WO1986002500A1

WO1986002500A1 - A method and device for detection of an electric load

Info

Publication number: WO1986002500A1
Application number: PCT/SE1984/000341
Authority: WO
Inventors: Jan Svensson
Original assignee: Jan Svensson
Priority date: 1983-04-18
Filing date: 1984-10-17
Publication date: 1986-04-24
Also published as: SE448590B; SE8302144D0; SE8302144L; EP0229044A1

Abstract

A method and a device for detection of an electric load (9) for the purpose of allowance of its connection to a power voltage from an electric voltage source over electric conductor means (7, 8). They are normally kept free from power voltage and when connecting the load to the conductor means (7, 8) a measurement of the resistance of the load is automatically carried out. Upon measured resistance within a first lower resistance interval, below the resistance of a human body the load (9) is allowed for connection to the voltage source. Upon a measured resistance within a second, higher resistance interval the presence of inductance of the load is detected. Upon detected inductance within the second resistance interval the load is allowed for connection to the voltage source and upon allowance of the load connection of power voltage to the connection point is made possible, when the load is connected.

Description

Title:

A Method and a Device for Detection of an Electric Load

Technical Field:

The present invention relates to a method for detection of an electric load for the purpose of allowance of its connec¬ tion to a power voltage from an electric voltage source over electric conductor means, said electric conductor means being normally kept free from said power voltage and upon the connec¬ tion of the load to said conductor means the resistance of the load is automatically measured.

The present invention also relates to a device for detec¬ tion of an electric load for the purpose of allowing its connec¬ tion to an electric voltage source at a connection point having electric conductor means.

The present invention relates to production against per¬ sonal injuries caused by a person's contact with voltage conduc¬ ting electric parts and builds upon the knowledge of that the human body is purely resistive and has a resistance which always exceeds a certain value. The Technical Problem:

The object of the present invention is to protect persons against injuries, caused by contact with voltage conducting parts by detecting a load connected to an alternating current mains. The Solution:

Said object is obtained by means of a method and a device according to the present invention. The method is characte¬ rized therein that upon a measured resistance within a first lower resistance interval, below the resistance of a human body, the load is allowed for connection to the voltage source, that upon a measured resistance within a second, higher resis¬ tance interval, the presence of inductance of the load is detec¬ ted, that upon a detected inductance within said second resis¬ tance interval the load is allowed for connection to the vol¬ tage source and that upon an allowed load connection of power voltage to the connection point is made possible, when the load is connected.

The device is characterized therein, that the device includes a connecting unit which is able to be changed-over between measuring condition, in which a measuring voltage, undangerous for the human body, is connected to the connection point, and a power condition, in which power voltage from said voltage source for the operation of said load is connected to the connection point, a resistance measurement unit in order to measure the resistance of the load in said measurement condi¬ tion and upon a measured resistance within a first lower resis¬ tance interval to allow the load, an inductance measurement unit in order to in said measuring condition upon a measured resistance within a second higher resistance interval detect the presence of inductance of said load and upon presence of inductance allow the load and that upon an allowed load enable the change-over of the connecting unit from measuring condition to power condition. Description of Drawings:

The invention will in the following be further described by means of an embodiment with reference to the enclosed drawings, in which:

Fig. 1 shows a schematical diagram over an embodiment of the device according to the invention; and

Fig. 2 shows a frequency diagram over the signal processing in the device. Preferred Embodiments:

The device according to the invention is based upon the idea to maintain certain voltage conducting parts in an alter¬ nating-current mains free from power voltage as long as no allowed load in the shape of an apparatus is connected to the mains. Upon connection of an allowed load electric power is supplied to the same, but on the contrary the supply of voltage is kept broken upon connection of a load in the shape of a part of a human body. This is accomplished principally by the device performs a measurement of the resistance of the load in the shape of a measurement of the ohmic resistance and in the shape of a measurement of inductance. As the human body is completely ohmic resistive according to accomplished scientific tests and furthermore exceeds with a high security a certain limit value of for example 500 ohms, a measurement of ohmic resistance takes place at a first lower resistance interval between for example 0 and 500 ohms, and upon measurement upon of resistance exceeding 500 ohms, for example between 500 ohms and 10.000 ohms the abovementioned inductive measurement takes place, that is to say measurement of the inductance of the load.

