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close this bookProtective Measures against Hazardous Contact Voltage - Course: Basic skills and knowledge of electrical engineering. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 23 p.)
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View the documentIntroduction
View the document1. The Necessity of Protective Measures against Hazardous Contact Voltage
View the document2. Potential Equalization
close this folder3. Protective Measure against Hazardous Contact Voltage
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View the document3.1. Protective Measures Preventing the Development of Hazardous Contact Voltage
View the document3.2 Protective Measures which Prevent that Hazardous Contact Voltages Continue to Exist

3.1. Protective Measures Preventing the Development of Hazardous Contact Voltage

Protective insulation

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 used?

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
- generators
- transformers with electrically separated windings
- accumulators
- galvanic cells

Where is the protective low voltage used?

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.

Protective isolation

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 used?

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.
If this level is not achieved, the supervisory equipment must switch off the plant and/or circuit.

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 V?