US2581188A - Oil burner safety control system - Google Patents

Description

Janl, 1952 F. H. HxBBARD 2,581,188

OIL BURNER SAFETY CONTROL SYSTEM INVENTOR 'FRANK H. HIBBARD ATTORN EY Jan. 1, 1952 F. H. HIBBARD on. BURNER SAFETY CONTROL SYSTEM 2 SHEETS-SHEET 2 Filed Aug. 2, 1948 FIGZ.

INVENTOR FRANK H. HIBBARD BY/ M ATTORNEY Patented Jan. 1, 1952 UNITED STATES PATENT oFFlcE OIL BURNER SAFETY CONTROL SYSTEM Frank H. mbbnra, Mountain Lakes, N. J.

Application August 2, 1948, Serial No. 42,023

13 Claims. (Cl. 15S- 28) This invention relates to improvements in safety devices. More particularly, it relates to improvements in safety devices for preventing certain dangerous conditions which may result from abnormal operation of oil burners.

It is well known that it is dangerous to allow unburned fuel oil to accumulate in the fire box of a heating plant. If the accumulated excess fuel is ignited, combustion will start with varying degrees of explosive violence. Hence, a delay in the operation of the ignition means, after oil has flowed, may, and often does, produce serious consequences.

In the most common kind of oil burner safety device of the prior art, the oil pump and the ignition means, usually an electrical spark, are turned on simultaneously. The power circuit for the pump includes a contact which normally opens within a predetermined time after the burner turns on thus limiting the period of deliberate risk-taking. If there is a flame within the predetermined time a stack switch responsive to the heat of the flame closes a. circuit around said contact before it opens. If the oil is not set aflre within said predetermined time the stack switch cannot operate and, therefore, at the end of this time the oil pump stops. In such devices oil is flowing throughout the risk-taking time and the sparking means, usually a high voltage transformer, is operating. For any number of reasons the spark may fail for a time, and then after oil has accumulated, it may appear, precipitating the dangerous conditions above described.

It has been known to reduce this hazard, by reducing the duration of the predetermined riskand difficult to service. Moreover, as described in U. S. Patent 2,416,781, many of the safety devices of this type are subject to an especially dangerous kind of failure. The high ambient tempera.- tures in a boiler room often will aiect the cathode coating in such a way as to cause the photo tube to develop internal leakage paths and the 'high ambient humidity which is prone to occur during periods when the burner is shut down often results in leakage paths around the tube base and socket. Internal and/or external leaks can cause spurious operation even through there is no ame, thus causing the pumping of unburned oil to continue indefinitely.

The safety device described in U. S. Patent 1,834,287, employs the mercury power switch which is operated directly by light from the oil ame without requiring the use of an amplifier. However... the simplication which this makes possible is offset by the fact that this special switch will only operate under carefully controlled conditions and therefore, cannot readily be applied as an attachment to existing oil burners. Moreover, this simplification does not alter the fact that the operation of this device is also taking time, but this is only a negative approach to the elimination of danger due to delayed ignition. If an operator undertakes to restart the burner, he may repeatedly introduce a flow of oil without combustion and thus accumulate a dangerous excess to be ignited when and if the spark does appear. It is a feature of the present invention that a positive means is provided for preventing any ow of oil until or unless a flow of spark current at the ignition electrodes has been established.

Photoelectric cells instead of thermal stack switches have been suggested t0 shorten the risktaking period. However, such devices like those described above also approach the problem negatively. Photoelectric cells have the added disadvantage that they require amplier circuits in order to utilize their small outputs for controlling electromechanical switches. Due to this requirement these safety devices are complex, expensive based on the negative approach of risking the unwanted accumulation of unburned oil for a predetermined period of time (during which the special mercury switch tests for the presence of flame).

From the foregoing it will be evident that two separate conditions of failureor abnormal operation are risked each time an oil burner is started: (l) failure to produce a spark, and (2) failure of combustion even though a spark is present and the fuel pump is operating. It is the object of the present invention to prevent a flow of oil in the flrst condition and to reduce the risk-taking time during which oil flows in the second condition.