For this purpose the device, as shown in fig. 1, is made by an ohmic resistance measurement unit 1 and an inductance measurement unit 2 and a connecting unit 3. Also a power supply unit 4 is included in order to supply to the device necessary voltage for measurement and power. The power supply unit 4 is connected to an alternating-current mains of for example 220 V, which forms power voltage for those apparatuses which are to be connected. The connections 5 and 6 symbolizes the connection of the alternating-current mains to phase conductor and zero conductor respectively. Two electric conductor means 7, 8 represent those connections to which said load 9 is to be connected for the purpose of its operation. The connections 7, 8 can be included in for example a wall socket 10 and consist of those parts which according to the invention are to be kept free from power voltage as long as an allowed load is not connected. The connecting unit 3 includes to anti-parallel connected diodes 11 and a measurement resistor

12 of for example 100 ohms, over which the voltage varies by the ohmic resistance of the load 9.

The connecting unit 3 further includes a connecting relay

13 which is provided to be switched over between a measurement condition, which is shown in the figure and connecting condi¬ tion, in which the power voltage for the load is connected to the connections 7, 8 of the wall socket 10. The relay 13 is normally kept in the shown measurement condition and is switched over to connecting condition upon supply of voltage to the coil 14 of the relay by making the transistor 15 conduc¬ ting.

The resistance measurement unit has an input 16, the voltage level of which is provided to be measured by means of said resistance measurement unit, and an output 17, which changes condition in dependence of the measured voltage at the input. The resistance measurement unit 1, is as the other circuits, substantially made by digital circuits and comprises substantially a high pass filter 18 in order to filter- away non-desired disturbances, a comparator 19 for comparing the voltage level at one input of the comparator, that is to say the input 16. A second input to the comparator 19 has a reference voltage 20, which is adjustable. Further, a load detector 21 is included, after which a current circuit is closed in one embodiment, alternatively is provided to be normally open, but to be instantaneously closed in a further embodiment. After the load detector a pulse shaping circuit 22 is connected. Further a zero pass detector 23, a gate circuit 24, a monostable multivibrator 25 having an adjustable resis¬ tance and a so called flip-flop 46 are included.

The inductance measurement circuit also has an input 25 and and output 26. The input to the inductance measurement circuit 22 is together with input 16 to the resistance measure¬ ment unit 1 connected after the low pass filter 18 so, that the same voltage level is detected in the two inputs. The input to the inductance measurement unit leads to one input to a second comparator 28 with a second input which is connected to a refe¬ rence voltage 29. This reference voltage differs from the reference voltage 20 which further will be described below. A first output 26 from the inductance measurement unit 2 leads to the power supply unit 4 in order to switch a switch transistor 30 in said unit which will be further described below. A second output 50 leads to the load detector 21. The inductance measurement unit further has a phase detector 32, a branch connection 33, a timer 34 for measurement time, an RS-multi- vibrator 35, a pulse generator 36, a pulse window circuit 37, a pulse counter 38, a so called missing detector 39 and a second RS-multivibrator 40. The circuits as included have a function which will be further described below.

The power supply unit includes a voltage converter and rectifier 41, from which required measurement voltage of for example +15 V direct voltage 1 at an output 42 and an operating voltage can be put out for operation of included circuits of for example +12 V at the output 43. By means of a so called power-up circuit 44 the supply of current for the circuit as included and the measurement current can be secured upon inter¬ ruption of supply of current.

