The present invention relates to a method according to the preamble of claim 1 for fault detection in electric glass panes.

The method also concerns an apparatus for fault detection in electric glass panes.

So-called selective glasses incorporating a conducting film have gained wide popularity also in applications to smaller buildings. In such glasses, the surface tempera¬ ture of the glass can be elevated to a desired value by feeding the conducting film with at a controlled level of electric power. The thickness of the conducting film must be selected very accurately so that the film is transmis- sive to the wavelength range of visible light, while on the other hand it must also be capable of effectively reflecting thermal radiation back to the interior spaces. An additional constraint related to production techniques is that only a few materials are suited for use as the conducting film. Therefore, the surface resistivity (sheet resistivity) values R^ of selective glasses fall in the range 15-25 ohm so that a typical value of the surface resistivity is 19 ohm. Accordingly, the end-to- end resistance of a single glass pane (height x width) can be computed from the formula

^R giaββ " Ro-k/1, where

k = interelectrode distance (e.g., height of glass pane) 1 = length of electrodes (typically width of glass pane).

On the other hand, the maximum surface power dissipation permitted for a window pane is typically in the range 50 - 600 W/m ² . Therefore, the maximum allowable input

power levels will be exceeded in a great number of cases for windows with normal dimensions if the window is directly connected to the mains voltage.

Accordingly, electrically heated glass pane elements are conventionally fed from the mains voltage either connect ed in series for a sufficiently high total resistance, o alternatively, having the glass pane elements connected to a lower voltage via a suitable transformer circuit. A novel type of heating power control is the time-switched input power feed control in which the mains voltage is applied in a time-limited manner, whereby a zero-power control cycle is inserted between the power-on control cycles.

Fault detection in heating systems based on continuous input power feed has not caused any major problem, because the occurrence of faults has been easy to monito by sensing the resistance of the power feed circuit. By contrast, no matter how cost-efficient the circuit may b otherwise, condition monitoring of a time-switched con¬ trol circuit is problematic as the power feed circuit is opened repetitively according to such a control scheme.

The goal of the invention is achieved by implementing th supply voltage feed to the fault-detection circuit by means of inductive coupling to the mains power feed con¬ ductor of the heated window pane so that said inductive coupling circuit is fed during the zero-power control cycles of the input power feed control period via an auxiliary power feed circuit connected in parallel with the terminals of the primary power switching circuit, whereby the auxiliary power feed is limited by means of series capacitor.

More speci ically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

Furthermore, the fault-detection apparatus according to the invention is characterized by what is stated in the characterizing part of claim 5.

The invention offers significant benefits.

The arrangement according to the invention provides con¬ tinuous supply voltage to the fault-detection circuit independently from the input power feed control status, which may be active or nonactive. The system may addi- tionally serve as a burglary alarm. The system can be installed in a place concealed from unauthorized people, whereby a person possibly attempting a burglary or vandalism has no means of knowing whether the electric glass pane system is provided with an alarm facility or not. The present arrangement is suited for use in con¬ junction with all conventional input power controller types and the function of the apparatus is not dependent on the power-on control cycle duration or the power- on/power-off duty cycle. The components of the apparatus are inexpensive, and moreover, the apparatus can be made small due to the small number of components required.

In the following the invention will be examined in great¬ er detail with the help of an exemplifying embodiment by making reference to the appended drawings in which

Figure 1 is a block diagram of a circuit arrangement according to the invention;

Figure 2 is a circuit detail of the arrangement shown in Fig. 1; and

Figure 3 is a circuit diagram of an apparatus according to the invention complemented with a backup power supply circuit.

Referring to Fig. 1, the fault-detection arrangement shown therein is intended for use in an environment having at least one electrically heated glass pane 1 con¬ nected in series with a power feed conductor 2, whereby electric input power feed to the glass panes can occur through switching the power feed on and off by means of a switch 7. Single-phase feed is typically used as the supply 8 for window pane heating. According to the inven¬ tion, the apparatus is provided with toroidal transformer unit 13 with the feed conductor 2 threaded through the opening of its magnetic circuit 3. The fault-detection circuit 6 obtains its supply voltage either directly via a winding 5, whereby the energy induced by the conductor 2 in the toroidal magnetic circuit 3 is transferred via the winding 5 to the circuit 6. In this mode, the trans- former structure 13 acts as a current transformer. By contrast, when the switch 7 is opened and the heating power feed to the window panes 1 is turned off, an auxil¬ iary power feed circuit connected in parallel with the switch terminals 7 feeds energy via a winding 4 to the magnetic circuit 3 that further transfers the energy to the electric input power feed detecting circuit 6. Herein, the transformer structure 13 acts as a voltage transformer. In this manner the fault-detecting circuit 6 in its simplest form can sense the presence of electric energy invoked by alternating electromagnetic induction in the magnetic circuit 3. If no electric power feed is sensed, the detecting circuit issues an alarm signal. The capacitor 9 of the auxiliary power feed circuit is dimen¬ sioned so that the auxiliary power feed circuit will not cause excessive load in the system while yet supplying sufficient energy for the detecting circuit 6 to trigger the circuit. A suitable capacitance value of the

