GB2124391A - Electricity supply fault detection - Google Patents
Electricity supply fault detection Download PDFInfo
- Publication number
- GB2124391A GB2124391A GB08221185A GB8221185A GB2124391A GB 2124391 A GB2124391 A GB 2124391A GB 08221185 A GB08221185 A GB 08221185A GB 8221185 A GB8221185 A GB 8221185A GB 2124391 A GB2124391 A GB 2124391A
- Authority
- GB
- United Kingdom
- Prior art keywords
- current
- frequency
- supply
- lines
- oscillator
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
Classifications
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H1/00—Details of emergency protective circuit arrangements
- H02H1/0007—Details of emergency protective circuit arrangements concerning the detecting means
- H02H1/003—Fault detection by injection of an auxiliary voltage
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/50—Testing of electric apparatus, lines, cables or components for short-circuits, continuity, leakage current or incorrect line connections
- G01R31/58—Testing of lines, cables or conductors
Abstract
An electricity supply fault detection apparatus is interposed between live and neutral mains terminals 10 and 11 and circuit wiring 13 and 14. The apparatus comprises a circuit breaker 12 arranged to react to an imbalance of current drawn on wires 13 and 14 and an oscillator 15 which generates an output voltage having a frequency significantly at variance with mains frequency. The oscillator output is connected by means of a series linked adjustable resister 18 and isolating capacitor 19 to the live line 13 downstream of the circuit breaker 12 and by means of a line 17 to the neutral mains line 11 upstream of the circuit breaker 12. When a current path is provided between lines 13 and 14 current flows at the output frequency of the oscillator to cause the circuit breaker to respond. By incorporating a choke 22 within the load 20 which presents a high impedance to currents at the oscillator output frequency the load 20 can draw mains current without causing circuit breaker 12 to respond. However, any non-protected load or current path between lines 13 and 14 will cause current to flow at the oscillator output frequency resulting in circuit breaker 12 responding. <IMAGE>
Description
SPECIFICATION
Electricity supply fault detection
The present invention relates to apparatus for and a method of electricity supply fault detection.
Earth leakage circuit breakers, also known as residual current circuit breakers, are well-known for detecting faults to ground in the lines of a mains electricity supply system. Such a residual current circuit breaker commonly comprises a differential transformer connected in the electricity supply lines and arranged to produce an output signal if there is an imbalance in the currents flowing in the lines resulting from current flowing from one of the lines to earth. In a simple domestic electricity supply comprising a single phase and neutral lines, such a residual current circuit breaker is capable of detecting faults to earth in either of the live or the neutral lines. Normally the circuit breaker reacts to an imbalance in the currents in the live and neutral lines by breaking the supply.Circuit breakers of this kind typically have relatively fast response times and can thus react quickly enough to prevent damage or injury in the event of a ground fault.
It can be seen however that such a residual current circuit breaker arrangement is not capable of detecting a fault between the live and neutral lines of the supply since such a fault would appear at the differential transformer to be a normal balanced current.
The present invention sets out to provide apparatus which detests faults between live and neutral lines of a mains electricity supply system or faults between phase and neutral of a multiphase supply.
According to the present invention electricity supply fault detection apparatus comprises an oscillator to generate a voltage at a frequency different from the supply frequency and connected to apply said voltage across the or a pair of the supply lines, and means responsive to current at said frequency flowing in the supply lines to signal a fault. Thus, a fault is signalled by the apparatus whenever a load is connected between the pair of supply lines which draws current at said different frequency. Accordingly, normally a legitimate load to be connected to the supply line is fitted with a filter arranged to block current at said different frequency to prevent the load drawing current at said frequency from the supply lines, whilst permitting the load to draw current at the supply frequency.In this way, only such legitimate loads fitted with the filter can be connected across the supply lines without a fault being signalled.
Typically the fault signalling may include means responsive to the fault condition to disconnect the electricity supply from supply lines, e.g. a circuit breaker. if the fault detection apparatus is sufficiently sensitive and fast acting, the circuit breaker may be operated to disconnect the supply from the lines before any substantial charge can flow through an illegitimate load connected across the power lines. Thus the system may prevent electric shocks being suffered by persons who accidentally touch both live and neutral lines.