With this object in mind I have provided a safety device in ywhicha probe associated with the spark gap is connected to the grid of a gas tube which in turn controls the oil burner. Thus depending upon whether or not a spark is present the oil burner will operate.

A feature of the present invention is that no oil is pumped into the nre box until the oil igniting means is operating properly.

Another feature of the invention is that a safety device for an oil burner of the type whose ignition means comprises a high voltage transformer and a spark gap, does not start pumping of oil until after the presence of a spark in the gap has been detected. The device after startthe secondary winding.

ing the lpump alsotests for the presence of flame and turns off the pump if no flame is detected within a predetermined time, whereby it completely prevents any accumulation of unburned oil is cases of spark failure and limits the accumulation of unburned oil in cases where ignition /spark is not produced at once and a visual indication cf spark failure is provided.

Other objects. features and advantages of the present invention will be apparent to those skilled in the art from the' following detailed description of an embodiment of the invention and from the drawings, in which:

Fig. l is a schematic representation of a circuit embodying the invention; and Y Figs. 2 and 2a represent end and side views of the spark-detector probe.

In Fig. 1 oil burner I comprises an electric motor driving both a pump which draws oil from a tank and forces it under pressure through an atomizing nozzle and a blower which projects a blast of air around the opening of the nozzle, all in a well-known manner. High voltage transformer 2 may also be a standard piece of equipment. It comprises a primary for receiving alternating current energy from a. power line and a secondary for stepping up the line voltage to some suitable value for. producing a suiiicient spark to ignite the atomized oil. Ordinarily, some means is employed for grounding the center' of The means shown in Fig. 1- consists of two condensers of equal capacity connected in series across the secondary and grounded at their point of junction to each other. The two ends of the secondary of transformer 2 are connected respectively to electrodes 3 and I, the ends of which are brought together to'form a spark gap. The exact size of the gap will dependr on the output voltage of transformer '2 and other conditions such as the average humidity and `the strength of the natural draught of air which passes through duct 5 and to which the gap is subjected.

In'most domestic oil burners this gap is of the order of a quarter inch Wide. In practice electrodes 3 and 4 are inserted through an opening in the back end pf air duct 5 and are pushed toward `the front end until they are positioned near the opening of the atomizing nozzle. Since this physical arrangement is well known it is not shown vin detail herein. Furthermore, in order 'to simplify the drawing, transformer 2 and electrodes 3 and 4 are placed in Fig. l in positions which are suitable primarily for laying out the electrical circuit. However, the physical relationship between the blast of vaporized oil delivered by burner I and the gap formed by electrodes 3 and 4 is indicated by a line of arrows used in Fig. 1 to represent the heat circuit. It

' may be grounded is permanently connected over conductor 9 to one of the inputs of the motor for burner I and to one side o`f the primary of transformer 2. Conductor 1 on the other hand will not be extended to deliver energy to these elements of the oil burner system unless heat is required. In the illustrative embodiment shown herein, an on-off manual switch I0 is inserted in series with conductor 1. Switch I0 is placed in its off position when no heat is required at all. and vwhen switch I0 is placed in its on position the heat requirements will be determined automatically by a thermostat represented at II. Thermostat I I may `be of any conventional kind and normally it will be manually adjustable. When the temperature in the room where the thermostat is situated drops below a value predetermined by its setting, it acts to extend a circuit from conductor 'I to elements of the oil burner system, either' doing so directly over its own contacts, which are closed by a suicient drop in room temperature, or indirectly over the contacts of a relay which it energizes over its own contacts. There is nothing novel herein about the arrangement or use of the thermostat.

