WO2008146040A1

WO2008146040A1 - System for identifying fire risk

Info

Publication number: WO2008146040A1
Application number: PCT/GB2008/050380
Authority: WO
Inventors: Kevin Jones
Original assignee: Kevin Jones
Priority date: 2007-06-01
Filing date: 2008-05-27
Publication date: 2008-12-04
Also published as: GB0710374D0; GB2449677A

Abstract

A system for identifying a risk of fire in a mains segment. The system comprises at least two monitors for measuring at least one of current and voltage at points in the segment. Each monitor is provided with a transmitter for transmitting measured data from the monitor to a controller. The controller is arranged to perform a fire risk analysis on the basis of the transmitted data, and the system further comprises means for, in the event that the analysis identifies a risk of fire, cutting a power supply to the segment.

Description

System for Identifying Fire Risk

TECHNICAL FIELD

The invention relates to the field of systems for identifying fire risk in an electrical power circuit.

BACKGROUND

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://www.interfire.org/features/electric_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 readily cause an electrical fire. The risk is easily understood. If the insulation in a cable carbonises such that there is a resistance of 1 kΩ between live and neutral, then (assuming a UK-standard 240vac circuit) the insulation will pass a current of 24OmA and will dissipate nearly 6OW power. A current of 24OmA is not sufficient to blow a standard mains fuse or trip a standard circuit breaker. However, dissipation of 6OW 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.] Natl. Bur. Stand., Gaithersburg, MD (1977), a good electrical connection passing 2OA 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. SUMMARY

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, power to the circuit can be cut off by operating a circuit breaker, in order to minimize the risk of a fire.

According to a first aspect of the invention, there is provided a system for identifying a risk of fire in a mains segment. The system comprises at least two monitors for measuring at least one of current and voltage at points in the segment. Each monitor is provided with a transmitter for transmitting measured data from the monitor to a controller. The controller is arranged to perform a fire risk analysis on the basis of the transmitted data, and the system further comprises means for, in the event that the analysis identifies a risk of fire, cutting a power supply to the segment.

One or more monitors may be incorporated into a controller. Furthermore, the system may be provided with multiple controllers to provided redundancy.

In one embodiment of the invention, the system comprises a first monitor arranged to measure a total current used by the mains segment, and further monitors, arranged to measure the current drawn by one or more appliances attached to the mains segment. In this case, the analysis comprises comparing the total current used by the mains segment with a sum of the currents measured at each further monitor. A fire risk is identified if the total current used by the mains segment is substantially greater than the sum of the currents measured for each appliance. In this embodiment, the system may also be used for detecting faults that are unlikely to pose a fire risk, where the total current used by the mains segment is substantially less than the sum of the currents measured for each appliance. In this instance, means may be provided for reporting a fault condition, or power may be cut to the segment. In an alternative embodiment, a risk of fire is identified if a voltage measured at a monitor is substantially lower than a voltage measured at the supply to the mains segment. In the instance that a voltage measured at a monitor is substantially higher than a voltage measured at the supply to the mains segment, then there may be a fault that is unlikely to pose a fire risk, in which case means are provided for reporting a fault condition or cutting the power supply to the segment.

Each monitor may measure electrical properties over time, and transmit either an average value, an integrated value, or multiple values of the measurements to the controller. Optionally, each monitor in the system may obtain a measurement at the same time. In this case, the controller may be provided with a transmitter to transmit a timing message to each monitor, the timing message being used by each monitor to synchronize when measurements are taken. Furthermore, each monitor may be arranged to transmit measured data in response to receiving a request message from the controller.

A portion of the segment may be monitored by performing the fire risk analysis by comparing the currents measured at at least two points in the mains segment, and determining if a significant imbalance is present between the two measured currents. In order to account for minor current loss that does not pose a significant fire risk, 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.

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 mains segment. The controller comprises a receiver for receiving messages from at least two monitors attached to the mains segment, the messages containing data relating to measurements of at least one of current and voltage. The controller further comprises 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, cutting a power supply to the mains segment.