The function of the device will now be described with reference to fig. 1 och 2, where fig. 2 shows changes in condi¬ tion and arising pulses in different points of the device. The relay contact 13 is consequently normally in position NC and consequently connected to the output 42 of the power supply unit over the current limitation resistor 31 and the transistor 30 which normally is conducting. When a load 9 at the time t (ohmic resistance load) or t~ (inductive load) see fig. 2, is connected to the connectors 7 and 8 in the form of an apparatus or a human body or a part of a human body which contacts the connectors intentionally or unintentionally, a current circuit is closed over the measurement resistor 12. The voltage arising over the measurement resistor is, as mentioned above, dependent of the ohmic resistance of the load. The voltage over the measurement is compared with the reference voltage 20 in the first comparator 19 in the resistance measurement unit 1. The load detector 21 changes the voltage condition at its output 45. The load detector 19 lower its output, if the voltage over the measurement resistor is higher than the reference voltage as set. Then the load detector has allowed a load which involves that the flip-flop 46 is released and activates the relay 13, 14 in a way which will further be described below. By means of the lowering of the output 45 furthermore blocking of the other comparator 28 in the induct¬ ance measurement unit 2 is accomplished which unit in this way will be blocked as a whole. Consequently this takes place at a resistance interval of the load 9 which is determined by means of the reference voltage 20 and consists of a first, lower resistance interval between substantially 0 and for example 500 ohms. Within this interval most current consumers generally fall, such as lamps, heating apparatuses, television apparatuses and similar things.

A direct activation of the relay 13, 14 (with a certain time delay) upon allowance of the load 9 by means of the com¬ parator 19 occurs at the times t₂ and t- respectively in those cases the connections x., x are clamped and consequently closed. If a so called outer activation is utilized the two connections x., x₂ are consequently normally open and provided to momentarily be closed at the times t₃ and t_g respectively by means of for example an opto switch and in this way an isolated weak current control can be established. The output 45 from the first comparator 19 will at this time be low as long as the load 9 is connected. A momentary closure over x, , x- results in a pulse of the time length of 30 ms to the gate circuit 24. The momentary closure occurs at the time t, in fig. 2, the 30 ms pulse is shown in fig. 2. During this time a spike pulse 48 will be emitted from the zero pass detector 23, said spike pulse trigging the multi-vibrator 25. The multi-vibrator 25 delays the actuation of the relay 13, 14 corresponding to the actuation time of the relay until zero voltage occurs. The multi-vibrator 25 clocks the flip-flop 46 and then the relay 13, 14 will be activated and means voltage will be connected by switching the relay contact 13 to the position NO. Over the diodes 11 a square pulse now arises having a length of 20 ms which is detected by the load detector and which maintains the output 45 from the comparator 19 continued low. A new momentary closure over x., ₂ repeats the same procedure as described above, however, with the exception from the zero pass pulses which now are taken from the output of the comparator 19. Control of this occurs from the flip-flop 46. The multi- vibrator 25 now delays the switching-off of the relay and in this connection time for switching-on and off should be the same. The multivibrator 25 can also be controlled by the flip-flop 46 if different time for switch-on and off must be utilized.

Consequently, the above described cuter activation involves that mains voltage will not be connected directly upon allowance of the load, but after that a switch will be manually activated.

Upon direct activation x-, x₂ consequently are closed, involving that the pulse 47 with the length 30 ms arises in the same moment as the output 45 from the comparator 19 goes low. When the load is disconnected by outer influence (by means of a cable switch, in connection with change of lamps without previously switching-off, in connection with interruption etc) the output 45 goes high and locks the flip-flop 46 in position off. This one can then not be activated as long as the output 45 is high. For shorter mains interruptions a back-up capacitor is present for the flip-flop 46, whereby the load detector 21 remembers if it was switched on or off. In connec¬ tion with longer interruptions the so called pcwer-up circuit 44 is in operation which always secures that the load detector 21 is kept in an off position.

