Protection concept

Surge protective devices from Phoenix Contact can be used to create an effective protective circuit around devices and systems and thus successfully prevent external surge voltages from being coupled in. Appropriate protective devices must be installed at all interfaces between cables and the protective circuit.

Divided into four main categories, you will find suitable surge protection for:

• The power supply
• Measurement and control technology
• Information technology
• Transceiver systems

Protection zones

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:

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

Effects of surge currents in cables

Surge voltages 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 current are discharged to ground potential, surge voltages 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.

Multi-stage protection concept for the power supply

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

Three stage protection concept with separately installed protection stages

• Type 1: Lightning arrester
  Protection level < 4 kV, typical installation location: Main distribution
• Type 2: Surge arrester
  Protection level < 2.5 kV, typical installation location: Sub-distribution
• Type 3: Device protection
  Protection level < 1.5 kV, typical installation location: Before the termination device

Protection stages 1 and 2 can also be implemented in an arrester combination. 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. Arrester combinations that operate according to the AEC principle have proven to be very effective. AEC stands for active energy control. Based on trigger electronics, AEC ensures that the energy from a surge voltage is appropriately distributed to the individual protection stages. This prevents overload of the individual protection stages and ensures the low protection level required.

Three-stage protection concept with type 1/2 arrester combination and separate type 3 arrester:

• Type 1/2: Lightning/surge arrester combination
  Protection level < 2.5 kV, typical installation location: Main distribution
• Type 3: Device protection
  Protection level < 1.5 kV, typical installation location: Before the termination device

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