On Tue, 04 Mar 2014 10:14:52 +0000 (GMT), "Dave Plowman (News)"
<
da...@davenoise.co.uk> wrote:
>In article <
456ffd11-bc2a-4207...@googlegroups.com>,
> <
meow...@care2.com> wrote:
>> > Do the regs requiring a galvanic isolated supply in a bathroom still apply
>> > where RCD house protection exists? At least some regs have been changed
>> > with this.
>
>> When an iso transformer is fed from rcd, its secondary output has no rcd
>> protection. Connecting 3 items to one ouput does therefore increase the
>> shock risk some.
>
>Which was why I asked if transformer isolated shaver sockets are still
>required on a new build with RCD protection.
>
>BTW, I used to work in TV studios, and before RCDs any non 'approved'
>mains equipment - like say a musicians' amplifier - was always fed via an
>isolating transformer. And each such amplifier had its own transformer -
>no sharing allowed. The reason being simple. If you parallel two bits of
>gear from the same transformer, and one has an 'L1' to case short, the
>other an 'L2' to case short and you touch both you get the full 240v. Now
>obviously two such faults are rare, but you have to allow for them.
>
>> It might be easier to plug tghe toothbrushes into a simple adaptor
>> outside the bathroom.
>
>I have a charging 'station' in the tank cupboard in my bathroom.
I know of no electric toothbrushes that use a direct galvanic
connection to the mains. They're either battery powered using
disposable primary cells (AA) or else chargable from an inductive
charging stand so you can discount the 110v toothbrush shock hazard.
This leaves (as it says on the tin) just the question of mains
powered shavers. In this case, as per the guitar amplifiers, there
remains the unlikely possibility of two people both using electric
shavers from a twin outlet shaver socket recieving an electric shock
via body contact _and_ both shavers being faulty as well.
The voltage hazard in this case being 110v, far less likely to prove
fatal (especially in this case of a "Buddy System" where it's more
than likely that at least one of the victims will shrug off the
effects quickly enough to assist the other).
If I was fitting such a shaver socket, I'd choose the much safer
option of a transformer that uses a centre tapped secondary winding
with the centre tap connected to earth. This limits the max live to
earth voltage to a mere 55vac rms, much less hazardous again.
It's still true that two people shaving from a double outlet
connected to such a transformer could, in the right (wrong?)
circumstances still experience a 110v shock (i.e. no worse than
before) but the more likely fault when only a single person is shaving
will reduce the shock voltage wrt earth.
The only way for someone to recieve a full 110v live to live shock
would be for the shaver to 'fall apart in their hand' exposing them to
two close spaced points of contact in said appendage with most of the
current confined to the local area. Any residual current across the
chest because they had been using their other hand to keep a tight
grip on a handy earthed 'grab rail' would be considerably reduced over
what they would experience with a simple 55vac live contact and no
worse (other than as a brief transient event when 'letting go').
I don't know what the VA requirements are for a typical shaver but
I'd imagine a single 50VA isolating transformer with centre tapped
110v secondary could power 2 or 3 shaver sockets wired in parallel
(once you're using the 'safe option' of a 55-0-55 vac transformer - if
it's good enough to allow the use of 110v power tools on an exposed to
the elements building site, it's equally as good for 'wet room' use),
there's no point in trying to galvanically isolate the sockets by
providing a seperate transformer for each.
If you want to make the system even safer by guarding against such
unintended live to live contacts between the sockets, you could go to
the expense of individual RCD breakers in each live feed (seperate
transformers won't make this any simpler) which, on the face of it
would require 6 such breakers for three sockets (there may interlocked
3 socket RCD/ELCB modules available, specifically for this usage case
but I doubt it).
Let's face it, it's only "Darwin Award Seekers" who are at risk of
the 'worse case 110v shock hazard scenario where the idiot in question
fails to notice that the shaver casing is on the point of 'exploding
in his hand' due to broken/damaged casing retention clips/missing
screws (and, at a mere 110v, the 'candidate' is unlikely to suffer
electrocution even then).
--
Regards, J B Good