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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> Disconnection times https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1216552965 Message started by cosbycarl on Jul 20^th, 2008, 12:22pm

Title: Disconnection times
Post by cosbycarl on Jul 20^th, 2008, 12:22pm

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

Title: Re: Disconnection times
Post by RabbitRabbit on Jul 20^th, 2008, 2:44pm

Because in typical cases 32A is used for 'portable appliances' whereas 45A is used, typically, for fixed wired devices (showers and cookers etc.) Thus where a 45A is used to feed say an cooker circuit one should NOT install a CCU having a 13A socket.

Title: Re: Disconnection times
Post by cosbycarl on Jul 20^th, 2008, 4:31pm

unless protected by an rcd,

Ok cheers

Title: Re: Disconnection times
Post by RabbitRabbit on Jul 20^th, 2008, 6:28pm

With or without RCD protection.

Title: Re: Disconnection times
Post by LSpark on Jul 20^th, 2008, 6:42pm

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

Title: Re: Disconnection times
Post by RabbitRabbit on Jul 20^th, 2008, 7:18pm

wrote on Jul 20^th, 2008, 6:42pm:

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

You can, the issue being discussed, I think ;), was disconnection times of 5 sec on ccts protected by MCB's greater than 32A LS.

Title: Re: Disconnection times
Post by cosbycarl on Jul 20^th, 2008, 7:26pm

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!!!!!

Title: Re: Disconnection times
Post by The_Trician on Jul 20^th, 2008, 7:36pm

I'd like to know about this - I don't have a new 7671 book yet, nor have I been on any new course.

Changes to permissible Volt drop values.

It seems that much of this new volt-drop bollerks is down to the Europhiles who have taken over the IET
I have yet to measure less than 240V at any supply terminals I have ever worked on.

So what's this 230V rubbish all about?

Disconnection times & reduced permissible Zs values.

Are we now saying that the protective devices in a 16th ed install won't disconnect in time because Zs has now been lowered due to the value of the nominal voltage value being changed?

Has anyone bothered to tell the mcbs that?

I'm really confused about this, but it literally means that any ctt protected by a mcb and whose disconnection time was to the old value, supposedly won't disconnect in time to clear a fault.

It seems to me that the 16th, which had more lenient values, is now undermined to a point where previously compliant installs are now supposedly dangerous.

Please tell me I'm bloody well wrong, and that it is just those wankers from Europe having a Silly-Ideas Session at the IET.

TT

Title: Re: Disconnection times
Post by sparxxxx on Jul 20^th, 2008, 8:16pm

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.

Title: Re: Disconnection times
Post by LSpark on Jul 21^st, 2008, 8:12pm

wrote on Jul 20^th, 2008, 7:18pm:

wrote on Jul 20^th, 2008, 6:42pm:

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

You can, the issue being discussed, I think ;), was disconnection times of 5 sec on ccts protected by MCB's greater than 32A LS.

To which you said a CCU with a socket outlet must not be installed ona circuit rated higher than 32A because it's 5second.

I get the feeling we've crossed lines, the regulation stating 5-seconds is there to make it easier for distribution circuits and the like to comply with the impedances to my knowlege, it's not there to impede you having a socket outlet on a circuit rated that high so long as it complies with the reduced disconnection time.

Title: Re: Disconnection times
Post by The_Trician on Jul 21^st, 2008, 11:05pm

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

Title: Re: Disconnection times
Post by dingbat on Jul 22^nd, 2008, 7:23am

All very straightforward, TT.

1. Zs values now based on Uo, not Uoc.
2. Insignificant, provided you actually understand why you do what you do.
3. It hasn't been changed. The nominal voltage has been 230V since 1995.

... do keep up at the back. ;D

Title: Re: Disconnection times
Post by The_Trician on Jul 22^nd, 2008, 9:21am

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

Title: Re: Disconnection times
Post by dingbat on Jul 22^nd, 2008, 9:55am

The_Trician wrote on Jul 22^nd, 2008, 9:21am:

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?

Brown book page 192.

Title: Re: Disconnection times
Post by The_Trician on Jul 23^rd, 2008, 8:41am

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

Title: Re: Disconnection times
Post by dingbat on Jul 23^rd, 2008, 10:23am

The_Trician wrote on Jul 23^rd, 2008, 8:41am:

Just to confuse matters further...

To paraphrase Al Murray, surely you were never confused, TT? ;D ;D

Title: Re: Disconnection times
Post by Goodsparks on Jul 23^rd, 2008, 3:55pm

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