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Posted By Topic: Concrete Chase

Davious
Mar 01 2018 09:54

Hello,
Has anyone heard of an X chase? I have been told by a shop fit out designer that it is 2x 10mm cuts in the shop floor (concrete) parallel to each other and that the sparkies on her previous jobs have just dropped the power and data cables into these cuts and concreted over!
My plan was to cut a chase wide enough and deep enough to accomodate a data and a power pvc conduit and have 50mm concrete over the top. The chase has been cut by someone else and is too shallow for this so when I pointed this out they have asked if a 3mm thick length of sheet steel can be placed over the top of the chase? Please can I get some input here. Thanks

   

Davious
Mar 01 2018 10:25

wiring rules 3.9.4.2 and 3.9.4.4 thoughts anyone?

   

AlecK
Mar 01 2018 10:26

If the chase is filled, then it's "underground"; and 50 mm cover is NOT enough. Needs to be min 75 mm, and the cable needs to be a Cat A cabling system or a cat B system with additional protection.. So NOT just TPS plastered in, and not just TPS in MD conduit either.

Regardless of what it may be called, in an open (ie unfilled) chase is not technically "underground"; so the minimum depth of cover [clause 3.11.4.4 & Table 3.6] doesn't have to be applied; and nor do the wiring system categories for u/g cabling.


But dropped in a chase makes the cable effectively fixed in position, so if cable is within 50 mm of surface, and unable to move to min 50 mm from surface, then protection from mechanical damage i.a.w. 3.9.4.2 is required. there are three acceptable options, and a 3 mm metal plate would probably meet option (a). If earthed (option (b)); could be thinner. Or fit RCD (option (c).
In fact neither (b) nor (c) protects the cable against damage, what they actually do is ensure supply is switched off if damage occurs - the cable still needs to be repaired.

Note that the 50 mm spacing in clause 3.9.8.4 is only for electrical safety; and is based on voice telephone; it doesn't deal with electrical cables interfering with data transmission. It's been a while since I swotted up on the TCF Premises Wiring Code (or current equivalent), but I'd be surprised if running data in a close parallel chase is acceptable. It certainly didn't used to be; the min separation for parallel runs for unshielded power cables & UTP data used to be 300 mm.


   

Davious
Mar 01 2018 11:26

Thanks AlecK. Now I just need to work out what to do from here with material costs, labour time and the best outcome legally. Cheers

   

mowgli
Mar 02 2018 17:23

3.11.2 NOTE Underground wiring systems do not include those that are- (i) embedded in a concrete floor, slab or pad;

Absolutely fixed in position and requires protection 3.9.4.2.

A 3mm fixed plate will act as an indicator for anyone drilling into the floor. It will protect against most things but not ramset which is likely in a shop fitout. So I'd be insisting on either the plate being earthed or cable on RCD as well.
   

AlecK
Mar 04 2018 08:29

well spotted mowgli;
I should have checked before posting