Bathroom Shower

.... not again, they shout ... Gentlemen, I'm new to the group and have read with interest loadsa stuff about bathroom electrics.
Situation: Mira Advance (9K) - 45A switch outside bathroom - 6mm twin & E cable - 45A MCB on the board. Earth connected at the board. Cable run (old house, 9 foot ceilings, board in the middle room): vertical, clipped, up to attic - about 5.5m, straight across attic (lying on the floor) - about 10m, down to switch for shower - about 1m.
I was advised (about 2 years ago) that 6mm was ok up to 16m, and I'm close to this, maybe a bit over.
I now notice that the cable gets warm (!); standard teenage daughter shower (30 minutes...) gets it warm enough to bother me. Full inspection of the cable, terminations etc shows no faults.
Looking through your site gave me a lot of info, but 3 questions. 1) should I go to 10mm cable ? 2) I have purchased a 30mA RCD - but after reading about 'nuisance trips' etc should I go for bigger ? 3) bathroom bonding is all ok, but one article recommends 'thickish' cable for the secondary bonding line back to the board. Do I then assume that the cable built-in earth wire ain't enough ?
Any comments/advice appreciated.
FYI I am a marine electronics engineer and as such cover all the electrics on board (up to the 6.6KV stuff). However, we work to different regs - and basically just use armoured 10mm as a minimum size.
Thanks in advance Peter Owen
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eto wrote:

Possibly.
No, higher tripping current than 30mA will not give useful protection against electrocution. You don't *have* to RCD an electric shower provided other conditions can be met, but it's easier just to put the RCD in.

Equipotential bonding doesn't have to go back to the consumer unit.

You'll be able to get the 10mm stuff cheap then :-)
Owain
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On 10 Mar 2006 08:47:24 -0800, eto wrote:

No, 6mm is fine if it's in air, see:
<http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

No, 30mA is right for power. You can split your consumer unit and put 100mA on the lights, but I think nuisance tripping is only a problem if you could fall down the stairs. Much better to put an emergency light in the hall, a rechargeable fluorescent is fine.

IMO, bonding is a load of bollocks if you have an RCD. Someone else will come along and disagree with me in a moment.
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Nigel M

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Nigel Molesworth wrote:

OK if you insist ;-)
So how do you guarentee that the fault causing your cold tap to sit at 240V is a result of contact with an RCD protected circuit then?
--
Cheers,

John.

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On Fri, 10 Mar 2006 20:56:59 +0000, John Rumm wrote:

No other way for it to happen.
--
Nigel M

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Nigel Molesworth wrote:

You sure?
What about a lighting circuit? Or is that on a RCD as well (which in itself may represent a significant injury risk)?
What about all your neighbours circuits - you sure they are all RCD protected? Or are you sure there is no stray earth connection to a neighbouring property via a shared water or gas main or even a bit of structural steel work?
What about when a daft JCB driver puts a bucket through the supply and causes a fault to earth on the supply side of your RCD, still happy you will be safe?
The technical content of BS7671 was not written by a bunch of guys with nothing better to do, who just get out of bed one day and thought of jolly ways to keep the electrical contracting trade busy. There is generally very solid reasoning to back up these concepts, but some of this stuff is quite subtle.
--
Cheers,

John.

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On Sat, 11 Mar 2006 01:01:14 +0000, John Rumm wrote:

Yup.
On 100mA, in a bungalow, and I've got emergency lighting.

My neighbour is 100m away. Water main is plastic. I've got a 25m copper pipe running to it in contact with the ground. My mains earth is bonded to it, & to another two 1.5m spikes 20m apart.

He'd have to be digging up in the air ;-)

I realise that, but perhaps over cautious in this day of RCDs? I'm not against bonding stuff together, mine is, but it's the fanatical way it is supposed to be done with earth wires connected behind the sink etc.
--
Nigel M

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Nigel Molesworth wrote:
One point that does occur to me is that your argument that "bonding is a load of bollocks if you have an RCD" seems to be based on the detail of *your* installation, however the statement itself is very general even if it were mostly true for your installation. Other installations may be very different and have consequentially different risks.

