|Protective Measures against Hazardous Contact Voltage - Course: Basic skills and knowledge of electrical engineering. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 23 p.)|
|3. Protective Measure against Hazardous Contact Voltage|
Criteria of the protective insulation
- The protective insulation is used in addition to the operating insulation.
- The protective insulation must stand an alternating current test voltage of at least 4 kV for one minute without any damage to the insulation being visible afterwards.
- Protection-insulated electrical equipment is not provided with safety plugs by the manufacturer. If a repair should require that such a plug is fitted and if the connecting line should have a protective conductor, this has to be connected to the plug - not to the device.
- Protection-insulated electrical equipment is marked by a double square on the casing.
Protection-insulate electrical equipment is not connected by a safety plug.
Forms of protective insulation are:
- protective insulation sheathing
- intermediate protective insulation
- reinforcing insulation a b c
Figure 4 - Forms of protective insulation a protective insulating sheathing, b intermediate protective insulation, c reinforcing insulation - 1 casing (metal or plastic), 2 protective insulation, 3 service insulation (normal), 4 live parts, 5 service insulation (reinforced)
Where is the protective insulation preferably
Checking of the protective measure of "protective insulation"
- visible defects of the protective insulation
- no connection of protective conductor
- marking by double square
- measuring of the insulation resistance between life parts and accessible conductive parts
The resistance of the insulation must be at least 1.5 megaohms with a measuring voltage of 100 V direct current voltage
Protective low voltage
Criteria of the protective low voltage
- The protective low voltage must not exceed an alternating current voltage of
· 25 V with load and
· 50 V without load.
- The parts generating the protective low voltage as well as the conductors must not be secondary-earthed.
- The secondary side must not be connected with leads of other circuits.
- Plug connections must have no secondary protective contacts.
- Electrical equipment operated with protective low voltage must have no connecting points for the protective conductor.
- Accumulators have to be disconnected from the circuit of protective low voltage by all poles during the process of charging.
For protective low voltage, only special plug-in devices must be used.
Devices for generating protective low voltage are:
- insulation transformers
- transformers with electrically separated windings
- galvanic cells
Where is the protective low voltage
Figure 5 - Generation of a protective low voltage by protective transformer or motor generator - 1 protective transformer, 2 symbol of protective low voltage, 3 symbol of protective insulation, 4 motor generator
Checking of the protective measure of "low voltage"
- By inspection whether the plant and devices meet the required criteria.
- By measuring whether the permissible voltage is not exceeded.
Criteria of protective isolation
- The protective isolation is accomplished if there is a galvanic isolation between the service circuit and the circuit supplying the energy. (The input end and the secondary end must be connected only through the magnetic flux!)
- When isolating transformers are used, only one device must be connected to each secondary winding.
- Circuits with protective isolation must not be earthed or connected with conductors of other circuits.
- Metal casings of electrical tools, which are used in boiler plants or other kinds of metal constructions, have to be connected in a conductive way with the latter. (Potential equalization has to be created!)
- The working voltage with load must not exceed 380 V three-phase current.
With the protective isolation, no electric connection must come into existence between the input end and the secondary end!
Figure 6 - Protective isolation - 1 isolating transformer, 2 symbol of protective isolation, 3 device
Devices for creating the protective isolation are:
- isolation transformers with a transformation ratio 1:1
- motor generators
- transformers with galvanically isolated windings
Where is the protective isolation used?
Checking of the protective measure of "protective isolation"
- By inspection whether the plant and devices used meet the required criteria.
Protective conductor system
Criteria of the protective conductor system
- The protective conductor system can only be used in plants with an own transformer.
- The neutral point of the transformer must be earthed through a fusible cut-out.
- All metallic parts and casings that do not belong to the service circuit, have to be connected with each other through the protective conductor.
- The protective conductor must not be connected with the neutral conductor.
- The maximum value of the earthing resistance of the protective conductor is 20 ohms.
- The supervisory equipment must switch off or signalize if the insulation resistance of current-carrying conductors towards the earth falls below 25 ohms/volt.
Figure 7 - Principle of the protective conductor system - 1 supervisory equipment, 2 alarm, 3 fusible cut-out, 4 protective conductor, 5 resistor, 6 water pipe system, 7 motor, 8 heating system, 9 metal construction
Where is the protective conductor system
Checking of the protective measure of "protective conductor system"
- The criteria of this protective measure are checked by visual inspection.
- By measuring, the insulation resistance between current-carrying conductor and earth as well as between the metallic parts is checked.
With the insulating test 25 ohms/volts are required.
Figure 8 - Testing process - 1 test resistance, 2 test prods, 3 continuity tester
What must be the value of the test resistance with a mains
voltage of 220