Step Potential and Touch Potentials are very important in substations because during ground faults all the ground current returns to the substation transformer (as the substation transformer is grounded). The current that returns through the earth can create a significant voltage gradient along the ground and between ground and conducting objects. A step potential in substation creates a path through the legs from one foot to the other. A touch potential in the substation is normally considered a hand-to-foot or hand-to-hand contact.
Step potential and touch potentials are of concern during normal conditions and during a ground fault.Under normal conditions, unbalanced currents can rise the neutral to earth voltage. This is not normally dangerous but it can cause shocks.
Step potential and Touch potentials during faults are more dangerous. Therefore it is important to reduce the step potential and touch potential to be within limits of the substation design.
Methods to Limit Step Potential and Touch Potential
Step potentials and touch potentials can be reduced by employing one of the following methods:
- By proving low resistance path to ground
- By providing insulation layer between operating personnel and earth
- By proper placing of ground conductors
Some of the methods employed to reduce step potential and touch potential in substation are
- Multi-grounded neutral helps to reduce the dangerous step potential and touch potential during line-to-ground faults. By creating low impedance path back to the source, faults are quickly cleared by fault interrupters. Multiple grounding electrodes tied together helps to reduce the touch potentials at the fault point. With multiple neutrals, step potentials are usually not dangerous since fault currents spread between several grounding electrodes.
- Using a reactor on the substation transformer neutral helps to limit the step and touch potentials. While utilities normally use the neutral reactors to limit the fault currents. the reduction of ground fault currents also reduce the step and touch potentials and reduces the current in grounding and bonding connectors.
- By wearing electric hazard shoes. These shoes when dry can have offer millions of ohms of resistance which can save the operating personnel against these dangerous potentials. By using insulating materials such as rubber gloves can protect the personnel.
- By providing resistive surface layers in and around the substation. It is often provided with the surface of crushed rock or pebbles which acts as insulation medium between the operating personnel and earth.