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Potential Control against Step & Touch voltages

Basic principles 


The earthing system is the basis for the safe function of every electrical system and its protection devices. It ensures operation and protects people against hazardous currents.

Buildings with IT systems and data cabling have high requirements for electromagnetic compatibility measures (EMC). To ensure the EMC shield and personal protection, a meshed equipotential bonding and a low-ohmic earthing system integrated in the structure are required.



Potential control against step voltages 


The step voltage is the voltage between a person’s feet placed 1 m apart. Here, the compensating current flows between the person’s feet through their body. In entrance areas or in front of lookout towers, a densely looped earthing system is installed to minimise step voltage and to protect people. The lightning current is distributed through the metallic earthing system. Earth rods route the current into the earth and the voltage drop on the earth’s surface and the resulting step voltage are reduced. Here, stainless steel of quality V4A (1.4404/1.4571) should be used as a material. 

Option 1: Potential control through ring earth electrodes Additional ring earth electrodes are routed around the foundation earth electrode and connected with one another in a grid format. As the distance from the rod or down-conductor increases, the ring earth electrode is routed 0.5 m deeper each time at the usual spacing of 3 m.

 Fig 1: Potential control on a streetlight pole 







Legend: 


1 Earthing voltage UE 

2 Controlled 

3 Uncontrolled, without potential control 


Option 2: Potential control through mesh grid 


In refuges or transport stops, potential control through a tight metal grid is wise. The mesh is installed at a small depth (max. 0.25 m) under the earth’s surface. The metallic mesh rods should have a minimum diameter of 3 mm and a maximum grid width of 0.25 x 0.25 m. To minimise the corrosion in the earth, the use of high-quality stainless steel V4A (1.4404/1.4571) is necessary. The mesh grids are screwed to each other using connection terminals and connected to the existing earthing system. This tightly meshed system greatly reduces the step voltage per metre and reduces the risk to people. 


Touch voltage 

If there is a lightning stroke, the lightning current is routed through the down-conductors into the earthing system and the earth. The resistance of the down-conductor and the earth causes a voltage drop, which can lead to so-called touch voltage. The touch voltage is the voltage between a component (e.g. the down-conductor) and earth potential. The current flows from the hand to the foot through the body. The potential hazard must be reduced by technical measures, e.g. a control earth electrode.

Fig 2: Electrical potential on the earth surface and voltages as current passes through the foundation earth electrode (FE) and control earth electrode (CE)


Legend: 


1 UE: Earthing voltage 

2 UB1: Touch voltage without potential control (on foundation earth electrode) 

3 UB2: Touch voltage with potential control (foundation earth electrode and control earth electrode) 

4 US: Step voltage (without control earth electrode) 

5 φ: Earth surface potential 

6 FE: Foundation earth electrode 

7 CE: Control earth electrode (ring earth electrode)

This article has been extracted from "Guide: Earthing systems" by OBO Bettermann 

The full free guide can be downloaded below

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