WO2009090382A1

WO2009090382A1 - An isolated mains power supply socket

Info

Publication number: WO2009090382A1
Application number: PCT/GB2009/000096
Authority: WO
Inventors: Fares Mayia
Original assignee: Oxford Radcliffe Hospitals Nhs Trust
Priority date: 2008-01-15
Filing date: 2009-01-14
Publication date: 2009-07-23
Also published as: GB0800667D0; EP2232650A1

Abstract

The isolated mains power supply socket has a housing, comprising an isolation transformer, an insulation sensor, a socket for receiving a plug from an electrical appliance, and a connector for connecting the device to a power supply.

Description

AN ISOLATED MAINS POWER SUPPLY SOCKET

For some electrical equipment, for example in laboratory, industrial, or medical environments, it is desirable to have an isolated power supply (IPS)/uninterrupted power supply (UPS) to ensure continuity under a "single fault condition" and provide an increased safety level against electric shocks. In medical settings, according to the International Electrotechnical Commission standards and the Institute of Electrical Engineering regulations, PSAJPS systems are required in Interventional Diagnostic and Therapeutic medical rooms to ensure continuity of the supply under single fault conditions and to provide an increased patient safety level for a patient who is connected to the electrical equipment

The use of a high quality shielded isolation transformer also has advantages for use with equipment which is sensitive to line transients and line noise, with medical equipment being a particular example. Although many manufacturers of voltage-sensitive equipment provide a measure of protection as an integral part of their equipment, this protection may not be adequate for frequent or serious disturbances. The shielded isolation transformer in an IPS provides a "clean" power source, which has greatly reduced noise

The use of IPS sources in hospitals began in the US in the 1920s. At present, IPS/UPS units are still provided centrally, for example one per room in medical settings. They are bulky and require space. Supply of isolated power to, for example, a room, requires a separate mains supply for the central IPS/UPS and new wall sockets or rewiring to provide isolated power to individual power points. In order to install the unit, the entire power supply for a room must be switched off, and hence the room is out of clinical use for, typically, several days. Providing isolated power in this manner is expensive, for example, the cost for a typical room, including equipment, is around £10 000 per room. Furthermore, if a fault is detected and an alarm sounded by the central PS system, where there is more than one piece of equipment connected to the PS system, the only way of establishing which piece of equipment is faulty is by unplugging each piece individually until the alarm stops. This is inefficient, but it is also particularly undesirable where the equipment connected to the PS/UPS system is critical to a patient's condition, for example a life support machine. Central IPS units are also liable to be damaged if many pieces of equipment are switched ON simultaneously, causing a high "surge" current Such a situation may occur, for example, when many pieces of equipment connected to the IPS unit are in operation and the circuit breaker is tripped, and then reset The resulting in-rosh current may be high enough to damage the IPS transformer.

The inventor has realised mat providing a self-contained unit may help to overcome many of these disadvantages.

According to the present invention there is provided an isolated mains power supply socket having a housing, comprising: an isolation transformer; an insulation sensor including a line isolation monitor arranged to measure current imbalance between two lines; an equipotential earth bar; a socket for receiving a plug from an electrical appliance; a connector for connecting the device to a power supply; and a switch to turn ON/OFF the power supply to the transformer, the switch being arranged to be moved automatically to the ON position when a plug has been received by the socket for receiving a plug from an electrical appliance and to the OFF position when a plug is removed from the socket

The switch may be in any suitable form, but may be a microswitch or, alternatively, may be an optically activated switch.

The socket may further include an alarm system and preferably the alarm system operates when a fault condition is detected.

The supply socket may be provided as part of dado trunking, but may also be in a floor box or wall socket, or it could be ceiling pendant mounted Alternatively, the supply socket may be mounted on a trolley. The supply socket may include a back plate.

Thus the present invention provides the benefits of a central IPS system, that is, an isolated power system and reduced or eliminated line transients and noise, but can be provided independently for an individual power point Therefore if a fault occurs, it is immediately evident which piece of equipment is faulty. When the wiring in a room is converted, for example, to an isolated mains power supply (IMPS) socket, no change to the existing electrical supply or wiring is required and thus the conversion time and cost is greatly reduced — typically, conversion of sockets requires a room to be out of use for only one day and costs £1000-£2000 per room. Converting existing power sockets also eliminates the need for a bulky central unit, and so is space saving.