The apparatus may include a filter in at least one of the lines or the pair of lines upstream, in relation to the source of supply, of the point of connection of the oscillator voltage to the supply lines, and then said responsive means may be responsive to current flowing from the oscillator.
The filter in this arrangement then prevents current at said different voltage from flowing through the electricity supply source which may in some cases have a relatively low impedance at the different frequency. However, the filter may not be required when the electricity supply source has a relatively high impedance at said different frequency.
In one arrangement said responsive means comprises a differential current transformer connected in the supply lines to detect a differential current flowing between phases of the supply lines or between a single phase and neutral, and the oscillator is connected across the transformer such that any output current from the oscillator produces a differential current at said different frequency in the supply lines through the transformer which is detectable by the transformer. Normally the transformer comprises part of a residual current circuit breaker which is arranged to respond to imbalance in currents flowing in the supply lines to disconnect the mains supply from the lines. The residual current circuit breaker operates normally in response to a fault to ground in one of the supply lines by sensing an imbalance in supply frequency current flowing in the supply lines.However, with the apparatus described above, the circuit breaker is also arranged to respond to a fault between live and neutral or phase and neutral by detecting the resulting imbalanced current in the supply lines at said different frequency, and responding thereto to disconnect the supply.
It may be found that some existing types of residual current circuit breaker do not respond well to imbalanced current at the selected different frequency of the oscillator output.
Accordingly, in another embodiment, said responsive means includes a differential current transformer as before together with switch means responsive to output current from the oscillator at said different frequency to provide a current path between the, or the pair of, supply lines across the current transformer to produce in the transformer a differential current between the lines at the supply frequency. With this arrangement, the switch means responds to an output current from the oscillator at the different frequency (indicative of an illegitimate current path between the supply lines) by in effect providing at least momentarily an alternate current path for a proportion of the supply current bypassing the normal current path of one of the supply lines through the current transformer.This bypass current produces an imbalance of current at the supply frequency in the supply lines as sensed by the current transformer and the circuit breaker reacts by disconnecting the supply.
The present invention also envisages a method of detecting a fault in electricity supply lines comprising the steps of applying a voltage at a frequency different from the supply frequency across the or a pair of the supply lines, fitting the or each legitimate load to be connected to the supply lines with a filter arranged to block current at said different frequency to prevent the load drawing current at said frequency from the supply lines, whilst permitting the load to draw current at the supply frequency, sensing current at said different frequency flowing in the supply lines and signalling a fault in response thereto.
Examples of the present invention will now be described with reference to the accompanying drawings in which:
Figure 1 is a schematic block diagram illustrating one example of the present invention; and
Figure 2 is a schematic block diagram illustrating a second example of the invention.
Referring firstly to Figure 1, an example of the invention is illustrated applied to the protection of a mains power supply typically a domestic supply, comprising just live and neutral lines 10 and 11.
The supply lines 10 and 11 are fed via a residual current circuit breaker 12 to the circuit wiring 13 and 14 in the usual way. The residual current circuit breaker 12 reacts to any imbalance in the supply currents flowing in the live and neutral lines 10 and 11 (corresponding to the circuit lines 13 and 14) by disconnecting the supply from the circuit wiring. The circuit breaker 12 illustrated is arranged to react to an imbalance current of 30 mA and has a rated response time of 30 msecs. Different residual current circuit breakers are available on the market having different tripping current and response times. It will be appreciated that a fast response is desirable to ensure the supply is disconnected before damage or injury can result from the fault.
In the normal way, the circuit breaker 1 2 reacts only to faults from one of the live and neutral circuit lines 13 and 14 to ground which would cause the detected current imbalance.
The example of the present invention illustrated in Figure 1 further includes an oscillator 1 5 connected to take its power supply from the live and neutral lines 10 and 11 of the mains upstream of the residual current circuit breaker 12. The oscillator 1 5 is arranged to produce a sinusoidal output voltage on output lines 1 6 and 1 7 having a frequency different from the supply frequency which in the United Kingdom is 50 Hz and in some other countries 60 Hz. Accordingly the output frequency of the oscillator may be for example 100 kHz. Such a high output frequency ensures good discrimination from the supply frequency.
