|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''|
The respective instructions on each working technique can be held in a teaching room. During these instructions, make sure that the trainees write down additional remarks or answers in the "Trainees' Handbook of Lessons". The "Trainees' Handbook of Lessons" is structured correspondingly to the introductory instruction. The main points have to be imparted. A precondition for learning the working techniques in the field of "protective measures against hazardous contact voltage" is that the trainees are able to lay cables, to install and connect devices and to carry out testing and measuring work. In imparting the above mentioned knowledge, therefore, these working techinques have to be refered to in the form of a repetition.
The "protective measures against hazardous contact voltage" should be imparted by making intensive of use all available teaching aids as well as practical experience on the spot.
Necessity of taking protective measures against hazardous contact voltage
In order to make the trainees aware of the necessity of taking protective measures against hazardous contact voltage, their attention should be drawn especially to the protection of human life, health and labour as a basic human concern. It presents itself in this context to point to the harmful effects of electric current on human organism.
In support, the representation of the sensation measurements with passing of alternating current may be used which is contained in the complex material "How to Act after Accidents Caused by Electric
Current". Also it can be dealt with known electricity-caused accidents due to inexpert installation or defective protective measures. With the help of examples, the trainees should be made aware of the importance of working carefully, reliably and free of faults.
It should also be mentioned that the manufacturer of electrical devices, motors, cables etc. is obliged to insulate them for the respective working voltage.
Protection of man has priority over functioning of any electrical device!
Here, the instructor together with the trainees should repeat the factors which may lead to the destruction of the insulation. Furthermore, the trainees must realize that by insulation defects voltages may be transferred to metallic parts that do not belong to the service circuit. Also it must be made clear that the thus created potentials between the metallic parts are dangerous for man. Starting from this, it is explained to the trainees how potential equalization and zero potential are created.
Therefore the knowledge of the zero potential the trainees have already acquired should be used. It is of paramount importance to draw the attention to the point that counting devices and plastic parts (non-conducting materials) in pipe systems have to be bridged.
The potential equalization must not be interrupted by the removal of certain parts.
Protective measures against hazardous contact voltage
Here, it is started from the fact that in electrical plants - except test and pilot plants - protective measures against hazardous contact voltage are required.
It is also important to explain to the trainees the division of the protective measures in two groups as well as the possibilities of further subdivision.
The fault-current circuit (Fig. 3 of the Trainees' Handbook...) should be explained in detail to the trainees with the help of a blackboard drawing or projected diagram, because the fault-current circuit is the basis of the understanding of the individual protective measures.
In explaining this protective measure, special emphasis should be put on the following points:
- fields of application
The criteria have to be mentioned and demonstrated by appropriate examples.
For the chapter of testing, the complex material of "Testing of Electrical Plants" may be used. Do not forget to point out that defects of the protective insulation must not be repaired by provisional mending.
Defects are repaired only by replacing the original parts.
Protective low voltage
Point out to the trainees that this is the most effective and safe protective measure. Its application is limited, though. This results from the reduced rated voltage and the involved operating currents which - with equal transferred power - are approximately 5 to 10 times greater than, for instance, with 220 V alternating voltage. For the trainees it is important to understand from this that for the low voltage plant and the service equipment required essentially greater conductor cross sections are necessary.
Protective low voltage plants are operated only up to approximately 4 kW.
Then, the criteria, required devices and fields of application are explained.
Plants of this kind, if available, should be demonstrated. Also give demonstration of inspection and measurement. If possible, the trainees should practise this at the plants shown to them.
Before explaining the main points of this protective measure, repeat the therm "galvanic isolation" together with the trainees.
Input and secondary end of the devices that create the protective isolation must be connected with each other only through the magnetic flux. There must be no current-conducting connection.
After this, the fields of application are deduced. Underline why not fault current cant flow with the protective isolation because of "non-earthing". Explain to the trainees what measures enable the operation of more than one electrical device with this protective measure. (Potential equalization leads and common base.)
Lacking potential equalization leads and the existence of two faults at different conductors of the supply line may lead to a dangerous contact voltage.
