Currently I only have RCD protected circuits in my house (One for sockets &
fixed appliances, and one for the lighting (Tripping one does not affect the
other)

(I am planning on re arranging this, putting the lighting, freezers and
alarm system on non RCD circuits)

Currently the shed feed is from a 20A MCB connected to the "sockets" RCD
(30mA) via a 4mm SWA Cable.
This cable is about 25m, however the shed is only about 4m from the house.
The house earth has been exported to the shed.

From the shed CU, via another 20A MCB (Connected to the shed 30mA RCD)
another 4mm SWA cable then feeds a smaller CU in the garage with yet another
30mA RCD (Only three MCB's)
This cable is about 30m. (The garages are right up the top of the garden, as
far away from the house as can be!)

The garage is then earthed with an earth rod. (It is about 35m from the
house)
(The SWA cable's armour is connected to the earth in the shed, but isolated
in the garage)

My questions are...
Should the garage feed simply piggyback the feed TO the shed CU, eliminating
the RCD and MCB there?

Should the feed from the house to the shed be via a 100mA RCD, separate to
the rest of the house?

I assume the earth in the garage should be isolated from the earth in the
house/shed, as it's earthing arrangements are different? (House earth is
provided from the electricity company (TN-S) where as the garage has it's on
rod (TT)

Another question is, why do I need to TT the supply in the garage? Can't I
just use the house earth?
(My logic is, the earth provided by the electricity company is at least 500m
long (That's where the substation is!)

(Patio is currently being re laid, so if I need to do any rewiring, now is
the time)

Sparks...

(I am planning on re arranging this, putting the lighting, freezers and
alarm system on non RCD circuits)

Sparks...

Good idea, you don't really want the lights on the RCD side.

Dave

--
For what we are about to balls up may common sense prevent us doing it
again
in the future!!

Another question is, why do I need to TT the supply in the garage? Can't I
just use the house earth?
(My logic is, the earth provided by the electricity company is at least 500m
long (That's where the substation is!)

(Patio is currently being re laid, so if I need to do any rewiring, now is
the time)

Sparks...[/quote]
The simple answer to this question is that unless you are an approved electrical contractor under part "P" you can't do this as sheds and garages are classed as special risk areas. If you wish to do it yourself you must be able to show competence and notify the local building inspectorate before carrying out the work and they will appoint an inspector / tester to verify the work before it is re -energised. This will cost the same as getting an approved electrician to do the job or even more than.

"Miketew" wrote in message
...

Another question is, why do I need to TT the supply in the garage? Can't
I
just use the house earth?
(My logic is, the earth provided by the electricity company is at least
500m
long (That's where the substation is!)

(Patio is currently being re laid, so if I need to do any rewiring, now
is
the time)

Sparks...
The simple answer to this question is that unless you are an approved
electrical contractor under part "P" you can't do this as sheds and
garages are classed as special risk areas. If you wish to do it yourself
you must be able to show competence and notify the local building
inspectorate before carrying out the work and they will appoint an
inspector / tester to verify the work before it is re -energised. This
will cost the same as getting an approved electrician to do the job or
even more than.


--
Miketew

On the other hand the cheapest, legal and therefore one has to assume the
most safest method, is to run an extension cable to the shed from a socket.
This is apparently called progress.

Miketew wrote:
The simple answer to this question is that unless you are an approved
electrical contractor under part "P" you can't do this as sheds and
garages are classed as special risk areas.

Unless he is in Scotland, or started the work before 31st Dec last and
is going to finish it this week, or doesn't give a fig about Part Pee
anyway.

(Or perhaps the OP is an approved electrical contractor and wants to
know how to do the job properly... :-)

Owain

Fred wrote:
Sparks...
The simple answer to this question is that unless you are an approved
electrical contractor under part "P" you can't do this as sheds and
garages are classed as special risk areas. If you wish to do it yourself
you must be able to show competence and notify the local building
inspectorate before carrying out the work and they will appoint an
inspector / tester to verify the work before it is re -energised. This
will cost the same as getting an approved electrician to do the job or
even more than.

On the other hand the cheapest, legal and therefore one has to assume the
most safest method, is to run an extension cable to the shed from a socket.
This is apparently called progress.

Presumably installing the whole thing, but sticking a normal 13A fused
plug on the feed to it would be OK ?

