current which might conceivably occur. This is
because any major high-current fault may in faet be, or be
~L~, 3~25
9D RG-11667
-- 5
accompanied by, a ground fault, which would trig~er the
gxound fault interrupter device~ Therefore, to merely
include a ground fault interrupter in a range, without the
present invention, would lead to a requirement to provide
relatively heavy contacts on any device employed as the
current-interrupting element of the ~round fault interrup-
ter, with an attendant relati~ely h~gh cost.
Acco~dingly, it is an object of the invention to
proyide an electrical load device such as a range with a
ground fault type o~ protective system which does not
re~uire excessively heavy current interrupting contacts.
It is another object of the invention to provide
a means for minimi~ing the current lnterxupting
requirements of a ground fault interrupter device~
Briefly stated, in accordance with the concept of
the invention, a GFI type device is employed to interrupt
the relatively low current ground faults which occur
durin~ the first class of failure. However, actuation
of the GFI-type deYice is avoided during relati~ely high
current failures o~ the second class, allowing such high
current failures to be interrupted by the overcurrent
interruption means associated with the power source.
Briefly stated, and in accordance with one
particular aspect of the inyention, to ensure that the
GFI~type device does not attempt to interrupt a high current
fault, a time-delay means is included which gives the
overcurrent interruption means of the power source
sufficient time to operate, if in fact the sensed ground
fault is accompanied by hi~h current.
Briefly stated7 and in accordance with another
aspect of the inYention, the electrical load device
comprises a controllable circuit interrupter which
intexrupts at least one of the conductors when activated.
~he contxollable circuit interrupter has a current
intexrupting capab~lity at least as high as the
predetermined current threshold of the power source
~ ~.3f~3`~
9D-RG-11667
-- 6 --
overcurrent interruption means, but a current
interrupting capability less than the maximum current
which may flow during a failure of the second clas~.
Additionally, there is a means for sensing excessive
current flow between at least one of the conductors and
the ground reference point. Lastly, there is a means
connecting the sensing means to the controllable circuit
interrupter for activating the controllable circuit
interrupter in response to excessi~e current ~low between
the conductor and the ground reference point. The
connecting means includes a time-delay for delaying the
activation of the con-trollable circui-t interrupter for an
interval su~ficient to allow the overcurrent in-terrupting
means to interrupt the power source in the event a failure
of the second class occurs.
~ hile the novel features of the in~ention are
set for-th with particularity in the appended claims, the
invention, both as to organization and content, will be
be-tter understood and appreciated, along with other
objects and features thereof, from the follo~ing detailed
description taken in conjunction with the drawings, in
~hich:
~ IGURE 1 is a circuit diagram of a system
including a protected electrical load device according to
the present invention; and,
FIGURE 2 is a detailed electrical schematic
diagram of a control circuit for use in the system of
FIGURE 1.
Re~erring first to FIGURE 1, an electrical load
device generall~ designated 10 comprises an electric range
having representative resistance heating elements 12, 14
and 16, which are the heating elements of representative
sheathed electrical resistance heating units. The heating
elements are adapted to be supplied through conductors
Ll, N and L2 from a standard single phase, three wire
power source 18, shown in phantom lines. The power source
r~
9D--RG-11667
-- 7 --
18 has a neutral terminal 20 connected to the N conductor,
and a pair of "hot" terminals 22 and 24 at opposite
potentials relattve to the neutral terminal 20 connected
to the conductors Ll and L2, respectively.
More particularly, the power source 18 comprises
a center tapped secondary winding 26 of a power
distribution trans~ormer. The secondary winding center
tap 28, in addition to being connected to the neutral
terminal 20, is typically connected to an earth ground 30.
~he c~rcuit protection in the event of a fault in
the electric range 10, the outer terminals 32 and 34 of
the secondary winding 26 are connected throu~h o~ercurrent
interruption means 36 and 38 to the terminals 22 and 24.
~hile the o~ercurrent interruption means 36 and 38 are
shown as thermal circuit breakers, it will ~e
appreciated that various specific means may be employed,
including simple fuses.
In a typical application, the power source 18
suppl~es 240 volts at 50 amperes between the L1 and ~2
terminals 22 and 24. Ll and L2 are balanced with
respect to N, and 120 volts is available between each of
the conductors Ll and L2 and the N conductor.