However, current consumers exist which the load detector 21 does not allow, as the resistance considerably exceeds the determined upper limit value in connection with a pure resistance measurement. This occurs in such cases where apparatuses of the type clock radio, games, timer etc are connected which have a relatively high resistance which in many cases corresponds to the resistance of the human body. In those cases not only a resistance measurement can distinguish apparatus from a human body. By this reason the device is provided with the inductance measurement unit 3 which supple¬ ments the resistance measurement unit 1 and is in operation at a second, upper resistance interval of for example 500 ohms and higher, for example 10.000 ohms. Those current consumers which lie within the upper measurement interval have namely some form of inductance connected in the shape of a transformer, motor, choke etc and therefore said apparatuses will be distinguished from the human body by detecting whether inductance is present. The presence of inductance is measured by detecting presence of phase difference between current and voltage upon switching of the measuring current over the measuring resistor 12 which according to the shown example takes place in the following manner.

The comparator 28 has a suitable reference voltage 29 so set that loads up to approximately 10.000 ohms are allowed by the load detector 21. By the fact that the comparator 28 is blocked at the resistance between approximately 0 and 500 ohms, the comparator 28 will allow loads between 500 ohms and 10.000 ohms. Then the output 29 from the comparator 28 goes high which consequently occurs at the time t-., when inductive load is connected. Then the RS-multivibratore 35 is set and the timer 34 is started in order to limit the measuring time to approximately 1 second and the pulse generator 36 is started. The pulse generator switches the transistor 30 which lies in series with the current limiting resistor 31 whereby the measurement current circuit is closed. Simultaneously a time reference pulse goes to the phase detector 32 and simul¬ taneously also the pulse window circuit 37 and the missing pulse detector 39 are triggered. The pulse which the transistor 30 has emitted will, if the load is an inductance, be somewhat delayed at the output 49 of the comparator. The time difference is detected as a difference pulse in the phase detector. In order that the gate circuit 33 will pass through this pulse a window must be opened in a correct time by means of the pulse window circuit 37 in order to prevent false difference pulses to clock the pulse counter 38. The missing pulse detector 39 will be started simultaneously with the pulse from the pulse generator 36 and will be reset by the different pulse. A correct number, for example 20, difference pulses must be clocked in a correct sequence (no difference pulse must be missing) and in a correct time in order that an allowance pulse will set the RS-multivibrator 40 which in turn allows the output 45 of the load detector 21 go low. Resetting the RS- multivibrator 40 occurs when the flip-flop 46 is activated. The procedure is then the same as described above in connection with the resistance measurement unit 1.

If any pulses are missing during the measurement or if a load having purely ohmic resistance has been connected, having a resistance within the upper resistance interval, there will never be issued any allowance pulse from the RS-multivibrator 40. The allowance pulse 50 is shown in fig. 2. The measure¬ ment would then be able to continue for an unlimited time. When the output 45 of the comparator 28 went high for the first time, the RS-multivibrator 35 was set, as previously described, and the measurement timer 34 was started. This timer 34 limits the measurement time to approximately 1 second and if no allow¬ ance pulse 50 has been received during this time, the RS-multi¬ vibrator 35 is reset and the whole measurement procedure stops. Please note that the output 49 from the comparator 28 lies constantly high as long as the load is connected, but the measure¬ ment procedure has been discontinued until the load has been removed and reconnected.

By introducing a further comparator which also is connec¬ ted after the low pass filter 18, a short circuit can be indi¬ cated before the relay is activated and the load is connected to the mains voltage. The reference voltage of this comparator is set so high that a short circuit involves a maximal voltage over the measurement resistor 12. Then this third comparator blocks the other comparators.

With reference to fig. 2 it will be mentioned that the signal 51 represents the voltage over the two diodes 11, the signal 52 represents the output from the comparator 19, the signal 53 consists of the input signal to the multi-vibrator 46 and the signal 54 consists of the output signal from the Q- output of the multi-vibrator 46. The signal 45 represents the change of condition of the relay 13, 14, the signal 56 repre¬ sents the output from the RS multi-vibrator 35, the signal 57 represents the output signal from the timer circuit 34, the output signal 58 represents the output signal from the pulse generator 36, the output signal 59 represents the output signal from the pulse window circuit 37, the output signal 60 represents the output signal from the phase detector 32 and the output signal 31 represents the output signal from the missing pulse detector 39. At the same time as the output signal 61 is constantly high the pulse counter 38 is set to zero position and locked. The time t. represents the time for switching off the connection relay 13.

