GB2449677A

GB2449677A - A system for identifying a risk of fire in a power network

Info

Publication number: GB2449677A
Application number: GB0710374A
Authority: GB
Inventors: Kevin Jones
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-06-01
Filing date: 2007-06-01
Publication date: 2008-12-03
Also published as: WO2008146040A1; GB0710374D0

Abstract

A system (4) for identifying a risk of fire in a power network. The system (4) comprises at least one monitor (5) for measuring at least one of current and voltage at a point in the network and a transmitter for transmitting measured data from the monitor (5) to a central controller (8). A central control monitor is provided for measuring at least one of current and voltage of the power network. At the central controller (8), a processor is provided for performing a fire risk analysis of the transmitted data and the measured data for the power network. The central controller (8) also comprises means for, in the event that the analysis identifies a risk of fire, arranging action to be taken to minimize the risk of fire which includes alerting the user and cutting power to the network. The systems measure the current or voltage at the various points in the network and compares to the values to the current and or voltage of the power network. If the difference is greater than a threshold then the action is taken to minimize the risk of a fire. A timing message is transmitted to each monitor to synchronize the recording of measurements.

Description

System for Identifying Fire Risk The invention relates to the field of

systems for identifying fire risk in an electrical power circuit.

According to a recent US study, "a sizeable fraction of ignitions of structures are due to electrical faults associated with wiring or wiring devices." (V.

Babrauskas, "How do electrical wiring faults lead to structure ignitions?", International Fire and Materials Conference, San Francisco, 2001, http:Ilwww. interfire.org!featuresfelectric wiring faults.asp). According to America's National Fire Protection Association, electrical distribution fires accounted for 336 civilian deaths, 1,446 civilian injuries and $643.9 million in direct property damage per year from 1993-1997. Electrical distribution was the fifth-ranked cause of fires, the fourth-ranked cause of fatalities and the second-ranked cause of property loss.

According to English Heritage, in England, there are over 400,000 listed buildings, including "virtually everything built before 1700 that survives in anything like its original condition". Most of them contain significant amounts of structural timber, and constant vigilance is required to ensure that fires do not break out in them. English Heritage generally requires listed buildings to be rebuilt after fire or other damage, at the owner's expense, provided that at least 40% of the original fabric remains. Many old buildings also have old wiring, since their owners often lack the financial resources to re-wire completely.

Electrical fires can result from various conditions. Some but not all of these conditions can be mitigated by the use of known electrical safety measures.

(i) Arcing Arcing occurs when there is a failure in the insulation surrounding the electrical carrier. A total failure of insulation, resulting in a short circuit between live and neutral, is not especially dangerous since in most power networks a fuse will blow or a circuit breaker will trip, causing a loss of power to the network and eliminating any further arcing. Even a partial failure of insulation between either live or neutral and earth can be mitigated by a residual current device (RCD). However, partial failure of the insulation between live and neutral may easily cause an electrical fire. The risk is easily understood. If the insulation in a cable carbonises such that there is a resistance of ikO between live and neutral, then (assuming a UK-standard 24Ovac circuit) the insulation will pass a current of 240mA and will dissipate nearly 60W power. A current of 240mA is not sufficient to blow a standard mains fuse or trip a standard circuit breaker.

However, dissipation of 60W in a confined space is easily sufficient to melt plastic or rubber insulation and cause ignition.

(ii) Excessive ohmic heating All electrical wiring and all electrical connections have some resistance, causing some voltage drop and power dissipation as current passes. In well-maintained circuits, the resistances are small fractions of an ohm per joint or per metre of cable, insufficient to cause any significant heating. However, loose connections have been identified as a cause of significant ohmic heating, leading to ignition. According to W.J. Meese and R.W. Beausoleil, "Exploratory Study of Glowing Electrical Connections" (NBS BBS 103), [U.S.] NatI. Bur. Stand., Gaithersburg, MD (1977), a good electrical connection passing 20A would dissipate only 0.08-0.2W power, but a glowing connection would dissipate 20-40W power. Even such a glowing connection would only cause a drop of 1-2v, which would not normally be noticed, but would dissipate enough power to melt plastic or rubber insulation and cause ignition.

