Sizing of the overvoltage limiters

The overvoltage limiters are sized based on a particular position in the network, i.e. whether they are to protect e.g. a line outlet, a line transition into a cable or transformer. Whatever their position, they have specific conditions for protection and overvoltage stress.

Selecting the operating voltage UC of limiters

The operating voltage, voltage-current characteristics and all voltage parameters of the limiter are dependent upon the height of column of blocks. By contrast, all voltage parameters are set by the selection of operating voltage UC. Incorrect selection of voltage UC may have a considerably negative effect on the limiter’s function.

Incorrect selection of voltage UC may have a negative effect on its function:
 
If a low UC is selected, the protective level URES and also the risk of failure of the protected device associated with it will be favourably low. On the other hand, however, there will be a risk of thermal stress on the limiters caused by temporary overvoltage, so the probability of them failing will be high.
 
If a high UC is selected, the risk of failure of the limiters owing to temporary overvoltage will be insignificant but a high protective level URES will imply a higher probability of the protected devices being destroyed.
 
The correct selection of continuous voltage UC of the limiters should mean optimum parameters of protection, hence a balanced risk of reliability of the supply for both causes.
 
The protection parameters can be improved by connecting the limiters as close to the protected device as possible with as short interconnecting wires as possible!
 
The limiters limit voltage to a limiter’s protective level UP. A limiter’s protective level UP is voltage on terminals with a given shape and peak value of current passing through. Values characterising a limiter’s protective level can be found in our catalogue. It is a limiter’s residual voltage URES.

Characteristics of overvoltage protection of LV and HV networks

In LV and HV distribution networks with overhead lines, it is necessary to protect equipment primarily from atmospheric overvoltage. Switching overvoltage reaches substantially lower current and voltage levels than atmospheric.

The greatest overvoltage in cable networks without connected overhead lines is caused by short circuits and/or switching.

The primary task of protective measures which are economically fully justified is to protect the equipment of LV networks from destruction by atmospheric overvoltage by installing surge arresters and, at the same time, to enable the protection of installation by adequately reducing overvoltage in the network.

Principles for positioning and connecting in LV networks

The overvoltage limiters in TN-C networks shall be connected between a phase conductor and a PEN conductor (in star) at the place of its earthing.
 
In the event that the overvoltage limiters are positioned at a place where there is no earthed PEN conductor, the earthing shall be carried out through a separate earth electrode. A 1-metre earthing rod or another equivalent earth electrode is considered to be sufficient. The magnitude of resistance of the earthing of the overvoltage limiters is not decisive for their functioning. When designing and earthing, the procedure as per PNE 33 0000‑1 shall be followed.
 
In absolutely exceptional and justified cases, overvoltage limiters connected between a phase conductor and PEN conductor are not to be earthed.
 
The overvoltage limiters in TT networks shall be connected between line conductors and the main protective PE conductor, from which discharge current is diverted to the ground via a test clip, earthing wire and earth electrode.