capacitor is, e.g., in the range of approx. 0.01 - 0.1 μF. In lieu of the toroidal magnetic circuit 3, any other suitable transformer core structure capable of inductively transferring the required sensing energy to the fault-detecting circuit 6 may be employed. A suitable triggering power level for the detecting circuitry is, e.g., in the range 200 - 500 m . The triggering power level is adjusted by varying the number of turns in the windings 4 and 5 as well as the number of turns of the conductor 2 wound about the core of the magnetic circuit 3. The following table gives typical guideline values for the different number of turns of the power feed conductor 2 for different current levels in the conductor.

Referring to Fig. 2, a typical fault-detection apparatus comprises a rectifier 10 followed by a low-pass filter, which in the present case is implemented as an RC circuit 11. A suitable time constant for the RC circuit 11 is, e.g., in the range 20 - 100 ms. The rectified, filtered sensing signal is in turn taken to a relay 12 arranged to open/close an alarm circuit at the loss of the sensing signal. Obviously, the relay 12 may be replaced by any digital sensing means based on solid-state devices.

Referring to Fig. 3, the circuit configuration of the apparatus is shown incorporating a backup power supply circuit. When the mains voltage (230 VAC) is switched on, the contactor K2 is energized thereby opening the con¬ tacts 21-22, 31-32 and 41-42. These contacts disconnect

the 12 V backup power supply circuit from the alarm cir- cuit(s). Simultaneously, the contact 15-14 is closed.

When the contact 15-14 of contactor K2 is closed, the contactor K3 is energized and contacts 21-22 and 31-32 open. These contacts give additional isolation in the disconnection of the backup power supply circuit. Con¬ tacts 23-24 and 15-14 are simultaneously closed, thereby switching the input power on to the electric heated glass pane system 1. Subsequently, the heating system which includes a thermostat for the electric glass pane and an alarm system commences operation in the same manner as the above-described electric glass pane system for burglary alarm.

With the mains feed switched on, the alarm relay Kl remains energized by the sensing voltage thus keeping the alarm circuit closed irrespective of whether the thermo¬ stat is open or close, provided that the electric glass pane circuit is intact.

Upon the disconnection of the mains supply, the contac¬ tors K2 and K3 release, whereby the electric glass pane system remains isolated from the mains on both the live and neutral ends of its feed circuit, and the backup power supply circuit is connected to feed the sensing circuit.

Now, the alarm relay Kl is connected to the positive potential of the backup power supply via its contact

K2:31-32 and to the ground potential of the backup power supply via the contacts K2:41-42 and K3:31-32 and the electric glass pane. The alarm relay Kl remains energized thus keeping the alarm circuit closed provided that the electric glass pane circuit is intact.

The purpose of the capacitor Cl is to prevent a direct short circuit over the voltage winding of the toroidal transformer Ml in the case that the mains supply is on while the thermostat contact is closed.

The purpose of the capacitor C2 is to block the DC path from the rectifier back to the toroidal transformer winding when the mains supply is disconnected.

The purpose of the capacitor C3 is to delay the release of the alarm relay Kl for the duration of contactor energization and thermostat contact switchover requiring a delay of less than one second typical.

The circuit 25 includes a mechanical antitamper switch serving to activate the alarm circuit if the enclosure of the apparatus is opened.

Elements 20 are fuses.

The invention is also suited for use in systems having the electric glass panes fed with a low-voltage supply via a transformer. Herein, the heating-power-controlling switch is typically located in the primary circuit of the step-down transformer.

The embodiment according to the present invention is particularly advantageously used in conjunction with a zero-voltage-switching power feed system in which each power-on cycle starts and stops at the zero-crossing point of sinusoidal AC supply voltage.

In the context of the present invention, the term magnet¬ ic circuit refers to any magnetic circuit suitable for use in a transformer such as a core made from a ferro¬ magnetic material.