No need then to lie in bed worrying that the IEE or Part P police will
come knocking on your door at 5am demanding to conduct an inspection.

--
Mark
Please replace invalid and invalid with gmx and net to reply.

No need then to lie in bed worrying that the IEE or Part P police will
come knocking on your door at 5am demanding to conduct an inspection.

--
There are no Part P police, thats a bit silly to say the least.
What does matter is that if you have reason to claim on your insurance and the work is not certified the Insurance company will just walk away, even if the claim is not related to the electrical work. the other thing is that after September ths year , the sale of a house will contain a statement from the current owner that any work carries out complies with the building regs. finally from January 2005 the house owner is legally responsible for the electrical safety of the property. I dont say the new law is good, in fact it has nothing to do with safety but a lot to do with tax collestion. I th real aim was safety they would have made it illegal for anyone but a qualified electrician to do any work, but that might lose votes!

Sparks wrote:

Should the garage feed simply piggyback the feed TO the shed CU, eliminating
the RCD and MCB there?

What you have is basically OK; all the rambling that follows is in the
realm of minor tweaks towards unattainable perfection. That said... the
cascade of RCDs and MCBs is not best practice - should you have a fault
in the garage, you could well end up with not only the garage power
being cut (which we want) but also the shed (pain) and the house sockets
(double-pain). What you want to get to is to have the cables to shed and
garage protected against short-circuit and overcurrent, while leaving
protection of the loads they feed to a dedicated RCD and closer-rated
MCB in each place (shed and garage). To achieve this, you want to feed
the 'submain' - the supply to shed and garage - from sthg like a 20A
*fuse* (not MCB: you actively *want* the slower reaction to overloads of
a fuse here) on the non-RCD side of the house CU. That should feed into
the little shed CU, and 'daisy-chain' (i.e. not passing through any way
of the shed CU) on to the garage. In both shed and garage your small CU
has its own RCD (or if you're often working at night, separate RCBOs for
lights and power in the garage, so that earth leakage fault from power
tool cable-slicing oopsie ;-) doesn't plunge you into darkness).

That's the outline of the 'best' solution. However, to install it and be
sure you've met required disconnection times for the supply cable will
need Real final-circuit design calculations - they're not hard, but do
require reference to tabulated values of cable characteristics.

As to earthing - again, what you have sounds about right (shed close to
house shares its earth, more distant garage has its own rod). If your
supply is PME (TN-C-S) there's an argument for making even the shed a TT
installation, but it's not an overwhelming one. The reason PMEness
matters here is that (in handwavy outline) PME creates one bigass
'equipotential zone' (kinda like supplementary bonding does in one
bathroom, on the whole-house scale), so we stop caring about 'true'
earth potential; and with metallic incoming services bonded to, the
installation earth is going to be v. close to 'true' earth anyway. Once
you're 30m away, like your garage is, and given that a single fault in
your buried feed could lose the bonding between N and E, it makes
increasingly little sense to rely on the (you-hope-it's-been-)exported
house earth; so making the garage its own little TT installation - as
you already have - is the Right Answer.

Should the feed from the house to the shed be via a 100mA RCD, separate to
the rest of the house?

It'd certainly be better for it not to share the house sockets' 30mA RCD
- doing that increases the probability of nuisance trips in the house
(both from full-on faults Outside, and just from outside appliances
adding a few mA of their legitimate leakage to 'preload' this shared RCD
close to its tripping point). As I whittered above, it's OK for the
supplies themselves not to have any RCD protection, provided the socket
circuits they feed do end up with RCD protection. If you feel happier
running through a 100mA RCD, you can, but you can't rely on that
necessarily discriminating with the garage RCD. (The strength of that
argument again differs with what you're doing in the garage: if you've
one light in there and one socket where you plug in a vac to clean the
car, a single whole-garage RCD is fine; if it's a uk.d-i-y.hardcore
garage, you've got no room for a car, but a couple of benches, two
pillar drills (one of which works and the other's going to Real Soon
Now), a boatload of other power tools, some unreasonably dangerous
electroplating lashup in the corner, oh and the feed to the combination
Thunderbirds launchpit-and-burglar-pit, and so on - in which case you'd
want the whole garage TT to have its own 100mA time-delay RCD, and
either a splitload 30mA-RCD CU or individual RCBOs for (at minimum) the
socket and fixed-equipment circs... In that case, you want the house end
of things to protect *only* the feed cable, leaving all the RCDing local
to the garage.