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 mains segment. The monitor comprises means for measuring at least one of current and voltage at a point in the segment, and a transmitter for transmitting measured data to at least one controller, the measured data arranged for use by controllers for performing a fire risk analysis of the transmitted data.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates schematically a mains electricity segment, with a controller and various monitors according to an embodiment of the invention;

Figure 2 illustrates schematically an electricity ring main in which two controllers are used to monitor the currents flowing into the two branches of a ring main; and

Figure 3 illustrates schematically an electricity ring main according to a further embodiment, with monitors used to protect only part of a ring main, the remainder of the ring main being "trusted".

DETAILED DESCRIPTION

It is known to measure accurately the voltage present at, and the current drawn by, any electrical appliance or any mains segment. Standard current measuring techniques include shunt resistors, current transformers, Hall effect sensors and Rogowski coils. 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 segment 4, with various appliances 1 , 2, 3 attached to it. These appliances 1 , 2, 3 draw currents h, i₂, i₃, etc. Monitors 5, 6, 7 are provided that measure the current drawn by each appliance. A further monitor 8 measures the total current i_totai drawn by the segment. Each monitor sends its measured data to a controller 9, which is operatively connected to circuit breaker 10 and can, if necessary, cause circuit breaker 10 to trip. In an embodiment of the invention, one or more monitors may be incorporated into the controller 9.

Under normal operation, the total current drawn by the appliances, h + i₂ + h +...., is equal to the current i_totai measured by the monitor 8. If i_totai is substantially greater than J₁ + i₂ + iβ +.... then this is an indication that arcing is occurring somewhere on the circuit. The additional current drawn i_totai - (h ⁺ h + iβ +....) 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.

In the event that i_totai is substantially less than J₁ + i₂ + iβ +.... then the controller 9 may report a fault condition, as this is unlikely to indicate a serious fire risk, or the controller alternatively trips the circuit breaker 10. It could be that there is an additional current source to the segment that has not been identified, that the monitors are not correctly configured (for example, monitor 8 might not be positioned so as to monitor the total current taken by the segment), or that there is a calibration error.

The controllers may additionally or alternatively measure the voltages, v-i, 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, v_suppiy.

If any of V₁, V₂, V₃ etc. is significantly lower than v_suppi_y, 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 9 will cut the power to the mains segment 4 by tripping the circuit breaker 10. If any of V₁, V₂, V₃ etc. is significantly higher than v_suppi_y, this is evidence of a configuration or calibration error. In this case, the controller 9 may report a fault condition or trip the circuit breaker 10.

The monitors 5, 6, 7, 8 communicate with the controller 9 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. Of course, in the scenario in which one or more monitors are incorporated in the controller, such communications are not necessary.

Each monitor 5, 6, 7, 8 can transmit voltage and current information read instantaneously, averaged over a period of time, integrated over a period of time, or as a series of samples.

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

The controller 9 may accept voltage and current information that has been read instantaneously, averaged over a period of time, integrated over a period of time, or presented as a series of samples. It may transmit time-stamps to the monitors 5, 6 7, 8 in order to synchronise clocks in the monitors 5, 6, 7, 8 and ensure that the same time periods are measured by each one. Time-stamps may be sent from the controller 9 either as a single broadcast message to all monitor units 5, 6, 7, 8 or as individual messages to each of them.

The controller 9 may have a remote connection facility, such as an Internet connection or a dial-up modem, to enable it to communicate with a remote system. The remote system may periodically upload data from the controller relating to currents and voltages measured over a period of time on the mains segment, and may analyse this data to provide additional early warning of potential faults.

Multiple controllers may optionally be used in a redundant configuration.

The monitors 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 a mains socket, disposed at a mains socket, disposed at a light fitting, or at any other point on the power network.