The inventor has devised a system and apparatus for detecting arcing across insulation and small voltage drops caused by excessive ohmic heating. If such conditions are detected, action will be taken to minimize the risk of fire. Such action may be cutting the power pending investigation of the cause, or alerting a user who can then decide what further action is required.

According to a first aspect of the invention, there is provided a system for identifying a risk of fire in a power network. The system comprises at least one monitor for measuring at least one of current and voltage at a point in the network. A transmitter is also provided for transmitting measured data from the monitor to a central controller. A central control monitor is provided for measuring at least one of current and voltage of the power network. The central controller has a processor for performing a fire risk analysis of the transmitted data, and further comprises means for, in the event that the analysis identifies a risk of fire, arranging action to be taken to minimize the risk of fire. Where the term power network is used herein, it can be taken to mean an entire power network, for example in a home, or a predetermined portion of a power network such as a ring main.

Any suitable means for arranging action to minimize the risk of fire may be used. These may include, in particular, means for alerting a user of the risk and cutting power to the network.

In one embodiment of the invention, the central control monitor measures the total current used by the power network. In this embodiment, the analysis comprises comparing the total current used by the power network with the sum of the currents measured at each monitor, such that a risk of fire is identified if the total current used by the power network is substantially greater than the sum of the currents measured at each monitor. By comparing the total current drawn by the network with the sum of the currents used at each monitor, the risk of fire cause by arcing can be detected. Furthermore, the analysis of fire risk may comprise comparing the currents measured at at least two points in the power network and determining if a substantial imbalance is present between the two measured currents. In the case of a ring main circuit, this would suggest that one branch of the ring main is displaying a significantly higher resistance than the other branch, which may be caused by a failing connection. A significant imbalance may be considered to be present between the two measured currents if one of the measured currents is greater than the other measured current by more than a predetermined factor.

A risk of fire may also be identified if the voltage measured at a monitor is substantially lower than a measured voltage at the supply to the power network. This detects a risk of fire caused by ohmic heating.

The monitor may measure electrical properties over time and transmit either an average or integrated value of the measurements to the controller.

Each monitor in the system may obtain a measurement at the same time. In this case, it is preferred that the controller further comprises a transmitter to transmit a timing message to each monitor, the timing signal being used by each monitor to synchronize when measurements are taken.

The controller may comprise means to measure either a current or a voltage at more than one point on the power network. Furthermore, a monitor may comprise means to measure a voltage at more than one point on the power network.

In order to prevent a non-compatible device without a monitor in the plug or device from being connected to the power network, a power outlet on the power network may comprise a projection arranged to co-operate with a corresponding recess on a plug.

According to a second aspect of the invention, there is provided a controller for use in a system for identifying a risk of fire in a power network, the controller comprising: a receiver for receiving a message from a monitor attached to the power network, the message containing data relating to a measurement of at least one of current and voltage; a processor for performing a fire risk analysis of the received data; and means for, in the event that the analysis identifies a risk of fire, arranging action to be taken to minimize the risk of fire.

According to a third aspect of the invention, there is provided a monitor for use in a system for identifying a risk of fire in a power network, the monitor comprising: means for measuring at least one of current and voltage at a point in the network; a transmitter for transmitting measured data to a central controller, such that the central controller can use the data for performing a fire risk analysis of the transmitted data.

In order that the invention may be more fully understood, the invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates schematically a mains electricity circuit, with a controller and various monitor units according to an embodiment of the invention; Figure 2 illustrates schematically an electricity ring main in which the controller comprises two components functionally connected together; Figure 3 illustrates schematically an electricity ring main according to a further embodiment, with a monitor unit that comprises two components functionally connected together; Figures 4 illustrates schematically a cross-section of a known standard mains plug and socket; and Figures 5 illustrates schematically a cross-section view of a modified plug and socket for preventing the connection of electrical appliances without monitor units in a system in which the monitor units are disposed at the plugs.

It is known to measure accurately the voltage present at, and the current drawn by, any electrical appliance or any electric circuit. Standard current measuring techniques include using the current being measured to drive an electric motor, feeding it through the primary winding of a transformer and feeding it through a low-value resistor. Chips for measuring the voltage present at, and current drawn by, individual appliances are now readily and inexpensively available.

