Motor earthing through structure bonding

[Locus]

Hi Everyone

I would like to have your views and whether it is acceptbale to BS 7671.

A new plant has been constructed and there are about 10 motors of different ratings (3 phases 400V). The Contractor runs a copper bar all along with the galvanised cable tray and the copper bar is connected to the main earth bus bar. The copper bar is acting as a bonding for the cable tray and is interlinked at each motor metallic support frame.

The 3 phase motors power cable is a 3 core (Brown Black Grey) or 2 x 3 cores for star delta. The is no CPC connected in the motor connection box. Therefore, the motor support metallic structure which is bonded by the copper bar is also indirectly bonded to the motor frame (metallic to metallic). There are 6 Nos. motor support structure where 2 Nos. motors are mounted on each structure.
Is it acceptable as per BS7671?

Thanks in advance.

 

[Geoff1946]

I would have expected that whoever designed the plant, and that earthing arrangement, should have specified bonding resistance tests to ensure adequate conductivity. The value(s) to be achieved would be part of the loop impedance calculation for proving adequacy of circuit protection.

 

[steptoe]

I often work on stuff with localised earth bars, with a lot more sensitivity than a few motors, although motors are often involved.
I can't really see an issue as long as, (has been stated above) , the design has allowed for such a scenario.
Some of the buildings I work on the whole structure has been designed as a Faraday cage.

 

[Andy™]

also worth noting that 7671 probably doesnt apply either

 

[binky]

Sounds like a clever and interesting design. Just because you are not used to a system like this doesn't make it wrong, a bit like using 2 core SWA with the armour as the earth - confuses the hell out of most younger sparks

 

[Andy™]

Sounds like a clever and interesting design. Just because you are not used to a system like this doesn't make it wrong, a bit like using 2 core SWA with the armour as the earth - confuses the hell out of most younger sparks

not just younger sparks, but many 'domestic' sparks. and its usually the same ones who will use 3 core and a core as a cpc, but not bother connecting the armour....

 

[binky]

not just younger sparks, but many 'domestic' sparks. and its usually the same ones who will use 3 core and a core as a cpc, but not bother connecting the armour....

Oi, I'm domestic! I'm also to tight to buy 3 core . I like the smaller physical size of using 2 core, apart from the financial savings, it's easier to work with, and looks neater when finished

 

[Old Timer]

not just younger sparks, but many 'domestic' sparks.

Or they use a 3 core Brown,Blue, Green/Yellow for a 3Phase supply!

 

[Andy™]

Oi, I'm domestic! I'm also to tight to buy 3 core . I like the smaller physical size of using 2 core, apart from the financial savings, it's easier to work with, and looks neater when finished

more those who only do domestic and have no idea of the world of commercial