The fact that individual IMPS units would typically support only one piece of equipment, compared with central units which supply power to many pieces of equipment, greatly reduces the maximum in-rush current that can occur, reducing the risk of damage to the transformer in the event that the main circuit is tripped.

Furthermore, individual IMPS units allow each power socket in a room to have an isolated supply, whereas the transformer in a central IPS unit has a limited capacity. The maYimiim power available from an individual IMPS unit is 3 kVA from a 13 A circuit, for example, and two units can be combined to give a power output equivalent to a central unit, whilst allowing the flexibility of two independent power sources.

As individual IMPS units can be located at a number of points around a room, equipment can be located close to the units, thus reducing the length of mains Cable and therefore reducing capacitive leakage effects. This is demonstrated by reduced line-to-earth voltage.. A lower line-to-earth voltage lowers the risk of electrocution.

The effectiveness of the transformer in reducing line-to-line and line-to-ground noise is increased when compared with central units as the isolation transformer is closer to the equipment being powered.

As the switch is moved automatically on inserting/removing a plug into/from the socket, the power supply to the isolation transformer, i.e. the mains power supply to the primary winding of the transformer, is disconnected automatically when the socket is not in use. Therefore, the primary coil will not be energised when the socket is not in use, thus saving energy. The combination of IMPS suitable for use with just a single piece of equipment, and thus with a smaller power supply than a central unit, and a switch to stop the primary winding receiving power when no equipment is plugged in, allows the power consumption in, for example, a particular medical room, to be proportional to the number of pieces of equipment which are plugged in. This contrasts with a central IPS, which must be fully energised, at a much higher power input than an individual IMPS, even if only one piece of equipment is plugged in. The in-rush current can be controlled by, for example, using a delay circuit. When the power to the primary coil of the transformer is switched on, the delay circuit will provide a gradual increase of the supply voltage rather than the sudden increase which results in an "in-rush" current. The delay circuit may be a positive thermal coefficient (PTC) component or any other suitable delay circuit The in-msh current limiter is inserted in series with the live line to the primary coil of the isolation transformer.

The transformer, in addition to isolating the power supply, may transform the voltage.

It may also be a 1:1 primary to secondary transformer. The transformer therefore provides only the function of isolating the power supply, and does not transform the voltage.

The transformer may be a medical grade shielded isolation transformer.

The insulation sensor may further include an insulation monitor. Insulation monitors and line isolation monitors monitor resistance and impedance, respectively, and both, therefore, can give an indication of the quality of insulation.

The socket may further include means of ensuring an uninterruptible power supply (UPS). Therefore, continuity of power is ensured in the event of a break in the mains power. UPSs are generally known in the art.

The socket may further include a power monitor.

The system may further include a device to monitor temperature. The device to monitor temperature may include a positive temperature coefficient sensor.

The alarm system may operate when the device to monitor temperature indicates that the temperature of the transformer has exceeded an acceptable value.

The system may include means of connection with an existing mains supply. The means of connection with an existing mains supply may comprise plug pins.

The connector for connecting the device to a power supply is suitable for forming a hardwired connection.

According to a second aspect of the present invention, there is provided a method of providing an isolated mains power supply socket by providing within a housing: an isolation transformer, an insulation monitor including a line isolation monitor airanged to measure current imbalance between two live lines; an equipotential earth bar; a socket for receiving a plug from an electrical appliance; a connector for connecting the device to a power supply; and a switch to turn ON/OFF the power supply to the transformer, the switch being arranged to be moved automatically to the ON position when a plug has been received by the socket for receiving a plug from an electrical appliance and to the OFF position when a plug is removed from the socket

According to a third aspect of the present invention, there is provided a method of providing an uninterruptible isolated mains power supply socket by providing within a housing: an isolation transformer; an insulation monitor; a socket for receiving a plug from an electrical appliance; a connector for connecting the device to a power supply; and means of ensuring an uninterruptible power supply.

Two embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:

Fig. 1 is a schematic representation of the isolated mains power supply socket of the first embodiment;

Fig. 2 is a schematic representation of the isolated mains power supply of the second embodiment, further including an uninterruptible power supply.