Figure 1 illustrates a general electricity mains circuit 4, with various appliances 1, 2, 3 attached to it. These appliances 1, 2, 3 draw currents i1, i2, i, etc. Monitors 5, 6, 7 are provided that measure the current drawn by each appliance and send the measured current data back to a central controller 8.

The central controller 8 measures the total current, total, drawn by the circuit.

Under normal operation, the total current drawn by the appliances, i1 + i2 + +...., is equal to the current measured by the controller 8. If tota is substantially greater than i1 + i2 + i3 +.... then this is an indication that arcing is occurring somewhere on the circuit. The additional current drawn total -(t + + will cause heat generation, with a consequent fire risk. Of course, insulators are not perfect and some leakage of current may occur, and so the system can take account of this.

The controllers may additionally or alternatively measure the voltages, v1, v2, v3 etc., present at each appliance 1, 2, 3. Under normal operating conditions, each measured voltage should be slightly less than the supply voltage, If any of v1, v2, v3 etc. is significantly lower than this is evidence that there may be excessive ohmic heating in the cabling or connections between the two different voltages.

If either arcing or excessive ohmic heating is detected, the controller 8 reports a fault condition and takes an appropriate action. This action may typically be selected from switching off the current to the circuit and alerting an operator to the fault.

The monitors 5, 6, 7 communicate with the controller 8 using any suitable means of communication. Suitable communication means include point-to-point connections such as RS-232, wired networking protocols such as Ethernet , wireless communication methods such as WiFi , and power line communication methods such as HomePlug . An advantage of using power line communication is that no additional connections are required.

Each monitor 5, 6, 7 can monitor voltage information either instantaneously or averaged over a period of time, and current information either instantaneously or integrated over a period of time. This enables the monitor unit to measure the total energy consumed by the associated appliance or group of appliances in any time period. This is useful where electrical appliances are used that draw varying amounts of current over time, for example those with electric motors. Integrating over a period of time increases the accuracy of the measurement.

Each monitor unit can transmit voltage and current information, including averaged voltage information and integrated current information, either according to its own internal clock or in response to a specific request from a controller.

The controller 8 may accept voltage information that has either been read instantaneously or averaged over a period of time, and current information that has been either read instantaneously or integrated over a period of time. It may transmit time-stamps to the monitor units 5, 6 7 in order to synchronise clocks in the monitor units 5, 6, 7 and ensure that exactly the same time periods are measured by each one. Time-stamps may be sent from the controller 8 either as a single broadcast message to all monitor units 5, 6, 7, or as individual messages to each of them.

The monitor units may be disposed at any suitable points in the network. For example, they could be disposed in the plug of an electrical appliance, in the electrical appliance itself, interposed between the electrical appliance and the power outlet of the power network, disposed at a power outlet of the power network, disposed at a light fitting, or at any other point on the power network.

In an alternative embodiment, the controller can measure electrical properties at multiple points on the network. This is useful, for example, with ring main circuits. Referring to Figure 2, the controller comprises a first measuring unit 9 and a second measuring unit 10. A measuring unit 9, 10, is disposed on each branch of a ring main circuit 11. In this case, tota = Ia + b, and the controller will report a fault condition if a + b!= i + + 3 In a further embodiment, a monitor unit may also comprise multiple components functionally connected together. Fig. 3 shows a ring main circuit 12 in which equipment at points C and D comprise a single monitor unit covering a segment 13 of the ring main circuit 12, passing a current of (i + Id).

Consequently, the controller will report a fault condition if Ia + lb!= c + Id + + + i3 +... -Under normal conditions, the current supplied to segment 13 should be reasonably well balanced between the two branches of the ring main. If nearly all the current supplied to segment 13 is supplied via either monitor C or monitor D, then the controller will be able to deduce that the segment of the ring main supplying monitor D or monitor C respectively has a substantially greater resistance, which might be caused by a failing connection within that part of the main. Consequently, the controller will report a fault condition if L is substantially different from Id. Of course, a modest imbalance may exist simply because of the amount of current being used by devices closer to either of monitors C and D, and the system can take account of this.

