Hi,
Sparks ) wrote:
: Probably, on the basis that there's no real need for the change so far
: as they are concerned and it costs them money to send a man round... I
: can't see any real justification for you to change from what sounds like
: a TN-S system (earth return via sheath) to a TN-C-S system (PME - earth
: return via neutral) - you'll still need the earth rod for operation from
: the generator in either case.
:
: Ok, but the reason for the change, was the earth cable from the incomer to
: the CU is small (looks about 5mm) - maybe this is adequate? (The
: reconnection bloke thought it was small, and suggested a PME connection)
For normal domestic installations a 16mm sq cable is usually specified
(a smaller cable may be acceptable in some circumstances and certainly
will have been in the past). The size should be the same irrespective
of whether the supplier provides a TN-S (earth via sheath) or TN-C-S
(PME) supply. There is no benefit to you to moving to a TN-C-S supply
compared to a TN-S supply.
: : Will it be ok to have the earth connected to both the neutral (Mains)
: and a
: : rod (Generator) at the same time?
:
: Probably yes, but again, this is something which I would confirm (in
: witing!) with your electricity supplier. Sketch out the proposed circuit
: arrangement (specifying cable sizes, etc.) and give them a bullet pointed
: list of the main features. Remember that it is not impossible (but
: fortunately quite difficult!) for you to electrocute the electricity
: supplier's staff if you get this wrong so make sure they are happy. They
: may not be willing to comment on your proposal but at the least should
: specify what standard you should meet - probably either an electricity
: supply industry standard or the appropriate parts of the IEE regs (or
: both).
:
: Okay, but with the transfer switch, there shouldn't be a problem!
I wouldn't trust a switch to switch an earth unless specially designed
to do so and recommended for that purpose by the manufacturer. There
are too many risks if the earth contact doesn't! You will also have
an extra link in the bonding between the CU and your earth (I assume
that if the generator earth is switch then the mains earth must also
be).
: Don't forget that the supply from the generator will need to be protected
: by a 100mA time delayed RCD unless you have something a lot more
: impressive than the average earth rod (standard earthing rules for a TT
: supply). It may also be necessary anyway due to high fault current loop
: impedance as a result of the generator impedance (almost certainly will
: be required for a little 800VA thingy). Hopefully it is obvious to you
: that the connection between neutral and earth for the generator supply
: should be on the generator (supply) side of the RCD.
:
: At the moment, it goes like this...
:
: Main supply 80A
: Generator 26A (Briggs & Stratton 7500LE) (
http://tinyurl.com/xikn)
Chunkier than I was expecting.
: Earth from incomer (supply sheath) to CU via 5mm cable
5mm is an unusual size. Is that 5mm diameter or cross sectional area?
Does the cable have any markings stamped into it?
: L&N from meter into DP isolator
:
: L&N from isolator to transfer switch (Double Pole, Three position switch
: (Mains, off & Generator))
:
: L,N&E from generator into transfer switch (Transfer switch has a 30mA RCD
: for the generator)
What is the earth connection for? Is it to bond the switch or is it
actually switched?
: (The generator neutral and earth are connected together in here, before the
: RCD)
For best integrity, I would make sure that all the earth connections
come together at a single main earthing terminal. This would include:
1. Earth rod.
2. Mains supply earth (see my previous post regarding confirming
this with the supplier - if you can't link the two earths you can't use
this arrangement)!
3. Connection to consumer unit(s).
4. Equipotential bonding of gas, water, etc.
5. Neutral connection (to generator terminal).
6. Earth connection for generator chassis.
I would also connect anything else that needs bonding to this terminal.
Make sure you get something designed for this purpose (its a relatively
common part but you will have more connections than normal).
: The earth rod is situated next to the generator, and is connected to the
: chassis of the generator (via the supplied earthing bolt)
There isn't a problem in keeping this arrangement (providing its
existance is clearly noted near the main earthing terminal!) but I would
ensure that I used the "star" arrangement with a single main earthing
terminal as well.
: There is also a power conditioner in between the generator and the transfer
: switch (Bloody heavy!)
I would check whether this provides isolation (i.e. contains a 1:1
transformer), you may need additional bonding and neutral referencing if
it does.
: L,N&E then run from the transfer switch into CU (L&N into the top of the
: 100A/30mA RCD, and earth the opposite end of the earthing strip) - there
: are then two additional L&N wires running from the top of the 100A/30mA RCD
: to the 40A/30mA RCD
I'm confused about your RCD arrangements. It sounds like you have two
separate RCD protected sub-supplies in your consumer unit which is
strange. Normal practise with TN-S or TN-C-S is to 30mA RCD protect
circuits where there is significant risk of shock (e.g. sockets) either
with a single RCD or individual RCBOs for each circuit and to leave
circuits which are a low shock risk (e.g. lighting) unprotected by RCD.
Usually this means that there is a single main switch in the consumer
unit which feeds lighting circuits via MCBs and feeds rings via a 30mA
RCD and MCBs or feeds rings via 30mA RCBOs.
Since when running on the generator supply you will be operating a TT
system, you need to protect all circuits with a minimum of a 100mA
time delayed RCD.
: The generator has a 30A thermal fuse on it's control panel
:
: Do I actually need the RCD in the transfer switch (on the generator circuit)
: as all circuits are protected via the RCD's in the CU? Will this cause
: problems having RCD's connected to RCD's? (I was thinking of swapping the
: RCD in the transfer switch for an isolator)
You don't actually need an additional RCD if all your circuits are
currently RCD protected in the consumer unit.
Since your current arrangement is a little strange I would be tempted
to sort out the consumer unit however so that it is more normal in its
arrangement with a split between non-RCD and RCD protected sides. I
would then put a 100mA time delayed RCD either on the input to the
consumer unit or, for extra reliability and since its only required
during operation from the generator, put it between the generator and
the transfer switch. As I noted in my previous email, the generator
supply must be connected to earth on the incoming side of the RCD
(you are doing this already but I will reiterate it because its
important!).
Hope this helps,
Dale.