In the latest edition of our technical blog series, we outline the key design considerations of designing an earthing system. LPI Group’s Technical Design Manager Mr Hadi Beik Daraei outlines some key considerations of bonding techniques to create a separated Lightning Protection Zone (LPZ) including:
- The importance of Knowing equipment ports (Internal and External)
- Realising the Key distance of preventing interference (Influence Distance)
- The importance of creating an appropriate Lightning Protected Zone for IT Equipment
If there is IT (Information Technology) equipment installed inside a certain zone of a data centre building, all measures need to be considered to avoid any damage to the internal and external ports that can result from flowing surge currents and the build-up of different voltage levels.
Knowing the type of Equipment ports (Internal or External) is essential to understanding the level of voltage that may damage the equipment.
Figure 1 shows the different types of Internal and External ports for equipment.
Generally, the external ports are connected to the service cable extended from the outside region of the building. For this reason, the external ports are in a higher exposure zone to the damaging voltages. In other words, if we could reduce or remove the damage sources which influence the external services and cable, the external ports will be automatically exposed to a lower level of damage risk.
Therefore, in technical guidelines and standards, there are tables that show the minimum distance requirements between IT cables and other services which could influence the performance or damage of the IT cable. Tables 1 and 2 below illustrate examples of the minimum distance requirements between IT cables.
Table 1 shows the minimum reference influence distance of an IT Cable which could be affected by the electromagnetic field of a power cable installed at a certain distance of it as shown in figure 2.
Table 2 shows the minimum distance that needs to be maintained between the earthing electrode of a power system (MV/HV) and IT Cables. These restrictions refer to the earth potential rise (EPR) which occurs once a line-to-ground fault happens in the MV/HV power distribution system.
As can be understood from the above-mentioned restrictions, the external ports for the IT equipment are at a higher level of exposure from their extended connection to an IT cable. Therefore, regarding the port type (External or Internal), the applicable tests to find the insulation strength of the Internal and external ports are as follows:
For External Ports:
- Use a Lightning Impulse Test (the voltage test is greater than 1.5kV)
- Power Induction and Earth Potential Rise Test (as a rule of thumb, the voltage test is 600V for a duration of 0.2 or 1 second and 230V for a sustained period)
For Internal Ports:
- Use a Lightning Impulse Test (the voltage test is less than 1kV)
Generally, the purpose of creating the LPZ is to achieve control of voltage magnitude and voltage difference to avoid any damage to critical IT equipment.
In terms of where IT Equipment is located ,they usually accommodated in LPZ 2. Moreover, if the interconnected ports between two interlinked pieces of equipment are of the Internal type, they are exposed to the lower level of voltages.
Figure 3 illustrates the typical Lightning Protection Zones and emphasises the boundary of the different LPZ.
Establishing the Lightning Protection Zone 2, dedicated to interconnected IT Equipment, will diminish the potential of creating voltage differences and take the surge entry point under control. Therefore, the way to build up the lightning protection zone is a crucial issue where IT Equipment is interlinked together.
How to Establish Lightning Protection Zones
- By installing coordinated surge protection devices at the input interface of all services (Power/ telecom/ data)
- By creating types of bonding backbone in IT rooms.
Figures 4, 5, and 6 demonstrate how two different electrical loads could be affected by inductive and conductive voltage differences through the protective bonding system.
As shown in figures 4 and 5 the resulting voltages could be of a conductive or inductive type, and this happens while a short circuit current flows through the bonding conductor.
Figure 6 shows the voltage difference that could be made in the case of a short circuit current that occurs in the backbone bonding network.
As shown in figure 6, if there is an isolating transformer inside the Power Distribution Units (PDUs) it establishes a ‘separately derived system’ at its downstream circuits. Therefore, the short circuit current comes back through the bonding conductor to its supply in the PDU panel. There could also be a voltage difference between the two pieces of electrical equipment. In this case, if these two pieces of equipment are interconnected by a signal cable, the signal ports would be affected by the voltage difference.
In General, the AC voltage (L-N) is 230 V ( in less than 0.1 seconds), therefore the maximum voltage difference between two interconnected IT Equipment will not be more than 230V
Figure 7 and 8 conveys the way a surge current could build surge voltage through the bonding system in a typical data centre where the IT equipment is interconnected to each other through the signal links.
According to IEC standards, if the building structure is utilised as a down conductor, the protection level of the building itself is enhanced. For physical building protection, utilising the steel structure will diminish the risk of sparks inside the masonry.
However, this issue could increase the surge current magnitude conducted through the common bonding network which the cable tray is a part of that.
Figure 8 shows the voltage difference that could be made if a fully isolated system has been installed and the lightning current is discharged to the underground electrode system without utilizing the steel structure. In this case, the propagation of the surge inside the above-ground bonding system due to the voltage difference (DV1) shown on the drawing is minimal.
- If there is an appropriate wiring system and an adequate bonding network has been offered for the IT Equipment, the risk of equipment damage is reduced. In other words, an appropriate lightning protection zone has been achieved for the IT Equipment.
- In IT Rooms, if the Loop surfaces are controlled by wiring methods, the internal ports of interconnected IT Equipment will not be damaged by any power frequency interference sources.
- In IT Rooms, if the following conditions are met the internal ports of interconnected IT Equipment will not be damaged due to the surge voltages:
- All incoming cables to the room are provided with overvoltage (primary) protection.
- The units of the equipment are well-interconnected by a ground connection network, which is not significantly influenced by the power installation or lightning. To achieve this goal The bonding practice between IT Equipment has been respected in accordance with the rules specified in standards BSEN 50310, TIA 607C.
- The electronic equipment has its power supply completely separated from the other equipment.
The Three above conditions will automatically create an LPZ, which is categorised as a class 2 according to IEC 61000-4-5, and Surge voltage may not exceed 500V.
LPI Group Design Services
LPI Group can offer a full design service for lightning protection systems, earthing systems and surge protection systems. The design of our high performance lightning protection systems are carried out by our experienced ATLAS accredited design team using specialist lightning protection system design software, CAD and AutoGroundDesign.
LPI Group Design Services:
- Design of coordinated lightning protection systems, earthing systems and surge protection devices to EN 62035, UNE 1186 and NFC 17 102.
- Writing of specifications and tender documents to EN 62305, UNE 1186 and NFC 17102.
- Drafting services, including CAD drawings with system layouts and details.
Contact our specialist team today for your earthing design solutions today.