Embodiment 1

Fig. 1 illustrates the isolated mains power supply socket 10 of this embodiment. The supply socket 10 is for use in a medical room. The supply socket 10 includes a 1:1 medical grade isolation transformer 12, with an earthed shield 14 which is earthed via an equipotential earth bar 16 to the earth 18. The transformer 12 is of a laminate type. The transformer 12 receives power at 230V from the mains power supply 22. Both the live secondary outputs 24, 26 from the secondary coil of the transformer 12, and the earth 18, are connected to a socket 28 for receiving a 3-pin plug (or S pin plug for 3 phase supply) from an electrical appliance. A positive temperature coefficient sensor (not shown) is embedded in the core of the transformer 12. A load monitoring current transformer 32 is provided on one of the live outputs from the secondary coil of the transformer 12. The socket 10 further includes an insulation monitor 34. This continuously monitors the insulation resistance of the socket 10 and any equipment connected to it The temperature coefficient sensor and the load monitoring current transformer 32 form an integral part of the insulation monitor 34. An alarm system 36 is connected to the insulation monitor 34. The alarm system has a green light 38, a red light 40, and an alarm 42.

A microswitch 44 is provided on the socket 28. The microswitch 44 is closed by the earth pin of a plug which is inserted into the socket and this operates the double pole switch 46 between the supply 22 and the transformer 12. The switch 46 is opened when a plug is removed from the socket 28, thus turning OFF the power supply to the transformer and preventing the primary winding of the transformer 12 from being energised when no equipment is plugged into the socket 28.

The supply socket is provided as part of the dado trunking in the medical room, and is suitable for supplying power to a single piece of equipment It is used as an alternative to a standard 13 A socket

When the supply socket 10 is connected to the mains power supply 22 the green light 36 operates to indicate normal functioning of the circuit. As the transformer 12 is a 1:1 ratio transformer, the power output to the socket 28 for receiving a 3-pin plug from an electrical appliance is 230V. If a fault is detected by the insulation monitor 34 the red light 40 operates and the alarm 42 sounds to indicate the fault A fault may be detected if the insulation resistance between the active phase conductors and the earth 18 drop below a threshold value, for example 50 kΩ, if the positive temperature coefficient sensor indicates mat the temperature of the transformer has reached an unacceptable level, or if the load measured by the load monitoring current transformer 32 reaches an unacceptable level.

Embodiment 2

Fig. 2 illustrates this embodiment. The arrangement of the isolated mains power supply socket 10 is similar to that of the first embodiment Only the differences from the first embodiment will be described. The same reference numerals will be used for equivalent features.

The transformer 12 is a toroidal type transformer. An uninterruptible power supply 70 is provided between the mains power supply 22 and the transformer 12 of the isolated mains power supply socket 10. The uninterruptible power supply 70 includes batteries 72, an AC to DC converter 74 between the mains power supply and the batteries 72, and a DC to AC converter 76 between the batteries 72 and the transformer 12 of the isolated mains power supply socket 10.

In use, if the power supply to the isolated power supply socket 10 fails, the batteries 72 in the uninterruptible power supply 70 will replace the mains power supply 22 in supplying power to the isolated power supply socket 10.

The embodiments given here are examples, but there are a number of alternatives which could be used.

The supply sockets described in the above embodiments are for use with medical equipment, and so a medical grade shielded isolation transformer has been used. However, a supply socket according to the invention may also be desirable in, for example, laboratory or industry settings, in which case a suitable grade of shielded isolation transformer may be chosen according to the proposed use.

In the above embodiments a transformer with a 1:1 primary to secondary ratio has been used, so that the only function of the transformer is to isolate the power supply. However, it would also be possible to use a transformer with a different primary to secondary ratio if transformation of the voltage was also required.

Although the socket for receiving a plug is suitable for receiving a 3-pin plug in the above embodiments, different types of socket could, of course, be used, to accommodate worldwide variations in plugs.

The embodiments described are replacements for a standard 13 A socket, but the invention could be used to provide power of higher or lower amperage rating, and thus different kVA ratings. The kVA rating is limited by the maximum current of the supplying circuit. The embodiments may also be used to supply 3-phase sockets.

