Electrical transmission or interconnection systems – Personnel safety or limit control features
Patent
1986-03-25
1987-11-24
Miska, Vit W.
Electrical transmission or interconnection systems
Personnel safety or limit control features
340650, 361 42, 361 56, H02H 316, G08B 2100
Patent
active
047091649
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a circuit for continuously watching the grounding resistor of electrical appliances whose touchable, electrically conductive structural members, such as especially the casing, are connected to a ground potential by a protecting conductor.
As is well known, the metal parts to be touched, like the housing of electrical household appliances are connected to ground so as to protect the user from electric shock. If an electrically conductive part of such an appliance should become hot because of faulty insulation, the user who touches that part still does not get an electric shock because the part is at ground potential.
With the state of the art two fundamentally different types of protective circuits are to be distinguished, namely the so-called "protecting conductor connection" and the so-called "neutralization".
With the protecting conductor connection there is no galvanic connection at or in the electrical appliance between the so-called neutral conductor of the power current network and the local reference ground. Rather, the appliance is furnished with a protecting conductor which connects all dangerous touchable metallic parts with the local reference ground. If a fault occurs in the insulation of the appliance, a fault current which reaches the casing of the appliance is shunted to the reference ground by way of the protecting conductor. If this shunted current reaches a certain threshold value, usually the current supply to the appliance is turned off by means of a fuse.
On the other hand, if the so-called neutralization is provided to protect the user, the neutral conductor of the supply network at the electrical appliance is connected to the local reference ground. The protecting conductor in this case need not be connected separately with the reference ground, instead it is connected to the neutral conductor of the supply network at the socket (or in the terminal box). If an insulation fault occurs in the appliance with this type of protective circuit, the current flowing through the outer metal parts which may be touched is led off through the protecting conductor to the neutral conductor. As soon as this shunted current exceeds a preselected current intensity, the feeding voltage of the appliance is interrupted.
The known protecting circuits do not afford one hundred per cent safety for the user if the connection of the protecting conductor is deficient. The occurrence of line and contact resistances at the terminal or plug connections is unavoidable both with the protecting conductor connection as well as the neutralization described above. Thus series resistance which is not recognized at once may occur between the reference ground and the touchable metal member of the appliance if the contacts are poor. This series resistance between the touchable metal member and the reference ground may have the consequence of a dangerous voltage being developed at the metal part if there is a fault in the insulation of the appliance.
For the sake of protecting operators, therefore, there is considerable interest in continuously watching the grounding resistor (resistance between touchable metal part of the appliance and local reference ground). If this ground resistance exceeds a certain critical value, the voltage supply to the appliance must be interrupted for reasons of safety.
For monitoring the grounding resistance mentioned, it is known to connect the casing of the appliance (or other touchable metal members) to ground potential by a separate additional conductor apart from the conventional connections (main conductor, neutral conductor, and perhaps protecting conductor).
High-ohmic voltage metering is effected between the protecting conductor and the additional lead. The voltage rises in case of an increase of the grounding resistance and an insulation fault, and the respective appliance is switched off. However, this separate lead to the local reference ground is not available in customary installations so that it cannot be used for every appliance.
It is the object of the i
REFERENCES:
patent: 3864581 (1975-02-01), Satyanarayana
patent: 3878458 (1975-04-01), Muska
patent: 4298864 (1981-11-01), Mahnke et al.
patent: 4447844 (1984-05-01), Schossow et al.
Elektromanufaktur Zangenstein Hanauer GmbH & Co.
Miska Vit W.
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