Protection concept

Effects of overvoltages

Representation of the protective circuit principle  

Representation of the protective circuit principle

The protective circuit principle describes a concept for complete protection against overvoltages. An imaginary circle should be drawn around the item to be protected. Surge protective devices should be installed at all points where cables intersect this circle. The nominal data of the relevant circuit should be taken into consideration when selecting the protective devices. The area within the protective circuit is therefore protected in such a way that conducted surge voltage couplings are prevented.

The protective circuit concept can be broken down into the following areas:

  • Power supply
  • Measurement and control technology
  • Information technology
  • Transmitter and receiver systems

Protection zones

Location of the individual protection zones using the example of a typical single-family home  

Location of the individual protection zones using the example of a typical single-family home

In order to achieve effective protection, it is important to determine where devices that are in danger are located and what influences represent a danger to the devices. The following figure shows a typical single-family home used as an example to illustrate the location of the individual protection zones.

The abbreviation LPZ stands for lightning protection zone and refers to the various danger zones. A distinction is made between the following zones:

  • LPZ 0A (direct lightning strike): Refers to the danger zone outside the building.
  • LPZ 0B (direct lightning strike): Refers to the protected danger zone outside the building.
  • LPZ 1: Refers to a zone inside the building where high-energy overvoltages represent a danger.
  • LPZ 2: Refers to a zone inside the building where low-energy overvoltages represent a danger.
  • LPZ 3: In this zone, overvoltages and other influences caused by the devices and cables themselves represent a danger.

Effects of surge currents in cables

Causes of induction voltages in cables  

Causes of induction voltages in cables

Overvoltages are limited by discharging high-frequency currents and therefore transient processes. This means that it is not the ohmic resistance but the inductive resistance of a cable that is of primary importance.

According to Faraday’s law of induction, when these types of surge currents are discharged to ground potential, overvoltages are created again between the coupling point and ground.

u0 = L x di/dt
u0 = Induced voltage in V
L = Inductance in Vs/A in H
di = Current change in A
dt = Time interval in s

The inductive resistance can only be reduced by shortening the cable length or connecting discharge paths in parallel. For this reason, mesh-shaped equipotential bonding that is as tightly meshed as possible is the best technical solution to minimize the total impedance of the discharge path and therefore the residual voltage.

Equipotential bonding

Equipotential bonding systems  

Equipotential bonding systems

Complete protection can only be achieved through complete isolation or through complete equipotential bonding. However, since complete isolation is impossible for many practical applications, only complete equipotential bonding remains.

To achieve this, all electrically conductive parts must be connected to the equipotential bonding system. Protective devices are used to connect live cables to the central equipotential bonding. In the event of an overvoltage, they are conductive and short-circuit the overvoltage. Damage from overvoltages can therefore be prevented effectively.

Various equipotential bonding systems can be created:

  • Line-shaped equipotential bonding
  • Star-shaped equipotential bonding
  • Mesh-shaped equipotential bonding

Mesh-shaped equipotential bonding is the most effective method, as all electrically conductive parts have a separate cable here and additional cables connect all end points via the shortest route. This type of equipotential bonding is suitable for particularly sensitive systems, such as computer centers.

Multi-level protection concept for the power supply

The measures required to protect devices and systems are divided into two or three levels depending on the protective devices chosen and the environmental influences to be expected. The protective devices for the individual levels differ with regard to the discharge capacity level and the voltage protection level depending on which protection level they belong to.

Three-level protection concept with separately installed protection levels:

  • Type 1: Lightning current arrester
    Voltage protection level <4 kV, typical installation location: Main distribution
  • Type 2: Surge protective device
    Voltage protection level <2.5 kV, typical installation location: Sub-distribution
  • Type 3: Device protection
    Voltage protection level <1.5 kV, typical installation location: Upstream of the end device

Protection levels 1 and 2 can also be implemented in a type 1+2 combined lightning current and surge arrester. This protective device meets the same requirements as type 1 and type 2 arresters. The main advantage is the easy installation. In addition, no special installation conditions have to be taken into consideration.

Three-level protection concept with type 1+2 combined lightning current and surge arrester and separate type 3 arrester:

  • Type 1+2 combined lightning current and surge arrester
    Voltage protection level <2.5 kV, typical installation location: Main distribution
  • Type 3: Device protection
    Voltage protection level <1.5 kV, typical installation location: Upstream of the end device

PHOENIX CONTACT (Ireland) Ltd

C6 The Exchange
Calmount Park
Ballymount
Dublin 12
D12 XE18
Ireland
(01)2051-300