Although in embodiment 1 the supply socket is provided as part of the dado trunking, it could also be provided at any height on a wall, in a similar manner to a normal wall socket, and could include a back plate. Alternatively, the socket could be provided in a utility box in the floor, or it could be ceiling pendant mounted. In addition, the supply socket may include suitable plug pins, so that the supply socket can be connected to existed power sockets, rather than being connected directly to the mains power. This allows the IMPS to be easily moved to different locations. In order to facilitate movement to different locations, the IMPS system could he mounted on a trolley. Inclusion of plug pins is only applicable for low kVA ratings, for example up to 0.5 kVA, as the transformers required for higher kVA ratings may be too heavy to be supported by the plug pins.

A microswitch is used in embodiment 1 to turn ON/OFF the power supply to the transformer, in response to a plug being inserted into/removed from the socket. Other means of detecting whether a plug is inserted into the socket, for example an optical sensor, can be used.

The switch 46 m embodiment 1 is a double pole switch, but a single pole switch may alternatively be used.

In embodiment 1, a laminate transformer was discussed, and in embodiment 2 toroidal transformer was discussed, but in fact either type of transformer could be used in both embodiments.

In both embodiments, a means of limiting the in-rush current by means of a delay circuit may be included, especially if a transformer of 1 kVA or higher is used.

Claims

1. An isolated mains power supply socket having a housing, comprising:

an isolation transformer,

an insulation sensor including a line isolation monitor arranged to measure current imbalance between two live lines;

an equipotential earth bar;

a socket for receiving a plug from an electrical appliance;

a connector for connecting the device to a power supply; and

a switch to turn ON/OFF the power supply to the transformer, the switch being arranged to be moved automatically to the ON position when a plug has been received by the socket for receiving a plug from an electrical appliance and to the OFF position when a plug is removed from the socket.

2. A socket as claimed in claim 1, wherein the switch is a microswitch.

3. A socket as claimed in claim 1, wherein the switch is an optically activated switch.

4. A socket as claimed in claim I, claim 2, or claim 3, wherein the in-rush current is controlled by a delay circuit

5. A socket as claimed in claim 1, wherein the socket further includes an alarm system.

6. A socket as claimed in claim 2, wherein the alarm system operates when a fault is detected.

7. A socket as claimed in any preceding claim, wherein the transformer, in addition to isolating the power supply, transforms the voltage.

S. A socket as claimed in any preceding claim, wherein the transformer is a medical grade shielded isolation transformer.

9. A socket as claimed in any preceding claim, wherein the insulation sensor further includes an insulation monitor.

10. A socket as claimed in any preceding claim, wherein the socket further includes means of ensuring an uninterruptible power supply.

11. A socket as claimed in any preceding claim, wherein the socket further includes a power monitor.

12. A socket as claimed in any preceding claim, wherein the system further includes a device to monitor temperature.

13. A socket as claimed in claim 12, wherein the device to monitor temperature may include a positive temperature coefficient sensor.

14. A socket as claimed in claim 12 or claim 13, wherein the alarm operates when the device to monitor temperature indicates that the temperature of the transformer has exceeded an acceptable temperature value.

15. A socket as claimed in any preceding claim, wherein the connector for connecting the device to a power supply is a connector for connecting the device with an existing mains supply.

16. A socket as claimed in claim 1 S, wherein the connector for connecting the device with an existing mains supply comprises plug pins.

17. A socket as claimed in any of claim 1 to claim IS, wherein the connector for connecting the device to a power supply is suitable for forming a hardwired connection.

18. A method of providing an isolated mains power supply socket by providing within a housing:

an isolation transformer;

an insulation monitor including a line isolation monitor arranged to measure current imbalance between two live lines;

an equipotential earth bar; a socket for receiving a plug from an electrical appliance;

a connector for connecting the device to a power supply; and

a switch to turn ON/OFF the power supply to the transformer, the switch being arranged to be moved automatically to the ON position when a plug has been received by the socket for receiving a plug from an electrical appliance and to the OFF position when a plug is removed from the socket

19. A method of providing an uninterruptible isolated mains power supply socket by providing within a housing:

an isolation transformer;

an insulation monitor,

a socket for receiving a plug from an electrical appliance;

a connector for connecting the device to a power supply, and

means of ensuring an uninterruptible power supply.