From thermostat II line 1 is extended over spring-loaded switch I2 which comprises a rest contact and a movable armature normally urged against it. Associated with switch I2 there is a non-conductive lock-out latch I3 which is mechanically linked both to a rotatable arm I4 of stack switch 6 and to armature I5 pivoted at I5' of a motor relay MR. It will be seen below that motor relay MR. becomes energized soon after the 'start of a cycle of operation of the burner and that if the cycle proves to be a normal one it will remain energized over a period of time during which the burner continues to operate until room temperature rises sufficiently to open thermostat II. Each actuation of relay MR in turning on burner I rocks its armature I5 in a direction to move latch I3 into engagement with the armature of switch I2. Whenever, thereafter normal prompt combustion takes place the heat of' the flame will operate stack switch 6 so that its rotatable arm I4 will act through a mechanical link I6 to raise latch I3 to a non-operating position where it cannot continue to engage the armature of switch I2. As a result a false lock-out of electrical power will be prevented when atfthe end yof the normal heating cycle the room temperature has risen sufficiently; thermostat I I has cut olf power to unit I and to relay MR; and relay MR, has released and drawn back latch I3. A false lock-out will be prevented since latch I3 will not be in position to pull back the armature of switch I2 so as to lock out the supply of electrical power to the system. On the other hand, for certain abnormal operational cycles latch I3 is intended to act to lock out electrical power. It will also be seen that after it has been energized for a predetermined time relay MR will be released if the cycle of operation which is under way proves to be not normal in that a flame is not produced within that time. In such a case the stack switch will not have operated and therefore upon the deenergization of relay MR and the return of its armature I5 to its rest position latch I3 will be drawn back yand will pull the armature-of switch I2 away from its rest contact to open the circuit and to lock out the supply of electrical power until latch I3 is manually raised to reset switch I2.

' After its extension over normally closed switch I2, conductor 1 is further extended over a pair of normally closed contacts I1 of stack switch 6 afa-anice' to the otherside of the primary of transformer 2. Therefore, when thermostat I closes its contacts (ordinarily this happens only when the furnace.

blower unit until a spark is produced. Accordingly, the cycle of the oil burner islplaced under the control of this device so that the oil pump and blower unit can only be started after the presence of spark at the spark gap has been determined by test; and when once started, the pump and blower can only operate for a maximum time interval, e. g. ten seconds, measured by this device. If combustion occurs within this measured time interval, the stack switch is actuated by the heat of combustion and the operation of the burner is continued.

The presence of spark is determined by spark testing probe 2| which is mounted in the gap between electrodes 3 and 4 and which is connected to th grid of tube I8 through dropping resistor 22 and voltage divider resistance 21. The

connection between probe 2| and the grid of tube I8 extends through a grounded shield represented at 23. The purpose of shield 23 is to prevent direct pickup from stray fields which may exist in the area through which this conductor must pass. By reason of this shielding the tube is prevented from firing by capacity coupling between other conductors and probe 2|. Tube |8 will be fired when a suitable voltage relative to ground is impressed on probe 2| connected to the grid of the tube. As will be described tube I8 must be fired to operate the relay GTR and relay MR and start the motor pump and blower. I have determined by extensive tests that when probe 2 I .is located approximately equidistant from electrodes 3, 4. the probe will have, impressed upon it, a sufficient voltage to operate tube I8 only if Ispark current is flowing between electrodes 3 and 4. I have found that when there is a spark between the electrodes the voltage impressed on probe 2| may vary from volts to 5000 volts (using a 10,000 Volt secondary transformer). The dropping resistor 22 and a voltage limiter 25 in association with voltage divider resistances 24 and 21, permit the passage of a sufficient current (approximately 1 microamperel to re the tube on the minimum of l5 volts; and to limit the ow of current at 5000 volts to a value low enough to avoid damage to circuit components.

By way of example, I have found that the probe and electrodes may be made from /.m square inch stock whose ends may be cut off square, and that an average gap width, A+B, (Fig. 2) of approximately 1/4" is satisfactory.

The appearance of the higher of voltages on the probe may be explained by considering the probe to be an approximate mid-point of a voltage divider formed by the two portions of the arc or ionized path on either side of the probe. This ionized path is naturally highly unstable in physical and electrical dimensions and the voltage divider formed by two such paths in series may be instantaneously and frequently greatly unbalanced. While the ionized path has too low a resistance to support such a voltage difference after gap breakdown, there is momentarily at the beginning of each cycle, a period of 6 high path resistance during which the 5000 volt instantaneous peak may occur.

