Step and touch voltages of transformers

Power distribution

Transformers are an essential part of the distribution system. They are easily accessible inside the substations since they (and other components of a substation) need regular maintenance. At the same time, the station itself will have a step-down transformer to mains voltage for the use of personnel. This makes it possible to use instruments that require mains power or other external power source to function, and also enables measurements using higher measurement voltage or current.

MI 3144 can be powered by battery, but requires an external supply to reach all of its specified measuring abilities. This makes it particularly useful in a substation setting where the mains supply is easily available.

Step, touch and contact voltages in case of a fault are a measure of safety or danger for people during this fault. They assure that the grid meets its design objectives.

Step voltage simulates a voltage between a persons’ legs when walking. It is particularly relevant outdoors when close to a grounding electrode of a high voltage system (power generation station, substation, distribution pylons/towers), or any grounded metal-work: meshes, fences, pipes, bridges, rails, etc. Fault voltage can travel very far through conductive or inductive connections.

Touch voltage simulates the touch of a person on exposed metal work that became live. It can be relevant indoors as well, but usually a different method is used for measuring it there.

Contact voltage is a variation to touch voltage that avoids the human resistance probe and uses a voltmeter with high input impedance. It gives different information, but can be used in a similar way.

MI 3144 injects a short pulse into the earthing system. Exact magnitude depends on the resistance of the grid and the user’s requirement on the load, but it can reach up to 300 A. The pulse only lasts half a cycle, which makes it safe for the environment and the operator. The measured step or touch voltage can then be scaled up to the prospective fault current and used as a measure of danger to life.

Additional accessories can be used to achieve good connection to different surfaces: S 2053 plates that the user can step on, A 1353 Step voltage probes for simulation by IEEE 81 standard and weight 25kg each, or copper probes that enable particularly good contact on concrete.