Disconnection time formula?

[JohnW2]

.... but I have been seeing very long 13A extension cables and (very long) temporary installations sometimes in 1.5mm cable with C16 overcurrent protection ...

The extension cable presumably has a 13A BS1362 fuse, and that is deemed to be adequate to protect a 1.5mm² cable.

Yes that's what I'm measuring the L-N loop impedance/PSSC (tester shows both at the same time). Sometimes it's higher than the minimum permitted Ze ...

What do you mean by the "minimum permitted Ze", since, if you actually mean Ze, there is surely no such thing? If you are talking about fault loop impedance (L-E or L-N) then, for a given OPD, there is surely a 'permitted maximum', not minimum?

Yes that's another way to write t = (k2*S2)/I2 I have both in the excel sheet I have been playing around with. It doesn't help with calculating disconnection time from fault current/loop impedance.

Indeed not - but, as I said before, that formula is only valid when adiabatic conditions apply - i.e. only for short disconnection times, certainly no greater than 5 sec. That's why Table 43.1 only gives values of 'k' for disconnection times up to 5 sec.

Kind Regards, John

 

[EFLImpudence]

Zs can be many even hundreds of ohms if the circuit is RCD-protected

I suppose it could be but - no it isn't.
Where have you found a Zs of hundreds of Ohms?
What difference does an RCD make?

but large L-N loop impedances (what I was calling Zx) mean low short circuit fault currents which means longer disconnection times and possible melting cables (you get 4-way adapters with 1mm cable for example).

What you say is factually correct but it doesn't happen. Call the Neutral loop Zn - Zx is not a term in this regard.
What difference does a 4-way adaptor make? The Zs of the socket and lead will either be compliant or not.

I applreciate that no one might check Zs at the end of your vacuum cleaner lead but why are you worrying about it.
The maximum Zs of a 13A fuse is 2.3Ω. With a socket Zs of 0.3Ω, that is a possible 0.75mm² lead of over 40m.

In a properly designed circuit volt drop limits mean you never get near that point but I have been seeing very long 13A extension cables and (very long) temporary installations sometimes in 1.5mm cable with C16 overcurrent protection. Even if the cable isn't going to melt I would still like to know approximately how long a fault will take to clear (without have to get the book out).

Then you will have to memorise a few numbers.

Yes that's what I'm measuring the L-N loop impedance/PSSC (tester shows both at the same time). Sometimes it's higher than the minimum permitted Ze, automatic disconnection has to come before the cable overheats (343.5.2). You have to check the graphs/tables.

Maximum Zs ???, but yes, so what?
I think you must have some fundamental faults somewhere if you are finding these things to worry about.

Yes that's another way to write t = (k2*S2)/I2 I have both in the excel sheet I have been playing around with. It doesn't help with calculating disconnection time from fault current/loop impedance.

It is but it tells you if the conductor is large enough.

 

[JohnW2]

I suppose it could be but - no it isn't. Where have you found a Zs of hundreds of Ohms? What difference does an RCD make?

I can but presume that he was talking about a TT installation in which earth fault protection was provided by an RCD but witht L-N fault (short-circuit) protection still having to be provided by an OPD.

This whole discussion does seem to be more than a little weird!

Kind Regards, John

 

[EFLImpudence]

I did think he was perhaps referring to TT but still, hundreds of Ohms?

But then continues with L-N large impedances.

Zs can be many even hundreds of ohms if the circuit is RCD-protected but large L-N loop impedances (what I was calling Zx) mean low short circuit fault currents which means longer disconnection times and possible melting cables (you get 4-way adapters with 1mm cable for example).

 

[skenk]

I'm not sure why you want to ignore the possibility of L-N faults. One normal designs (or does tests/checks) on the basis of the EFLI, since (in a TN installation) that will virtually always (if there is any T+E involved) be higher than the L-N fault loop impedance - so if one satisfies the disconnection time requirement for L-E faults, one will inevitably also satisfy those requirements for L-N faults

Some installations aren't designed properly, but they are still put in to service and used. For RCD protected circuits the EFLI can be a lot higher than was traditionally permitted (1667 ohms is bandied around). I'm not ignoring the possibility of LN faults my query is about exactly that. For the hypothetical scenarios in question an earth fault would be take care of by the RCD.

