Risk assessment for surge protection - what you need to know
By Robin Earl of DEHN UK LTD.
Now that amendment 2 of BS7671 the 18th edition has been released, we can look at the revisions to section 443, the risk assessment for surge protection.
There is a work flow to the process. In the first instance any structure that has a fitted lightning protection system (LPS) is required to have surge protection devices (SPDs) to meet part 4 of BS EN 62305 the lightning protection standard. See regulation 443.1.1.
After those structures have been protected, the standard turns to structures without the fitted LPS but satisfies one of the following conditions in 443.4.1.
Protection against transient overvoltages shall be provided where the consequence caused by the overvoltage could result in:
(i) serious injury to, or loss of, human life
(ii) failure of a safety service, as defined in Part 2
(iii) significant financial or data loss.
In the above circumstances SPDs shall be fitted to prevent injury/death, to maintain the availability of any safety services, and ensure no loss of commercial interest or data.
Continuity of service and an acceptable risk of failure
Further into section 443, we are required in 443.6.1 to look at the availability of the equipment with regard to continuity of service and an acceptable risk of failure. We need to identify the most sensitive equipment where a temporary loss of function of the equipment is unacceptable, and to do this we need to know the impulse immunity voltage of the equipment.
So, if the sensitive equipment is for example, mains powered smoke detectors in a dwelling, to prevent any loss of function because any loss of function is unacceptable, those devices will need SPDs if a voltage beyond the withstand level (Uw) is possible. We are referred to BS EN 61000-4-5 the immunity test standard.
Datasheets of smoke detectors and LED lights should show compliance to BS EN 61000-4-5. In this example, the value could be shown as 2kV on the detector datasheet.
This 2kV Uw value falls between the equipment categories 1 and 2 in table 443.2. - they have 2.5kV and 1.5kV values. The target value in section 534 is 2.5kV so a SPD may be required locally to limit the safety circuit overvoltage to less than 2kV if the last SPD is located further than the protective distance of 10m.?
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The 10m rule
This 10m rule is in 534.4.4.2. Bear in mind a poorly installed SPD with long circuit cables, can also result in Up levels beyond the withstand voltage of the installed equipment.
The total protection level (Up) of the SPD circuit is dependent on the cable length - refer to 534.4.8. Excessive cable lengths over 500mm and never over 1000mm will result in higher Up levels requiring additional SPDs to correct this to lower than the Uw.
In summary
The three idents in 443.4.1 are only the start of the risk assessment. It is incumbent upon the designer /installer to identity the most sensitive safety equipment so as to ensure the highest level of protection is installed where continuity of services must be maintained.
Where such data is not available for the end equipment's impulse withstand level (Uw) or that the Uw level is above the protection level (Up) of the last upstream SPD, then additional SPDs may be required to lower this Up level locally.
TechTalking podcast
This episode of the TechTalking podcast covers everything you need to know about SPDs – why we need them, what the regulations say and how to use them to protect different zones. Darren and Dave spark up the chat with Robin Earl of DEHN UK LTD.
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