I agree - I much prefer isolators after the meter - it makes a lot more sense to me for the demarcation point to be a pair of accessible terminals than a captive end of the consumer's meter tails. But the DNO's position is that the cut-out provides all the isolation that's necessary (which would be the case if you need to work on the isolator itself - in a way we're just moving the edges of the problem a few cm back rather than soundly solving it).

During the debate about smart meter rollout it was suggested that isolators were fitted at the same time (or more likely a design of smart meter with a built-in isolator be used - at marginal extra cost) - but even then the powers that be couldn't be persuaded. 

   - Andy.

The powers that be need to be refered back to

Regulation 12(1)(b) of EAWR states ‘where necessary to prevent danger, suitable means shall be available for […] the isolation of any electrical equipment’, where ‘isolation’ means the disconnection and separation of the electrical equipment from every source in such a way that the disconnection and separation is secure.

Yup and they say the cut-out does that - remove the fuse carrier, remove the cartridge fuse from the carrier, re-insert the carrier and re-seal to secure. Isolation is adequate (several cm air gap) and secure as it can't be re-energised without the use of a tool (wire cutters). and possession of a suitable link (Most people even keep the cartridge fuse in a pocket to make it even harder for anyone else to re-connect). Pretty much the same procedure for isolating/making safe final or distribution circuits fed from fuses (other than the ownership/permission bit).

   - Andy.

Safe isolation is on the LIVE conductors which means Line and Neutral.  The cartridge fuse does not do that.  The cartridge fuse is only on the LINE.  Also if there was a broken PEN down the road (broken PENs have been on the rise for the last 15 years) the potential could rise up either the Neutral or the Earth allowing electrocution.  A DP (DoublePole) isolator will disconnect both LIVE conductors.  This is why we have to prove dead after safe isolation. 

Excuse the capitals.  They are to highlight the point that some poeple call it Live and some call the conductor Line.  This normally depends on age.  Over 40s will call it live as that is how were we taught.  Anyone under the age of 25 call it Line as that is what they are taught. 

Except again, on the DNO side neutral/ PEN being at or near earth potential, by the rules of the ESCQR is not considered a live conductor - though somehow by a stroke of the pen it becomes a live conductor on  the BS7671 side.... 

In any case would you want the option to be able to break the neutral in a 3 phase supply without breaking the phase connections first ?

Of course if you consider a broken PEN to be a credible risk, (and I think we really should) then really to be truly safe, you need an isolator that breaks the earth (CPC) as well as Phase(s) und Neutral.

Perhaps new houses should all plug in with a 63A plug that could be withdrawn for servicing and repair - a similar idea works for caravans after all ;-)

Safe isolation is on the LIVE conductors which means Line and Neutral. 

Not really - even BS 7671 generally allows N to remain intact for isolation (at least on TN systems - see reg 461.2). Single pole MCBs are often used for isolating individual circuits.

Also if there was a broken PEN down the road

In which case even DP isolation won't help much as the N-PE link will still be upstream of the isolator - so the MET and everything connected to it will still be at the hazardous voltage - in that situation it doesn't matter much if the N is disconnected if the c.p.c. is hazardous live.  It's not a particularly easy condition to verify ether without some independent 0V reference (fork in a flower bed or haven forbid a neon screwdriver) - tests between c.p.c. and live conductors, or between live conductors will often been fooled.

   - Andy.

I do think in time people will refer to LIVE conductors in a single pahse install as

Line Neutal Earth.

Under normal conditions Line will carry most current with Neutral not carrying as much current.

Earth and CPC will carry some earth leakage more so over time as things go from Electric to electronic espcially with LED lamps and other DC circuitry discharging to earth naturally as part of their design function.

Under fault conditions all 3 conductors could be carrying a potential so a 3 pole isolator would be the best solution in the meter cubboard in the long run. 

However due to cost I assume a 2 pole isolator in the meter cupboard is where we should start and then in maybe 10 years or so upgrade the BS7671 regulation to a 3 pole isolator. 

Sergio Fernandez said:

Under normal conditions Line will carry most current with Neutral not carrying as much current.

I hope, in a single-phase installation, they carry approximately the same current - in reality barring leakage currents which would ordinarily be at least a couple of orders of magnitude lower than the line conductor current unless everything is switched onto standby and we only have standing leakage currents?

Sergio Fernandez said:

Under fault conditions all 3 conductors could be carrying a potential so a 3 pole isolator would be the best solution in the meter cubboard in the long run. 

I wouldn't recommend switching a protective conductor unless it's absolutely necessary (for example, transfer of supply, or an occasion such as open-PEN protection device).

Sergio Fernandez said:

10 years or so upgrade the BS7671 regulation to a 3 pole isolator

Not sure that would go down too well, or, in general, be wise to use under most circumstances. Not all installations are PME in any case.

Part of the problem is that to disconnect the supply earth it needs to be on the supply side of the equipotential bonding, as things like gas and water pipes, shared with the neighbours, will be bonded not only to your earth, but theirs as well.  It is probably safer not to disconnect the earth as a matter of course, but if really needed, to have some death or glory arrangement, like a 400a fuse, that lets go just before the building burns down, but is otherwise a solid link.  Like switches in the CPC, fuses are not considered good, and are prohibited by regulation  in protective earthing, but are occasionally seen, along with RF chokes, in purely functional earthing paths inside equipment.

Mike.

Can I put the cat amongst the pigeon.

Scenario 1. Single phase in a dwelling working normally
Scenario 2. Single phase in a dwelling working in ISLAND mode

The supply side has now changed.  Is the DNO/DSO/MO worker down the street doing an install or repair still safe?  Could the worker get a shock from a reverse feed?

Can I put the cat amongst the pigeon.

Scenario 1. Single phase in a dwelling working normally
Scenario 2. Single phase in a dwelling working in ISLAND mode

The supply side has now changed.  Is the DNO/DSO/MO worker down the street doing an install or repair still safe?  Could the worker get a shock from a reverse feed?

Sergio Fernandez said:

Scenario 2. Single phase in a dwelling working in ISLAND mode

The supply side has now changed.  Is the DNO/DSO/MO worker down the street doing an install or repair still safe?  Could the worker get a shock from a reverse feed?

Please could you describe how the "reverse feed" might happen under these circumstances (get you from your inverter down the street to the worker, then back to your inverter again to complete the circuit? The live conductors (line and neutral) are disconnected, per BS 7671, but not the protective conductor.

Yes, I can see small amounts of current being shared under very unusual circumstances (electricity usually takes the easiest path), but this is no different to current shared through extraneous-conductive-parts from installations of other DNO distribution circuits that are still energized in the area. Consider that, if we disconnect the distributor's earthing terminal from an islanded installation, extraneous-conductive-parts are still connected, which are:

(a) still connected to the ground, and therefore still connected to the PME earth electrodes, albeit via earth resistances; and
(b) if shared metallic services, still connected to the distributor's earthing arrangements through other installations via main protective bonding.

In summary, in switching the link to the distributor's earthing terminal when moving to island mode, you'd be introducing a potential point of failure in the installations' predominant earthing arrangement (connected mode, via distributor's earthing terminal), for little or no realized benefit.