By the same variations it is obvious that the voltage divider may at other instants and without predictability, be substantially perfectly balanced, with the result that no impressed voltage with respect to ground,` would appear on the probe 2|. I have determined, however, that whether or not such a condition of complete balance occurs, no condition of zero potential on the probe is found when spark current is flowing through gap 3-4, This may be explained by phenomena known as anode and cathode drop, or the voltage consumed when electrons enter or leave the probe through the boundary layers at the surfaces of the probe metal. The complete circuit from electrode 3 to electrode 4 may be described as composed of the following elements in series, the ionized path from electrode 3 to the surface of the probe, the probe surface boundary layer which presents an anode drop (electrons entering), negligible internal metallic resistance of the probe, boundary layer of the probe in the direction of lexit where the cathode drop is consumed, and nally, the second ionized path from probe surface to electrode 4. If the two ionized paths are exactly equal and if, as I believe, the anode drop is greater than the cathode drop, the probe will experience a difference of potential, even in the balanced gap condition, equal to the difference of anode and cathode drop. In this way I account for the presence of a minimum of about I5 volts potential on the probe relative to ground whenever spark current is flowing between electrodes 3 and 4. While this potential may in many cases approach 5000 volts, whenever spark current is flowing there will be a voltage on the f probe relative to ground of not less than l5 volts which is sufficient to trigger the tube. The probe positively detects the presence of spark at the electrodes by transmitting sufficient energy to the grid of tube I8 to fire the tube.

I have found that in the absence of a spark, the voltages that might be capacitatively induced on probe 2| are unable to drive suilcient current through the impedances connected to the probe to energize tube I8.

In general, the following additional requirements should be considered in determining the value of resistor 22. It should be of such high resistance that the spark will tend to occur directly from one electrode to the other rather than from each of them to groundvia the probe and over the grid circuit of tube I8. To generalize, for this purpose its resistance should be high with respect to the output impedance of transformer 2 at line frequency. On the other hand to avoid direct capacitive pickup it should be low with respect to the capacitive reactance between the probe and either electrode at the line frequency.

The heater-Winding in th tube is in series with the anode-to-cathode discharge path. However. due to its low resistance its limiting ell'ect on the discharge current is negligible and. on the other hand, due to the smallness of the dischangev current as compared to the heater current, any change in cathode temperature which may occur when the tube is red will be negligible. In the present embodiment no filament transformer is required for providing heater current for tube I8. Instead, an incandescent lamp I9 is employed as a voltage dropping resistor so that the heater may be energized directly from the 7 line over this lamp. I have found that a 100 watt incandescent lamp has an appropriate resistance value if the heater of tube I8 requires 6.3 volts. This arrangement has the additional advantage that the fact of a spark failure will be signalized by continued burning of lamp I9. For, as will be seen below, for each starting cycle in winch a sparky is produced successfully the operation of tube i8 and lamp I9 will only last, a few seconds (first line voltage will be applied to the heater circuit, a few seconds will pass while the tube heats up, the tube will fire, its discharge I current will actuate the gas tube relay which,

its cathode heated, for detecting a delayed spark.

This is particularly advantageous as it is a common phenomenon for the spark to fail at first only to be produced a little later after the transformer has warmed upA somewhat. The safety device disclosed herein will give it plenty of time and yet during none of that time will any oil be pumped.

It will be seen that tube I8 after it has fired, will continue to draw anode-to-cathode current for a short period of time after itsheater circuit is opened and that this delay is utilized to serve a useful purpose in the present arrangement. .If the heater is brought up to its full operating temperature before the tube is red, this delay will always be very close to ten seconds. However, in some experimental models it was discovered that the tube could be fired before the cathode was fully heated if a suiiciently strong signal were picked up from the spark gap and that in such an instance the delay between the opening of the heater circuit and the deionization of the tube could be very materially reduced. In order to prevent this a means is employed for grounding the control grid to eliminate all signals for a period of time during which the cathode may become fully heated. This means comprises a normally closed bimetallic switch 2l) over which the grid is connected to ground. The contacts of this switch do not have to carry large currents and therefore it may be of such light construction that it requires but a small amount of heat to operate it. In a preferred embodiment of the invention bimetallic switch 20 is mounted adjacent to lamp I9 and is actuated by the heat from this lamp.

