I'm planning to rework the distribution system on the farm and have a
supply in a meter house near the road, some distance from the nearest
farm buildings. Basically I wish to provide a 300mA time delayed RCD
at the meter house with an extended time delay so that I can pick off
sub-systems along the route of the 25mm SWA main (backbone) feed
cables which runs down the site and have tap-off points for each
building, employing "type S" 100mA time delayed RCDs at each building
distribution board incomer, followed by 30mA general purpose (fast)
RCD protection for socket outlets etc. on the appropriate final
circuits. Normal type S RCDs disconnect in typically less than 0.2
seconds I understand. General purpose disconnect in 0.04S at 150mA
leakage current.
The general purpose RCDs are freely available, the normal time delayed
type S 100mA units also easy to source. Where I am finding a problem
is sourcing a 300mA RCD with a time delay greater than the 100mA units
have, so discrimination can be assured as I don't want to take out the
whole system if an earth fault occurs on one of the distribution
system cables protected by the 100mA type S units.
Without going into fantastically priced speciality units does anyone
know of reasonably priced 300mA RCD with a delay disconnection time
between 0.2 and 1 second as allowed by 411.3.2.4 and suitable for a
100A TPN service.
The service is TN-C-S so I need to convert to TT, not extend the
suppliers earth into the farm systems.

On Sep 11, 9:36*pm, cynic wrote:
I'm planning to rework the distribution system on the farm and have a
supply in a meter house near the road, some distance from the nearest
farm buildings. Basically I wish to provide a 300mA time delayed RCD
at the meter house with an extended time delay so that I can pick off
sub-systems along the route of the 25mm SWA main (backbone) feed
cables which runs down the site and have tap-off points for each
building, employing "type S" 100mA time delayed RCDs at each building
distribution board incomer, followed by 30mA general purpose (fast)
RCD protection for socket outlets etc. on the appropriate final
circuits. Normal type S RCDs disconnect in typically less than 0.2
seconds I understand. General purpose disconnect in 0.04S at 150mA
leakage current.
The general purpose RCDs are freely available, the normal time delayed
type S 100mA units also easy to source. Where I am finding a problem
is sourcing a 300mA RCD with a time delay greater than the 100mA units
have, so discrimination can be assured as I don't want to take out the
whole system if an earth fault occurs on one of the distribution
system cables protected by the 100mA type S units.
Without going into fantastically priced speciality units does anyone
know of reasonably priced 300mA RCD with a delay disconnection time
between 0.2 and 1 second as allowed by 411.3.2.4 and suitable for a
100A TPN service.
The service is TN-C-S so I need to convert to TT, not extend the
suppliers earth into the farm systems.


The French use much higher trip current RCDs for distribution than
over here. Trying to discriminate just by using a large difference in
current ratings isnt ideal, but it can frequently work, if not all the
time.

These 2 are just for interests sake really... In principle its also
quite possible to make your own high trip current RCD using relays,
but I dont think that would be usful for you. The other possible is to
forego RCD protection on some of the distribution by using armoured
cable and a good enough earth, like a Ufer earth. Not exactly trivial
though!


NT

On 12 Sep, 07:58, NT wrote:
On Sep 11, 9:36*pm, cynic wrote:





I'm planning to rework the distribution system on the farm and have a
supply in a meter house near the road, some distance from the nearest
farm buildings. Basically I wish to provide a 300mA time delayed RCD
at the meter house with an extended time delay so that I can pick off
sub-systems along the route of the 25mm SWA main (backbone) feed
cables which runs down the site and have tap-off points for each
building, employing "type S" 100mA time delayed RCDs at each building
distribution board incomer, followed by 30mA general purpose (fast)
RCD protection for socket outlets etc. on the appropriate final
circuits. Normal type S RCDs disconnect in typically less than 0.2
seconds I understand. General purpose disconnect in 0.04S at 150mA
leakage current.
The general purpose RCDs are freely available, the normal time delayed
type S 100mA units also easy to source. Where I am finding a problem
is sourcing a 300mA RCD with a time delay greater than the 100mA units
have, so discrimination can be assured as I don't want to take out the
whole system if an earth fault occurs on one of the distribution
system cables protected by the 100mA type S units.
Without going into fantastically priced speciality units does anyone
know of reasonably priced 300mA RCD with a delay disconnection time
between 0.2 and 1 second as allowed by 411.3.2.4 and suitable for a
100A TPN service.
The service is TN-C-S so I need to convert to TT, not extend the
suppliers earth into the farm systems.

The French use much higher trip current RCDs for distribution than
over here. Trying to discriminate just by using a large difference in
current ratings isnt ideal, but it can frequently work, if not all the
time.

These 2 are just for interests sake really... In principle its also
quite possible to make your own high trip current RCD using relays,
but I dont think that would be usful for you. The other possible is to
forego RCD protection on some of the distribution by using armoured
cable and a good enough earth, like a Ufer earth. Not exactly trivial
though!

NT- Hide quoted text -

- Show quoted text -

Given the BS7671 requirement for a 300mA head end RCD on farms it is
surprising that a more versatile time delayed version is not listed as
readily available, especially one with sufficient time delay to
discriminate with a normal type S or with an adjustment available. It
seems uneccessarily complex and expensive going for a distribution
network scale leakage detection system.
I could I suppose use the suppliers earth to protect the backbone
armoured cable which is for the most part underground but there is
always the risk of future workers not under my control who might take
the TN-C-S armour sheath earth into the vulnerable areas. John Rumm
has made a useful suggestion which I will follow up.

On 12 Sep, 14:38, John Rumm wrote:
cynic wrote:
I could I suppose use the suppliers earth to protect the backbone
armoured cable which is for the most part underground but there is
always the risk of future workers not under my control who might take
the TN-C-S armour sheath earth into the vulnerable areas.

That would be a viable option, but you would need to ensure the cable
armour is isolated from all the distribution boards. I warning notices
on the cable and dis boards might help re-enforce the point.

Personally I would go with no 100mA RCDs at all, and 30mA RCBOs on the
circuits where you need them. That way you will discriminate with most
faults being cleared by the RCBO since you are unlikely to have much in
effect directly protected by the head end 300mA RCD (probably the odd
sub main etc).

--
Cheers,

John.


If I went down that route I would terminate the armoured cables with
the TN-C-S earth connections into plastic boxes and link the i/c and o/
g backbone cable sheath earths then come out of the plastic box with
meter tails as offtakes to the local distribution boards. I could then
install sub-meters to monitor the individual building usage. This
would be a useful addition if I rent out one or more of the sheds at a
future date. For the cost saving by not using a 300mA type S at the
head end I could purchase a few certified kWh meters. There will also
be at least two 32A sockets for welders so 100mA non delayed RCDs
needed there.
Ho hum time for further thoughts.

If you want to isolate the PME earth via a "plastic" SWA gland then
search for the CCG glands. Nickel-plated brass base & cone nut, with a
tough plastic shroud that screws up tight to the enclosure and hand
locking seal nut on the rear. Nothing metal can be touched.

You can argue it was tightened by hand so can be undone without the
use of a tool, but pretty immaterial. If you find assembly is
difficult (the plastic shroud will not screw up against the washer on
the enclosure) then simply back off the cone-nut & re-tighten. UK SWA
is a little different to other countries and thus some of ours can be
at the edge of specification (same with any gland).

Cheap and off the shelf compared to the "true" all plastic SWA glands
which are impossible to find and probably some min-qty.