11 Point test for Safe isolation on 3-phase Electrics in the UK

Chris Pearson said:

Were we not discussing safe isolation?

No, not really. We are discussing "unsafe " isolation, or more pedantically, failed isolation and faulty installations.

And, pedantically, it could be 6 connections and rogue fault currents once the external ground and diverted currents are included...

should add that it's the checking and diagnosis of the safety of the isolation, rather than a presumption of safety through simple procedural compliance....

4 step - much quicker and 100% foolproof.

1. Id cct, & switch off,

2. Disconnect from  fuse/MCCB and Neutral bar. if Neutral present.

3. Disconnect cpc.

4.Insulate disconnected ends.

Done!

I suspect that most UK householders would not like the extra bulk. 3-phase: that's scary stuff.

Not really scary, think of it as 3 lots of single phase and a common neutral..

In some EU countries it is common, for example most German houses have a 3 phase supply (Drehstrom - rotating current) of 32 or 40A per phase, and divvy up one phase per floor except for cookers and larger fixed water heaters that take all 3.

Quite handy for the basement workshop also.

It does require a bit more knowledge for the house basher sparks and the DIYer, but the info is out there.  The neutral cancellation and lower PSSC conspires to give your substations a slightly greater reach.

Mike

and 100% foolproof.

I wouldn't bet my life on that ... that procedure wouldn't have spotted the borrowed neutral I found from the landing light when working on the upstairs lighting circuit (perhaps better described as borrowed L - the landing light was fed from the downstairs lighting circuit L via the hall switch, and upstairs lighting circuit N) - sadly not an uncommon arrangement. Ditto with wall lights and N going to a handy socket rather than back to the original lighting circuit. (They joys of insulated & sheathed singles that were popular up to the 1980s.)

It does raise the point of re-testing after disconnecting N ....

   - Andy.

A lad received a nasty shock when stripping out accessories from an empty house. He thought he was safe as the DNO meter and tails were removed. Unknown to him squatters had patched into the supply next door via an extension lead neatly drilled through window frames.

The foolproof method mentioned above would not have prevented the injury.

My daughter's house was full of death traps: e.g., 3 circuits feeding kitchen sockets, outdoor light with switch downstairs fed from the upstairs circuit.

So, if you cannot identify the circuit with 100% confidence, the only safe approach seems to be to test dead at each accessory. Does anybody do that?

Does anybody do that?

Probably not, but some of us work like it is live even when we are fairly sure it isn't, and that approach (of don't touch the metal if you can avoid it) probably ought to be encouraged!!

I have probably related this tale before as it is from the last century, when I had longer, darker hair and the immortal attitude of youth.  Some 30 odd years ago in a house where I was lodging, I was doing the owner a favour and replacing a non-working hallway light fitting. I duly removed the downstairs lights fuse, and smiled as I verified, as the living room and kitchen plunged into darkness that it was the right one (more fool me..). With that fuse in my back pocket I climbed the wooden steps and removed and replaced the offending fitting. Much to my surprise, when I put the bulb in at the end, it came on in my hand.  Of course unbeknown the power for the downstairs hall light came down from the landing light above, the upstairs lights, which fuse I had left in. My gast had never been more flabbered.... Simultaneous feelings of horror at my stupidity, and relief that my working style meant I had not been bitten , but it could have been a lot worse. 

Don't assume its safe, sometimes it won't be, so in general work like it isn't.
Mike

Chris Pearson said:

My daughter's house was full of death traps: e.g., 3 circuits feeding kitchen sockets, outdoor light with switch downstairs fed from the upstairs circuit.

You'd love the outside light switch in my kitchen. It's a double switch for two lights.

Completely different circuits. One is a fused spur off one of the rings in the kitchen. The other is a switch drop from a light powered from the garage. 

I haven't worked out why there are two runs of SWA from the meter cupboard to the garage. I think one may be for that light only. No idea which breaker feeds it.