Online assistance for electrical trade people Login  |  Register  |   Forgot Password
Assistance for electrical trade people
 

 

 

 


Click here to send Ron a pdf document for publication on this Topic

Documents must be less than 200k in pdf format

Posted By Topic: Bonding conductive reinforcing (domestic)

Sarmajor
Jun 04 2017 18:11

A question for all out there.
Inspectors, have any of you noticed in the course of your work electricians providing a bonding wire to the conductive reinforcing in a house floor slab?

Electricians, are you aware of this requirement?


   

DougP
Jun 04 2017 18:43

Yes.
5.6.2.5
Doesn\'t require inspecting though...
   

Sarmajor
Jun 04 2017 21:10

Not looking at it from any inspecting point of view. Just trying to get a feel I got for why so many sites that I look at don't have it done.
If I point it out to a builder they have no idea that it is a requirement.
If I point it out to an electrician they generally give me a blank look. When I mention 5.6.2.5 they get the idea that I am not making it up.
Then we progress to when did it get changed to require the bonding of the conductive reinforcing. Seems to have appeared in Ammendment 1 so around July 2009.

Not really my problem as:
1, I didn't sign the COC.
2, it is not part of a high risk inspection.
3, Even if it was it would be hard to inspect and verify after the fact.

But it is disappointing that those specialist electricians that wire up domestic installations appear to not know enough of the rules that affect things that they do on a regular basis.


   

SaintAlan
Jun 05 2017 15:25

They regs are not specific about how this connection should be made; the industrial earth bonds are quite expensive and probably over-kill for the job.
I have used earth-rod clamps to join a 10mm earth wire onto a main re-bar before the slab is poured, and covered the joint in denso tape to prevent corrosion. It seemed to work okay until a (**#$* idiot) building inspector ordered the builder to remove it or cancel the concrete pour.
On the last job we put a 12mm threaded rod through the boxing and clamped it against the re-bar inside with SS hose clips, it looked more like a 'proper job'. When the concrete was set and the boxing removed it leaves a SS stud located near the incomer, for bolting the bonding wire with a lug.

And while we are on the subject of earth bonding, not many sparkies seem to know or care about connecting a bond to the frame of steel-frame houses. It's easy if you do it before the cladding but a PITA afterwards.
   

Sarmajor
Jun 05 2017 16:01

When I lived in the Far North I knew the local building inspector. He was aware of the requirement to bond the conductive reinforcing and would mention it to all the builders that he interacted with. He got similar responses from me but had copies of the page in AS/NZS3000:2007 showing the requirement helped him out.

The fact that the method is not specified leaves the electrician with a lot of leeway to carry out the connection.

One of the simplest ways to acheive bonding of the conductive reinforcing would be to use the conductive reinforcing as the earth electrode as per AS/NZS3000:2007 section 5.3.6.2.

Of course then you would get some inspectors who would not accept it because it is not normal.
   

DougP
Jun 05 2017 18:55

Except that most new house slabs have insulation and a polythene moisture barrier layer, making them unsuitable as main earth electrode.
   

Narfnarf
Jun 05 2017 19:21

Starter bars in a block strip are typically connected to the footings on the other side of the polythene and bent over to tie into the slab mesh. so you could then use the slab as the main earth.

Or if its a full slab with no block work then couldn't you just cut a hole in the polythene and drive a stake into the ground and seal the polythene around the stake?
   

Sarmajor
Jun 05 2017 19:29

Most floor slabs I see have deeper poured sections that are full of rebar and are generally not insulated from the ground. Mostly the plastic is under the sections of the floor that are between / over the top of the beam type sections.

Fully insulated (electrically) concrete slabs would I think be rare in NZ.

And even then bonding the floor slab and using a driven electrode would fulfil the requirements of the Regs.
   

DougP
Jun 05 2017 20:37

You would need to check the slab insulation on a case by case basis. I doubt that any slab with a polythene moisture barrier would be suitable.
   

DougP
Jun 05 2017 20:38

In any case, it isn't difficult to drive a standard earth stake. And the Wricon slab bond kits are expensive. Around $150 list price and very little discount as far as I know.
   

DougP
Jun 05 2017 20:40

The one in my photo I made myself. About $10 worth of material.
http://www.transnet.co.nz/shop/Earthing/Exothermic+Connections/Wricons.html
   

DougP
Jun 05 2017 20:41

The other difficulty with using the slab as the main earth, is that the connection to the MEC must be accessible for testing (unlike the 5.6.2.5 bonding which doesn't need to be accessible).

