Cable derating

[Martin Carroll]

Just wanted to check that I was reading the OSG correctly!

If I use Reference method A (i.e. enclosed in conduit in insulated wall) then 2.5mm flat T+E cable will be sufficient for a ring circuit.

Presumably I cannot put 2 cables in the conduit (i.e. to and from the socket) as this will derate the circuit under 20A and I would need to increase the size of the cable.

The alternative is to ensure that the cable is touching the inner surface of the plasterboard and that the two cables do not run in close proximity to each other.

Cheers

Martin

[dennis]

You can't do that unless the cable is protected or you have 2" plasterboard.

[Martin Carroll]

In article , dennis@home writes

Or the cable run is in the zone horizontally/vertically to the socket.

[dennis]

That doesn't satisfy the new regs AFAIK

[Martin Carroll]

In article , dennis@home writes

Why not? AIUI as long as a cable runs in a line horizontally or vertically to an accessory or within 150mm of top of wall or angle formed by two walls.

Martin

[Andy Burns]

Used to be, now 17th edition requires either

cable is buried more than 50mm

or

cable is not buried at all

or

conductors are fully enclosed by an earth (e.g. T&E within earthed metal capping, or a cable such as SWA, or BS8436 cable with foil screen, or pyro etc)

or

protected by 30mA RCD or RCBO

or

something else I missed?

[Martin Carroll]

In the 17th Edition OSG, (page 60)

A cable installed in a wall or partition must:

i be at least 50mm from surface ii have earthed armouring..... iii be enclosed in earthed steel conduit.... iv be provided with mechanical protection...... v be installed either horizontally.........

In domestic and similar installations, cables not installed as per i, ii, iii or iv but complying with v shall be protected by 30mA RCD.

So as long as there is an RCD in CU protecting the circuit cable can be unprotected in the "safe" zones.

Cheers

Martin

[BigWallop]

To comply with the 30mA RCD ruling, you won't be able to run a fan heater or anything else of that nature on the final ring. In fact, one PC may be enough to trip a 30mA RCD because of the feedback to earth from the switching PSU. Try it, though.

[Toby]

I have a total of 6 PC's and numerous other things with switched mode power supplies, all connected to a single RCD, no problems!

Toby...

[John Rumm]

Huh?

You can run a fan heater, or PCs, or anything else you want on a 30mA RCD protected circuit. (fan heaters are not exactly renowned for high leakage anyway. Heater elements with hygroscopic mineral insulation that are immersed in water - perhaps more so). You would also need quite a significant number of (non faulty) electronic devices with mains input filters to fill the leakage budget of the RCD.

17th Edition installs are less likely to trip under aggregated leakage anyway, since they will typically have the loads spread over at lease two RCDs rather than one in the past.

[BigWallop]

I much prefer a simple MCB on the final circuits. With RCBO switch gear on the supply. RCD is fine outside, but not on an internal domestic ringer. Maybe it's me who still doesn't trust them, seeing them develop over the years, and seeing them fail miserably for most of that time. Even if they think they have got the engineering sussed on them, now. I still prefer the old days of reliability with safety built in. Not safety designed around stupidity.

OK. I'm old fart. :-)

[John Rumm]

Well bear in mind that generally (TT install excepted) one still designs such that conventional protection works as before, and the RCD is only providing additional protection. So there is no down side in that sense.

One could argue that nuisance tripping is the weakness of RCD installations, but in most real world situations they are tripping either as a result of a genuine fault which otherwise might not have been noticed, or a circuit/protection design weakness.

With regard to reliability, the 96% effectiveness reported in the link Andy gave above is quite impressive I thought:

OK. I'm old fart. :-)

Well that goes without saying ;-)

[tony sayer]

Must be a really duff PC to be leaking that much!.....

[Dave Plowman (News)]

Eh? The type of element normally used in fan heaters isn't associated with earth leakage which might cause an RCD to trip.

The value of 'feedback' to earth is such that many SMPS can be used on one RCD - far more than likely in a domestic environment. The effect is additive. You might well have problems in an office, though, with lots of them.

[John Rumm]

Indeed, I think the amount of leakage from such things is often overstated. I must have at least a dozen SMPS in this room alone, and there are two other socket circuits on the RCD that serves this circuit as well. In addition I can still stick at least another 15mA leakage into the thing with my RCD tester and not trip it. (under test it trips in 26ms at 30mA leakage)

[Dave Plowman (News)]

The figures have been pointed out elsewhere - it's effectively the thick end of 10 watts needed to trip an RCD. That is a lot of heat to dissipate unnecessarily from a power supply which would add to its cost. And such high current resistors ain't cheap compared to what's used - as well as bulky.

[Tim S]

Dave Plowman (News) coughed up some electrons that declared:

Shouldn't that be "10VA" - the leakage might be largely reactive, and probably will with things that employ mains conditioning filters.

Just a thought.

Cheers

Tim

[dennis]

I recommend you get the appliance tested. No PC or fan heater or fridge or anything else I have has every tripped a 30 ma rcd and I don't see why they should.

[Dave Plowman (News)]

There are all sorts of ways - but many SMPS do use just simple resistors.

[Pete Verdon]

Rubbish. My whole house is on a 30mA RCD. I wouldn't do it like that if I were rewiring, but it is there and it doesn't give me nuisance trips.

Pete