The overcurrent interruption means 36 and 38
have a predetermined current threshold above which the
power source 18 is interrupted. Typically, circuit
interruption is not instantaneous for moderate overloads.
The higher an overload current, the more rapid is the
circuit interruption.
The electric range 10 is connected to the L1,
N and L2 conductors through a terminal strip 40 mounted
on the metal frame 42 of the range. Normally, the
texminal strip center terminal which is connected to the
N conductor is also connected directly to the range frame
42, as indicated by the representative ground
connections 44. This provides a safety factor in that
the frame 42 is normally held near earth ground potential
.3~
9D-RG-11667
-- 8 --
at all times, preventing electric shock by contact
therewith.
The representative resistance heating elements 12,
14 and 16 are supplied from the -terminal strip 40 through
a set of conductors Ll', N' and L2' t and throu~h another
set of conductor~ Ll", N" and L2". In the particular
connect~on of the heating elements 12, 14 and 16, the
element 16 is operated ~rom 240 volts and therefore
connected directly across the conductors Ll" and L2"~ The
other two heating elements 12 and 14 are lower po~er
heating elements and accordingly are operated from 120
volts by each being connected bet~een one of the
conductors Ll!l and L2" and the N'l conductor. For clarity
o illustration, conventional selector switches for the
heating elements 12, 14 and 16 are not shown.
The range lO is subject to a ~ixst class of
failure characterized by excessive current flow between
at least one of the internal conductors Ll" or L2" and
the ground re$erence point 44, but with the current drawn
20 ~rom the power source 18 within the normal current -.:
supplying capability of the power source 18, or in other
words, at or below the predetermined current threshold
o~ the overcurrent interruption means 36 and 38. In
FIGURE 1, such a failure of the ~irst class is
illustrated by a leakage resistance 46, shown in daeh
lines, bet~een an intermediate portion of the resistance
heatin~ element 16 and the range from 42 (which is
connected to the ground reference point 441. Such a
~ailure may be eithex an incipient breakdown of the
ceramic insulation (not shown) bet~een the heating element
16 and the grounded conductive outer sheath (not shown),
or a more substantial breakdown of the insulation near a
mid-point o~ the heating element 16 such that suf~icient
heating element resistance remains in the circuit to
limit the current belo~ the predetermined current
threshold.
~3~
9D-R(~ 667
g
The range 10 is also subject to a second class of
failure characterized by a relatively high current.
Specifically, during a ~ailure of the second class the
current through at least one of the conductors is in
excess of the predetermined current threshold of the
overcurrent interruption means 36 and 38. Under such
circumstances, at least one of the overcurrent
interruption means 36 and ~8 interrupts the power source
18, removiny power from the range 10. Such a failure of
the second class may ~e a short circuit anywhere within
the range 10. One possibility is a breakdown of a heating
unit near one of the ends thereo~ such that insu~ficient
heating element resistance re~ains in the circuit to limit
the current. It will be appreciated that failures of the
second class as employed herein include both ground fault
and non-ground fault failures. As previously mentioned in
the "Background of the Invention", instantaneous current
may be as high as 3,000 amperes (for a direct short
across Ll" and ~2") before the overcurrent interruption
means 36 and 38 have time to react.
Lastly, the range 10 includes a general means
47 for interrupting at least one of the conductors Ll, Ll',
Ll", L2, L21 or L2" in response to excessive current
flow between the conductors Ll" or L2" and the ground
reference point 44. In accordance with the invention,
the means 47 for interrupting includes means for
preventin~ conductor interruption by the means 47 in the
event a failure of the second class occurs.
More paxticularl~, the means 47 for interrupting,
and therefore the range 10, includes a pair of
controllable circuit interrupters 48 and 50 which are
interposed between the conductors Ll' and Ll" and the
conductors L2' and L2", respectively. These conductors
are interrupted when the controllable circuit interrupters
48 and 50 are activated. The current interrupting
capability of the controllable circuit interrupters 48
3~
9D-RG~11667
- 10 --
and 50 are activated. The current interrUp~ing
capability of the controllable circuit interrupters 48
and 5n is at least as high as the predetermined current
threshold of the ovePcurrent interruption means 36 and
38 of the power source 18, but less than the maximum
current which conceivably may flow du.ring a failure
of the second class. Typically, the current interrupting
capability of the circuit interrupters 48 and 50 is
reasonably in excess of the predetermined current threshold,
but well below the maxi:mum current which may flow.