The output voltage at this frequency from the oscillator 1 5 is applied via a variable resistance 1 8 and a mains isolating capacitor 1 9 between the neutral line 11 upstream of the circuit breaker 12 and the live line 13 of the circuit wiring downstream of the circuit breaker 12. The high frequency output voltage of the oscillator 1 5 is thus applied between the live and neutral lines of the supply. In the absence of any current path between the lines 13 and 1 4 of the circuit wiring, no current flows at the output frequency of the oscillator. However, if there is a current path at the output frequency between the lines 1 3 and 14, then current flows from the oscillator in the supply lines.It can be seen that as a result of the selected connection from the output of the oscillator to the supply lines that any current drawn at the output frequency of the oscillator produces an imbalance current at this frequency in the residual current circuit breaker 12. In fact current at the oscillator output frequency flows only through the neutral line winding of the differential transformer in the circuit breaker 1 2.
Normally, the circuit breaker 12 will react to the resulting current imbalance at the oscillator output frequency in the normal way by disconnecting the mains supply. Thus, any current path at the oscillator output frequency between the lines of the circuit wiring causes the circuit breaker 1 2 to trip out.
In order to enable legitimate loads to be connected across the lines 1 3 and 14 of the circuit wiring, such legitimate loads are each protected by a filter so that they do not draw current at the oscillator output frequency but can still draw current at the mains supply frequency.
An example of a legitimate load is illustrated in
Figure 1 at 20 where the load itself is illustrated by the box 21 and the filter is illustrated by a choke 22 in one of the supply connections to the load 21. The value of the choke 22 is selected to present a high impedance to currents at the oscillator output frequency but negligible impedance at the mains supply frequency. A value of 6 mH is suitable in most cases where the oscillator output frequency is of the order of 100 kHz.
It can be seen therefore that when protected as illustrated, a legitimate load 20 can be connected to the circuit wiring and draw mains current without causing the circuit breaker 12 to trip out.
However any non-protected load or current path between the lines 13 and 14 of the circuit wiring will cause current to flow at the oscillator output frequency resulting in tripping of the circuit breaker 1 2. This arrangement provides a very effective protection of the circuit wiring against live to neutral faults.
Referring now to Figure 2, it has been found that, although the arrangement described in
Figure 1 works well for some types of residual current circuit breaker, problems can be experienced with resonance effects resulting from the connection of the differential current transformer windings of the circuit breaker 1 2 across the output of the oscillator.
In the arrangement of Figure 2, mains supply lines 24 (neutral), 25 (live) and 26 (earth) are connected to circuit wiring with corresponding lines 27, 28 and 29 in the usual way via a residual current circuit breaker 30. Also, an oscillator 31 corresponding to the oscillator 1 5 is again
connected to be supplied by the mains voltage
upstream of the circuit breaker 30. However, in
this arrangement, output lines 32 and 33 are
arranged to apply the output voltage of the oscillator via a potentiometer 34 and a mains
isolating capacitor 35 directly across the neutral
and live lines 27 and 28 of the circuit wiring downstream of the circuit breaker 30. As before,
no current at the output frequency of the oscillator
is drawn from the oscillator by a legitimate load 36 which is protected by a choke 37.However, current is drawn from the oscillator by an
illegitimate load or current path established between the live and neutral lines 27 and 28. A signal proportional to any current drawn from the oscillator at the oscillator frequency is fed from the slider 38 of the potentiometer 34 via a diode 39 to the gate input of a voltage controlled rectifier, thyristor or triac 40. The thyristor 40 has its cathode connected back to the output line 33 of the oscillator 31 and its anode connected via a variable resistor 41 to the neutral line 24 of the electricity supply.
