Do I need to do anything about single pole Isolator owned by DNO

On a TN system, certainly on the DNO side, there is no need to break the neutral to isolate for work such as changing a meter- indeed on 3 phase it is strongly discouraged. The DNO will not be breaking any of their rules.The customers need to meet BS7671 is met by the switch in the CU, much as it would be if no such isolator was present and you were working with the (single pole!!) DNO fuse pulled to swap the CU.

I suggest no action required. The DNO is likely to respond that they are not obliged to provide means of switched isolation at all, unless you offer to pay for it, and then they  will charge accordingly.

Mike.

Generally DNO responsibility will only be upto the head now. The meter and possibly the isolator will be down to whoever they buying the electric from. The single pole isolator could well be classed as the consumers if after the meter. Best practice is to get a double pole isolator fitted before replacing the consumer unit where possible. 

It's not that uncommon to see SP supplier's isolators - I've even seen pictures of dual tariff supplies with the 24h supply on one pole and the off-peak on the other pole of a DP isolator.

BS 7671 wise there's no need to isolate the N on a TN system (421.6) unless it's the main switch intended to be operated by ordinary persons (household etc.) when the N does need to be switched as well (462.1.201). So it's all down to whether the supplier's switch is being used as the 'main switch' - which is possible, e.g. if there are multiple CUs connected by Henley blocks directly after the supplier's switch, otherwise the CU incomer is likely to be deemed to be the main switch and so it wouldn't appear to be an issue.

   - Andy.

I agree with what Andy says ... but with the further thought that the supplier's equipment is outside the scope of BS 7671. So, provided there's BS 7671-compliant means of isolation within the consumer's installation, I guess that's all that can be done.

Why do you need to isolate a conductor which is connected to all the extraneous-conductive-parts of the installation in a TN earthed installation?

In an installation with PME earthing arrangements, as follows:

• Equipment outdoors (where there is no "equipotential zone", although we don't use that term now).
• Installations with no extraneous-conductive-parts (because all incoming services are non-conductive).
• Where a measurement of external earth fault loop impedance is required (Regulation 643.7.3), or an earth fault loop impedance tester is used to measure the earth electrode resistance of a consumer's earth electrode [permitted by Note to Regulation 411.4.2, and may be used for other purposes, such as described in Regulation 722.411.4.1 (ii)] before energisation (Regulations 643.1 and 643.7.2).

In a TN-S installation ... well, there may be a break in the PE conductor of the supply to the installation.

All things in the "toolbox" for risk assessment according to EAWR ... although with skilled persons at least, the disconnection may be made "after the fact" and this is generally recognised by BS 7671.

In TN-C-S installations, I'm not sure the requirement in dwellings and similar for isolation of the neutral achieves much, though ... as you say.

By my hob and oven in my kitchen there is a double pole switch to isolate both the live and neutral in the supply to them, but when they are isolated the metal cases of the appliances are earthed to the DNO PEN conductor so are hardwired to the neutral.

Similarly in my garage there’s a double pole main switch in the consumer unit that isolates the whole installation, but all the earthing system within the house along with the exposed-conductive-parts and extraneous-conductive-parts are hardwired to the DNO PEN conductor.

There is a double pole switch to isolate the central heating system, but the water and gas pipework is hardwired to the DNO PEN conductor as as the enclosures of the boiler, pump, three port valve, etc.

There is a lot of emphasis on installing double pole switches as over the place, but they only really serve the purpose of achieving isolation from the DNO network in TT earthed installations.

The main purpose of double pole switches is isolating faulty hardwired appliances which are tripping a RCD, other than that they aren’t particularly useful are they?

There was an IET video on YouTube filmed in a meter operative training school showing the dangers of thinking that because you have “isolated” an installation using a double pole switch or even by removing the tails it is safe.

The last time I looked for the video I couldn’t find it, which is a shame because it was one of the best electrical training videos available on the internet.

It may also stem from the fact that many of our continental cousins do not really consider L-N  reversal to be a serious fault on single phase equipment, and so for them double pole disconnection for things like changing a light bulb is a useful thing. Less so here I think, but our regs are to a degree sychronised if we like it or not. (though not our distribution practices)

However DP isolation  is also important if you wish to isolate faulty sections from an RCD protected part of an installation - RCBOs that break the neutral as well as the live (wylex crabtree etc) are a very useful improvement in this regard.

I suggest the DP isolator has its merits, but I agree especially on PME set-ups the neutral is never far away from touch. Personally I'd argue on that basis we ought to permit split concetric to be routed like SWA in TN-x' settings, the regs disagree....

Mike.

Thinking again, perhaps 2-pole isolation has its uses.

Testing of appliances. Can't do ccp to PE insulation resistance test if the Neutral is still connected.

Also, you'll trip RCDs when testing, fault-finding or repairing if you don't disconnect L and N on single-phase.

Finally, Neutral is considered a live conductor according to BS 7671.