AccOrdingly, the controllable circuit interrupters 48 and
50 may comprise relatively smaller contacts 9 and therefore
be less expensive, than would otherwise be the case.
The means 47 for interrupting, and therefore the
range 10, additionally includes a means 52 for sensing
excessive current flow between either of the conductors
Ll" or L2" and the ground reference point 44. This
sensing means 52 comprises a conventional differential
current transformer 53 having an apertured core 54 and a
secondary winding 56. In conventional fashion, the
conductors Ll', N' and L2' pass from appropriate .
terminals on the terminal strip 40 through the core
aperture.
Additionally, there is a means 58 for connecting
the sensing means 52 to the controllable circuit
interrupters 48 and 50 for activating the controllable
circuit interrupters 48 and 50 in response to excessi~e
current flow between either one of the conductors Ll" and
L2" and the ground reference point 44O In accordance with
a particular aspect of the invention, the connecting means
includes time-delay means for delayi:ng the activation of
the controllable circuit interrupters 48 and 50 for an
interval sufficient to allo~ the power source overcurrent
interrupting means 36 and 38 to interrupt the po~er source
18 in the event a failure of the second class occurs.
~ore particularly, this connecting means 58
~ 3~
9D-RG-11667
-- 11 --
co~prises a control cixcuit 60 which has an input
connected to the differential current -transformer
secondary winding 56, and an ou-tput connected to a heater
62 which, together wi-th the cir-uit interrupters 48 and 50,
comprises a hot wire relay 64. A suitable hot wire relay
64 is described in the above-mentioned commonly assigned
U.S. Patent Wo. 4,054,857 ~ dated October 18, 1977 -
Bowling. It will be appreciated that the thermal means of
the heater 62 may advantageously be utilized to pro~ide
the necessary tlme delay. ~owever, the delay may also be
prov~ded by the control circuit.
In the absence of any ground fault such as the
leakage resistance 46, the net differential current
carried by the conductors Ll', Nl and L2' passing through
the core 54 is zero. However, in the event of a yround
Eault in which a portion of any of the resistance heating
elements 12, 14 or 16 develops excessive leakage current
to the range ~rame 42, and therefore to the ground terminal
44, an unbalanced condition exists because a leakage
current travels through the frame 42, bypassin~ the
differential current transformer 53. In other words, the
di-f~erential current transformer 53 senses more current
flowing into the load resistances than out.
~hile suitable circuitry for the control circuit
60 is described belo~ ~ith reference to FIGURE 2~ it will
be apprec~ated that it may take many forms. ~hile the
controllable circuit interrupters 48 ~nd 50 are illustrated
as the contacts of the hot wire relay 64, any suitable
electromagnetic relay ha~ing noxmally closed contacts may
be employed. ~dditionally, since the maximum current which
the circuit interXupters 48 and 50 will be called upon to
interrupt is limited by the present in~ention, a solid
state switching means, such as a triact may readily be
employed, rather than relay contacts. In fact, the same
contacts which are cycled to regulate khe heating elements
commonly called the "infinite hea-t control" may be made
to respond additionally to the yround fault. ~ccordingly~
9D-RG-11667
- 12 ~
the connecting means 58, which in the illustrated
embodiment comprises ~he control circuit 60 and the
heater 62, may be any suitable circuitry for interconnect-
in~ the sensing means 52 and the controllable circuit
interrupters 48 and 50. Moreover, the means for sensing
excessive current flow between one of the conductors Ll"
or L2" and the ground reference point 44 need no~ be a
d~fferential current transfo~mer. As illustrated by the
above-mentioned U.S. Patent No~ 4,044,224 - dated August
23, 1977, Jenkins et al, other leakage current sensing
means are possible.
Referring to ~IGURE 2I the exemplary control
ctrcuit 60 will now be described. In FIGV~E 2, the
differential current transformer secondary winding 56
and the hot ~ire relay heater 62 of ~IGURE 1 are shown
in dash lines for clartty.