So you have a TT install by the sounds of it...
(i.e. Note that these are often installed as PME these days with the earth and neutral bonded at the head end of the install in your property. This has the advantage of giving a very good earth compared to an earth spike, but also opens up the very real dangers from a disconnected neutral outside of the property)

Out of interest what impedance do your multiple spikes etc give you?
(not dissimilar to the arrangement here - but the best impedance I get is about 11 ohms)

Well, when you think about it - that is an even more likely fault scenario. Something tall trying to drive under a power line (or worse into a power pole), or a tree falling etc. Perhaps even just a high wind.

I think one of the problems here is that faults of any kind in fixed installs are exceedingly rare - hence most of us (including me) can legitimately say "well I grew up in a house without any bonding at all and never had any problem". However all that highlights is that the likelihood of an incident is small, not that the risk should one occur is low. What the bonding seeks to do is to massively reduce the risk of injury from these rare events when they do occur - not change the frequency of the fault events.
RCDs certainly help in many cases (and can hinder in others), but as I attempted to demonstrate with a few examples in my previous post, they are not a panacea for all ills. There are several fault conditions for which they will be unable to assist.
You also need to consider that the nature of the supply delivered to properties these days is also often different from the past. There was a time you almost always saw TN-S or TT set-ups and PME was rare. These days it is often the default it seems. This brings a number of advantages, but also much bigger liability if not correctly bonded.

There is nothing to stop it being done under the floor by the sink if you prefer or anywhere else in close proximity.
It is also the case that you (being aware of the issues) could also get your own setup "safe enough" if you had soldered copper pipes and more remote bonding - however that does not give any future proofing wrt to piping changes (something the regulated way needs to address for completeness since not all people making changes will be aware of the implications)
--
Cheers,

John.

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On Sat, 11 Mar 2006 14:55:32 +0000, John Rumm wrote:

Now I didn't know that was OK, so perhaps I'm out of order.
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Nigel M

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Nigel Molesworth wrote:

I expect behind the sink is common simply because it is quick and non disruptive as a retro fit.
--
Cheers,

John.

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On 10 Mar 2006 08:47:24 -0800 someone who may be "eto"

As a rough guide, unless it is too hot to touch it is not warm enough to cause a problem. Many electric cables in houses are grossly over-rated for their duty, because people have selected the cable from a chart rather then doing the calculations.
Note that I'm not objecting to them being over-rated, just commenting on it. Many years ago I demonstrated that it is possible to connect a 3kW immersion heater via 1.0mm cable. The calcs showed that the cable, in the particular installation, was just capable of meeting all the requirements. It did get hot, but not excessively. Had I selected the cable I would have thought that 1.5mm was the very minimum size, with a preference for 2.5mm.
--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
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On Fri, 10 Mar 2006 17:28:02 +0000, David Hansen wrote:

I've seen a bathroom heater connected with bell wire.
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Nigel M

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Indeed. 2.5mm is better for voltage drop. Given that electricity is so expensive to begin with, better to heat the water than the cable!
Christian.
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Technically, 6mm can run this current, but only when clipped direct. Almost any other installation method, the capacity is much less. The real problems occur if you install within insulation. This is most likely if the cable is buried in your loft insulation.
So it appears that the 6mm is not dangerous (assuming loft insulation burial). However, it will get warm over a long period, as you have found, as it is running so close to its maximum. It is temperature rise that limits the cable current capacity, as the PVC insulation has to remain below 70C.

Yes, if the run is easily accessible. You will waste a bit less electricity and, more importantly, cable will run much cooler which is less scary and will last longer.

No. Use 30mA.

The article is misinformed. You should not intentionally bond the secondary equipotential bonding to the system earth. It may be implicitly bonded by electrical or plumbing fixtures. This is usually unavoidable, but actually decreases safety in some cases. (It does provide more reliable disconnection of live<>earth faults, but unfortunately provides a low earth loop impedence, resulting in higher potential human leakage).
Christian.
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Of course this should be "assuming NO loft insulation burial".
Christian.
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eto wrote:

Warm does not actually indicate a fault - the rating of the cable is dictated by how much current it can carry while its conductors remain below 70 deg C. So even if if felt quite hot to the touch there is a fair chance it is still in spec.
Having said that you will be near the limit.

You don't have to, but you could if you wanted (it ought to extend the life of the cable a bit, and wil allow a bigger shower to be fitted in future)

What are you proposing to protect with the RCD? (and no you are unlikely to want bigger unless you have some sort of special situation like a TT earthing setup)

No this is wrong - all you are trying to do is create an equipotential zone.
--
Cheers,

John.

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