Another question is, why do I need to TT the supply in the garage? Can't I
just use the house earth?
(My logic is, the earth provided by the electricity company is at least 500m
long (That's where the substation is!)

Hope I've outlined the reasoning above. For TN-S, the exporting issue is
less sharp than for TN-C-S, but for a 'remote' garage 30m away you're
increasing the earth loop impedance a fair bit from what the supplier's
big-ass incomer gave you as you pass it down your wimpy little 4mmsq SWA
to the end of the garden. And with PN-C-S becoming more widespread, even
as a 'retrofit' as suppliers meet increased local demand from all that
good brownfield development, it makes sense to make new/upgraded
installations 'PME-aware' ;-)

HTH - Stefek

What you have is basically OK; all the rambling that follows is in the
realm of minor tweaks towards unattainable perfection. That said... the
cascade of RCDs and MCBs is not best practice - should you have a fault in
the garage, you could well end up with not only the garage power being cut
(which we want) but also the shed (pain) and the house sockets
(double-pain). What you want to get to is to have the cables to shed and
garage protected against short-circuit and overcurrent, while leaving
protection of the loads they feed to a dedicated RCD and closer-rated MCB
in each place (shed and garage). To achieve this, you want to feed the
'submain' - the supply to shed and garage - from sthg like a 20A *fuse*
(not MCB: you actively *want* the slower reaction to overloads of a fuse
here) on the non-RCD side of the house CU. That should feed into the
little shed CU, and 'daisy-chain' (i.e. not passing through any way of the
shed CU) on to the garage. In both shed and garage your small CU has its
own RCD (or if you're often working at night, separate RCBOs for lights
and power in the garage, so that earth leakage fault from power tool
cable-slicing oopsie ;-) doesn't plunge you into darkness).

Well, the garage is basically for the car (at the moment!)
The shed is my "work room" for fixing PC's (My job) so no real risk of me
wielding a huge angle grinder in there, then getting plunged into darkness!
I have installed an emergency light in the shed anyway, as I had one going
spare ;-)


As to earthing - again, what you have sounds about right (shed close to
house shares its earth, more distant garage has its own rod). If your
supply is PME (TN-C-S) there's an argument for making even the shed a TT
installation, but it's not an overwhelming one. The reason PMEness matters
here is that (in handwavy outline) PME creates one bigass 'equipotential
zone' (kinda like supplementary bonding does in one bathroom, on the
whole-house scale), so we stop caring about 'true' earth potential; and
with metallic incoming services bonded to, the installation earth is going
to be v. close to 'true' earth anyway. Once you're 30m away, like your
garage is, and given that a single fault in your buried feed could lose
the bonding between N and E, it makes increasingly little sense to rely on
the (you-hope-it's-been-)exported house earth; so making the garage its
own little TT installation - as you already have - is the Right Answer.

Excellent!
Is there any value in having the house earth in the garage as well, and also
connecting this to the rod? - or is this a definite no?

Should the feed from the house to the shed be via a 100mA RCD, separate
to the rest of the house?

It'd certainly be better for it not to share the house sockets' 30mA RCD -
doing that increases the probability of nuisance trips in the house (both
from full-on faults Outside, and just from outside appliances adding a few
mA of their legitimate leakage to 'preload' this shared RCD close to its
tripping point). As I whittered above, it's OK for the supplies themselves
not to have any RCD protection, provided the socket circuits they feed do
end up with RCD protection. If you feel happier running through a 100mA
RCD, you can, but you can't rely on that necessarily discriminating with
the garage RCD. (The strength of that argument again differs with what
you're doing in the garage: if you've one light in there and one socket
where you plug in a vac to clean the car, a single whole-garage RCD is
fine; if it's a uk.d-i-y.hardcore garage, you've got no room for a car,
but a couple of benches, two pillar drills (one of which works and the
other's going to Real Soon Now), a boatload of other power tools, some
unreasonably dangerous electroplating lashup in the corner, oh and the
feed to the combination Thunderbirds launchpit-and-burglar-pit, and so
on - in which case you'd want the whole garage TT to have its own 100mA
time-delay RCD, and either a splitload 30mA-RCD CU or individual RCBOs for
(at minimum) the socket and fixed-equipment circs... In that case, you
want the house end of things to protect *only* the feed cable, leaving all
the RCDing local to the garage.