Protective conductor system
Here, the criteria and conditions of this protective measure should be dealt with first. Then, the mode of action is explained, which may be repeated by some trainees. The fields of application should be determined together with the trainees. Point out the high degree of safety of this protective measure. As the reason of this mention the great extent of this measure.
Also, demonstrate the testing of this protective measure. Demonstration should be followed by the trainees exercising the testing.
The mode of action of the measure of protective earthing is explained for both possibilities:
- closing of the fault-current circuit by the ground and
- closing of the fault-current circuit through the water pipe system.
For this purpose, prepare appropriate diagrams on the blackboard. Then, explain the criteria of the measure of protective earthling and demonstrate one calculation of the protective earthling resistance for the two possibilities, each.
When explaining the fields of application of the measure of protective earthling, make sure that the trainees understand that this measure has some disadvantages, though. When coming to the testing of the protective measure, explain to the trainees, that at first the earthling resistance has to be calculated and then checked by measuring.
Connection to neutral
This protective measure can be elaborated together with the trainees on the basis of Fig. 11 of the "Trainees' Handbook of Lessons".
In doing so, the difference to the measure of protective earthling must become clear.
Most of the criteria of this protective measure, tool, can be found out together with the trainees with the help of the illustration. The rest of the criteria is explained by the instructor.
Then, the fields of application and the testing of this measure are dealt with.
In the context of this protective measure, put the main emphasis on the explanation of the following points:
- If the conductors are very long, the resistance has to be kept as low as possible. (This is achieved by earthling the neutral conductor at the network ends).
- The neutral conductor must not be fused or separately switched.
- This protective measure becomes inefficient, if too strong fuses are used.
- In the case of fracture of the conductor before the plant, all casings of the devices that are switched on carry the full mains voltage in the absence of potential equalization.
Fault-voltage protective system
To explain the mode of action of this protective measure, a blackboard diagram for projection may be prepared, too.
Fig. 12 of the "Trainees' Handbook of Lessons" would also be suitable. It is also important to show a foult-voltage protective switch and demonstrate its mode of action.
This is followed by the explanation of the criteria and application of this protective measure.
In doing so, put special emphasis on the following points:
- The resistance of the auxiliary earthling must not exceed a certain maximum admissible value.
- The cross section of the protective conductor must not be less than a definite value.
- Testing of the serviceability.
- Measuring the resistance of the auxiliary earthling. (Therefore, use the Fig. 13 of the "Trainees' Handbook of Lessons". It is suitable to support the explanation of the measuring circuit.)
Fault-current protective system
For explaining the mode of action start from the fact that with normal operation the sum of all conductor currents including the neutral conductor amounts to zero.
(Unequal intensity in the external conductors is of no importance in this respect!)
Only if a fault current flows, the total of these currents is unequal to zero and the protective system comes into action.
While afterwards the criteria and application of this protective measure are elaborated with the trainees, it is absolutely necessary to calculate an example of the resistance of the protective earthling system.
Whit this protective measure, too, the functioning of the protective system has to be demonstrated and then practised by the trainees.
Put special emphasis on the following points:
- Fault-current protective switches are manufactured, for instance, with rated fault-current intensities of 15 mA, 30 mA, 80 mA, 100 mA, 200 mA, 300 mA and 500 mA.
- The fault-current protective system can be combined with the connection to neutral.
- Switches are manufactured for rated current intensities up to 80 A. (Beyond this limit, they are used in combination with transducer cores, trip coils and contactors.)
- The fault-current protective system needs a good insulation value. (Otherwise, currents caused by insulation damages lead to the switching off.)
- The fault-current protective system may also be used in three-phase systems without earthed net point.
Isolating fault-current protective system
In explaining this protective measure, start from the fact that its mode of functioning is similar to that of the fault-current protective system.
In contrast to the fault-current protective system, this system is operated with the help of an isolating transformer the neutral point of the secondary side of the transformer being earthed over a coil.
In elaborating the criteria of this protective measure, special value should be attached to the breaking fault current, the resistance of the protective earthling and the insulation resistance between conductor and protective conductor as well as to the capacity between conductor and earthling.
The functional testing has to be demonstrated especially carefully because, for this purpose, the input end of the isolating transformer has to be disconnected from the supply system.
It is not allowed to work at the alive system, except for certain special conditions and observing all required protective measures.