Possibly changing the 5 Second Disconnection Limit

Ah well at some point in your lists do cover the UKs unique 110v folly of the 55-0-55 Reduced Low Voltage (RLV) service for building sites.  (sometimes actually 2 phases at 66 volts and 120 degrees apart, but the 110V drill does not care so much)

When you do the sums on that neither RCDs nor fuses seem to be that important.


Note that 5 seconds is as good as forever in terms of electrocution - the big change in the graph is around half the period of a human heartbeat, for very good reasons when we think about how the muscles do their thing.

As far as I know in the UK, at least for the last 50 years or so,  the intention was never that someone would be holding an item with a 5 second disconnection time, as that was not really intended for final circuits supplying earthed portable equipment.

Mike.

But, There is no evidence that 5 seconds is in any way a problem, so why change it? How many deaths last year? None, well worth redesigning the Electricity system for that. Good Luck, but I will vote against it!

My understanding, is that it's a traditional "custom and practice" that's absolutely necessary for selectivity or to prevent devices operating due to inrush or starting currents.


At present, there is no direct evidence to my knowledge of an incident where the 5 s disconnection time has been called into question, and so I support David's view.

It`s not very long since at all we had final ccts at 5.0 or 0.4 seconds and lighting was usually 5.0 except bathrooms etc

A long time ago there wasn't any particular disconnection time required at all in the regs - just a a requirement that an earth fault (of negligible impedance) should produce a fault current of at least 3x the rating of the protective device - so a fault couldn't persist indefinitely, but shock protection (as we now look at it) wasn't really a consideration.


Later it was tightened to 5s - rumour has it that 5s was decided upon because one member of a committee did a back of an envelope calculation based on a (factory?) installation he was responsible for and worked out that was the best it could achive without major alteration. So still no significant mathematical basis for shock protection,


As ebee says, we then had 0.4s - but initially only for final circuits supply hand-held equipment (and perhaps bathrooms and outdoors) - more reliance was placed on bonding rather than ADS for everything else - but the benefits of main bonding were never quantifiable (and often quite small where Ze is low - which became increasingly common where the suppliers provided an earth facility).


Gradually the 0.4s (and later 0.2s for TT) spread to encompass most small final circuits, and additional protection by 30mA RCD similarly increased. But we still have the 'heritage' of longer disconnection times on larger and non-final circuits. As others have noted, the actual risks remaining seem relatively small - chances are when a hand-held appliance suffers an earth fault it's being moved or used - so it's quite likely someone is touching a hand-held appliance when that sort of hazardous event occurs, but Earth faults on distribution circuits are far far less frequent, and much less likely that someone will be holding on to a metallic part at that particular moment. Nothing is 100% - ADS fails if the c.p.c. has previously become open circuit, or possibly if the fault isn't of negligible impedance, even PME earthing poses its own risks of shock (broken CNE supply conductors) so striving for mathematic perfection in disconnection times might be a bit pointless if the bigger picture still undermines it.


   - Andy.

I want to thank everyone here for letting me have this discussion. Freedom of debate is truly a beautiful thing. Trying to have this discussion on US based forums in my experience leads to hostility or being banned. Few want to admit that a safe electrical instalation requires and effective ground fault current path, or that table 250.122 will not be adequate under delayed clearing. Regardless of the persecution I face for wanting to protect life and property, NFPA-70 needs to revise article 250. Since no one is willing to do it, that is where I will come in to fulfill my civic duty toward humanity.

This is a good forum because it is well run. Abuse etc. is not tolerated, and you have a number of very expert people here, some of whom work on the regulations themselves. We also try to present reasonable arguments and information right across the board, from the simplest question to the largest problems. Everyone is allowed to ask or answer and we have a large readership who are silent. Please recommend!


Thanks for your comment and questions.


Regards

David CEng etc.

Indeed, discussion is not normally a problem here, and is usually quite civilised, if sometimes terse,  though some err, "mission creep"  / " topic drift" is not unknown. ? 

I think we can all be guilty of forgetting the assumptions that underpin the way we do stuff, and seemingly 'basic' questions like 'why exactly 5 seconds ?' are often not at all easy to answer, comparisons to other places are often very interesting.

This does not mean we are going to suddenly change the mains voltage to 110v or anything, but it does not hurt to keep the door open to 'but what if ?' 

See you further down the log I hope.
LATE Edit

Looking at 250-122 (I do not normally look at 110V land regs so had to find a copy) it is the CPC size for various breaker ratings. This seems, unlike real cable ratings, to have current rating scaled linearly with area,

so 

6kA calls up 800 kcm ~ 450mm2

and 3kA calls up 400kcm  ~220mm2

1500A calls up 4/0 or about 120mm2

???

This makes ratings for lower currents a bit oversized - 30A (6mm2) or 20A (4mm ) look quite big to UK eyes

 This constant scale factor is not what we would do in the UK  - larger cables do not cool as well as small ones, (though they do have more thermal inertia so yo umay be OK for your long-ish breaking times),  and that on AC the current is not using the full cross-section.

Instead, the preferred approach where we have have no choice but to to throw around that many amps (and really we strongly prefer to get closer to the load at a higher voltage if possible, so high rise buildings may have HV LV transformers part way up) is to go for multiple bunches of thinner conductors in parallel, these would be  grouped into phase triplets 'trefoils'  or phase and neutral 'quarofoils' so the magnetic effects are somewhat cancelling.
    


Mike.