DISCONNECTION TIMES
For TN Systems, a 0.4 s
disconnection time is
required for all final
circuits up to 32 A rating
with a 5 s disconnection
time allowed for
distribution circuits and
final circuits of rating
exceeding 32 A.

Why would a circuit exceeding 32a have a max discon time of 5 sec, compared to a circuit up to 32a having a max discon time of 0.4 sec? Surely more amps means more danger, or more amps would create a quicker discon time, depending on earth resistance?

C

unless protected by an rcd,

Ok cheers

I'm sure you can supply a CCU with a socket outlet on a circuit <32A provided your disconnection times meet 0.4s.  

a lighting circuit is 5 secs and thats under 32a, the way things are worded contradict themselves sometimes. I know a lighting circuit is fixed and doesnt supply portable equipment which is why its 5 sec, but looking at the original statement it should have a 0.4 sec discon time. Showers fall under the 0.4sec as well doesnt it? not 5 sec for fixed equipment, even though it is a piece of fixed equipment (u dont plug ur lawn mower off it do ya) i dont think the 16th stated u had to rcd protect a shower either, but in the manufacturers instructions they said you should! (or was it the other way around?) i know with the 17th all circuits need to be protected by rcd under certain conditions!

There is sooooo much f ing stuff to learn in electrics, so many different terminology, I wish i worked for a better firm, hate domestic!!!!!

The reason for the difference in disconnection times is that fixed applliances and lighting are considered to be less of shock risk to a person.

A Cooker outlet without a 13a s/o should have a disconnection time of 5 secs, if it has a 13 a s/o the disconnection time must be o.4 secs.

The voltage in this country was and is 240volts and it will never change, because to change it would effect the design characteristics of every previous installation and put the onus for any problems caused onto the supply companies.

I have read the 17th edition and think it is a document produced by politicians who think there will is our command. The great danger is that it can be ridiculed in many ways and will end up being completely ignored.

Seriously,

I'd like to know

1)why last year's Zs values have been reduced.

2) What the physical effects are if we just carry on complying with the old Zs values?

3) Why the mains voltage value has been changed (when we all know that we STILL get 240v)

Reasoned answers only please.

TT

All very straightforward, TT.

1. Zs values now based on Uo, not Uoc.  

Why has it been changed?


2. Insignificant, provided you actually understand why you do what you do.

Which is exactly why I am asking the question.

So if it is insignificant as you put it, then why bother changing it?

3. It hasn't been changed. The nominal voltage has been 230V since 1995.

That isn't the figure originally quoted in the Brown Book, nor is it the value I measure on a regular basis, so who is kidding who?

To me, none of this makes any sense at all, but it does rather reek of Europe voiding fecal material all over what were, at one time, reasonably argued and straight forward regulations.

TT

Just to confuse matters further (well, confusion one of the few things I'm good at, so I might as well stay in practice - the choice of Uo or Uoc is partly political.
In the 'good old days' when mains was real mains - i.e. 240/415V - all calculations were done at 240. AFAIK there was no concept in the regs of Uoc in those days.

Then came voltage harmonization and our declared voltage became 230V. In fine Euro style nothing of consequence changed, no change of taps at the substation, no lowering of HV network voltages, nothing. Just the bit of paper now said 230V +10% -6% instead of 240V +/- 6%. By mathematical slight of hand almost all of the old voltage range is contained within the new one.

But, and this is an awkward but, if you re-did your 240V calculations at 230V, you'd find that all your Zs tables would now be wrong, some installations that were previously OK when calculated at 240V would now not comply with the regulations. It can be difficult to "sell" changes in the regulations to the wider world at the best of times, but how do you explain a claim that a circuit that was perfectly safe yesterday is unsafe today, when the only thing that has changed is a couple of digits on a piece of paper?

Answer? - easy, just carry on using 240V for Zs calculations just like before!
Come up with a new concept to explain it away, and Hey Presto! problem solved.

If we were really going to use Uoc, wouldn't 253V be a more realistic value? Is Uoc of 240V a safe assumption if we were dealing with a supply that could really go as low as 230V-6% = 216.2V at the whim of the supply company?

The more things change, the more they remain the same, so ferk em, I shall continue to deal in Real World Numbers and just record the fact I have used 240V on the 'Departures' form,  along with Zs values based on this number, since as 'Designer', I believe this to be a more accurate reflection of Real World Physics.

Ferk the Euro- Harmonisation shiiite

TT

This was my point earlier in the year for installations that operated through a 'voltage optimiser' (www.powerperfector.co.uk)

If loop tests were carried out as part of a PIR and the new measured efli figure was higher than that permitted in the 17th but still complied with that in the 16th, what would happen ?

Paul