I have been planning on adding a second, smaller CU to the house, with just
an isolator and a few MCB's for the freezer and alarm system, I think I will
just add the outside stuff to this new CU (On a 32A Type1 MCB)

Another question is, why do I need to TT the supply in the garage? Can't
I just use the house earth?
(My logic is, the earth provided by the electricity company is at least
500m long (That's where the substation is!)

Hope I've outlined the reasoning above. For TN-S, the exporting issue is
less sharp than for TN-C-S, but for a 'remote' garage 30m away you're
increasing the earth loop impedance a fair bit from what the supplier's
big-ass incomer gave you as you pass it down your wimpy little 4mmsq SWA
to the end of the garden. And with PN-C-S becoming more widespread, even
as a 'retrofit' as suppliers meet increased local demand from all that
good brownfield development, it makes sense to make new/upgraded
installations 'PME-aware' ;-)

Fair point, makes sense to future proof these things as much as possible, as
there is already a rod, I wont take it away!

HTH - Stefek

Thanks for taking to trouble to explain this, I did think it was basically
there, just needed a few minor adjustments!

I have just done some calculations for the house - shed run... using
http://www.kevinboone.com/cablecalc.cgi

House to Shed
Generated by CableCalc V1.0 (c)2000 Kevin Boone, all rights reserved
Basic cable properties:
Cable type: PVC-insulated two-core-and-earth 4 mm2 with 1.5 mm2 earth
conductor
Installation method: clipped to a surface
Uncorrected nominal current: 36 amps at 30 degrees celcius
Rated full load temperatu 70 degrees celcius
Ambient temperatu 30 degrees celcius
Current rating temperature correction factor: 1
Semi-enclosed fuse correction factor: 1
Grouping method: no grouping; cables widely separated
Cables in group: 1
Grouping correction factor: 1
Ring correction factor: 1
Length in thermal insulation: 0
Thermal insulation correction factor: 1
Corrected nominal current: 36 amps
Specific resistance of power conductor: 0.00461 ohms/metre at 20 degrees
celcius
Specific resistance of earth conductor: 0.0121 ohms/metre at 20 degrees
celcius
Temperature cofficient of resistance: 0.004 K-1
Specific resistance of power conductor: 0.005532 ohms/metre at 70 degrees
celcius
Specific resistance of earth conductor: 0.01452 ohms/metre at 70 degrees
celcius

Voltage drop:
For a total cable length of: 25 metres
Volt drop at end of cable: 8.8512 V when carrying 32 amps and cable
temperature is at maximum value of 70 degrees celcius
Basic compatibility of overcurrent device with cable and design current
Selected over-current device: 32-amp type-1 MCB
Nominal current rating of overcurrent device is compatible with design
current
Nominal current rating of overcurrent device is compatible with corrected
cable current rating
Current required to disconnect device in 5 seconds: 128 amps
Effective worst-case earth conductor resistance of cable: 0.363 ohms at
maximum rated temperature
Worst-case shock voltage after five seconds: 46.464 volts
Disconnection times
Total fault resistance: 1.3013 ohms
Fault current: 176.746 amps
Over-current device will disconnect in 5 seconds
Over-current device will disconnect in less than 0.4 seconds
This cable/device combination is suitable for all allowed applications,
including bathroom and outdoor systems



Now from the shed to the garage (Assuming it's on a 20A MCB in the shed):-


Shed to Garage
Generated by CableCalc V1.0 (c)2000 Kevin Boone, all rights reserved
Basic cable properties:
Cable type: PVC-insulated two-core-and-earth 4 mm2 with 1.5 mm2 earth
conductor
Installation method: clipped to a surface
Uncorrected nominal current: 36 amps at 30 degrees celcius
Rated full load temperatu 70 degrees celcius
Ambient temperatu 30 degrees celcius
Current rating temperature correction factor: 1
Semi-enclosed fuse correction factor: 1
Grouping method: no grouping; cables widely separated
Cables in group: 1
Grouping correction factor: 1
Ring correction factor: 1
Length in thermal insulation: 0
Thermal insulation correction factor: 1
Corrected nominal current: 36 amps
Specific resistance of power conductor: 0.00461 ohms/metre at 20 degrees
celcius
Specific resistance of earth conductor: 0.0121 ohms/metre at 20 degrees
celcius
Temperature cofficient of resistance: 0.004 K-1
Specific resistance of power conductor: 0.005532 ohms/metre at 70 degrees
celcius
Specific resistance of earth conductor: 0.01452 ohms/metre at 70 degrees
celcius

