Why TT on Temp Building Supply?

[rolyberkin]

Okay, I know you have to use TT on a temporary building supply but cannot understand why? If the supplier is able to supply a TNCS why is this not preferable to a TT?

 

[Leo_chaos]

Googled a lot, from what I'm finding (in small amounts of information)
"the practical difficulties ofbonding all accessible extraneous conductive-parts" so I think that since there may be a difference in potential, and since it's temporary maybe something could happen to break the earth continuity, making sure a TT is being used would mean they know it's being earthed and without the supplier being at fault if you mess something up with it since it's not their earth.

Though this is the first i've heard of this, so I could be miles off but this is what I'm assuming from what I found.

 

[Richard Burns]

A construction site is very hard on electrical equipment and prevention of electric shock is difficult to implement.
As Leo_chaos says there may be a myriad of items that are extraneous conductive parts, even if only temporarily, such as a digger, and so the effective bonding of these is practically impossible.
Where the site design is such that it is possible to effectively bond all extraneous metal work TNCS is permitted.
Since the risk of damage to electrical equipment is high the risk of contact with earth live parts is high and the presence of true earth everywhere in mud and so on means that all measure should be taken to reduce risk of electric shock.
The reduced low voltage and electrical separation supplies in use reduce the risk but having anything relying on a single connection such as the TNCS earth / neutral link presents more of a risk.

 

[Simonslimline]

Section 704. 704.411.3.1

A PME earthing facility shall not be used for the means of earthing for an installation falling within the scope of this section unless all extraneous-conductive parts are reliably connected to the main earthing terminal in accordance with Regulation 411.3.1.2.

NOTE If the PME earthing facility is considered for use, see also BS7375.

 

[rolyberkin]

Section 704. 704.411.3.1

A PME earthing facility shall not be used for the means of earthing for an installation falling within the scope of this section unless all extraneous-conductive parts are reliably connected to the main earthing terminal in accordance with Regulation 411.3.1.2.

NOTE If the PME earthing facility is considered for use, see also BS7375.

Yes those are the regs, but that isn't what I asked, why is this a safer alternative to PME, due to potential cable damage, if you have a TT installation the extraneous metal would still be present and not bonded, perhaps I haven't got my head around earthing issues?

 

[Simonslimline]

This is worth a read.

http://file:///Users/simonaragon/Do...ters_earthing_your_questions_answered (4).pdf