Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
2000-06-09
2003-12-02
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C702S065000, C702S071000, C702S079000, C702S185000
Reexamination Certificate
active
06658360
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a fault current detection system. More particularly, the present invention relates to a fault current detection system that is implemented via software and relates to a method employed by the detection system.
BACKGROUND OF THE INVENTION
In many applications, electrical currents are supplied to one or more electric devices to provide power for the devices. For example, electrical currents are supplied from a power company to one or more electrical outlets in a residential home, and a user can connect an electric device to an outlet to supply power to the device. If the electric device malfunctions or is mishandled by the user, a potentially dangerous situation may arise. For example, if the user contacts a portion of the electric device that receives electrical currents from the power company, the electrical current will pass through the user to the ground and may cause the user's heart to suffer from a cardiac arrest. Also, if the portion of the electric device that receives electrical currents is improperly grounded due to faulty insulation, a current will be supplied to the electric device and may start a fire in the user's home. The additional surge of current that is supplied to the user's home when the electric device malfunctions or is mishandled is known as a fault current.
In order to prevent fire in the user's home or to prevent the user from being harmed, a circuit breaker has been developed that detects fault currents and that blocks the supply of electrical current to the one or more electrical outlets in the user's home if the detected fault currents exceed certain levels.
FIG. 7
shows an example of such a circuit breaker
1
that comprises a sum-current transformer
2
, a power supply
4
, a triggering circuit
5
, a triggering relay
6
, a switching mechanism
7
, and a switch
8
.
The electric currents are supplied from the power company to the user's home via a conductor network LN, and the network LN includes three active conductors L
1
, L
2
, and L
3
and a neutral or ground conductor N. The conductor network LN is wrapped around a core
3
of the sum-current transformer
2
to form a primary winding N
1
of the transformer
2
. Also, a secondary winding. N
2
is wrapped around the core
3
of the transformer
2
, and the triggering circuit
5
is connected to the winding N
2
. Specifically, the triggering circuit
5
is connected across the output terminals of the winding N
2
, and the triggering relay
6
is connected across the output terminals of the circuit
5
. The triggering relay
6
controls the switching mechanism
7
to selectively open and close the switch
8
, and the switch
8
is provided in the path of the conductor network LN between the power company and the electrical device.
When the electrical device in the user's home is operating or being handled under normal conditions, no fault currents exist. As a result, the vector sum of the currents flowing through the core
3
via the conductor network LN is zero. However, if a fault current If is generated, the vector sum of the currents is not zero, and a voltage U
e
is generated across the secondary winding N
2
. The characteristics of the voltage U
e
correspond to the characteristics of the fault current I
f
, and the triggering circuit
5
generates an output voltage U
a
based on the input voltage U
e
. The output voltage U
a
causes a current I
a
to flow through the triggering relay
6
, and the relay
6
triggers. The triggering of the relay
6
causes the switching mechanism
7
to open the switch
8
and block the supply of current from the power company to at least one outlet in the user's home. Accordingly, when the user contacts a conductive portion of an electric device connected to an outlet and causes a fault current I
f
to be generated, the relay
6
triggers, and the switching mechanism
7
opens the switch
8
. As a result, the dangerous fault current If is no longer supplied to the user's home and will not harm the user.
The value of a triggering fault current I
&Dgr;trigger
of the circuit breaker
1
(i.e. the value of a fault current If that will trigger the relay
6
) is determined based on the rated residual current (or nominal fault current) I
&Dgr;n
. The nominal fault current I
&Dgr;n
corresponds to the sensitivity of the circuit breaker
1
and is selected based on the electrical standards of the electrical system in which the circuit breaker
1
is incorporated. An example of how the triggering current I
&Dgr;trigger
is selected will be described below in conjunction with the graph illustrated in FIG.
8
.
The graph shows an example of a fibrillation limit curve G
1
and a fire prevention limit curve G
2
. The fibrillation limit curve G
1
represents the maximum value of the fault current If that will not cause a user's heart to fibrillate if the user contacts the current If, and the values in the curve G
1
are,dependent upon the frequency of the fault current If. For example, if the fault current If has a frequency of 100 Hz and is less than or equal to about 30 mA, the user will not suffer ventricular fibrillation, but if the fault current I
f
is greater than approximately 30 mA, the user will experience fibrillation. On the other hand, if the fault current I
f
has a frequency of 1 kHz, the user's heart will not fibrillate if the current I
f
is less than or equal to approximately 420 mA but will fibrillate if the current I
f
is greater than such value.
While the maximum current values in the fibrillation limit curve G
1
are dependent on the frequency of the fault current I
f
, the maximum current values represented by the fire prevention limit curve G
2
are not frequency dependent. In particular, if the value of the fault current I
f
(at any frequency) is less than or equal to approximately 420 mA, a fire will not occur in the user's electric device or home, but if the value is greater than 420 mA, a fire will likely occur. In the present example, the current value of 420 mA is selected for a power system with a voltage of 230 V (with respect to ground) in order to prevent a power dissipation that is greater than 100 Watts at the location of the fault.
As indicated above, the specific values and characteristics of the limit curves G
1
and G
2
are governed by the electrical standards of a particular electrical system. For example, the limit curve G
1
is determined according to the international standard IEC
479
. If the circuit breaker
1
were operating according to different standards, the specific values of the curves G
1
and G
2
would be different.
The triggering fault current I
&Dgr;trigger
, which causes the circuit breaker
1
to trip, should be selected based on both the fibrillation limit curve G
1
and fire prevention limit curve G
2
on the graph shown in FIG.
8
. Specifically, the triggering fault current I
&Dgr;trigger
should be selected such that, when a fault current I
f
occurs, the circuit breaker
1
will trigger before the fault current I
f
rises to a level that can cause injury to the user of an electric device or to a level that can cause a fire. Therefore, if the circuit breaker
1
is operating in an environment in which fault currents having low frequencies may be generated, the triggering fault current I
&Dgr;trigger
may be set to a value that is below the fibrillation limit curve G
1
at low frequencies. In the example shown in
FIG. 8
, the triggering fault current I
&Dgr;trigger
would be less than approximately 30 mA if harmful fault currents I
f
having frequencies of 50 Hz may possibly be generated. However, as shown in
FIG. 8
, the maximum values of the limit curve G
1
significantly increase as the frequency of the fault currents I
f
increases.
In addition, several types of fault currents I
f
may occur that can cause harm to a user of an electric device or than can cause a fire in the user's home. The different types of fault currents include an alternating fault current, a pulsatin
Gies Stefan
Schmid Reinhard
Hoff Marc S.
Siemens Aktiengesellschaft
Suarez Felix
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