As is apparent from Figs. 2 and 2a the probe has quite large areas in planes transverse to the spark. This is to assure that it will intercept the spark. Since the natural draft of air through the blower duct will displace the spark forward and upward, the center of the probe area which should intercept the spark is similarly displaced with respect to the gap.

To insure that the average engergy passing through the winding of GTR is adequate, a circuit is provided for applying a reliable and properly phase firing signal to tube I8 once it has fired. This signal is a portion of the line voltage and it is applied to the grid of tube I 8 over normally open contacts 29 of relay GTR.

This circuit comprises a voltage divider including grid resistor 24 which. of course. is ground'- ed at one end, and in series therewith, resistors 21, 28 and normally open contacts 29. When relay GTR is first energized by a signal received from the spark gap contacts 29 are closed and the ungrounded end of the voltage divider is connected to the live extension of line conductor The relative values of these series resistors are such that enough of the line voltage will be applied to the grid of tube I8 to maintain the firing of the tube.v

From the foregoing it is seen that tube I8 will only fireiif the ignition means is functioning properly and that when it res it operates its associated relay GTR. At this time relay GTR moves its armature 30 from its back contact 9| to its front contact 32 and in so doing it opens the heater circuit of tube I8 and at the same time closes a loop applying line voltage across the winding of motor relay MR. Relay MR rocks its armature I5 closing the make contacts 33 thereof to apply line voltage to the motor of unit I. As already described above armature I5 at the same time moves latch I3 into engagement with the armature of switch I2. Since the igniting means must be functioning properly before burner l can be turned on, the start of operation of the burner will immediately be followed by combustion for a very large percentage of starts. However, it is known that a small percentage of combustion failures is attributable to other causes than a failure of the ignition means. For example, there may be a temporary oil stoppage or there may be impurities in the oil or even improper atomizing of the oil. And, of course, if burner I continued to operate, where there is combustion failure for any of these reasons, the result can be an accumulation of unburned oil. For this reason, in the present safety device, means are provided whereby, even after the spark has been checked and found to be satisfactory, the pump and blower unit cannot continue to operate for more than a very short time unless a ame is detected in the fire box within that time. It is to this end that relay GTR is arranged to open the heater circuit of tube E8 at the same instant that it applies energizing potential to the motor relay. Tube i8 will not be de-ionized immediately as its cathode has a cooling time of about ten seconds. Therefore, for ten seconds relay GTR will continue to be energized over tube i8 and will continue to apply energy to energize the motor relay. However, only for this long will the motor relay be energized by GTR. Within these ten seconds it is essential that stack switch 6 be operated by heat from the flame so that it may act to provide an alternate circuit for energizing the motor relay. For this purpose stack switch 6 is provided with make contacts 34 which it will shut within the ten seconds provided a flame is produced immediately after unit I is turned on. When stack switch 6 operates line 'I is extended over make contacts 34 to the ungrounded side of the winding of motor relay MR via a make contact 35 of this relay. Since normally closed contacts I1 will be opened when stack switch 6 operates, line I will cease to be extended to transformer 2 and to tube I8 and the heater circuit will not be re-energized when armature 30 falls back on its rest contact 3|.

There are three possible operating cycles: a normal cycle in which everything performs as the ignition means operates properly but there is a combustion failure for some other reason.

