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Demystifying surge protection standards

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Leading manufacturer and distributor of electrical and electronic devices, Weidmuller throws light on European and Australian standards governing the use of surge protection devices.

An effective surge protection system will increase safety and productivity in fully functioning equipment, while helping to minimise risk to equipment in the event of a surge.

Although the benefits of surge protection are well documented, and are especially prevalent now that Australian businesses and homes are inundated with electronic products, the differences between the European and Australian standards that govern the use of surge protection equipment still cause confusion.

As a manufacturer and distributor of European electrical and electronic devices, Weidmuller has worked with Australian Standards for all products, while also ensuring compliance with the European CE and C-Tick (RCM) as well as ATEX and IECEX. As a manufacturer of surge protection devices, the company works with current AS/NZS1768 standards. In the future, AS1768 may follow AS/NZS5033 for photovoltaic systems and adopt IEC standards.

The AS1768:2007 and IEC61643-11:2011 standards covering point of entry to a building is where these two standards primarily differ. Under Australian standards, Category C protection requires an Imax value (1 shot peak current) of between 40-100kA depending on the classification C1-C3. IEC Class 1 devices all define Iimp values that are considerably lower. The difference lies in the waveform used for testing. 

Under the rigorous IEC scheme, all testing must be performed by a certified test laboratory. The IEC 61643-11 uses a test waveform of 10/350μs for building entry protection specified for Class 1 products. Both the Iimp and Imax are tested, with the test stating that the Iimp value (impulse discharge current) is lower than the Imax due to the waveform size and duration time, which exceeds the 8/20μs waveform. 

The Iimp value and Imax value rating are normally documented on datasheets and product face. The Iimp will nominally state a 25 to 35kA rating, which equates to an Imax of 100kA in tests, with Iimp of 12.5kA equating to Imax of 50kA. So when using IEC tested Class 1 products, the use of the Imax rating enables easy identification in line with AS1768.

Two other important tests that Class 1 surge protection devices are required to meet under IEC standards are the Temporary Over Voltage (TOV) and safe failure mode for end of life. These tests help reduce fires and damage resulting from the overheating of surge protection devices.

A major issue for surge protection devices, Temporary Over Voltage can be catastrophic. Class 1 devices under IEC61643-11 must pass a TOV test of 335V for 5 seconds, and 435V for 120 minutes based on a neutral breakage. Passing these tests ensures there is no physical damage to the housing, no electrical hazards are created, the unit is still IP20 finger safe and can disconnect safely if failed, and that it displays a visual indication. 

In the case of safe disconnection, the Australian surge protection standards require protective fusing of the device to ensure safe disconnection at end of life, when the device could provide a short circuit to neutral/earth. This also provides safe replacement or disconnection if the device is not pluggable and requires wiring changes. This is no different for a Class 1 device under IEC rules, with the added assurance the device will also safely disconnect in a manner that ensures the surge protection device cannot cause a fire, or damage the housing exposing electrical components while providing indication of failure.

With regards to fusing, an update to the Australian standards nominated fusing needs to be specified by the manufacturer. However, IEC standards actually require surge protection devices to be tested for correct fuse rating.

Tests with NH type fuses and 8/20μs waveform indicated a 125A fuse was required to pass a 40kA surge. Likewise for a 70kA impulse, a 200A fuse was required. Yet most 40kA surge diverters are installed with 125A or smaller main fuses; therefore it is important for the customer or designer to use a suitable fuse rating for the protection required in the geographical area to be installed. 

A surge protection device fuse cannot be larger than the main fuse and should be a factor of 1:1.6 or approximately two sizes smaller. Too small a fuse will be damaged or melted by a high surge current, disconnecting the surge protection. Too big a fuse may cause the main building fuse to fail instead of the surge protection device at the end of its life.

It should also be noted that surge protection devices can operate effectively for many years but will eventually fail. A contact output on surge protection devices will help alert users when it’s time for replacement. 

Category B/Class II and Category A/Class III models are also rigorously tested under IEC requirements. Category B/Class II models are tested to the 8/20μs test pulse and rated in Imax value. Class II models still provide features of safe disconnect with indication, over temperature, and TOV. Category A/Class III models are tested by 8/20μs pulse and the Class D 1, 2/50μs.

Rest assured that the Imax can be used to determine the required value and whether an IEC class I, II or III device can be used safely under Australian standards. Ideally, a Class I device is suitable for use in a Category C application for the best protection of point of entry, a Class II device for a Category B application, and a Class III for Category A.

Standards play an important role in ensuring safety in design and installation of products; therefore installation of surge protection should always be in accordance with AS/NZS 3000:2007 wiring rules and AS/NZS 768:2007 surge protection standard.

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