![]() | Protective Measures against Hazardous Contact Voltage - Course: Basic skills and knowledge of electrical engineering. Methodical guide for instructors (Institut für Berufliche Entwicklung, 13 p.) |
![]() | ![]() | 3. Recommendations for Practical Vocational Training in the Working Techinques of ''Protective Measures against Hazardous Contact Voltage'' |
For consolidating and testing the knowledge and skills acquired, questions are composed in this section.
To each question the respective answer is given.
Questions contained in the "Trainees' Handbook of Lessons" are marked with the letter "A".
1. What is the aim and purpose of protective
measures?
(Protection of human life, health and ability to work)
2. Why is it necessary to take protective measures even if
cables, electrical devices and plants are sufficiently insulated?
"A"
(Natural ageing, destruction due to inexpert use, premature ageing by climatic
influences and chemical effects may lead to dielectric breakdowns that make
conductive parts alive.)
3. Why must the potential equalization not be broken when
certain parts are removed?
(Because in this case a potential may be
created which, when bridged by the body, may cause a mortal current flow.)
4. How can a potential equalization to zero potential be
achieved?
"A" (When all metallic parts, such as water pipes, gas mains,
central heating, metallic sheathings of cables and other metal constructions,
are connected with one another and with the ground in a conductive way.)
5. What line cross-sections should be used for potential
equalization?
"A" (10 mm2 copper, 25 mm2 aluminium,
50 mm2 steel.)
6. Where is the protective insulation preferabely
used?
"A" (household appliances/except thermal appliances, cosmetic and
massage as well as do-it-yourself equipment.)
7. Where is the protective low voltage used?
"A"
(Electrical toys, cosmetic and massage appliances without protective insulation,
inspection lamps and electrical tools for boiler and large pipe systems,
vibrating devices.)
8. Where is the protective isolation used?
"A" (In all
plants and pans of plants in which especially great danger may be evoked by
contact with large metallic constructions, furthermore, for operating
physiotherapeutical equipment for medical purposes as well as for shaving
sockets in washrooms of passenger trains.)
9. Where is the protective conductor system used?
"A"
(In factories where the failure of one machine leads to a disturbance of the
entire manufacturing process; with electrical plants in operating theatres; in
miming.)
10. What value must the test resistance of a protective
conductor system have with a mains voltage of 220 V?
"A" (5.5 kiloohms)
11. Where is the protective earthling used?
"A"
(Mainly in all nets which cannot be connected to neutral.)
12. Why can the protective earthling give a limited
protection only?
(Because the earth transistor resistance - due to
climatic influences - is subject to fluctuation; because the required earthling
resistances are difficult to achieve; because the single-pole switching-off of
three-phase current consumers may cause defects at these consumers.)
13. Where is the connection to neutral used?
"A" (In
all nets with sufficiently earthed neutral conductor/zero conductor; because of
the simple and cheap application in all fields of electric current.)
14. Why does the protective measure of "connection to
neutral" become inefficient if a too strong fuse is connected
before?
(The defective external conductor does not switch off quickly
enough so that the fault voltage remains at the defective equipment.)
15. Where is the fault-voltage protective system
used?
(In order plants, where it has not yet been replaced by the
fault-current protective system. By fault-voltage protective switches, neutral
conductors can be controlled.)
16. Where is the fault-current protective system
used?
"A" (It can be installed in all electrical plants with three-phase
or alternating current and can be combined with the protective measure of
"connection to neutral".)
17. Where is the isolating fault-current protective system
used?
"A" (Mainly in ship
building.)