Typically, the oscillator 31 has a transformer output stage so there is in effect a low impedance path at the mains supply frequency between the output line 33 of the oscillator and the output line 32 through the output winding of the output transformer of the oscillator. It can be seen therefore that firing of the thyristor 40 provides a current path from the live line 28 downstream of the circuit breaker 30 through the secondary winding of the output transformer of the oscillator 31, the thyristor 40, the variable resistor 41, to the neutral line 24 upstream of the circuit breaker 30.
The potentiometer 34 is set so that the thyristor 40 first in response to a desired low threshold current at the oscillator output frequency. It can be seen that on firing of the thyristor 40, an alternate current path is established between the live line 28 downstream of the circuit breaker and the neutral line 24 upstream thereof which bypasses the normal current path for the neutral line through the current transformer of the circuit breaker 30. Thus, firing the thyristor 40 produces an imbalance current between the live and neutral lines in the current transformer of the circuit breaker 30 at the supply frequency. The output of the oscillator 31 is also superimposed on this imbalance current. As a result, the residual current circuit breaker 30 trips to isolate the circuit wiring.
The variable resistor 41 may be set to have a relatively high resistance so that the current actually flowing via the thyristor is relatively low but only sufficient to ensure tripping. With one form of circuit breaker 30, a current of the order of 30 mA is sufficient.
Figure 2 further shows switch contacts in series between the resistance 41 and the neutral line 24 and in series with the live line 28 of the circuit wiring. These switch contacts comprise additional contacts of the circuit breaker 30 and are arranged to become open on tripping of the circuit breaker 30 to further isolate the circuit wiring from possible current paths via the oscillator 31 and thyristor 40.
It will be appreciated that for each of the examples illustrated in Figures 1 and 2 of the drawings, substantially no current (or at least current below a predetermined threshold level) is drawn from the oscillator at the oscillator output frequency except when there is a fault or illegitimate current path between the live and neutral lines downstream of the circuit breaker. In some arrangements, the impedance of the supply source may be sufficiently high at the output frequency of the oscillator that no substantial current can flow between the supply lines through the supply source. However, if the impedance of the supply source is such that current at the oscillator output frequency can flow through the source, then a blocking choke can be incorporated in the live line upstream of the circuit breaker as shown at 45 in Figure 1 and 46 in Figure 2.
Claims (6)
1. Electricity supply fault detection apparatus comprising an oscillator to generate a voltage at a frequency different from the supply frequency and connected to apply said voltage across the or a pair of the supply lines, and means responsive to current at said frequency flowing in the supply lines to signal a fault.
2. Apparatus as claimed in Claim 1 together with a legitimate load for connection to the supply lines wherein the load is fitted with a filter arranged to block current at said different frequency to prevent the load drawing current at said frequency from the supply lines, whilst permitting the load to draw current at the supply frequency.
3. Apparatus as claimed in Claim 1 or Claim 2 and including a filter in at least one of the lines or the pair of lines upstream, in relation to the source of supply, of the point of connection of the oscillator voltage to the supply lines, wherein said responsive means is responsive to current flowing from the oscillators.
4. Apparatus as claimed in any preceding claim wherein said responsive means comprises a differential current transformer connected in the supply lines to detect a differential current flowing between phases of the supply lines or between a single phase and neutral, and the oscillator is connected across the transformer such that any output current from the oscillator produces a differential current at said different frequencies in the supply lines through the transformer which is detectable by the transformer.
5. Apparatus as claimed in any of Claims 1 to 3 wherein said responsive means includes a differential current transformer connected in the supply lines to detect a differential current flowing between phases of the supply lines or between a single phase and neutral, and switch means responsive to output current from the oscillator at said different frequency to provide a current path between the or the pair of supply lines across the current transformer to produce in the transformer a differential current between the lines at the supply frequency.
