17th Edition test question

[zak325]

evening

i came across this question on a 2382-20 (17th Ed update course) sample paper:

"Which one of the following systems is not used on construction sites, with the exception of a fixed building on the site?

a) TNCS

B) TT

c) TNS

d) Reduced low voltage"

From BS7671:2008, 704.411.3.1 (page 181) i figure the answer to this question is (a) - TNCS.

Am intrigued as to why this Regulation has been made? Any explanation welcomed.

thanks

Zak

 

[spinlondon]

It's basically down to the network operators.

They do not like providing earthing if they are not certain that the installation earthing and bonding arrangements are adequate.

With PME, they require that all extraneous-conductive-parts are bonded.

As a building is constructed, often no earthing and bonding is done untill some way into the construction.

If there were to be a fault on the network PEN, which would be the responsibillity of the network operator.

It could introduce a dangerous potential between the network earthing point and any unbonded extraneous-conductive-parts of the construction that have a connection to local earth.

As such, they want whoever is in charge of the installation to provide the earthing arrangements by using rods. If there is any earthing faults, it would not be the responsibility of the network operator, but of whomever is in charge of the installation.

 

[sparkytim]

Ditto

Construction site installations can be TNS but normally TT never TNCS (Except for Canteens Office etc if normal type Building) , due to higher risk if PME supply looses its nuetral.

 

[zak325]

thanks a lot for the responses, much appreciated.

 

[Andy™]

quick search for 'exporting PME' will get you lots of info

its also the same with caravans