Voltage drop:
For a total cable length of: 30 metres
Volt drop at end of cable: 6.6384 V when carrying 20 amps and cable
temperature is at maximum value of 70 degrees celcius
Basic compatibility of overcurrent device with cable and design current
Selected over-current device: 32-amp type-1 MCB
Nominal current rating of overcurrent device is compatible with design
current
Nominal current rating of overcurrent device is compatible with corrected
cable current rating
Current required to disconnect device in 5 seconds: 128 amps
Effective worst-case earth conductor resistance of cable: 0.4356 ohms at
maximum rated temperature
Worst-case shock voltage after five seconds: 55.7568 volts
Disconnection times
Total fault resistance: 1.40156 ohms
Fault current: 164.103 amps
Over-current device will disconnect in 5 seconds
Over-current device will disconnect in less than 0.4 seconds
This cable/device combination is suitable for all allowed applications,
including bathroom and outdoor systems

All looks OK to me, and that would be exporting the earth to the garage via
1.5mm cable (I have 4mm)

Sparks...

Sparks wrote:

Is there any value in having the house earth in the garage as well, and also
connecting this to the rod? - or is this a definite no?

A definite no. I believe you wrote you had the SWA armour connected to
the house earth at the house end, but isolated from the garage CU -
that's just what you want to make the garage a pukka TT system. If you
did connect the house earth to the garage rod, under fault conditions
(not necessarily in your own house, but for example another place close
by on the same phase of the same substation) you might end up with quite
substantial fault currents coursing through your earth wiring; not
really desirable...

I have just done some calculations for the house - shed run... using
http://www.kevinboone.com/cablecalc.cgi

House to Shed
Generated by CableCalc V1.0 (c)2000 Kevin Boone, all rights reserved
Basic cable properties:
Cable type: PVC-insulated two-core-and-earth 4 mm2 with 1.5 mm2 earth
conductor

Liar, liar, pants on fire ;-) You said in your original post it was SWA.
If 3-core (L,N,E all get an inner core of their own, armour earthed
Because It Should Be rather than to carry fault currents) then the earth
conductor's the same size - 4mmsq - as the L & N. If it's 2-core with
the sheath being used as the E conductor, you need to use the right
Table (or hope that CableCalc has access to its contents) to use the
impedance of the armour.

Voltage drop:
For a total cable length of: 25 metres

That's the second time you've said that ;-) I take it, then, that the CU
is more or less in the opposite corner of the house to the shed, so that
although the shed's only 4m from the nearest bit of house, the total SWA
run is 25m as you say...

Volt drop at end of cable: 8.8512 V when carrying 32 amps and cable
temperature is at maximum value of 70 degrees celcius

So at (unrealistic) full load, that's just about your entire voltage
drop budget eaten in this first 25m (conventionally we allow up to 4% as
cable loss - for the 240V supply that's 9.6V). Unlike the earth
impedance calc, this isn't affected by SWA-versus-T&E, since the live
conductors are the same size (4mmsq).

Then you take another 25m (and you lie again about the cable
construction ;-) for the segment from shed on to garage. But the calcs
from CableCalc believe this is a 25m segment from the origin of the
supply; so you need to (a) add the voltage drop for this segment to the
already-established 8.8V drop for the first run - putting you over the
voltage drop limit for drawing a heftier load in the garage; and (b)
establish clearly what the earth fault path actually is for an
L-to-E-conductor fault right at the end of the garage run. If that path
relies on the house CU breaker, you need to factor in the full 50m of
the combined run to do your fault-path calcs (which I now notice you
mention, at the end of your message, do assume the reduced-cross-section
protective conductor of T&E, while you know you have 4mmsq.

