Surge protection in the 17th edition
BS7671 amendment 1
Robin Earl
Business development Manager
Surge Protection
DEHN UK Ltd
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SECTION 443 of BS7671 amendment 1
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Section 443
The protection against overvoltage's of atmospheric origin or due to
switching (SEMP)
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Section 443
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Section 443 part 443.2.1
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This is just lightning surges no mention yet of man made surges
No overhead lines could apply to any modern housing development or
retail park.
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Table 44.3
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To know if an SPD is needed you also need to know what category the
installation is and then refer to table 44.4.
A typical industrial installation may have all these and a domestic
property only some of these
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Table 44.4
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Each installation will have elements of all 4 categories.
Category I examples, hospitals, military sites, power stations
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Application of SPDs in BS/IEC Overvoltage
Categories
Withstand surge voltage and overvoltage category in
accordance with BS7671
6 kV
IV
SE
4 kV
III
2.5 kV
II
1.5 kV
I
M
230/400 V
II
III
I
SPD Class in accordance with IEC 61643-1
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01.08.06 / 608e_a
Selection of SPDs
So we can say that if we know what the category is
for each part of the installation we can then know if we
need to fit SPDs and what type it will be at that location.
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The AQ method of SPD selection in BS7671
The AQ1 figure enables us to determine if a SPD is needed if there is an
over head line.
If AQ1 is less than 25 active days a year then no SPD is needed if the
category is such that the impulse is in accordance with table 44.3 .
This is true for all of the UK no part of this country is >25 active days
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The next AQ method of SPD selection or AQ2
Here if AQ2 is greater than 25 active days then a SPD is
fitted.
No part of the UK > 25 active days per year.
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Ceraunic levels.
This is the numbers of days that thunder can be heard at any given
location per year. For the UK this is between 10-20 days, not 25!
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Selection of SPDs
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The risk assessment model 443.2.4
Here the selection is clearer. We need to understand the application.
If there are consequences to (i) ,(ii) or (iii) just fit the SPDs
For (iv) and (v) we now go to calculations.
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Section 443.3.4 calculations
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d=d1+d2/Kg+d3/Kt an example
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Using the risk assessment in BS7671 section 443
d=d1+d2/Kg+d3/Kt
d1= LV overhead line
d2= LV buried line
d3= HV overhead line
d=0+150/4+1000/4
d=287 or 0.287 Km
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The next part of the calculation
I need another map then as Ng is not the same as Ceraunic
the factor we needed in the AQ values before !
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Lightning flashes per year/ Km2 or the Ng value
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Back to my maths
d=0.287km and Ng =2 (worse case on the coloured map!)
dc = 1.0/2 for the consequences (iv) if groups of people =0.5
dc =2/2 for the consequences (V) if a person at risk.
=1
At my home d<dc as 0.287<1
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Other example
If we considered a TT supply over head line right up to the house
and the transformer on the pole in a rural context
d=d1+d2/Kg+d3/Kt
d= 1000+0/4+1000/4
d=1.25Km
Then dc= Ng/2 for a house dc=2/2 =1
So d> dc as dc =1 so SPD is needed, no surprise there!!
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Site inspection report forms
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Site inspection report form
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Site inspection report form conclusion
This site was large residential with two home office functions, so
was this really a cat (iii) installation or commercial?
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after all the maths…
So we have used section 443 to determine if measures to protect
against lightning are required or not.
The calculation has not factored in any other threat.
Man made surges
Road repairs
Trees falling on line
Wind damage to overhead lines
Power outage due to maintenance work
Local inductive loads or industry
The effects of proximity to a LPS fitted nearby
Other conductors entering the building not covered by BS7671
Etc…
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