Protective device, in particular a fault current protective...

Details Protective device, in particular a fault current protective... Protective device, in particular a fault current protective...

2002-02-25

2003-11-18

Riley, Shawn (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

With specific current responsive fault sensor

C361S047000

Reexamination Certificate

active

06650523

ABSTRACT:

FIELD OF THE INVENTION
The invention generally relates to a protective device. Preferably, it includes at least one detection system with a core balance transformer whose secondary winding is followed in a tripping circuit by a tripping relay for operating a switching mechanism which switches a conductor network. A tripping circuit is a circuit along which an electrical monitoring variable is produced and is assessed. Further, such a circuit produces an electrical tripping signal which activates a tripping relay (causes it to trip), for example, when a tripping condition is satisfied. A protective device can include a fault current protective device and/or a differential current protective device, for example.
BACKGROUND OF THE INVENTION
Such a fault current protective device is used to ensure protection against a dangerous body current in an electrical system. Such a body current occurs, for example, when someone touches a live part of an electrical system. The fault current (or else differential current) then flows via the person as a body current to ground. The protective device that is used for protection against the dangerous body currents disconnects the relevant circuit from the main system quickly and safely when the so-called tripping fault current is exceeded.
The construction of known fault current protective devices is known, for example, from “etc” (1986), Issue 20, pages 938 to 945. FIGS. 1 to 3 there show outline circuit diagrams and functional principles of a fault current circuit breaker (Fl circuit breaker) and of a differential current circuit breaker (DI circuit breaker). FI and DI circuit breakers are constructed, in a similar way from three assemblies. When a fault current occurs, a voltage signal is induced in the secondary winding of a core balance transformer, through whose transformer core all the current-carrying conductors of a conductor network are passed. This voltage signal actuates a tripping relay, which is connected to the secondary winding via tripping circuit electronics, or via a tripping circuit. The tripping relay then operates a switching mechanism, via which the conductors of the conductor network are disconnected. In this case, the tripping circuit of the FI circuit breaker is coupled purely inductively via the core balance transformer to the conductor network. It thus takes the energy required for tripping from the fault current itself, irrespective of the main system voltage. In contrast, in a DI circuit breaker, tripping takes place as a function of the main system voltage via an amplifier circuit, which is conductively connected to the conductor network.
The tripping fault current is defined in DIN VDE Standard 0664 Part 10 (=German translation of EN Standard 61008). This is the value of the fault current which causes an FI or DI circuit breaker to trip in defined conditions. In this case, the tripping fault current for, for example, sinusoidal alternating fault currents is 0.5 to 1.0 times the rated fault current, which is in turn a measure of the tripping sensitivity of the FI or DI circuit breaker. Thus, for example, in order to protect personnel coming into direct contact with active parts, the rated fault current must not exceed 30 mA, while an FI protective device with a rated fault current of more than 30 mA offers protection only against indirect contact.
The tripping response of the circuit breaker is furthermore normally also matched to a specific frequency, for example to 50 Hz, or to a specific frequency band, for example from 50 Hz to 400 Hz. Nevertheless, despite this matching, these protective devices can also offer personnel protection at higher frequencies, provided the tripping fault current is below the specified limit curve for ventricular fibrillation, in accordance with IEC Standard 60479. In accordance with this limit curve, the tripping current may rise to approximately 420 mA at 1 kHz, in order still to offer personnel protection.
In order to ensure, furthermore, that such a protective device provides fire protection as well, a maximum electrical power of 100 W must not be exceeded, irrespective of the frequency, in order to avoid fires. If the voltage between an outer conductor and ground is assumed to be 230 V, this results in a maximum tripping fault current of 430 mA, which must not be exceeded, in order to avoid fires. Other limit values for the tripping fault current for other main system voltages correspond to this.
However, DI and FI protective devices until now have had the problem that their tripping fault current rises continuously as the frequency increases and, at high frequencies, in particular in the kilohertz band, exceeds the maximum acceptable value for fire protection of, in this example, 430 mA. For applications in electrical systems in which frequency changers and appliances with pulsed power supplies are used, fault currents having fault current frequencies up to about 20 kHz, furthermore, can also occur in the event of a fault. Thus, the tripping fault current of the protective device or of the circuit breaker rises beyond the limit value, in the described manner, and fire protection is no longer ensured in all cases. As a result of the greatly increasing number of equipment items which can generate such fault currents at a relatively high frequency in the event of a fault, this problem is becoming increasingly important.
SUMMARY OF THE INVENTION
An embodiment of the invention is based on an object of specifying a protective device, in particular a fault current protective device, via which it is also possible to detect fault currents at a fault current frequency of more than 1 kHz. The tripping fault current should not exceed the respective predetermined limit values for personnel protection and fire protection, in particular over a frequency band from about 50 Hz to at least 20 kHz.
According to an embodiment of the invention, this object can be achieved by a protective device.
An embodiment may be based on the idea that, in the case of a protective device such as this and even over a wide frequency range from, for example, 50 Hz to at least 20 kHz, firstly, fire protection is ensured provided the tripping fault current is less than 430 mA for 100 W and 230 V and, secondly, at the same time, personnel protection is provided as long as the tripping fault current is always below the limit curve for ventricular fibrillation in accordance with IEC 60479. Since, as is known, the limit curve for ventricular fibrillation reaches a value of 420 mA at a fault current frequency of about 1 kHz, and is thus virtually equivalent to the limit value for fire protection, the tripping fault current would need to be restricted to this limit value only above a corresponding changeover frequency, while the tripping fault current for fault current frequencies below this changeover frequency should have a profile below the limit curve for ventricular fibrillation and, in this case, should follow this profile as closely as possible.
This can be achieved in a reliable manner via two detection systems of different design, one of which provides the tripping function below this changeover frequency, while the other provides the tripping function above this changeover frequency which is, for example, between 1 kHz and 5 kHz, and preferably in the band between 1 kHz and 2 kHz.
A first detection system, which provides the tripping function below the changeover frequency, can be expediently matched to a fault current frequency of 50 Hz, or for fault currents in the comparatively broad frequency band from, for example, 50 kHz to 400 kHz. At this frequency, or in this frequency band, the tripping conditions can be satisfied, in accordance with EN Standard 61008 (which corresponds to VDE 0664 Part 10) for a protective device which is sensitive to pulsed currents (corresponding to Type A in accordance with EN 61008). The first detection system may have added to it a detection system for smooth direct fault currents whose fault current frequency is 0 Hz, so that, overall, this results in a protective device wh

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