All looks OK to me, and that would be exporting the earth to the garage via
1.5mm cable (I have 4mm)

Though we're not actually 'exporting' the house earth to the garage, are
we? (The garage end is TT). But the house earth is what an L-to-E fault
at the end of that supply cable will go back through (and you've got the
worst-case 0.8ohm earth impedance of your TN-S supply to allow for in
calcultating the fault current, touch voltage, and disconnection time,
therefore...)

HTH, rather than confusing - Stefek

"Stefek Zaba" wrote in message
...
Sparks wrote:

Is there any value in having the house earth in the garage as well, and
also connecting this to the rod? - or is this a definite no?

A definite no. I believe you wrote you had the SWA armour connected to the
house earth at the house end, but isolated from the garage CU -

Yup, you belive correclty :-)

that's just what you want to make the garage a pukka TT system. If you did
connect the house earth to the garage rod, under fault conditions (not
necessarily in your own house, but for example another place close by on
the same phase of the same substation) you might end up with quite
substantial fault currents coursing through your earth wiring; not really
desirable...

Yea, but, when I earth my metal gas pipe and metal water pipe, like the good
little boy I am, does this then make my house installation partly TT, thus
potentally unsafe as you describe!
(This is why I asked :-) )


I have just done some calculations for the house - shed run... using
http://www.kevinboone.com/cablecalc.cgi

House to Shed
Generated by CableCalc V1.0 (c)2000 Kevin Boone, all rights reserved
Basic cable properties:
Cable type: PVC-insulated two-core-and-earth 4 mm2 with 1.5 mm2 earth
conductor

Liar, liar, pants on fire ;-) You said in your original post it was SWA.

Yep, I did, however, the site doesnt have an option for SWA, so I assumed if
it was OK with the 4mm+1.5mm earth, then is must be OK with a 4mm earth!

If 3-core (L,N,E all get an inner core of their own, armour earthed
Because It Should Be rather than to carry fault currents) then the earth
conductor's the same size - 4mmsq - as the L & N. If it's 2-core with the
sheath being used as the E conductor, you need to use the right Table (or
hope that CableCalc has access to its contents) to use the impedance of
the armour.

I am using three core SWA, the armour is connected only at the house end.
However the thrid earth conductor from the shed - garage is not connected
(as it is earthed via a rod)

Voltage drop:
For a total cable length of: 25 metres

That's the second time you've said that ;-) I take it, then, that the CU
is more or less in the opposite corner of the house to the shed, so that
although the shed's only 4m from the nearest bit of house, the total SWA
run is 25m as you say...

Yea, the CU in under the stairs, in the middle of the house - the run was a
pain in the arse, goes up to the loft, then back dowm - however I should be
able to shorten this soon as the kitchen is being ripped out.
(New SWA will now be run under the patio, not overhead with 8 coax, 5UTP and
a couple alarm cables (It looks crap, but it was the only way without
digging uo the crazy paving (Yea, I know, its all been dug up now!)))

Volt drop at end of cable: 8.8512 V when carrying 32 amps and cable
temperature is at maximum value of 70 degrees celcius

So at (unrealistic) full load, that's just about your entire voltage drop
budget eaten in this first 25m (conventionally we allow up to 4% as cable
loss - for the 240V supply that's 9.6V). Unlike the earth impedance calc,
this isn't affected by SWA-versus-T&E, since the live conductors are the
same size (4mmsq).

Then you take another 25m (and you lie again about the cable construction
;-)

Same excuse as before ;-)

for the segment from shed on to garage. But the calcs from CableCalc
believe this is a 25m segment from the origin of the supply; so you need
to (a) add the voltage drop for this segment to the already-established
8.8V drop for the first run - putting you over the voltage drop limit for
drawing a heftier load in the garage; and (b) establish clearly what the
earth fault path actually is for an L-to-E-conductor fault right at the
end of the garage run. If that path relies on the house CU breaker, you
need to factor in the full 50m of the combined run to do your fault-path
calcs (which I now notice you mention, at the end of your message, do
assume the reduced-cross-section protective conductor of T&E, while you
know you have 4mmsq.

Yea!, See! I was paying attention :-)

All looks OK to me, and that would be exporting the earth to the garage
via 1.5mm cable (I have 4mm)

Though we're not actually 'exporting' the house earth to the garage, are
we? (The garage end is TT).

