No Earth from Supply on front of terraced house

[scotsparky]

No RF if i see a 2.5 as a Main Earth i will code 1 it.

 

[securespark]

As I said before:

How can you justify giving a code 1 to an installation that complies fully with BS 7671 2008?

You will look a bit of a berk.

Now, before you get upset, I did exactly the same & I ended up looking a real berk in front of my client.

I made an error in judging an installation & marked it down for earthing conductors being undersized.

Before I could say Ohm's Law, the NIC spark who did the job was on the other end of my mobile berating me for classifying his installation as not complying with regulations. Then he kindly proceeded to explain where I had gone wrong. It was a very good lesson learned and showed me that you just can't generalise and mark down an installation if it doesn't have 16/10mm².

 

[aptsys]

No RF if i see a 2.5 as a Main Earth i will code 1 it.

But there may not be any reason to. As securespark said, it should require a quick calculation and an assessment of the installation method to determine whether it complies rather than an automatic code 1.

 

[Spark123]

Where, in a TT system, disconnection is achieved by an overcurrent protective device and PE bonding is connected to all ex-c-p's within the installn in accordance with 411.3.1.2, the max disc. times applicable to TN system may be used. In other words, you can scrap the 0.2s and use 0.4s instead.

Most TT systems require RCDs to provide disconnection as opposed to overcurrent protective devices.

 

[RF Lighting]

Regardless of the supply type, an undersized earth is not a code 1 anyway.

 

[scotsparky]

RF can you explain why an undersized main earth is not a code 1

 

[RF Lighting]

Which of the following statements would you say most accurately describes an undersized earth?

"It poses an immediate danger for persons or property"

or

"It does not pose an immediate danger, but it should be rectified as soon as possible"

 

[securespark]

Where, in a TT system, disconnection is achieved by an overcurrent protective device and PE bonding is connected to all ex-c-p's within the installn in accordance with 411.3.1.2, the max disc. times applicable to TN system may be used. In other words, you can scrap the 0.2s and use 0.4s instead.

Most TT systems require RCDs to provide disconnection as opposed to overcurrent protective devices.

Doh!


RF: depends whether the earthing conductor will vapourise when the fault current passes down it or not.

 

[bongos]

Just to add my tupence worth (and because i'm slightly confused by all this!)

The reference for using 2.5mm on an earthing conductor buried in the ground (ie. TT) comes from table 54.1 of BS7671.

The requirements for establishing csa sizes for all protective conductors continues in section 543 and involves using the adiabatic to calculate minimum csa.

The fault current flowing along the protective conductor will be less in a TT system (because resistance is higher) therefore a conductor with a csa of minimum 2.5mm can be used as long as the figures from the adiabatic allow.

AFAICT 2.5mm is fine

 

[Spark123]

Yep, (it is acceptable to bury a 2.5mm conductor in the ground as long as it is protected against mechanical damage.)

 

[ElectricDunc]

I use a good calculation for sizing main earths for TT installations. It's usually the 'what have I got on the van' calculator meaning that most of mine are oversized as I generally only carry 10mm!

 

[bongos]

Yep, (it is acceptable to bury a 2.5mm conductor in the ground as long as it is protected against mechanical damage.)

Think I just about got my head round it! I could have probably written my post a bit clearer though.

Also just re-reading other posts seems like everyone else got there a while ago.

I'm going to use the 'i'm not a spark' (yet) excuse. I did score 58/60 on my 17th though but unfortunately I think that is just damning evidence of the 'difficulty' of the exam.

 

[Spark123]

I got 29/30

 

[RF Lighting]

59/60

 

[securespark]

59/60

Me too. Am I narked I got one wrong!!

Still, least I rank alongside the mighty RF.