On 01/07/2018 07:08, wrote:
On Sunday, 1 July 2018 00:36:23 UTC+1, John Rumm wrote:
On 30/06/2018 23:44, tabbypurr wrote:
On Saturday, 30 June 2018 15:57:00 UTC+1, John Rumm wrote:

If discrimination against N-E faults is required, why are single pole RCBOs sold? There's something I'm not clear on yet here.

For TN-C-S installs there is unlikely to be any significant voltage
difference between N & E at the origin of the installation (since they
are joined there). So once the loads are disconnected there is unlikely
to be any significant current flow in the CPC. What there is, will be
conducted by the suppliers PEN conductor which is designed to be a
current carrying conductor in normal operation (i.e. it is a "live" wire)

With TT, the neutral at the point of supply could be at a different
potential to that of the local earth spike. So a persistent N E fault
could drive significant current through the local earth spike (which in
turn could cause heating, loss of moisture and rising external earth
impedance).

So on balance a SP RCBO will still result in a significant reduction in
serious injury risk for most installations - its just not as suitable
for TT installs

gotcha
Presumably for the same reason it wouldn't be well suited to TN-S either, only for PME supplies.

I don't recall DP being a requirement for TN-S. While you are likely to
see a bigger voltage between N & E at the premises, they are still tied
together typically within a few hundred yards. The supply cable earth is
also properly bonded to a substatial earth at the sub station, so there
are not the same risks as there might be with a puny local earth rod.

ok cheers. So the TN-S place can use the SP RCBOs for the final circuits. Do they still need to be fed from 2x DP RCDs to provide N-E fault protection?

On TN-S NE fault currents are likely to be small in comparison to LE
(from an infrastructure point of view they are typically a RCD nuisance
trip hazard rather than a fire risk) so don't require explicit protection.

(obviously NE shorts can pose a shock risk to users and maintainers of
the system - and may lay undetected on non RCD systems).


--
Cheers,

John.

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