I'he operation of the apparatus shown herein during a normal cycle is as follows: withswitch I in its on position, thermostat II is operated by a drop in room temperature and extends line conductor 1 through normally closed switch I2 and normally closed contacts I1 of the stack switch to the ignition transformer. the anode circuit of tube I8, and the heater circuit of this tube. Incandescent lamp I8 will heat bimetallic switch 2li taking a longer time to operate it than is required for the cathode of tube I8 to become adequately heated. After a delay of twenty or thirty seconds, bimetallic switch 20 will unground the grid of tube I8 permitting this tube, in cooperation with probe 2I, to test for the presence of a spark in gap 3, 4. u Tube I8 will fire and operate relay GTR. which will apply a 60 cycle holding signal to the grid of tube I8, will open the heater circuit of this tube and will close an energizing circuit for motor relay MR. While the cathode of tube I8 is cooling, i. e., for about ten seconds, relay GTR. will continue to be energized by discharge current of the tube and in turn it will energize the motor relay. When it is energized the motor relay will close the power circuit for the motor of burner I and at the same time it will prepare its own holding circuit by closing its contacts 35. Combustion will commence and in less than ten seconds the stack switch, over its normally open contacts 34, will complete the holding circuit for relay MR and, by opening its rest contacts II, will sever the extension of line conductor 'I to the ignition transformer, and the anode and heater circuits of tube I8.

In the present arrangement the risk-taking period is very short because of the novel means employed herein for determining this period by the relatively short cooling off period of tube I8 (instead of by the relatively slow action of an electrically heated bimetallic switch). The period of operation of the stack switch is from two to ve seconds, this being easily attained by switches which are commercially available.

When the stack switch operates it also raises latch I3 so that when eventually the operation of the heating plant raises the room temperature sufficiently to cause thermostat II to sever the extension of line conductor 1 thus deenergizing motor relay MR and turning olf the burner, the armature of normally closed switch I2 will not be pulled open by latch I8 and the system will not be locked out from the supply of electrical power.

In the case of a'spark failure the cycle commences in the same manner as that described above. However, tube I8 does not flre and none of the events which follow the firing of this tube can take place. Since tube I8 does not fire, relay GTR can neither act to energize the motor relay nor to open the heater circuit of tube I8. As a result tube I8 remains in a prepared condition for detecting a spark, lamp I8 remains lit to signalize a spark failure, and no pumping of fuel takes place. If after a time a spark is produced, the cycle will continue in the manner described above.

When there is a combustion failure despite the presence of a spark the stack switch will not operate during the cooling oil of the cathode of tube I8. The result will be that about ten seconds l0 after motor relay MR became energized and applied power to the motor of burner I, it will be deenergized and cut off this power. The amount of unburned fuel would be very hunted and insufflcient to cause any damage if ignited. At the same time since latch I3 will have remained in .its operating position due to the failure of the stack switch to raise it, the rocking back of armature I5 upon the de-energization of motor relay MR would pull open the armature of normally closed switch I2 to lock out al1 power to the` burner. It would be impossible for the system to re-start `ritself until latch I3 is manually raised to reset switch I2. The lockout of switch I2 telltales the nature of abnormal condition of the burner, i. e., that there has been a combustion failure for some reason other than a spark failure. If desired, another signal lamp may be used, for example, the armature of switch I2 in its locked out position may engage a normally open contact and thereby close a circuit for a second signal lamp not shown.

It is seen that while the present arrangement makes use of a discharge tube, it does so without requiring a separate heater transformer or a direct current power supply for energizing the anode circuit and that the tube employed is of the gas filled type which has very positive switching action and does not require any amplifiers in order for it to operate a relay. Moreover, in the particular arrangement shown herein the discharge tube is employed so that in addition to performing its novel function in testing for the presence of a spark in the ignition means, it also serves at no additional expense as the means for determiningr the risk-taking period and it is capable of doing so more reliably and for shorter periods than has been economically possible by the means generally used in the prior art.

It is evident that if tube I8 should fail for any reason, for example, if its filament should burn out, if its envelope should be broken or if its emission should decrease so that it does not re readily, the effect will be merely to prevent the oil burner from operating. Therefore, though its failure will entail some inconvenience it will not result in decreased safety.

Obviously, stack switch 6 may be replaced in the present device by photoelectric means for detecting a flame. In fact, due to the shortness of the risk-taking period in systems according to the present invention, such photoelectric means are especially appropriate in that they have the advantage of being capable of continuing the supply of power to the motor of burner I in cases where combustion occured very late in the short risk period, whereas this would not be true of a thermal switch even if it were unusually fast-acting.

