|Protective Measures against Hazardous Contact Voltage - Course: Basic skills and knowledge of electrical engineering. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 23 p.)|
The voltage of a conductor or body to earth is called the "potential" of this conductor or body. The earth is electrically neutral and thus has the potential "zero". The unit of measurement for the potential is volt.
A conductor or body has the potential "zero" - earth potential - if it is conductively connected with the earth.
As a consequence of damages to the insulation, voltages may be transmitted to metal parts which do not belong to the service circuit.
This causes the development of potentials between the metal parts which might be dangerous for man.
Figure 1 - Potential development between metallic parts with insulation damage - 1 defective device, 2 pipe conduites, 3 central heating, 4 metallic constructions, 5 potential developed, 6 current through human body
If a man touches two different metal parts at a time - for instance an electrical device and a water pipe - he bridges the existing potential and a current flows through his body.
If an electric current flows through the body of a human being, this might be mortal.
A remedy to this is a potential equalization to zero potential.
How can a potential equalization to zero potential be achieved?
The potential equalization must not be broken by the removal or demounting of parts.
What line cross-sections should be used for potential
Figure 2 - Potential equalization - 1 service box, 2 gas main, 3 water pipe, 4 water meter, 5 heating pipe, 6 leads for further metallic parts, 7 potential equalization bar, 8 potential equalization leads