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Posted By Topic: Underground Services

Apr 13 2019 13:53

I noticed that almost every one I've seen do services use the method of the same trench to save using separate ones but normally they just chuck in underground rated NS cable, buried earth, water pipe and fibre conduit

Reading exception 2 it says it's ok to do so provided that each service has a separate enclosure

Does this mean if you want to do mains in this way that it has to be in conduit or a similar enclosure? And that it doesn't comply if it's the the bare cable? Or is the cable sheathing itself counted as an encloser?

I take it the buried earth is fine in this way as long as none of the enclosures are conductive?

Apr 13 2019 16:27

AS/NZS 3000 Table 3.5 details the type of cables and for some types of cables if an enclosure (conduit, galvo pipe) is required.

For example a neutral screen marked as suitable for underground can be direct buried with no enclosure the the depths detailed for Type A. Then 3000 clause and table 3.6 detail the minimum depth of burial required.

Apr 13 2019 17:07

Sorry I left out my references

3.11.5 Exception 2 is what I was referring

Says they may be in the same trench provide they are in enclosures that identify what service it is

Upon reading that I think I answered my question because it's about being able to identify what is what and clearly if you saw a unenclosed cable you'd know what service it is

Apr 13 2019 19:34

When you look at an exception; have to look at what it's an exception to.
In this case the default oi 100 mm separation between different services. You can come closer than 100 mm IF each service is in a separate enclosure.

Apr 13 2019 20:17

Yes I had read this and saw it said provided each is enclosed in a system that identifies the service

So wouldn't it not matter as well if the cable is unenclosed because you can identify it by that means?

Apr 14 2019 05:20

The need for an enclosure when services are close together is primarily for additional protection against damage. If someone digs down to do something with their service and the other service(s) are close they could damage it.
You can't expect a muppet from a telco, or a plumber, or an electrician, to distinguish a black plastic N/S cable from a black plastic water pipe. Sure there will be a marker tape at half depth for electric[], so they'll know one of the services is electrical, but which one? The min separation in Table 3.7 helps with that, and also reduces risk of damage to cables from people working on pipes / gaslines / telecoms.

So, if you want to lay them closer;enclose to protect from digging risks, and use a form of enclosure that provides identification. For electrical, that would be not just conduit but orange conduit - even for a cable type that is normally allowed to be buried direct.