With 127A through a 13A BS1362, "virtually instantly" would be an awful lot less than 0.4s - seemingly something around 0.005s according to the curves I have.

Ok fine but the point is the graph has to be looked up, it wont slot into a cell in excel without lots of hassle!

That is a re-arrangement of an adiabatic equation, and hence is only valid if adiabatic conditions apply (i.e. if the disconnection time is less than about 5s) - and I thought the whole point of what you seem to be trying to do was to determine disconnection times which could well be quite prolonged? In any event, even for disconnection times ≤5s, it would not tell you what the temp rise was - merely whether or not it was regarded as 'safe'.

It's the equation from 434.5.2. Yes values of K are for up to 5 seconds. If you had the actual disconnection time you would be able to compare it with the maximum value calculated from the equation. I guess values of k for faults longer than 5s are hard to come by but I am still interested to know how long a fault will take to clear if it is under or around 5 seconds.

 

[skenk]

The extension cable presumably has a 13A BS1362 fuse, and that is deemed to be adequate to protect a 1.5mm² cable.

It was a 1.25 I think, the most typical size for 13A extensions. It was the C16 protected temporary installation that has the 1.5. The relatively common melted cable reel extension proves that the fuse will not protect the cable in all situations.

What do you mean by the "minimum permitted Ze", since, if you actually mean Ze, there is surely no such thing?

yeah typo i meant max Zs!

 

[JohnW2]

It was a 1.25 I think, the most typical size for 13A extensions. .... The relatively common melted cable reel extension proves that the fuse will not protect the cable in all situations.

A 13A fuse will adequately protect a cable whose CCC (as installed/used) is no less than 13A. The CCC of 1.25mm² cable coiled up on a reel is a lot less than 13A, so a 13A fuse will not necessarily protect it against melting.

I really don't see how the calculations you appear to want to do would help at all in that situation.

Kind Regards, John

 

[JohnW2]

For RCD protected circuits the EFLI can be a lot higher than was traditionally permitted (1667 ohms is bandied around).

An EFLI of 7,667Ω is low enough for a 30mA RCD to respond to an L-E fault. However, a Ze above 200Ω is not considered acceptable for a TT electrode (and ≤100Ω really desirable),

I'm not ignoring the possibility of LN faults my query is about exactly that.

Apologies. As should be apparent from what I went on to write, that was a typo (noe corrected). I should have typed "L-E fauklts".

Ok fine but the point is the graph has to be looked up, it wont slot into a cell in excel without lots of hassle!

You don't seem to understand why the magic formulae you so much want could not be produced. What matters is how real OPDs actually perform, and that can only be determined empirically (as per the published graphs). Any formulae one attempted to produce on a theoretical basis would almost certainly fail to reflect accurately how the real-world devices actually perform.

It's the equation from 434.5.2. Yes values of K are for up to 5 seconds. If you had the actual disconnection time you would be able to compare it with the maximum value calculated from the equation. I guess values of k for faults longer than 5s are hard to come by but I am still interested to know how long a fault will take to clear if it is under or around 5 seconds.

Again, you don't seem to understand. It's not just that values of k for longer duration fault currents "are hard to come by". The point/problem is that the equation itself (which, at best, is an approximation) ceases to be valid for currents of longer duration. It is an "adiabatic" equation - which means that it only applies in the very simple situation in which heat is produced (current flows) for such a short period of time that there is no time for any significant 'movement' of heat to occur. If the duration of heat production (current flow) becomes more than a few seconds (usually said to be about 5s), heat will start moving from the conductors, and the calculation becomes dramatically more complicated, because it depends upon such factors as the geometry of the cable, the properties of the insulation and sheath, the orientation and path of the cable, the ambient conditions etc. etc. etc.

Kind Regards, John

 

[skenk]

I suppose it could be but - no it isn't.
Where have you found a Zs of hundreds of Ohms?

I haven't myself but under 200 is considered stable for TT, and I've measured 20-80odd before myself. I've have had a 7ohm Ze (and various other very wrong readings) on TNS supplies. The DNO come out pretty quick and dig up the road usually. I found a 44Ω TT supply protected only by 3x100A fuses once (about 100m length under a park from supply to DB, all signed off with a ze of 44Ω and 100A fuse written on the EIC!). You were saying the Zs being within spec was virtually all that mattered, and I'm pointing out basically exactly what it says at 411.4.4 NOTE 1 first sentence

What difference does an RCD make?

