PARAMETERS OF A PROTECTOR: Uc, Imax, Up

MAXIMUM PERMANENT VOLTAGE (Uc)
In order to select the voltage at which the protector will work the rated voltage of the line to be protected needs to be considered.

For networks with 380V and 220V.

1º- The protectors with Uc=440V are selected for installation between the conductors and earth (common mode).

2º- The protectors with Uc=250V are selected for installation between phases and neutral (differential mode).

MAXIMUM DISCHARGE CURRENT (Imax)

The maximum discharge current (Imax) is the value in amps which the protector is able to derive to earth one time without suffering damage.

The rated current (In) is the value in amps which the protector is able to derive to earth 20 times being of waveform 8/20 µS.

According to the standard NF C 61-740/1995, the manufacturer of the surge protector has to specify these values.

A surge voltage generated by lightning effects is characterized by very high energy levels and short duration (<100 µS) compared to the frequency of 50 Hz in the electrical network (half a period 10ms=10.000 µS).

The model for the waveform of the current produced by lightning effects is given by the 8/20 µS wave. The time to raise up to 90% of its amplitude is 8 µs, and the time to decrease up to 50% of its amplitude is 20 µs.

CRITERIUM FOR THE SELECTION OF Imax
There are several factors which may be considered when selecting Imax of the protectors. The geographic area, the proximity of lightning rods, the cost of the equipment to be protected, and most important the cost of not having the equipment working, where one may consider both the financial cost and the loss of prestige in the customer's eyes.

The diagram may help to select P1 depending on Imax where P1 is the protector to be installed in the mains input.

P1 is in charge of deriving the mayor part of the current produced by surge voltages to earth.

NIVEL DE PROTECCIÓN (Up)
The protection level (Up) is the value of the voltage withstood by the equipment to be protected without suffering damage.

It has to be verified that the voltage between both ends of the protector (Up) appearing during the discharge of current is less than the maximum voltage which can be withstood by the equipment to be protected.

In order to determine Up the various subcircuits are considered, which may require different protection levels (Up) depending on the equipments connected to them. Normally, within the same subcircuit similar equipments are connected.

Examples for typical Up values

Coordination of protectors:
It is not possible to achieve a high discharge capacity and low residual voltage with a single protector, so using only one protector may not assure the protection of the whole installation where sensitive equipment exists. For such cases one installs 2 or more coordinated protectors. The first of them (P1) is installed in the mains input, while the following protectors (P2, P3, etc.) depending on the required Up are installed in the more sensitive lines and as close as possible to the equipment to be protected.

P1 is selected based on the required Imax, and the following protectors are selected in order to achieve a reduced Up. For a correct functioning of the coordinated protectors a minimum distance of 10 m between the protectors has to be guaranteed, since the inductive behaviour of the electrical cable in case of surge voltages provides a delay of the current. Thus P1 is first activated and derives the largest part of the energy to earth while the following protectors reduce the residual voltage of the first protector.

In the panels where two protection steps are installed together and the separation distance between the protectors of 10 m does not exist, a inductor of 15µH is used, whose inductivity simulates a 15 m cable distance.