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Article Date: 14th July 2020

Kingsmill Industries - Seven Simple Steps to Select your Surge Protection Device

Kingsmill Industries, surge protection

Here, at Kingsmill, we take Lightning Protection and Earthing systems seriously. To protect against lightning, it is important to provide a system that encompasses both structural and electronic systems protection.

Kingsmill are passionate about protecting infrastructure from the devastating impact of lightning activity. Not only do Kingsmill offer a wide range of protection solutions, we also offer systems designed to monitor activity.

In order to select a protector, the following information has to be determined:

STEP 1: Carry out a risk assessment to determine the Lightning Protection Level (LPL)

STEP 2: Assign the Lightning Protection Zones (LPZ x) This will involve determining the locations of distribution boards and equipment to be protected

STEP 3: Determine the voltage protection level (Up)

STEP 4: Determine the number of metallic services entering the building and establish the kA rating of the device

STEP 5: Determine the earthing system type into which the SPD will be connected

STEP 6: Establish the positioning of each device (taking into account protective distances)

STEP 7: Assess cable routeing and other considerations

STEPS 1 & 2

Lightning Protection Zones

BS:EN 62305-4 employs a principle of using Lightning Protection Zones (LPZ) to progressively reduce a potential 6,000 volt transient overvoltage to a safe voltage. This voltage must be below that of the withstand voltage of the equipment to be protected. SPD’s are located at the boundaries of these zones.


Determine the voltage protection level

It is important that a protector does not ‘let through’ harmful voltages to the equipment that it is protecting. In the table below, “withstand level” equates to Up or voltage protection level. In the case of everyday electronic equipment, this is 1,500 volts.


Selection of mains Surge Protection Devices

Once we have determined:

• The Lightning Protection Level (LPL) and Lightning Protection System (LPS), see Risk Assessment

• Whether a structural Lightning Protection System is required or not, and

• The Lightning Protection Zones in which to locate the SPDs, together with the purpose of the SPD

• The number of metallic services entering the structure

When evaluating the existence of a metallic service, it is important to establish whether it is continuous and provides a solid path to earth.

NOTE: some metallic services connect to non-metallic or insulating material close to the structure (ie water pipes, gas pipes, fibre optics etc)


The next task before a final SPD part number can be selected, is to determine the earthing system used in the building. This will be either TN-S, TN-C-S, TN-C or TT. The differences between the various systems are in how the Neutral and Earth conductors enter the building, and whether, as in the case of TN-C-S. A combined Neutral and Earth, is separated out in the Main Distribution Board.


Protective Distances

We learnt from pages SPD:15 and 16 and figure 7, that protectors need to be installed at the service entrance position and as close as possible to the equipment being protected.

If the distances between SPDs or the SPD and the equipment being protected are too long, reflected voltages may appear on the line which could destroy the connected equipment or cause breakdown of the cable insulation. Such reflections can cause the up-stream SPD “let-through voltage” or Up (voltage protection level) to double. This effect occurs if the equipment is disconnected inside or its input impedence is high.

If the distance between the SPD and the equipment being protected is less than 10m, such reflections can be ignored. However, if the distance is greater than 10m additional SPDs must be installed.


Assess cable routeing and other considerations

Cable routeing and the connection of SPD’s can affect the performance of the SPD and the level of protection that it can provide:

• Cable routeing should avoid proximity to lightning protection down conductors

• Large inductive loops between communication and power cabling should be avoided

• Cable screening should be considered

• Connecting leads must be as short as possible

• Avoid long distances (over 10m) between the SPD and the equipment being protected to avoid oscillations

• Examine use of electromagnetic shielding on cables

• Determine locations of distribution boards and the connected equipment to be protected

• Determine length of circuit cables

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