The last one I did, I had the MEC connection accessible behind a blank plate in the garage. Others I have seen, have the rod extending horizontally outside the slab above ground level, and use a standard earth stake and clamp.
   

pluto
Jun 05 2017 20:56

Doug P

The horizontal earth kit shown in the reference
does not comply with 3000 if claims to be 6 mm2 bare copper table 5,2 requires 25 mm2 Copper for 7.5 metres in the horizontal plane.

I general a horizontal earth electrode is many times better than a 1.8 m vertical rod, and in river valleys, large stones for the vertical rod to dodge. Use the mains cable trench for the horizontal earth electrode.
   

DougP
Jun 05 2017 21:02

Not sure which one you are referring to Pluto. The link was for reinforcing bond or earth kits.

TNC-6-CK and TNC-6-BT
   

Flashman
Jun 06 2017 11:04

Pluto, that bare earth one you mentioned looks to me like its 10mtrs of 6mm conduit wire exothermically welded to 25mm bare conductor. No length given but assuming its at least 7.5mtr.
   

pluto
Jun 06 2017 12:07

My reading of the item on the website is this


" Designed & manufactured in NZ to meet the requirements of AS/NZS 3000
Cadweld exothermic connection blocks water and is resistant to corrosion as required in 5.5.5.3
10m of 6mm² earth wire allows for easy placement in any situation
Additional sizes and lengths available on request"

No indication of wire sizes and the picture does not show clearly that it may be some other wire size.

Why not use 25 mm2 CU in one piece of cable? Sleeve the bare wire in the switchboard enclosure and fit crimp lug for termination on earth bus bar of main switchboard. No join needed.

   

DougP
Jun 06 2017 13:16

Ok Pluto, so you're referring to the horizontal earth kit in the link below.

They don't specify the size of the bare copper, but one would presume it is the correct size. Permanently connected to the uninsulated 6mm2 MEC.

Their 10m length of 6mm2 MEC, would allow it to at least reach the house, and in some instances, reach the MSB.

Your suggestion of the bare 25mm2 running all the way to the MSB wouldn't comply IMO, as earthing conductors are required to be insulated (with some exceptions - but a MEC is not exempt).

Another option would be to run the bare 25mm2 up into the building, and connect the MEC to it in an accessible location.
http://www.transnet.co.nz/shop/Earthing/Exothermic+Connections/Horizontal+Earth+Kit.html
   

AlecK
Jun 06 2017 14:34

Now we're into electrodes as opposed to bonding the slab; so somewhat off-topic.

Pluto's correct that horizontal electrodes are generally superior to vertical electrodes; so yes they make some sense when mains are in a trench. Bearing in mind that we are under no obligation to adopt "best practice, but need only meet minimum acceptable practice. So where "better" is also "more expensive", commercial pressures must be considered as well as compliance issues.

However his suggested methodology of extending the bare copper electrode to MSB has several issues that need to be dealt with, and dealing with them limits application.

1 the section of cable within the structure is generally several sizes larger than required for an MEC, so amounts to additional expense.

2 While that section can be regarded as an extension of the electrode; it is actually an MECD; which as DougP says must be insulated. And that means either spending time stripping an insulated 25 mm2, or spending time and materials applying sleeving to a bare 25 mm2 - and then avoiding damage when installing it. Time is money. For a typical house, 6 mm2 is just so much cheaper and easier to install.

3 Pluto's suggestion that adopting this method means there is no MEC is simply wrong and non-compliant; because both ESR 4's definition of "main earthing system", and clause 5.3.1's list of parts of the earthing system make it clear that there MUST be an MEC. There is simply NO option of omitting the MEC.

4 Accordingly there is always a (at least nominal) point of "connection", and - as per 5.5.1.2 - that "connection" must be accessible for testing. It also needs a label, unless the method of connection is such that it cannot be disconnected (cadweld would count, so would Pluto's theoretical-only connection. Which is easy enough done, eg in a flush box with (labelled) blank plate. Saving not much: 1 label and 1 line-tap.

It follows that the "straight-through" MEC + electrode in one is only useful where either a) the MEC run is very short; or
b) the MEC size is required to be as big as (but no bigger than) the 25 mm2 acceptable size for an electrode.