The invention is not limited to the embodiments which are described above and shown on the drawings, but can be modified in a plurality of ways within the scope of the accompanying claims. For example it is imaginable that the relay 13, 14 is replaced by an electronic relay. Further the circuits as inclu¬ ded can be replaced by other circuits for a corresponding princi¬ pal function. The circuit can be made as an integrated circuit by for example CMOS technology and be made extremely current saving and space saving. The device can then as a whole be built within a substantially conventional wall socket or other installation components, such as a lamp socket, a switch or similar. The device can also be integrated with other units, such as relay central units. Common for the applications is to secure that in connection with electric apparatuses, which can be switched on and off, hold the electric connections free from voltage with respect to the power voltage for the apparatuses as long,as no apparatus is connected whereafter the device upon connection of an apparatus connects the power voltage not until when the device detects that an apparatus and not a human body contacts certain chosen connections.

Claims

Claims:

1. A method for detection of an electric load (9) for the purpose of allowance of its connection to a power voltage from an electric voltage source over electric conductor means (7, 8) said electric conductor means (7, 8) being normally kept free from said power voltage and upon the connection of the load to said conductor means (7, 8) the resistance of the load is auto¬ matically measured, c h a r a c t e r i z e d t h e r e i n that upon a measured resistance within a first lower resistance interval, below the resistance of a human body, the load (9) is allowed for connection to the voltage source, that upon a measu¬ red resistance within a second, higher resistance interval, the presence of inductance of the load is detected, that upon a detected inductance within said second resistance interval the load is allowed for connection to the voltage source and that upon an allowed load connection of power voltage to the connec¬ tion point is made possible, when the load is connected.

2. A method according to claim 1, c h a r a c t e r i z e d t h e r e i n that said first interval lies between 0 and approximately 500 ohms and that said second, interval exceeds approximately 500 ohms.

3. A method according to claim 1, c h a r a c t e r i z e d t h e r e i n, that a resistance of approximately 0 ohm, involv¬ ing a short circuit, no allowance of the load (9) will be made.

4. A device for detection of an electric load for the pur¬ pose of allowance of its connection to an electric voltage source at a connection point having electric conductor means, c h a r a c t e r i z e d t h e r e i n, that the device in¬ cludes a connection unit (3) which is to be changed over between a measuring condition, in which a measuring voltage, undangerous for the human body, is connected to the connection point, and a power condition, in which power voltage from said voltage source for the operation of said load is connected to the connection point, a resistance measurement unit (1) in order to measure the resistance of the load in said measurement condition and upon a measured resistance within a first lower resistance interval to allow the load, an inductance measure¬ ment unit (2) in order to in said measuring condition upon a measured resistance within a second higher resistance interval detect the presence of inductance of said load and upon presence of inductance allow the load and that upon an allowed load enable the change over of the connecting unit from measuring condition to power condition.

5. A device according to claim 4, c h a r a c t e r i z e d t h e r e i that the connection unit (3) includes a measuring resistor (12), which is connected to the connection point (7, 8) and over which the voltage depends on the resistance of the connected load, and that resistance measurement unit (1) includes a first comparator (19) , provided to enable the changeover of the connection unit in dependence of arising voltage over the measurement resistor relatively to a chosen first reference voltage and that the inductance measurement unit (2) includes a second comparator (28) , provided upon arising voltage over the measurement resistor relatively to a chosen second reference voltage to bring said measuring voltage over the connection point (7, 8) to vary, whereas arising phase shift over the load and consequently the inductance is detected and will be made possible that the connection unit is changed over to power condition upon presence of inductance.