Main protective earth bonding conductors

[cliffed]

We are now told no bonding is required if the Water enters via plastic insert, same with the Gas..
So why are we bonding the Lighting protection conductor to the Main earth terminal bringing an extraneous conductive inside the building,when we are actually trying to prevent this happening.

 

[Robson689]

This is a requirement of BS 6651 / BSEN 62305

 

[westward10]

The reason we now fit a main protective bond to the lightning protection is to prevent a flash over to the internal electrical system. If no bond is in place and a strike occurs it can jump through building fabric into the electrical system causing damage, a bond should prevent this by diverting it straight to earth. It's purpose is different than a bond to other services.

 

[cliffed]

The reason we now fit a main protective bond to the lightning protection is to prevent a flash over to the internal electrical system. If no bond is in place and a strike occurs it can jump through building fabric into the electrical system causing damage, a bond should prevent this by diverting it straight to earth. It's purpose is different than a bond to other services.

Really,it makes me think about the satellite dish tv aerial, a strike of lighting would put it straight into the electrical system with or without an earth.

 

[Robson689]

Really,it makes me think about the satellite dish tv aerial, a strike of lighting would put it straight into the electrical system with or without an earth.

You can get surge protectors for pretty much anything including TV antennas if it's something you want. A complete lightning protection system will cover all angles and incorporates all the necessary surge protectors in addition to the physical conductors.

 

[pc1966]

We are now told no bonding is required if the Water enters via plastic insert, same with the Gas..

In those cases they are not extraneous.

So why are we bonding the Lighting protection conductor to the Main earth terminal bringing an extraneous conductive inside the building,when we are actually trying to prevent this happening.

As above, you don't want a voltage difference to be able to cause problems. For the usual 'extraneous' cases it is around the 230V under fault conditions.

But if you get a lightning strike you can see 10-100kA appear in a few microseconds. It takes very little inductance to result in 100's of kV or more and that can flash over, potentially even through common building materials.

 

[cliffed]

You can get surge protectors for pretty much anything including TV antennas if it's something you want. A complete lightning protection system will cover all angles and incorporates all the necessary surge protectors in addition to the physical conductors.

Do we really need lighting protection,on normal domestic properties we don’t.
Also not all properties have this, & maybe require it.

 

[pc1966]

But if you get a lightning strike you can see 10-100kA appear in a few microseconds. It takes very little inductance to result in 100's of kV or more and that can flash over, potentially even through common building materials.

To put some numbers to this:

The average strike current from Wikipedia is 30kA (Lightning - Wikipedia - https://en.wikipedia.org/wiki/Lightning#Positive_and_negative_lightning) and if we take the 10us rise time of the direct SPD test waveform we have 3.0E9 A/sec rise time. For a typical straight conductor the inductance is about 1uH/m (see Inductance - Wikipedia - https://en.wikipedia.org/wiki/Inductance#Inductance_of_a_straight_single_wire) so a 2-story building with a conductor of around 10m length will see 10 * 1.0E-6 * 3.0E9 = 30kV appearing along the length of the conductor simply from L*dI/dt, without even considering the cable resistance or the ground rod impedance.

Assuming an Earth rod impedance for the lightning protection system of 5 ohms then your 30kA peak is going to add another 150kV (assuming it is still 5 ohms under such a violent discharge, of course...)