What is Step and Touch Voltage?
Step voltage between the feet of a person standing near an energized grounded object. It is equal to the difference in voltage, given by the voltage distribution curve, between two points at different distances from the electrode. A person could be at risk of injury during a fault simply by standing near the grounding point.
Touch voltage between the energized object and the feet of a person in contact with the object. It is equal to the difference in voltage between the object and a point some distance away. The touch potential or touch voltage could be nearly the full voltage across the grounded object if that object is grounded at a point remote from the place where the person is in contact with it.
Why should be measured?
1. Personnel safety
2. Requirement from national standards or safety regulations
Grounding System tests must be performed after construction of electric facilities and on a regular basis every 4-5 years. This is to guarantee personnel safety during a single phase fault and to check the quality of the grounding system’s construction according to the dimensioning during the planning period. Therefore, it must be proven that no hazardous step & touch voltages in and around the substation or at the pole of an overhead transmission line are caused.
Ground Impedance, Step and Touch Voltage Measurements
Our company Intertest Ltd provides tests for Step and Touch voltages using the Omicron CU1 and HGT1 devices. With these devices, we can measure the ground impedance, step and touch voltage.
GROUND IMPEDANCE
For the determination of a ground impedance a test current is injected into the soil via a remote ground electrode. Usually the remote ground electrode is another substation where the current is injected via an existing power line between the substation under test and the
remote substation. If no line is available for testing purpose the test can also be injected via a current probe.
The test current is driven by an AC source from CU1 device which causes a potential rise of the grounding system as it would be the case for a real fault. The voltage is initially measured between a grounded reference point in the substation and a location at the edge of the ground grid.
The voltage referring to the initial measurement is supposed to be quite small since the rod is
close to the grounding system which theoretically has the same potential at each location. For the next measurements, the rod’s distance to the grounding system increases in direction of 90̊ from the grounding system.
Increasing the rod’s distance results in an increase of the impedance resp. the voltage. The measurement can be stopped as soon as the results for impedance and voltage do not change anymore as it is the case for the last 3 points. The value of the impedance curve’s flat part is the ground impedance, the value of the voltage curve’s flat part is the ground potential rise.
STEP AND TOUCH VOLTAGE
For step & touch voltage measurements the injection of the test current remains the same as for the ground impedance measurement. The only difference is the voltage measurement which is now performed at selected locations in and outside the substation.
EN 50522 suggests the personnel simulation method by measuring the touch voltage across a 1 kΩ resistor and using a metal plate which is simulating bare feet 1 m apart from the object. The plate must have dimensions of 20 cm x 20 cm and be loaded with at least 50 kg, ideally a person who steps on it. EN 50522 also recommends to wet the soil under the metal plate in order to simulate the worst case.
To obtain complete and quality measurement, you should measure the touch voltage to each steel construction mounted in the substation.