9.2.7. Fault-current Protection
In the fault-current protection system, a special switching
device - the FI protective switch or the FI relay - is connected in series with
the loads. All accessible conductive parts must be earthed Fig. 9.7. shows the
fault-current protection device.
A summation transformer is arranged in the FI protective switch
which monitors the current flowing into and out of the installation. In the
faultless condition, the sum of these currents is always equal to 0 according to
the first Kirchhoffs law, even in the case of an unsymmetrical load.
The magnetic fields produced in the current transformer
neutralise each other and the secondary of the transformer is not excited. If,
in case of a fault, body contact occurs, the fault current does not flow via the
summation transformer but via ground.
Fig. 9.7. Fault-current protection
system with earthing the protective conductor connection
1 - Summation transformer in fault-current
2 - Fault-current protective switch
3 - Pipe lines as
RS = earthing resistance
Consequently, the sum of the inflowing and out flowing currents
in the transformer is unequal to 0. The voltage generated in the secondary of
the summation transformer causes the triggering of the switch and the all-pole
switching off of the installation within a very short time (about 20 ms).
The FI protective switches differ by the height of the tripping
current (rated, fault current Ifn) Ifn should be
designed, in such a way that it is three times the leakage current to be
expected. The earthing resistance must be so small that the rated fault current
causes a maximum voltage drop of UB perm at the most (formula 9.3.)
RS = UB perm/Ifn
maximum permissible contact voltage
rated fault current of the FI protective switch
For a protective switch with Ifn = 50 mA and UB
perm = 65 V, an RS of 2.15 kW is
obtained. This value can be reached without great difficulties. Also, for a
protective switch with Ifn = 500 mA, an earthing resistance of 130
W is sufficiently small. When a tool and the like
is with necessity connected with an earth lead (water pump, electrical thermal
storage water heater), this method of earthing will suffice when the required
earthing resistance is ensured.
In connection with the protective measure known as connection to
neutral, the FI protective system can be used to advantage according to an IEC
recommendation (Fig. 9.8.). The advantage over the connection to the neutral
consists in the fact that - in case of a relatively low fault current which is
considerably lower than the rated current of the fuse connected in series - a
quick switching off of the faulty installation is effected. The problems of
connection to the neutral associated with the realisation of the switching-off
factor k (equation 9.2.) are avoided, the total switching-off time is shorter.
In order to protect man from the dangerous effects
of an electrical shock, various protective measures can be taken in dependence
of the concrete conditions given. Besides the protective measures without
protective conductor (extra-low voltage, protective insulation), there are
protective measures with protective conductor (protective isolation, protective
conductor system, protective earthing, connection to the neutral). The measures
of the second group differ with respect to protective effect and costs. It is
possible to apply several protective measures at the same time (connection to
the neutral with FI protective system). The selection of the suitable protective
measure is dependent on the type of three-phase network given and the dangers
that may occur in the handling of electrical tools and the like. Further, it
must be decided whether or not several tools and the like may be switched off in
cause of a fault current (e.g. in case of connection to neutral) or only the
defective tool (e.g. in case of separate FI protective switch for each tool and
the like). Further, it must be decided whether in case of simple body contact it
should only be signalled and the work can be finished without endangerment (e.g.
protective conductor system with monitoring of the insulation
Fig. 9.8. Connection
to neutral combined with FI protective switch
1 - FI protective switch
2 - Protective contact
3 - Connection of PE to conductive parts in the building
1. Explain the protective effect of the various
2. What protective measure is suitable for dwelling
installation? Start from the consideration of the different
three-phase current mains!
3. What are the advantages of the connection to neutral with FI
protective switch over the connection to neutral?
4. What has to be observed when using the protective isolation?
5. Why have special connection to be used for the protective
measure known as extra-low protective voltage?
6. Why should the earthing resistance not exceed a maximum value
when the protective measure known as protective earthing with FI protective
system is used?
7. Why should the breaking current value be higher than the
rated current of the fuse connected in