The circuit 60 includes a power supply section
66 comprising a pair of rect:ifier diodes 68 and 70 having
their cathodes connected together and to a node 72, and
their anodes connected to be supplied by the conductors
Ll" and L2". ~ccordingly, there is supplied to the node
72 full ~ve rectified AC power. Fo~ operation from a
240 volt AC grounded neutral source, the peak voltage at
the node 72 is approximately 160 volts referenced to the
N" conductor, ~h,ich serves as a common re~erence point in
the circuit 60. For protection against power line
transients, a suitable metal o~ide Varistor (MO~) 73 is
connected bet~een the node 72 and the N" conductor.
In order to supply a lower rectified DC voltage
to a supply conductor 74l a Yoltage dividex comprising
resistors 75 and 76 is supplied from the node 72 and has
its midpoint output connected to the anode of an
isolation diode 78, which ;n turn has its cathode
connected to one texminal of a filter capacitor 80 and to
a voltage dropping resistox 82. The other end of the
resistor 82 is connected to the supply conductor 74
9D-RG-11667
- 13 -
and to the cathode of a vol-tage regul~ting Zener diode 84.
The anode of the Zener diode 84, the other terminal of the
capacitor 80, and the lower terminal of the voltage
divider resistor 76 are all connected to the N" circuit
reference po~nt.
Considering now the functional part of the
control circuit 60, an AC amplifier 88 having an
approximate voltage galn of sixty comprises an operational
amplifier 90 with an input network 92 connected to the
terminals of the differential current transformer secondary
winding 56. To establish the voltage gain of sixty, a
negative feedback resistor 94 is connected between the
output terminal and the inverting (-~ input of the
operational amplifier 90, and input resistors 96 and 98
are connected to the inverting (-) and non-inverting (-
~
i.nputs. The other ends of the input resistors 96 and 98
are connected through conductoxs 100 and 102 to the
terminals of the differential current transformer
secondary winding 56. To complete the input network 92,
oppositely-polarized voltage limiting diodes 104 and 106
are connected across the conductors 100 and 102, along
with a biasing resistor 108, and a noise suppxession
capacitor 110. For proper biasing of the amplifier 90,
another resistor 111 is connected between the operational
amplifier non-invertiny (~) input and the N" reference
conductorO
The output of the operational amplifier 90 is
connected through a signal rectifier diode 112 and a
current limiting resistox 114 to a voltage comparator
generally designated 1160 Additionally, a biasing
resistor 118 connects the junction of the signal rectifier
diode 11~ and the resi:stor 114 to the N" circuit reference
point, and a filter capacitor 119 is connected between
the right-hand end of the resistor 114 and the circuit
reference point. In operation, a DC voltage appears at
th.e ~unction o~ the resistor 114 and the capacitor 119,
9D-R~-11667
- 14 -
with the magnitude of this voltage depending upon the
degree of current imbalance as sensed by the differential
current transformer 53.
The voltage c.omparator 116 comprtses another
operational ampli~ier 120 of which ~he non-inyertiny (t~
input is the sense input and accordingl~ is connected to
the junction of the resistor 114 and the capaci-tor 119.
The invertin~ input i5 the reference input and
accordingly is connected to the midpoint 121 of a voltage
divider compxising resistors 122 and 123 connected between
the DC supply conductor 74 and the N" circuit reference
point.
In the particular application o~ a range, due to
capacitance effects, there is a fairly high AC leakage
current, in the order o~ 200 Milliamperes, which exists
during normal operation. Accordingly, the comparator
threshold established by the voltage divider resistors
122 and 123 is selected in v~ew of the characteristics of
the differential current transformer 53 and the gain of
the amplifier 88 such that the interrupting means 47 is
not actiyated for leakage currents less than 200
milliamperes.
To complete the circuit 60, the output of the
operation~l amplifier 120 is connected through a current
limiting resistor 124 to the gate terminal 126 of a
silicon controlled recti~ier ~SCR) 128. The cathode of
the SC~ 128 is connected to the N" circuit reference
terminal, and a biasing resistor 130 is connected between
the SCR gate 126 and anode terminals.
