The 30/30 rule comes up constantly in searches about lightning, so it is worth setting out what it is and, more usefully, what it is not.
The rule has two halves. If you see lightning and cannot count to 30 before you hear the thunder, the storm is close enough to be dangerous and you should get inside. Once the storm appears to have passed, wait 30 minutes after the last thunder before going back out.
The first 30 is about distance. Sound travels roughly one kilometre in three seconds, so a 30 second gap puts the strike about ten kilometres away, which is within the range at which the next strike could reach you. The second 30 exists because the trailing edge of a storm still produces strikes, and those are the ones that catch people who thought it was over.
It is guidance for people, outdoors, in the moment. It says nothing about buildings, and it is not a substitute for any part of the process that decides whether a structure needs protecting.
That decision is a calculation, set out in BS EN IEC 62305-2. It weighs the likelihood of a strike affecting the structure against the consequences across four categories: loss of human life, loss of service to the public, loss of cultural heritage and economic loss. The output is whether protection is required and, if it is, to which of the four protection classes.
If you are asking about lightning because of a building rather than because of a walk, the questions worth asking are different. Is there a system on the structure already? When was it last tested? Is there a current certificate and a log book? Has anything been added to the roof since the system was designed?
That last one catches the most people. Roof mounted plant, masts and solar panels get installed by other trades, do not get bonded into the lightning protection system, and nobody records it. The system quietly stops complying until it is next tested.
If any of that applies to a building you are responsible for, call us on 01892 667258.
