|Electrical Installation - Basic Vocational Knowledge|
|1. Protective measures against too high a contact voltage and conduct following electrical accidents|
|1.4. Mode of action, conditions and application of protective measures against too high a contact voltage|
This protective measure is mostly applied in ship-building. It works similarly to the current-operated earth-leakage protection, the difference being in principle that the system is operated via an isolation transformer, the neutral point of the transformers secondary side being earthed via a coil, This coil is connected as a voltage divider limiting the fault current to 7 mA.
In case of fault, the fault current will flow via the current coil of the isolating current-operated earth-leakage circuit breaker which will be excited thereby and disconnect the system. The circuit breaker will disconnect the system at a fault current of 6 mA alternating current and 10 mA direct current resp. on all poles.
Live service conductors must not be earthed when applying this protective circuitry.
On the other hand, all conductive parts of the plant not pertaining to the operating circuit via which a fault current can flow are to be connected to protective earthing.
The protective conductor is to be earthed in the vicinity of the supply unit. For systems on ships it is to be connected to the metallic hull of the ship.
In principle the protective conductor is to be isolated from the lines in front of the isolation transformer!
In case of this protective circuit attention has to be paid further to
- the resistance of the protective earthing is not to exceed 500 ohms
- the insulation resistance between a conductor and the protective conductor must not be less than 20 kOhms and
- the capacitance between conductor and earth is not to exceed 100 Nanofarad.
1 supply unit, 2 isolating transformer, 3 capacitance, 4 resistor, 5 voltage dividing coil, 6 current coil of the isolating current-operated earth-leakage circuit breaker, 7 test resistor, 8 test key, 9 protective conductor, 10 socket for testing purposes, 11 earth resistance, 12 motor.