Nope, we are not, but the calcs were assuming I was, so I thought with a
better earthing arrangement, it was even better than OK!

But the house earth is what an L-to-E fault at the end of that supply
cable will go back through (and you've got the worst-case 0.8ohm earth
impedance of your TN-S supply to allow for in calcultating the fault
current, touch voltage, and disconnection time, therefore...)

Okay, if I was to connect the garage to a 20A MCB, connected to a 30mA RCD
in the shed (No not Ideal I know!), would I still need to calculate from the
house?

If not, what would you suggest I do (I Really don't want to have to replace
the cables!!)

Sparks...

Sparks wrote:

Okay, if I was to connect the garage to a 20A MCB, connected to a 30mA RCD
in the shed (No not Ideal I know!), would I still need to calculate from the
house?

With an RCD at the shed end, we can ignore earth loop impedance stuff,
as the RCD will cut out given an L-E fault. You're left with the voltage
drop needing calculating for the full 50m run (and the 4mm T&E figures
are reusable for 4mm SWA, neglecting differences in temperature rise);
if you do shorten the CU-to-shed run, and since you don't 'really' run
heavy stuff in the garage, you're likely to be OK (but a calc wouldn't
hurt).

If not, what would you suggest I do (I Really don't want to have to replace
the cables!!)

Of course you don't (who would?)...

that's just what you want to make the garage a pukka TT system. If you
did connect the house earth to the garage rod, under fault conditions
(not necessarily in your own house, but for example another place close
by on the same phase of the same substation) you might end up with quite
substantial fault currents coursing through your earth wiring; not really
desirable...

Yea, but, when I earth my metal gas pipe and metal water pipe, like the
good little boy I am, does this then make my house installation partly TT,
thus potentally unsafe as you describe!
(This is why I asked :-) )

Can any one shed any light on this question!?

(Another reason I ask this, is, if a generator is installed, this has to
have a rod (AIUI), but if this has a rod, connected to the house earth
supply, does this them break the earthing rules on my TN-S system?)

Transfer switches only seem to switch the L&N (Via an "all off" position)

Sparks...

Sparks wrote:

Yea, but, when I earth my metal gas pipe and metal water pipe, like the
good little boy I am, does this then make my house installation partly TT,
thus potentally unsafe as you describe!

S'far as I understand it... for one thing, there's a balance of risk:
the possible differences in potential between the supplier's earth in a
TN-S setup and the 'local' earth of buried metallic services doesn't
arise in 'normal' conditions, but rather in the case of faults, while
keeping all your inside-the-house pipework at the same potential
protects against likelier, local faults. ('Better still' from this p-o-v
is for the incoming services to be non-metallic, so it's just the
within-house pipework which is bonded.) A second factor is how low a
reistance the local earthing has: the connection to earth that you get
from a substantial length of buried cast-iron water main and gas supply
is usefuly lower than that from one lil' earth rod.

But I've neither read deeply nor thought hard about this, so I'd welcome
anyone better-informed weighing in...

Stefek Zaba wrote:

But I've neither read deeply nor thought hard about this, so I'd welcome
anyone better-informed weighing in...

I haven't been following this thread that closely, but the recent
questions seem to be ...

Yea, but, when I earth my metal gas pipe and metal water pipe, like the
good little boy I am, does this then make my house installation partly TT,
thus potentally unsafe as you describe!
(This is why I asked :-) )

An installation can't be partly TT. If there's a metallic earth path
back to the substation transformer then it's TN, if there isn't then
it's TT. (IT doesn't apply here.)

Can any one shed any light on this question!?

(Another reason I ask this, is, if a generator is installed, this has to
have a rod (AIUI), but if this has a rod, connected to the house earth
supply, does this them break the earthing rules on my TN-S system?)

Transfer switches only seem to switch the L&N (Via an "all off" position)

A generator installation has to have its own means of earthing, in case
the connection to the supply distributor's mains earth has been severed.
The earths can be connected in parallel - there's no need for switching.

