Various types of surge voltages can occur in electrical and electronic systems. They differ mainly with respect to their duration and amplitude which, depending on the cause, can last anywhere from a few hundred microseconds to hours or even days. The amplitude can range from a few millivolts to some ten thousand volts. Lightning strikes are a special cause of surge voltages. Direct and indirect strikes can result not only in high surge voltage amplitudes, but also particularly high and sometimes long current flows, resulting in some very serious effects. In this article, we take a look at what these two systems are and how they work together.
A Surge protection device is designed under standards to limit electrical surges and transients to a safe level suitable to protect the equipment installed in that location. Surges, transients and protection methods are described in AS/NZS1768 with sizes suggested depending on the location. Surge protection installation is categorised into 3 zones; “Category C” (1,2,3) for building entry (i.e. main switchboard) “Category B” for >10m from main switchboard but prior to sensitive electronic equipment (i.e. distribution board) and “Category C” for final circuits.
How is surge protection different to lightning protection?
In the electrical sense, surge protection is designed to withstand an electrical surge with a rise time of 8 micro seconds with a 50% decay time of 20 micro seconds and rated in Imax. A lightning arrestor is designed to withstand an impulse with 10 micro seconds rise time with a 50% decay time of 350 micro seconds and rated in Iimp. A lightning arrestor would normally be installed in the main switchboard of a building with a mechanical lightning protection system installed, or a smaller value model installed in an adjacent building to one with a lightning system installed.
Selecting a surge protection device
When selecting devices for protecting electrical systems from surges, there are ratings associated that assist in determining the most suitable model.
- Imax: “one shot” rating of the device and will be or close to spent if this occurs. Used as a guide to selection only.
- Inom: 15 x rating of the device. This value is more realistic in normal operational use. Use this value in conjunction with the Imax to determine the best product.
- Fuse size: A good size fuse ensures repeated protection. Fuse size will be in the order to 125A for 40kA, 250A for 100kA. Note: the manufacturer specifies the maximum fuse size suitable for their product.
Fuse sizing and wiring is critical when installing lightning and surge protection. Please follow the manufacturers installation guidelines or AS/NZS1768 standards.
Pacific Automation stock a range of surge protection devices from leading electronics and electrical connectivity manufacturer Weidmuller. For help with selecting the most suitable Weidmuller surge protection device for your electrical system, contact the team at Pacific Automation.