Electricity: electrical systems and devices – Safety and protection of systems and devices – Load shunting by fault responsive means
Patent
1991-04-01
1992-10-20
Pellinen, A. D.
Electricity: electrical systems and devices
Safety and protection of systems and devices
Load shunting by fault responsive means
361111, 361120, 361127, H02H 322
Patent
active
051575728
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention is concerned with apparatus suitable for use in protecting electrical installations from transients, especially mains power installations.
BACKGROUND ART
Electrical transients on known power circuits are a major cause of problems especially in sensitive electronic equipment. The higher the voltage level of an electrical transient which reaches equipment to be protected, the more likely the equipment is to be damaged or adversely affected by the transient; for example, on 240 volt A.C. mains supply, transient over voltages can be caused by switching, remote lightning strikes and the like. The mains supply may carry transient over voltages of as much as 10 kilovolts, for example, and it is necessary to ensure that the over voltage which reaches the load equipment is reduced substantially, preferably below 1000 volts, suitably below 700 volts.
Various proposals have been made to protect installations against transient over voltages but these have often been expensive and/or not altogether effective: for example in some proposed protective apparatus, a leakage of a small current from the live conductor to earth may occur, which is undersirable. A voltage dependent resistor, known as a varistor, rated for operation on 240 volt mains will leak a steady-state current of less than 1 mA. However whilst diverting a 1 kA transient the varistor will develop in excess of 900 volts across its terminals. A varistor (or combination rated in total for 190 volt A.C. will develop only about 620 volts for the same transient.
Although the 190 volt rated component clearly offers better protection, it cannot be used on 240 volt mains since the steady-state leakage current would lead to over-heating and destruction of the varistor.
It has been proposed to protect electrical equipment against electrical transients by devices including gas discharge tubes and voltage dependent resistors. For example GB2175156A describes one such apparatus but in this apparatus voltage dependent resistors are connected directly between the live and neutral supply leads and, therefore, will suffer from current leakage as outlined above, or provide poor protection against live
eutral transients. Furthermore, the protection against transients occurring between live and neutral conductors is relatively ineffective, relying on the voltage dependent resistors. U.S. Pat. No. 4,677,518 describes another such apparatus but this is designed to give protection between only two conductors (live and neutral) and no direct protection between live and earth nor between neutral and earth conductors is envisaged.
DISCLOSURE OF THE INVENTION
One of the objects of the present invention is to provide improved protection to mains power equipment against electrical transients.
Another object of the invention is to provide apparatus effective to reduce large amplitude transients to a low level.
In one aspect the invention may be considered to provide a device suitable for use in protecting electrical installations from transients comprising a network including voltage dependent resistors connected to live, earth and neutral conductors, and a gas discharge tube disposed so that there can be no connection between the live conductor and the neutral and earth conductors other than through the gas discharge tube.
In preferred embodiments, the gas discharge tube is connected with the live conductor either directly or via a voltage dependent resistor but there is no connection between live and earth or live and neutral conductors, within the device, other than through the gas discharge tube.
In a device in accordance with the invention the spark-over level of the gas discharge tube (the voltage at which it becomes conductive) represents the minimum let-through voltage of the device i.e. transients smaller than the spark-over voltage will not be attenuated by the device. This voltage is preferably less than 600 Volts, more preferably about 500 volts.
Gas discharge tubes may be from any suitable source. Suitable tubes are supplied by Beswick,
REFERENCES:
patent: 3816802 (1974-06-01), Crask et al.
patent: 4677518 (1987-06-01), Hershfield
Jackson S.
Pellinen A. D.
W. J. Furse & Co. Ltd.
LandOfFree
Apparatus suitable for use in protecting electrical installation does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus suitable for use in protecting electrical installation, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus suitable for use in protecting electrical installation will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-197560