.... not again, they shout ...
Gentlemen, I'm new to the group and have read with interest loadsa
stuff about bathroom electrics.
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
Thanks in advance
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
Equipotential bonding doesn't have to go back to the consumer unit.
You'll be able to get the 10mm stuff cheap then :-)
No, 6mm is fine if it's in air, see:
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.
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.
On Sat, 11 Mar 2006 01:01:14 +0000, John Rumm wrote:
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 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
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
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).
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
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
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.