|Introduction to Electrical Engineering - Basic vocational knowledge (Institut für Berufliche Entwicklung, 213 p.)|
|9. Protective Measures in Electrical Installations|
|9.2. Measures for the Protection of Man from Electric Shock|
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 protective switch
2 - Fault-current protective switch
3 - Pipe lines as earthing resistance
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 resistance).
1 - FI protective switch
2 - Protective contact socket
3 - Connection of PE to conductive parts in the building
1. Explain the protective effect of the various protective measures!
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 series?