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Posted By Topic: Cable Burial to gate

Apr 24 2019 11:34

Trying to get a power supply to an electric gate. Digging through the lawn and garden no problem until a depth of around 250 mm striking what we locally call Papa rock which is a clay pan. A LOT of chiseling with a kanga hammer will get us to 300 mm but my reading of 3000 only allows that depth if under continuous poured concrete ie a patio etc or is there a way to work it that we could pour concrete along the whole trench. Or I have toyed with investigating if they do a low voltage system. Thoughts?

Apr 24 2019 12:14

The internals of the gate openner are probably 12/24 VDC. Can you bypass the transformer and run the supply at ELV?

Apr 24 2019 12:15

Or run a battery and small solar charger.

Apr 24 2019 14:54

Another oprion is to use a category C underground wiring system clause of AS/NZS 3000:2007 refers

Apr 24 2019 20:32

Just looked up category C underground cable. What is a "served" cable?

Apr 24 2019 20:47

Figured it out. It's the outer layer of an armoured cable. From the inside: conductor, primary insulation, sheath, armour, serving.

Apr 25 2019 06:31

What about putting PVC conduit inside galvanised water pipe?

Apr 25 2019 10:15

The galvanised pipe would be Column 6 of Table 3.5, so would amount to Cat a (with or without the PVC conduit). Cat A still requires 500 mm depth of burial.

Going to ELV will involve significant volt drop issues over any length.

You could pour 75 mm of concrete above ground level along the entire trenchline - basically make it a path. But unless they want a path, it's an ugly solution.

I'd follow Pluto's suggestion. Nothing says a Cat C system has to be on top of the ground.
Chase into your "rock" layer, use apprpriate cable, and pour concrete over it.
Then back-fill the trench. I'd also include signal strip as an "extra".

Apr 25 2019 18:30

Thanks again for the nudge in the right direction. I'd kind of discounted the "C' solution as the drawing had shown the top of the concrete to be level with the rock. As long as a small amount of soil can go over the top of everything I think the customer will be pleased. Cheers. It's not quite "rock" in the traditional sense but I presume around the country everyone will be familiar with how solid these clay pans are!

Apr 25 2019 20:30

Run 6mm solar cable to the motor. (Depends on distance) Remove transformer from motor & put in IP rated enclosure, fuse primary & secondary sides.

Apr 26 2019 08:52

True Fig 3.16 shows concrete flush with upper surface, with cable min 50 mm below that.
However the point of it all is protecting the cable from damage; and have to remember that the Figs for this clause are "examples". Words "example" or typical" in title of a Fig mean it sets NO requirements, just illustrates the clause. And there's nothing in clause to say how deep the chase can be, nor to prohibit burying the concrete.

Have another look at the clause, and read the Note that says you can use a Cat A system laid in a channel & covered with 50 mm of concrete; and it is deemed to be Cat C. So you don't need armoured cable if you give it Cat A protection.

but you can't do the same with a Cat B. So actually your earlier idea of galv pipe would be OK, provided you chase it into the "rock" to give it give it 50 mm of poured concrete below rock surface. Or even HD PVC conduit.

If 50 mm of concrete visible on surface is deemed OK; then putting it actually underground must be better actual protection, however less visible so potentially people won't know anything's there.

Hence suggestion of using (extra)signal strip at half depth as for Cat A or B systems. If anyone digs, they'll find the tape, and instead of wondering what the narrow strip of concrete is for they;ll actually know there's a cable under it.

Looked at another way, if you chase-&-pour for Cat C at that depth, and left the trench open; it would clearly comply with "3000"; but set up a WHS issue of the open trench.

So while not specifically mentioned that it's OK to bury a Cat C system; I believe that's due the writers believing that their readership has at least some smarts, and don't

Apr 26 2019 18:39

Word back from the suppliers gives the go ahead for a 24 volt supply remote from the gate. Given the blood sweat and tears I've experienced in similar situations its great to have this knowledge to store away. Appreciated!