Garden ring main

[ac1951]

Anybody see any particular issues with this:-

6mm twin&earth cable from 30A RCBO in CU Cable runs to a IP65 rated juction box outside of house. Two 2.5mm SWA cables run from juction box to 4 outdoor rated 13A sockets around the garden forming a ring main. The ring also passes through a detached garage connecting to a 2 way CU fitted with a 5A fuse suplying lighting inside and 2 150W halogen lights outside the garage.

Total length of ring is circa 90M

Earthing makes use of the SWA which in turn is conected to the 6mm cable earth and back to the CU..

Incomming supply to house is overhead with PME earth.

all comments welcome. thanks Andy

[Andrew Gabriel]

Minor nit: ring _circuit_.

I would make it a TT circuit (own earth rod(s) with 30mA RCD, and not exporting the house earth). I also have an indoor double pole switch (it's actually a 45A cooker switch) on my outdoor socket circuit so I can isolate it from indoors.

I would use a fused connection unit for the lights rather than a CU. (Actually, I would use a separate one for the external halogens and the internal garage light if you would otherwise have had two switches for them anyway.)

The length needs checking against whatever circuit protection type the circuit has, but I don't have any tables on me to look that up.

[ac1951]

There are two points where the earth could easily be connected to an Earth Rod(s). One at the junction box where the T&E meets the SWA cables and the other at the Garage. They are 25M apart in terms of cable distance is there an issue with using both ? or should you choose one ?

thanks Andy

[John Rumm]

I would be reluctant to connect garden sockets to a PME earth. If you are proposing having a CU in the garage anyway, you might find it simpler to run a single T&E/SWA combo to there from the house, then have CU there configured as a TT system, and ruin the sockets from that.

The earthing question gets difficult with PME systems and outbuildings:

also that from a current carrying capacity point of view 90 deg C XLPE SWA is able to take the full RCBO current without needing a ring configuration - however you will need to check voltage drop is within limits for the design loads.

A double pole switch in the house to isolate the external feed would be handy (check your RCBO is two pole). You may find you get more flexibility without having the RCD function at the head end (although note the new cable protection requirements of the 17th edition if you do this).

[Tim S]

John Rumm coughed up some electrons that declared:

It's a complicated situation. I'd agree that exporting PME is generally a bad idea and may be even against DNO's rules under some circumstances.

I've just signed up to a 4-weekend course leading to a Part P Domestic Installer's VRQ, with an option to spend a further 3 day weekend doing the City and Guild's 2382**. I'll make a point of asking the question there and see what the instructor's opinion is. I'll feed it back here, though it will be in a months time, bit late for this thread.

Cheers

Tim

** Why spend the time and money? Twofold: 1) The BCO apologised to me that the LABC couldn't manage Part P in the way the ODPM had envisaged. I was given two options which were: a) Get a PIR done at 200-300 quid, my expense; b) Be qualified to the 16th or better and my EIC would be accepted. Given his amazing helpfulness on other more traditional issues, I decided not to make a big deal of it.

Given that I'll be doing planned extentions to the installation over the years, b) works out just cost effective, discounting cost of test instrument because I'd get one either way.

Secondly, it seemed a wise investment of time and money given I have the opportunity to design a whole new installation and I want it right.

[John Rumm]

or run the sockets even ;-)

[Andy Wade]

Bear in mind though that the conductor temperature should be limited to

70 deg. when connecting to normal wiring accessories like 13 A sockets, so in reality you should use the 'PVC' current ratings.

The proposed design here with a 90 m ring on the end of a 6 mm^2 tail of unspecified length is unlikely to comply on voltage drop if all the load is in the middle of the ring. Either the ring needs to be wired in 4 mm^2 SWA (which should be manageable in outdoor sockets if there are no spurs) or else the circuit could be downrated to 20 A, if that's enough for the intended load. Recommended max. voltage drop has changed in the

17th ed. and is now 3% for lighting and 5% otherwise (and previously was 4% for everything).

[Tim Lamb]

In message , John Rumm writes

use the same duct for data and signal cables......

When routeing for alarm cables was discussed, I thought this was OK provided the signal cable insulation met the requirements of the highest voltage encountered?

regards

[Owain]

I think you are correct, but also note electromagnetic compatability and the possibility of interference.

Owain

[Andy Wade]

That's OK in general for ELV circuits running in proximity to mains, and there are other options that will comply, such as compartmented trunking systems. [Reg. 528.1 in the 17th ed.]

*However* the 17th ed. introduces a new requirement where underground power and /telecommunications/ cables meet or cross [Reg. 528.2]. Verbatim transcript follows: [Heading] 528.2 Proximity of communications cables

In the event of crossing or proximity of underground telecommunications cables and underground power cables, a minimum clearance of 100 mm shall be maintained, or the requirements of (i) or (ii) shall be fulfilled:

(i) A fire retardant partition shall be provided between the cables, e.g. bricks, cable protecting caps (clay, concrete), shaped blocks (concrete), protective cable conduit or troughs made of fire retardant materials

(ii) For crossings, mechanical protection between the cables shall be provided, e.g. cable conduit, cable protecting caps or shaped blocks.

Note 1: Special considerations of electrical interference, both electromagnetic and electrostatic, may apply to telecommunications circuits, data transfer circuits and the like.

Note 2: Segregation requirements for communications services are given in BS 6701 and BS EN 50174 series.

[ac1951]

Andrew G, Thanks for the advice and I will look to make this a TT supply

John R, Thanks for the advice, I agree that it would be best to distribute everything from the garage and make that the base of the TT supply. BUT (there always is one) part of the distribution already exists i.e. the 6mm T&E cable and the two 2.5mm SWA cables, one running off to an out-door socket on the garden patio the other runs to a fused spur (feeds lights) in the garage. Changing these cables would involve digging up paths and part of the drive but it would be fairly easy to run a cable from the garage to the patio and hence create the ring.

Andy W, Thanks for the advice and will down rate the circuit to 20A

Tim, good luck on your course, all feedback is most welcome irrespective of how long after the question.

All, assuming I go ahead as above is there a preference where I establish the Earth point(s) rod(s). I have two options, either at the garage or the dis box where the T&E meets the SWA's.. (or both ?) Or is the whole idea crazy, illegal, or down right bad practice .....

thanks Andy

[Tim Lamb]

Hmm.. I think I am going to interpret alarm cables as being neither telecoms nor data:-)

The protection suggested implies they are concerned about cable faults leading to dangerous voltages or interruptions appearing in telecoms tackle. Roll on fibre optics!

regards

[John Rumm]

Could you draw this to make it a little clearer?

At the garage I would have thought. As long as you have RCD protection somewhere to cope with the increased earth loop impedance of the TT setup, and you don't expose the PME earth outside then you should be ok.

no, probably, not really. ;-)