It may be that a user wishes to prevent any electrical devices from being connected to a power circuit unless the electrical devices have an associated monitor unit. One way to ensure this is to use a plug with a specific shape for electrical appliances with monitor units. Figure 4 illustrates schematically a known plug in which pins 17, 18 are arranged to interact with corresponding sockets 19, 20. Referring to Figure 5, a power outlet is provided with a projection 21, and a plug of an electrical device having a power monitor comprises a corresponding recess 22 arranged to receive the projection 21. In this way, an electrical device with a standard electrical plug as shown in Figure 4 will not be connectable to a power outlet on a circuit that has a projection, as shown in Figure 5.

It will be appreciated by those of skill in the art that various modifications may be made to the above described embodiment without departing from the scope of the present invention. For example, a system may monitor voltage only, current only, or a combination of voltage and current.

Claims

CLAIMS: 1. A system for identifying a risk of fire in a power network,

the system comprising: at least one monitor for measuring at least one of current and voltage at a point in the network; a transmitter for transmitting measured data from the monitor to a central controller; a central control monitor for measuring at least one of current and voltage of the power network; a processor at the central controller for performing a fire risk analysis of the transmitted data and the measured data of the power network; and means for, in the event that the analysis identifies a risk of fire, arranging action to be taken to minimize the risk of fire.
2. A system for identifying a risk of fire in a power network according to claim 1, wherein the action to be taken is selected from one of alerting a user of the risk and cutting power to the network.
3. A system for identifying a risk of fire in a power network according to claim 1 or 2, wherein the central control monitor measures the total current used by the power network; and the analysis comprises comparing the total current used by the power network with the sum of the currents measured at each monitor, such that a risk of fire is identified if the total current used by the power network is substantially greater than the sum of the currents measured at each monitor.
4. A system for identifying a risk of fire in a power network according to claim 1, 2 or 3, wherein a risk of fire is identified if the voltage measured at a monitor is substantially lower than a measured voltage at the supply to the power network.
5. A system for identifying a risk of fire in a power network according to any one of the preceding claims, wherein said monitor measures electrical properties over time and transmits either an average or integrated value of the measurements to the controller.
6. A system for identifying a risk of fire in a power network according to any one of the preceding claims, wherein each monitor in the system obtains a measurement at the same time.
7. A system for identifying a risk of fire in a power network according claim 6, wherein the controller further comprises a transmitter to transmit a timing message to each monitor, the timing signal being used by each monitor to synchronize when measurements are taken.
8. A system for identifying a risk of fire in a power network according to any one of the preceding claims, wherein the controller comprises means to measure a current at more than one point on the power network.
9. A system for identifying a risk of fire in a power network according to any one of the preceding claims, wherein a monitor comprises means to measure a current at more than one point on the power network.
10. A system for identifying a risk of fire in a power network according to claim 8 or 9, in which the analysis of fire risk comprises comparing the currents measured at at least two points in the power network and determining if a significant imbalance is present between the two measured currents.
11. A system for identifying a risk of fire in a power network according to claim 10, wherein it is determined that a significant imbalance is present between the two measured currents if one of the measured currents is greater than the other measured current by more than a predetermined factor.
12. A system for identifying a risk of fire in a power network according to any one of the preceding claims, wherein a monitor comprises means to measure a voltage at more than one point on the power network.
13. A system for identifying a risk of fire in a power network according to any one of the preceding claims, wherein a power outlet on the power network comprises a projection arranged to co-operate with a corresponding recess on a plug.
14. A controller for use in a system for identifying a risk of fire in a power network, the controller comprising: a receiver for receiving a message from a monitor attached to the power network, the message containing data relating to a measurement of at least one of current and voltage; a processor for performing a fire risk analysis of the received data; and means for, in the event that the analysis identifies a risk of fire, arranging action to be taken to minimize the risk of fire.
15. A monitor for use in a system for identifying a risk of fire in a power network, the monitor comprising: means for measuring at least one of current and voltage at a point in the network; a transmitter for transmitting measured data to a central controller, such that the central controller can use the data for performing a fire risk analysis of the transmitted data.
16. A system for identifying a risk of fire in a power network substantially as described herein, with reference to Figures 1, 2, 3 and 5 of the accompanying drawings.
17. A controller for use in system for identifying a risk of fire in a power network, the controller substantially as described herein, with reference to Figures 1, 2, 3 and 5 of the accompanying drawings.
18. A monitor for use in system for identifying a risk of fire in a power network, the controller substantially as described herein, with reference to Figures 1, 2, 3 and 5 of the accompanying drawings.