In an alternative embodiment, the current entering a mains segment is measured at multiple points. This is useful, for example, with ring main circuits. Referring to Figure 2, the current drawn by the two branches of the ring main 11 is measured by monitors 12 and 13. In this case, i_totai = i_a + ib, and the controller will trip the circuit breaker if i_a + ib > h ⁺ h + h +■ ■■ Furthermore, a fault condition may be reported, or the circuit breaker tripped, if i_a + ib < h + h + i₃ ⁺...

In a further embodiment, the system may monitor only part of a ring main, rather than all of it. Fig. 3 shows a ring main circuit 11 in which the wiring between points C and D is known to be safe. The total current taken by the appliances between these points is (i_c + id)- Consequently, the controller will trip the circuit breaker if i_a + ib > i_c ⁺ id ⁺ h ⁺ h + h +■ ■■ Furthermore, a fault condition may be reported, or the circuit breaker tripped, if i_a + ib < i_c ⁺ id + h + h + i₃ ⁺...

Under normal conditions, the current supplied to segment 14 should be reasonably well balanced between the two branches of the ring main. If nearly all the current supplied to segment 14 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 i_c is substantially different from i_d

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 mains segment, the system comprising: at least two monitors for measuring at least one of current and voltage at points in the segment, each monitor having a transmitter for transmitting measured data from the monitor to a controller; wherein the controller is arranged to perform a fire risk analysis on the basis of the transmitted data; and means for, in the event that the analysis identifies a risk of fire, cutting a power supply to the segment.

2. The system according to claim 1 , wherein the controller comprises a monitor for measuring at least one of current and voltage at a point in the segment.

3. The system according to claim 1 or 2, comprising a first monitor arranged to measure a total current used by the mains segment; and a plurality of further monitors, arranged to measure the current drawn by one or more appliances attached to the mains segment; wherein the analysis comprises comparing the total current used by the mains segment with a sum of the currents measured at each further monitor, such that a risk of fire is identified if the total current used by the mains segment is substantially greater than the sum of the currents measured for each appliance.

4. The system according to claim 3, further comprising means for reporting a fault condition or cutting the power supply to the segment in the event that the total current used by the mains segment is substantially less than the sum of the currents measured for each appliance.

5. The system according to any one of claims 1 to 4, wherein a risk of fire is identified if a voltage measured at a monitor is substantially lower than a voltage measured at the supply to the mains segment.

6. The system according to any one of claims 1 to 5, further comprising means for reporting a fault condition or cutting the power supply to the segment in the event that a voltage measured at a monitor is substantially higher than a voltage measured at the supply to the mains segment.

7. The system according to any one of claims 1 to 6, wherein each monitor measures electrical properties over time and transmits either an average value, an integrated value, or multiple values of the measurements to the controller.

8. The system according to any one of claims 1 to 7, wherein each monitor in the system obtains a measurement at the same time.

9. The system of claim 8, wherein the controller further comprises a transmitter to transmit a timing message to each monitor, the timing message being used by each monitor to synchronize when measurements are taken.

10. The system according to any one of claims 1 to 9, wherein each monitor in the system is arranged to transmit measured data in response to receiving a request message from the controller.

11. The system according to any of claims 1 to 10, in which the analysis of fire risk comprises comparing the currents measured at at least two points in the mains segment and determining if a significant imbalance is present between the two measured currents.

12. The system according to claim 11 , 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.

13. The system according to any one of claims 1 to 12, further comprising a plurality of controllers, the plurality of controllers providing redundancy in the system.

14. A controller for use in a system for identifying a risk of fire in a mains segment, the controller comprising: a receiver for receiving messages from at least two monitors attached to the mains segment, the messages containing data relating to measurements 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, cutting a power supply to the mains segment.

15. A monitor for use in a system for identifying a risk of fire in a mains segment, the monitor comprising: means for measuring at least one of current and voltage at a point in the segment; a transmitter for transmitting measured data to at least one controller, the measured data arranged for use by controllers for performing a fire risk analysis of the transmitted data.