Here's a generator wiring diagram I've posted befo


L o----------------o
\ SW1a
Mains o---------- L to CU
from
meter +-------o
| SW1: DPCO changeover switch
| 100A break before make!
N o----------------o
| \ SW1b
| o---------- N to CU
|
| +---o
| |
| |
Supplier's E o-----------------------------o Main earth terminal
earth | | /
| | |
| | 16mm^2 |
| | earth |
------- |
| | |
| G.P. | | G.P. = Generator
| | | protection,
------- | fuse or CB
| | | plus 100mA RCD
| | |
-- | - | -- |
| G( | | |
| G( | | |
| G( | | | G = generator winding
| G( | | |
| G( | | |
| | | | |
Generator | +---.B | |
frame - | | | | B = bonding connection
-------.-- |
B|\ |
| \-------------+
|
|
|
-----
--- Your earth electrode(s)
- Re 200 ohm


NB 1. You are supposed to consult with your electricity supplier /
distribution network operator before installing switched standby
generating plant.

NB 2. Reg. 551-02-03 requires automatic load shedding arrangements if
the generator is not rated to supply the whole installation.

--
Andy

"Andy Wade" wrote in message
...
Stefek Zaba wrote:

But I've neither read deeply nor thought hard about this, so I'd welcome
anyone better-informed weighing in...

I haven't been following this thread that closely, but the recent
questions seem to be ...

Yea, but, when I earth my metal gas pipe and metal water pipe, like the
good little boy I am, does this then make my house installation partly
TT, thus potentally unsafe as you describe!
(This is why I asked :-) )

An installation can't be partly TT. If there's a metallic earth path back
to the substation transformer then it's TN, if there isn't then it's TT.
(IT doesn't apply here.)

Can any one shed any light on this question!?

(Another reason I ask this, is, if a generator is installed, this has to
have a rod (AIUI), but if this has a rod, connected to the house earth
supply, does this them break the earthing rules on my TN-S system?)

Transfer switches only seem to switch the L&N (Via an "all off" position)

A generator installation has to have its own means of earthing, in case
the connection to the supply distributor's mains earth has been severed.
The earths can be connected in parallel - there's no need for switching.


Okay, the origanal question was why shouldt I connect the house earth to the
garage TT earthing arangement - why must they be kept seperate?
(The cable run up to the garage is about 50m in 4mm SWA)


NB 2. Reg. 551-02-03 requires automatic load shedding arrangements if the
generator is not rated to supply the whole installation.


So it has to be impossible to switch on too much?
or can there just be some instructions to turn off some MCB's?


Sparks...

Sparks wrote:

Okay, the origanal question was why shouldt I connect the house earth to the
garage TT earthing arangement - why must they be kept seperate?
(The cable run up to the garage is about 50m in 4mm SWA)

Well if you connected them together it would no longer be TT, but nobody
should have said that isn't allowed. It depends what you're intending
to do in the shed and whether the equipotential zone concept can be
properly applied there. If you search Google groups for "shed
electrics" you'll find plenty of previous threads to muse over.

NB 2. Reg. 551-02-03 requires automatic load shedding arrangements if the
generator is not rated to supply the whole installation.

So it has to be impossible to switch on too much?
or can there just be some instructions to turn off some MCB's?

What it says is "Means shall be provided to automatically disconnect
such parts of the installation as may be necessary if the capacity of
the generating set is exceeded."

--
Andy

"Andy Wade" wrote in message
...
Sparks wrote:

Okay, the origanal question was why shouldt I connect the house earth to
the garage TT earthing arangement - why must they be kept seperate?
(The cable run up to the garage is about 50m in 4mm SWA)

Well if you connected them together it would no longer be TT, but nobody
should have said that isn't allowed. It depends what you're intending to
do in the shed and whether the equipotential zone concept can be properly
applied there. If you search Google groups for "shed electrics" you'll
find plenty of previous threads to muse over.

(It's the garage, the shed uses the house earth, as it is close to the
house)
In the garage, the power is just for a couple of flurecent lights, a 12v PSU
for some CCTV cameras & alarm sensors
and a few sockets for general use (battery chargers, lead lamps, very
occational DIY tools like drills/ jigsaws
possibly a pressure washer once in a blue moon.

There isn't going to be any fixed appliances, or anything high demand.

Would it be better/safer etc, to have an earth rod in the garage, AND
connect it to the house earth (Via a 4mm conductor)?
Or just use the house earth
Or just use the rod?

Ta!

Sparks...