What is claimed is:

1. An oil burner safety system including a high voltage transformer, two electrodes connected to the transformer to form a spark gap, a gas discharge tube having a cathode, a cathode heater,

an anode, and a control grid, a probe supported within the gap substantially in the center of the pathway traversed by a spark produced therein, said probe of suitable area with respect to the gap such that when a spark is produced across the gap the probe picks up a signal voltage suicient in magnitude to fire said tube, a shielded connection from the probe to the control grid over a resistance of higher value than the output impedance of the transformer at the frequency of the line voltage supplying the transformer, a relay in series with the anode-cathode discharge end of the heater over a dropping impedance for applying a fraction of the alternating current line voltage across the heater to produce both heat and bias for the cathode, and normally closed contacts opened by said relay when the tube is fired to sever the connection which applies a fraction of the alternating current line voltage across said heater whereby once thetube fires the relay will be energized and will remain energized until the tube de-ionizes due to cooling of its cathode.

2. A spark testing apparatus as in claim l, in which said dropping impedance is the filament of an incandescent lamp for signalizing a condition in which the cathode is heated but the tube is not ionized.

3. A spark testing apparatus as in claim 1, in which said dropping impedance is the filament of an incandescent lamp, a bimetal thermal switch which has normally closed contacts and is responsive to heat from the lamp to open the contacts, and a circuit connecting the control grid to Y said other conductor of the line over the contacts of the thermal switch.

4. A spark testing apparatus as in claim 1, in which a cold cathode discharge device is connectedbetween 'said shielded connection and said other conductor of the line on the side of said resistance which is furtherest from the probe, the cold cathode discharge device having the'characteristic that the voltage drop across it while itis flredis large enough to overcome said cathode bias of said first-mentioned gas tube to an extent suicient to re said tube.

5. An oil burner safety system including an electrically operated oil pump, a source of power for the pump, a high voltage transformer, two electrodes connected to the transformer to form a spark gap for igniting oil, a probe made from similar material as the electrodes and located approximately in the center of said gap adapted to pick up an electrical triggering signal from a spark in the gap, control means connected to the probe for receiving said signal and for responding thereto to provide a connection for a limited period of time between said source of power and the pump, and means responsive to the heat produced by the ignited oil for maintaining the connection between the source of power and pump.

6. A safety system as in claim 5, in which said control means includes a gas discharge tube having a cathode heater, having a cooling time'of approximately ten seconds during which the tube provides a connection between the source of power and the pump.

7. A safety system as in claim 5, in which said means responsive to the heat include a motor relay, a thermally controlled stack switch, normally open contacts on the relay included in the circuit of the pump and source of power, and interlock means between the relay and switch whereby the connection between the source of power and pump is maintained only while the switch is operated. l

8. A safety device as in claim 5, in which said control means comprises a hot cathode gas discharge tube having a control grid, and an input circuit for receiving said triggering signal and applying it to the control grid to re the tube.

9. A safety device as in claim 5, in which said lov means responsive to the heat comprises a noi'- mally-open holding circuit and a thermal stack switch located in the path of the heat from the burner to close the holding circuit in substantially less than said limited period of time thereby continuing the connection between the source and the pump.

10. A safety device as in claim 5, in which said control means comprises a hot cathode gas dis-v charge tube having a control grid, an input circuit for receiving said triggering signal and applying it to the control grid to fire the tube, a relay which is in series with the anode-to-cathode dischargepath of the tube and is energized by its discharge current, normally closed contacts for the relay which are opened when it is energized, a power supply circuit for the cathode heater of the tube which circuit is over the normally closed contacts and therefore is opened when the tube res, and a circuit, which is completed by said relay between the time when the tube lires and the time when it de-ionizes due to the cooling off of its cathode, for providing said connection between said source of power and said pump for said limited period of time.

11. A safety device as in claim 5, in which said control means comprises a hot cathode gas discharge tube having a control grid, an input circuit for receivingsaid triggering signal and applying it to the control grid to re the tube, a.

