Dimensioning 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 limitersThe operating voltage, voltage-current characteristics, and all voltage parameters of the limiter are dependent on the height of the column of varistors. By contrast, all voltage parameters are set by the selection of operating voltage Uc. An incorrect selection of voltage Uc may have a considerably negative effect on the limiter's functionality.
An incorrect selection of voltage Uc may have a negative effect on its functionality:
If a low Uc is selected, the protective level URES and also the risk of failure of the protected device connected to it will be favourably low. On the other hand, however, if there would be risk of thermal stress on the limiters caused by temporary overvoltages, so the probability of them failing will be high.
If a high Uc is selected, the risk of failure of the limiters due to temporary overvoltages will be insignificant, but a high protective level URES will imply a higher probability of the protected devices being damaged or even destroyed.
The correct selection of continuous voltage Uc of the limiters shall give optimum parameters of protection while offering a balanced risk of supply reliability loss due to both above-mentioned 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. The values typical fora limiter's protective level can be found in our catalogue. It is the residual voltage URES of the limiter.
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 overvoltages. Switching overvoltages reach substantially lower current and voltage levels than atmospheric ones.
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 damage or destruction, caused by atmospheric overvoltages by installing surge arresters, and at the same time to protect the installation, by required reduction of overvoltage in the network.
Principles of placing and connecting limiters in LV networksThe overvoltage limiters in TN-C networks should 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 will 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 is followed as per PNE 330 0001.
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 should be connected between line conductors and the main protective PE conductor, from which the discharge current is diverted to the ground via a test clip, earth wire, and earth electrode.