   

mrsparky
Jun 07 2017 12:43

What I used to do to comply with this was use a line tap with 6mm2 earthwire onto the rebar then wrapped in self amalg and run it up next to a pipe, these were normally nearby as the requirement is for bonding in wet areas. I would leave enough for a coil to either run it to the roofspace or to a flushbox(once installed) for joining and extending to the switchboard.

I found this a fairly cheap and easy way of doing it
   

Braden
Jun 07 2017 17:16

Hi,
Would guys bond each specific 'wet' area as the reg would suggest, or just bond general reinforcing in a continuous slab?
   

gregwires
Jun 07 2017 17:26

Hello,

5.6.2.5, note 3 states "one point of connection of the bonding conductor to the reinforcement is satisfactory where bonding is required at more than one location"
   

gregwires
Jun 07 2017 17:28

I'll add to that the proceeding sentence, from he same note.

Provided that the reinforcement is satisfactorily electrically connected together,
   

Sarmajor
Jun 07 2017 18:32

I am not sure why you would bother to attempt to insulate a line tap with self amalgamating tape when it is going to be encased in concrete.

As the notes say as long as the insulation is wire tied together attaching the earth wire to any point of the slab is sufficient.

You could also just bond the section in the shower area if it is poured after the rest of the floor slab to create a sloped or recessed shower area. I have seen a few done that way.

Good to see discussion on this topic is back on track after Pluto attempted to derail it to buried strip earth electrodes..

So come on guys how do you do it and why are so many not doing it.
   

Apprentice
Jun 07 2017 20:36

What if you get the floor guys to tie a piece of steel vertically, that extends into awall cavity, then you can bond to that.

Then it isn't prescribed electrical work that the floor guys are doing, and they are just allowing you the means of doing the prescribed work without having to be there at a time when you don't really need to be there. Win win.
   

Narfnarf
Jun 07 2017 21:15

Nekminnit turn up after the pour and the rebar isn't there. Sometimes you just can't rely on other trades.


   

AlecK
Jun 08 2017 08:38

There's also the problem that a deformed rebar is not easy to make an electrically sound long-term connection to; also plain steel and subject to corrosion when not enclosed in concrete.

I use approx 1/3 of a standard "earth stake" (16mm galv rod) bent 90 deg & wire-tied to the rebar within the slab.
I don't see why anyone would sub this off to another trade, as there are always things to do before the pour - like making sure nobody has moved your conduits, or forgotten to plug the duct you intend to use for mains.

And if you don't see it for yourself, how can you possibly "know" it's compliant for CoC? The bond connection has to be tested, ie from the soon-to-be-covered rebar to somewhere accessible - or are you going to sub out the testing as well?

   

SaintAlan
Jun 08 2017 15:23

"I am not sure why you would bother to attempt to insulate a line tap with self amalgamating tape when it is going to be encased in concrete"
It is really to stop galvanic corrosion. The joint between dissimilar metals inside the concrete must be taped over (I use Denso tape)or the steel will rust rapidly.

We used to weld flat steel tags onto the rebars for a bolted lug connection but welding is no longer allowed becauese it affects the strength of the bar. Pity, that was a neat solution from a sparky point of view.
   

AlecK
Jun 08 2017 15:34

Galvanic corrosion isn't a big issue, because there's no moisture present (after the relatively short period while the new concrete dries). No moisture, no oxygen, so no corrosion.

Where we need to take steps against galvanic corrosion is pour exposed / accessible connections, especially eg to earth electrodes (sorry Sarmajor, veered off-topic again).
   

pluto
Jun 08 2017 16:51

Have any of you considered what this connection does and how it was included in the Wiring Rules?

It was brought in by AU Electricity distributors as an "arse protection"in the event of HV fault in there distribution system. But the important issue rarely brought up is that in AU all the water pipes in the street to each house are copper, ideal for transferring the HV fault shock voltages. Also for the waste water from the shower tray to the local drainage connection is typically copper in AU.

In NZ all our water service and waste pipes are in general done in plastic which don't conduct so the need for bonding is not necessary and disappears.

There is general review (to include some new materials, practices, etc) of the about to be published AS/NZS 3000:2017 and I will be submission that where the water service and waste pipe are plastic this bonding of the concrete slab and the shower taps and shower head be deleted.

Don't stop doing your current installations until told, but it sure needs to be reviewed with the view of deletion of the requirement. It provides very little extra safety, it is the same as for the old practice of bonding the stainless steel bench top to the taps with copper piping used for all services which was discontinued many years ago.