Lwhen the tube i'lres, said supply circuit for the heater including an incandescent lamp in series with the heater for signalizing a condition in which the tube has its cathode heated but has not fired thereby to indicate that there is no spark in the gap, a circuit, which is completed by said relay between the time when the tube iires and the time when it de-ionizes due to the cooling off of its cathode, for providing said connection between said source of power and said pump for said limited period of time, means for shortcircuiting any trigger signals received for a period of time while the cathode is being heated including a normally closed thermal switch connected in a low impedance circuit between the grid and cathode, the thermal switch being near to said incandescent lamp for receiving' heat therefrom when the heater supply circuit is closed and for responding thereto after a predetermined time to open the low impedance circuit.

l2. A safety device as in claim 5, in which said control means lcomprises a hot cathode gas discharge tube having a control grid, an input circuit for receiving said triggering signal and applying it to the control grid to iire the tube, a relay which is in series with the anode-to-cathode discharge path of the tube and is energized by its discharge current, normally closed contacts for the relay which are opened when it is energized, a power supply circuit for the cathode heater of the tube which circuit is over the normally closedl contacts and therefore is opened when the tube fires, said supply circuit for the heater including an incandescent lamp in series with the heater for signalizing a condition in which the tube has its cathode heated but has not fired thereby to indicate that there is no spark in the gap, a circuit which is completed by said relay between the 13 time when the tube res and the time when it deionizes due to the cooling of its cathode, for providing said connection between said source of power and said pump for said limited period of time, a source of alternating curent energizing potential for the tube, means for connecting the last mentioned source across the anode-tocathode discharge path of the tube, normally open contacts for said relay, and means including a circuit which is closed over the normally open contacts when said relay is energized, for applying to the grid as a ring voltage, of known phase and magnitude appropriate for firing the tube, a portion of saidA alternating current energizing potential.

13. A safety device as in claim 5, in which said control means comprises a hot cathode gas discharge tube having a control grid, an input circuit for receiving said triggering signal and applying it to the control grid to fire the tube, a relay which is in series with the anode-to-cathode discharge path of the tube and is energized by its discharge current, normally closed contacts for the relay which are opened when it is energized, a power supply circuit for the cathode heater of the tube which circuit is over the normally closed contacts and therefore is opened when the tube fires, said supply circuit for the heater including an incandescent lamp in series with the heater for signalizing a condition in which the tube has its cathode heated but has not red thereby to indicate that there is no spark in the gap, a circuit, which is completed by said relay between the time when the tube fires and the time when it de-ionizes due to the cooling off of its cathode, for providing said connection between said source of power and said pump for said limited period of time, a motor relay which is 14 actuated temporarily over said last-mentioned circuit and thereafter is held during an operating cycle of the burner over an alternate energizing circuit controlled by said means responsive to the heat, the motor relay having normally open contacts which are closed when it is energized to provide said connection between said source and said pump, a normally closed switch in series with one of the line conductors of said source of power for the motor, the motor relay comprising a latch for engaging a movable element of said normally closed switch when the motor relay is energized and for moving it and opening the switch to lock power out from the burner when the motor relay is deenergized, and a link between said latch and said means responsive to a flame by which the latter means in responding to a ame will disengage the latch from said movable element to prevent it from opening the normally closed switch.

FRANK H. HIBBARD.

REFERENCES CITED The following references are of record in the le of this patentz UNITED STATES PATENTS Number Name Date 1,688,697 Fischer et a1 Oct. 23, 1928 1,688,864 Fischer et al Oct. 23, 1928 1,738,299 Kille Dec. 3, 1929 1,899,744 Briesky et al Feb. 28, 1933 1,904,549 Scognamillo et al. Apr. 18, 1933 2,162,501 Draper June 13, 1939 2,181,970 Fallon Dec. 5, 1939 2,231,420 Gille Feb. 11, 1941 2,276,803 Suits Mar. 17, 1942 2,301,446 Ott Nov. 10, 1942

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