It allows earth loop values to go way higher than was traditionally possible (jeez do i have to spell it out!?) this can result in having to look more closely at L-N disconnection scenarios.

What difference does a 4-way adaptor make? The Zs of the socket and lead will either be compliant or not.

I'm looking at what will happen to the 1mm cable (which might happen to be on the end of a 4-way, at the end of 100m+ of extensions, resulting in a large loop impedance and resultant small fault current) under L-N fault conditions. The Zs could be anything up to 1667Ω and be compliant. If the cable was long enough the circuit may not disconnect at all under short circuit conditions.

I appreciate that no one might check Zs at the end of your vacuum cleaner lead but why are you worrying about it.

I know it's late but I don't think vaccum cleaners have been mentioned? I'm not worrying about it, I was playing around with some formulae in excel and I was wondering if there was one for calculating disconnection time for a given fault current.

The maximum Zs of a 13A fuse is 2.3Ω. With a socket Zs of 0.3Ω, that is a possible 0.75mm² lead of over 40m.

Great but the question was is it possible to calculate disconnection time from fault current, a simple no would have done instead of offering answers to questions which haven't been asked.

I think you must have some fundamental faults somewhere if you are finding these things to worry about.

I'm not worried, just expanding my knowledge and understanding. I have seen some rather long circuits recently though and am looking in to what happens when there is a fault on such a circuit.[/QUOTE]

 

[EFLImpudence]

Ok fine but the point is the graph has to be looked up, it wont slot into a cell in excel without lots of hassle!

Just a small point and not the modern way, I suppose, but why is looking at a graph more onerous than entering details into excel?

 

[skenk]

I suppose it could be but - no it isn't.
Where have you found a Zs of hundreds of Ohms?

I haven't myself but under 200 is considered stable for TT, and I've measured 20-80odd before myself. I've have had a 7ohm Ze (and various other very wrong readings) on TNS supplies. The DNO come out pretty quick and dig up the road usually. I found a 44Ω TT supply protected only by 3x100A fuses once (about 100m length under a park from supply to DB, all signed off with a ze of 44Ω and 100A fuse written on the EIC!). You were saying the Zs being within spec was virtually all that mattered, and I'm pointing out basically exactly what it says at 411.4.4 NOTE 1 first sentence

What difference does an RCD make?

It allows earth loop values to go way higher than was traditionally possible (jeez do i have to spell it out!?) this can result in having to look more closely at L-N disconnection scenarios.

What difference does a 4-way adaptor make? The Zs of the socket and lead will either be compliant or not.

I'm looking at what will happen to the 1mm cable (which might happen to be on the end of a 4-way, at the end of 100m+ of extensions, resulting in a large loop impedance and resultant small fault current) under L-N fault conditions. The Zs could be anything up to 1667Ω and be compliant. If the cable was long enough the circuit may not disconnect at all under short circuit conditions.

I appreciate that no one might check Zs at the end of your vacuum cleaner lead but why are you worrying about it.

I know it's late but I don't think vaccum cleaners have been mentioned? I'm not worrying about it, I was playing around with some formulae in excel and I was wondering if there was one for calculating disconnection time for a given fault current.

The maximum Zs of a 13A fuse is 2.3Ω. With a socket Zs of 0.3Ω, that is a possible 0.75mm² lead of over 40m.

Great but the question was is it possible to calculate disconnection time from fault current, a simple no would have done instead of offering answers to questions which haven't been asked.

I think you must have some fundamental faults somewhere if you are finding these things to worry about.

I'm not worried, just expanding my knowledge and understanding. I have seen some rather long circuits recently though and am looking in to what happens when there is a fault on such a circuit.[/QUOTE]

 

[skenk]

well my last reply is awaiting moderator approval apparently, what's with that?

 

[skenk]

In response to your last question ELFI, excel fits in my pocket, the regs book does not and I find it quicker to enter things into excel than flick through the regs book to remind myself of formulas, rewrite them, and work things out with a pencil and calculator, but each to their own!

 

[JohnW2]

well my last reply is awaiting moderator approval apparently, what's with that?

Only the mods could tell you that ... it presumably must mean that they have been 'watching you' for some reason!

Kind Regards, John

 

[skenk]

nah i think i screwed up a multi-quote and it was a bot wot did it, you're awfully paranoid!