In this particula~ circuit, the operatlonal
amplifiers 90 and 120 are included within a single
integrated circuit package, and accordingly only one set
o~ DC power supply connections is shown~ Speciftcally,
the operational ampli$ier ground connection is made
through a conductor 131 to the N" circuit reference point
and the positive DC supply connection is made through a
9D-RG-11667
- 15 -
concluctor 132 to the supply conductor 74.
To provide an ou-tput from the circuit 60 to the
hot wire relay heater 62, the heater 62 is connected in
series wi-th a resistor 134 between the anode o~ the SCR
128 and the supply node 72. Accordingly, whenever the SCR
128 is gated into conduction, the heater 62 is energized,
and after a thermal time delay -the circuit interrupters
48 and 50 (FIGURE 1) to interrupt the conductors Ll" and
L2". It should be noted that the node 72 has unfiltered
full~rave rectified power such that the hot wire heater 62
responds in a series of relaxation steps as described in
the above-mentioned U.S. Patent No~ 4,054,857 - dated
October 18, 1977 - Bowling such that the switch contact
opening is substantially synchronized w~th the zero
current crossing so as to minimize arcing. Diode 78
serves to commutate the SCR 128 by isolating the DC
voltage on capacitor 80 from the SCR.
The following component values have been found
suitable ~or use in the circuit of FIGURE 2. These values
are provided merely by way of example, and are not
intended to limit the scope of the claimed invention.
~ ~1.3~
9D-RG-11667
- 16 -
~ESIS ~ORS
75, 76 10 K Ohm, 1 Watt
82 1 K Oh~
94 330 K Ohm
96, 98 5.1 K Ohm
108 1.8 K Ohm
111 100 K Ohm
114 51 K Ohm
118 1 Meg. Ohm
122, 123 39 K Ohm
124 5.6 K Ohm
130 2 X Ohm
134 24 Ohm, 15 Watt, wirewound
CAPACITORS
~ r
lJ 80 10 mfd., 25 ~olt
110 0.001 mfd.
119 0.22 mfd.
SEMICONDUCTOR DEVICES
68, 70, 78 lN4004 silicon diode
73 G.E. Type No. V150LAlOA MOV
84 8.1 Volt, ~ Watt Zener Diode
90, 120 Included within National
5emiconductor Type No.
LM358 dual op. amp. I.C.
104,106,112 lN4001 silicon diode
128 G.E. Type No. C106Y SCR
In the particular system disclosed herein, the
time-delay for the interval which permits the overcurrent
interruption means 36 and 38 of the po~er source 18 to
interrupt large faults o$ the second class is proYided by
the thermal ~ass o~ the hot wire relay heater 62, and
depends upon a prope~ selection of the ~alue of the current
limiting resistor 134 so as to limit the heater current.
The lower the heater current, the longer will be the time
~d~ ~?~
9D-RG-11667
- 17 -
delay.
However, it will be apparent that many other
time-delay methods are possible, such as a simple RC time
delay. If a conventional fast acting electromagnetic relay
or a triac is employed for the controllable circuit
interrupters 48 and 50, then the convenient time-delay
effected by the combination o~ the hot wire relay and
current limiting resistor 134 could not be used~ In this
situation, another time-delay method would be employed.
Por example, a digital timer may be started upon the
sensing of the ground ~ault while the trigger pulse to open
the contacts is generated a preselected time thereafter.
The p~ec~se time delay required is not critical
and must be selected as a compromise between such factors
as the overcurrent versus trip time characteristics of
the power source interruption means 36 and ~8~ the actual
current-interruption capability of the interrupters 48
and 50, and the degree of protection desired against
incipient ground faults. As an example, a 200 millesecond
time delay after exceeding a 200 milliampere current
threshold has been ~ound suitable.
It will be apparent, therefore, that there has
been pro~ided an effective and low-cost means for
protecting a range or other load de~ice characterized by
drawing potentially heavy fault currents, wherein the
ground fault interrupter device may have relatively low
current interrupting capabili~y.
While a speci~ic embodiment of the invention has
been illustrated and described herein, it is realized
that modi~ications and changes will occur to those s~illed
in the art~ It is therefore to be understood that the
appended claims are intended to cover all such modifications
and the changes as fall within the true spirit and scope
of the ~n~ention.
,,