6. A method of detecting a fault in electricity supply lines comprising the steps of applying a voltage at a frequency different from the supply frequency across the or a pair of the supply lines, fitting the or each legitimate load to be connected to the supply lines with a filter arranged to block current at said different frequency to prevent the load drawing current at said frequency from the supply lines, whilst permitting the load to draw current at the supply frequency, sensing current at said different frequency flowing in the supply lines and signalling a fault in response thereto.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08221185A GB2124391A (en) | 1982-07-22 | 1982-07-22 | Electricity supply fault detection |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08221185A GB2124391A (en) | 1982-07-22 | 1982-07-22 | Electricity supply fault detection |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| GB2124391A true GB2124391A (en) | 1984-02-15 |
Family
ID=10531827
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08221185A Withdrawn GB2124391A (en) | 1982-07-22 | 1982-07-22 | Electricity supply fault detection |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2124391A (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2239530A (en) * | 1989-12-29 | 1991-07-03 | Megger Instr Ltd | Testing AC Installations |
| CN109725232A (en) * | 2017-10-27 | 2019-05-07 | 半导体组件工业公司 | Fault detection circuit and correlation technique |
| US11635474B2 (en) * | 2019-09-30 | 2023-04-25 | Pass & Seymour, Inc. | Frequency dependent ground fault interrupt |
Citations (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB383971A (en) * | 1930-06-05 | 1932-12-01 | British Thomson Houston Co Ltd | Improvements in protective arrangements for electric transmission lines |
| GB388362A (en) * | 1930-05-16 | 1933-02-23 | British Thomson Houston Co Ltd | Improvements in and relating to protective systems for electric transmission lines |
| GB960931A (en) * | 1961-01-10 | 1964-06-17 | Charbonnages De France | Electric network protection system |
| GB963229A (en) * | 1962-02-22 | 1964-07-08 | Ferranti Ltd | Improvements relating to apparatus for protecting electrical power lines |
| GB1172683A (en) * | 1966-07-28 | 1969-12-03 | Electricity Council | Improvements in or relating to Apparatus for and a Method of Locating Faults in Electrical Conductors. |
| GB1390016A (en) * | 1971-05-08 | 1975-04-09 | Refsum A | Method of fault location on electric transmission and distribution systems |
| GB1491286A (en) * | 1975-06-19 | 1977-11-09 | Standard Telephones Cables Ltd | Monitoring radiating cable systems |
| GB1518300A (en) * | 1974-12-10 | 1978-07-19 | Us Commerce | Impedance multipliers |
-
1982
- 1982-07-22 GB GB08221185A patent/GB2124391A/en not_active Withdrawn
Patent Citations (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB388362A (en) * | 1930-05-16 | 1933-02-23 | British Thomson Houston Co Ltd | Improvements in and relating to protective systems for electric transmission lines |
| GB383971A (en) * | 1930-06-05 | 1932-12-01 | British Thomson Houston Co Ltd | Improvements in protective arrangements for electric transmission lines |
| GB960931A (en) * | 1961-01-10 | 1964-06-17 | Charbonnages De France | Electric network protection system |
| GB963229A (en) * | 1962-02-22 | 1964-07-08 | Ferranti Ltd | Improvements relating to apparatus for protecting electrical power lines |
| GB1172683A (en) * | 1966-07-28 | 1969-12-03 | Electricity Council | Improvements in or relating to Apparatus for and a Method of Locating Faults in Electrical Conductors. |
| GB1390016A (en) * | 1971-05-08 | 1975-04-09 | Refsum A | Method of fault location on electric transmission and distribution systems |
| GB1518300A (en) * | 1974-12-10 | 1978-07-19 | Us Commerce | Impedance multipliers |
| GB1491286A (en) * | 1975-06-19 | 1977-11-09 | Standard Telephones Cables Ltd | Monitoring radiating cable systems |
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2239530A (en) * | 1989-12-29 | 1991-07-03 | Megger Instr Ltd | Testing AC Installations |
| GB2239530B (en) * | 1989-12-29 | 1993-07-21 | Megger Instr Ltd | Circuit checking |
| CN109725232A (en) * | 2017-10-27 | 2019-05-07 | 半导体组件工业公司 | Fault detection circuit and correlation technique |
| CN109725232B (en) * | 2017-10-27 | 2023-05-23 | 半导体组件工业公司 | Fault detection circuit and related method |
| US11635474B2 (en) * | 2019-09-30 | 2023-04-25 | Pass & Seymour, Inc. | Frequency dependent ground fault interrupt |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |