Nigel Molesworth wrote:


One point that does occur to me is that your argument that "bonding is a
load of ******** 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.

What about a lighting circuit? Or is that on a RCD as well (which in
itself may represent a significant injury risk)?


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

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)

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?


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.

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)

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?


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

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.

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.


I realise that, but perhaps over cautious in this day of RCDs? I'm not

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.

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.

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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