Feature Surge Protection Your questions answered
It’s been 18 months since Amendment 1 to the 17th Edition Wiring Regulations, yet there is still a level of uncertainty for many specifiers and installers when it comes to surge protection. However, in this article, Hager’s Paul Collins is on hand to answer some of the more burning questions
Q: Why do we need surge protection? A: Everyday activities at both home and work rely on electronic equipment. Products such as computers, printers, flat screen TVs, industrial control equipment, alarms and microwaves are all common- place. These can all be vulnerable to tran- sient overvoltages, which can significantly reduce the equipment’s lifespan.
Q: What are the different types of surge protection device? A: They are classified into three types: Type 1 can discharge partial light- ning current with typical waveform 10/350µs. Type 2 can prevent the spread of overvoltages in the electrical installa- tion and protects equipment con- nected to it. It is characterised by an 8/20µs wave form. Type 3 has a low discharge capacity. They must only be installed as a supple- ment to a Type 2 SPD and are usually installed close to the sensitive load.
Q: How do I decide if SPDs are needed? A: A quick way is to ask three questions. 1. Does the installation have a light- ning protection system?
2. Is the installation adjacent to any tall structure, tall trees or near a hill- top in a lightning prone area? 3. Does the installation contain
equipment where higher reliability from overvoltages is required? If the answer is yes to the first two questions then you should install a Type 1+2 device. This will provide protection against surges caused by direct lightning strikes and provide protection against transient overvolt- ages caused by indirect lightning strikes or by switching events. If the answer is yes to the third ques- tion then you should install Type 2 devices and perhaps also consider Type 3 devices.
Q: Where are the different types installed? A: Type 1 SPDs are used at the origin of the installation, Type 2 SPDs are used at the distribution boards, and a Type 3 SPD is used near the terminal equipment.
Q: How do I decide what level of pro- tection is needed? A: Surge protection needs to be selected such that its voltage protec- tion level (Up
) is lower than the
impulse withstand capability of the equipment to be protected.
Referring to table 44.3 and 44.4 of BS 7671, this suggests that this value should be referred to category II. For a 230/400V installation it suggests that the value should not exceed 2.5kV. However, to protect sensitive and criti- cal equipment, you should consider reducing this to that required for cate- gory 1 equipment, which is 1.5kV. BS 7671 534.2.3.4 also gives guid- ance. You need to select Type 1 devices so that its impulse current (Iimp is not less than that calculated in
)
Q: What inspection and testing is needed for SPDs? A: During both the initial verification and as part of a periodic inspection and test, the SPD should be inspected to ensure its operation.
There is usually some kind of visual indication on the SPD that the device is still working, this is often shown by a green visual indicator. If this window is red then the device has reached its ‘End of Life’ and needs replacing. With some products, this will involve replacing the device, while for others you can simply replace the removable cartridge. It is also worth noting that an insula- tion test of 500V DC carried out with the SPD connected will produce incor- rect readings. This is due to the SPD starting to conduct as the value of Uc may have been exceeded. You should conduct the test with the SPD discon- nected or you can do the test at the reduced voltage of 250V DC. Hager has produced a technical guide to surge protection devices on its website.
Hager
www.hager.co.uk T: 01952 677 899
Electrical Engineering SEPTEMBER 2013
accordance with BS EN 62205-4. If this cannot be calculated then this value must not be less than 12.5kA. Also due to the connection method, the value of Iimp
between the neutral
Above: Paul Collins, Hager’s technical manager
Left and below: surge protection devices offer security to vulnerable
electrical equipment
and the protective conductor shall not be less than 50kA for 3-phase systems and 25kA for single-phase where the value cannot be calculated. For Type 2 devices the value of the surge current of the 8/20µs waveform (In
)
shall not be less than 5kA. The value between the neutral and protective con- ductor shall not be less than 20kA for 3- phase systems and 10kA for single-phase. Larger values may be required as classified in BS EN 61643-11.
Q: How do I ensure protection throughout the installation? A: Cascading combines several levels of surge protection devices into one installa- tion. This increases the current diverting capacity of the SPD system whilst main- taining a low voltage (Up
) to ensure the best protection for valuable equipment.
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