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Terminology & selection criteria

Surge protection devices are classified according to their standard into different types

Type 1

SPD which can discharge partial lightning current with a typical waveform 10/350 μs. Usually employs spark gap technology.

Type 2

SPD which can prevent the spread of overvoltages in the electrical installations and protects equipment connected to it. It usually employs metal oxide varistor (MOV) technology and is characterized by an 8/20 μs current wave.

Type 3

These SPDs have a low discharge capacity. They must therefore only be installed as a supplement to Type 2 SPD and in the vicinity of sensitive loads. Type 3 SPD’s are characterised by a combination of voltage waves (1.2/50 μs) and current waves (8/20 μs).
Iimp – Impulse current of 10/350 μs waveform associated with Type 1 spd’s
In – Surge current of 8/20 μs waveform associated with Type 2 spd’s
Up - The residual voltage that is measured across the terminal of the SPD when In is applied
Uc - The maximum voltage which may be continuously applied to the SPD without it conducting

Selection of suitable devices

BS 7671 section 534 gives the requirements for correct selection of devices against overvoltages.

Regulation 534.2.1 prescribes that where required by Section 443 or otherwise specified, SPD’s shall be installed:

(i) near the origin of an installation, or
(ii) in the main distribution assembly nearest the origin of an installation

The notes to this regulation give further guidance, stating that a Type 1 or a Type 2 SPD may be used at the origin whilst Type 2 and Type 3 are also suited for locations close to the protected equipment. Type 1 SPD’s are often referred to as equipotential bonding SPD’s and are fitted at the origin. A lightning protection system employing these devices only, offer no effective protection against failure of sensitive electrical and electronic systems. In order to achieve this, additional coordinated devices will have to be employed. In summary, a Type 1 SPD is used at the origin of the installation, a Type 2 SPD is used at distribution boards and a Type 3 SPD is used near terminal equipment. Surge protection needs to be selected such that their voltage protection level (Up) is lower than the impulse withstand capability of the equipment to be protected. 534.2.3.1.1 suggests that this value should be referred to category II of Table 44.3. This for a 230/400V installation suggests that the value should not exceed 2.5kV. However 534.2.3.1.2 suggests that to protect sensitive and critical equipment, then consideration should be given to reduce this value to that required for category 1 equipment (ie 1.5kV).

534.2.3.4 also gives guidance as to the selection of an appropriate device.

The specifier should ascertain from BS 7671 which connection type is preferable (CT 1 or CT 2). Hager manufacture devices with connection type CT2. Type 1 devices need to be selected such that the value if Iimp is not less than that which shall be calculated in accordance with BS EN 62305-4. However if this cannot be calculated then this value shall be not less than 12.5kA. Also, due to the connection method, the value of Iimp between the neutral conductor and the protective conductor shall be not less than 50kA for three phase systems and 25kA for single phase where the value cannot be calculated. For Type 2 devices the value of In shall be not less than 5kA and the value between the neutral and protective conductor shall be not less than 20kA for three phase systems and 10kA for single phase. Larger values may be required as classified in BS EN 61643-11.

Cascading

Cascading is the term used to describe the method of combining several levels of surge protection devices into the one installation. This takes advantage of the best features of each device to improve the protection level for the equipment.
Hager recommends using a high surge current capacity device to divert the bulk of the transient over-voltage at the origin of the installation. In the case of a Class 1 & 2 device this would be either the spark gap arrester or a high current capacity MOV. Should finer protection be required, the next step is to install a Class 3 device SP202N near the terminal equipment.

Cascading increases the current diverting capacity of the SPD system whilst maintaining a low voltage (Up) to ensure the best protection for valuable equipment. Selecting SPD of the same manufacturer or make will ensure correct co-ordination between devices