Dad has a kiln in a workshop at the bottom of his garden, rated at
11.25kW, so maximum current 49A assuming 230V (perhaps notionally 47A
given its age, the rating plate mentions 240V/415V rather than
230V/400V, it can be wired for 1ph or 3ph, he runs it on 1ph).

The main part of the run from the house is 35m of 16mm^2 SWA, so
expected to give a voltage drop around 5V.

He's keen to reduce the voltage drop (thinks this will reduce the duty
cycle, I suppose it will, but I have my doubts if it will be a
significant effect).

I know it is normally frowned on to parallel-up conductors to give
increased current carrying capacity ...

But given that the SWA that is installed actually is four core, would it
still be bad practice to parallel the live and neutral cores *just* to
achieve equivalent of 32mm^2 thereby reducing voltage drop? To clarify,
the MCB(s) would not be uprated, just done to halve the current in each
core, if somehow one core were to go open-ciruit, the current in the
remaining core would end up the same as it presently is.

I can envisage that physically getting 32mm^2 of strands securely into
the henley blocks at the house end and the terminals of the consumer
unit in the workshop might be a challenge.

This would also involve switching the CPC from one of the cores to the
steel armour. MCB trip time calculations certainly aren't in my day
job, given this is not handheld equipment and is effectively a submain,
does a 5s disconnect time apply, rather than 0.4s?

I can see that 35m of 16mm^2 gives
R1 as 0.403 ohm for single core (or 0.202 ohm for double cores)
R2 as 1.085 ohm
PME supply assumed Ze 0.35 ohm

Assume worst case where one core has failed
prospective fault current = 230V / (0.35 + 0.403 + 1.085) = 125A

For a 50A type C MCB, a quick glance at the curves, I think this gives a
disconnect time of approximately 70 seconds, oh dear, I suppose that
alone is enough to rule out the changes he has in mind !!!

Seeing that, I went on to check the existing situation which uses one
16mm^2 core each for live, neutral and CPC
fault current = 230V / (0.35 + 0.403 + 0.403) = 198A
from the 50A type C curve this still gives a disconnect time of 20s !

So even the existing CPC doesn't meet the disconnect time for a 50A type
C MCB, even getting an actual measurement of Ze rather than assuming
0.35 ohms, couldn't possibly result in a low enough Zs to allow a type C
MCB.

But reverting to type B would meet the disconnect time (I think he only
recently changed to type C at the house end because of poor fault
discrimination).

OK, so I think the disconnect time issue has trumped the original
question about using parallel conductors, any thoughts (or mistakes in
my calculations)?

Andy Burns wrote:

the existing situation which uses one
16mm^2 core each for live, neutral and CPC
fault current = 230V / (0.35 + 0.403 + 0.403) = 198A
reverting to type B would meet the disconnect time

Ooops, I think I picked up the wrong curve on the type B MCB graph, 198A
would still take 20s to trip a 50A type B (the same time as a 50A type
C, is that correct?)

So is the only way to make this safe change to a 50A fuse instead of MCB?

On 21/11/2011 02:47, Andy Burns wrote:
Dad has a kiln in a workshop at the bottom of his garden, rated at
11.25kW, so maximum current 49A assuming 230V (perhaps notionally 47A
given its age, the rating plate mentions 240V/415V rather than
230V/400V, it can be wired for 1ph or 3ph, he runs it on 1ph).

The main part of the run from the house is 35m of 16mm^2 SWA, so
expected to give a voltage drop around 5V.

He's keen to reduce the voltage drop (thinks this will reduce the duty
cycle, I suppose it will, but I have my doubts if it will be a
significant effect).

Well its not going to be much even if you can halve the drop...

I know it is normally frowned on to parallel-up conductors to give
increased current carrying capacity ...

The reason for this is that it can be very difficult to ensure fault
protection for the individual conductors. Since in this scenario you are
not using the parallel conductors to achieve a higher design current,
but only reduce the voltage drop, its not going to be an issue here
(assuming the fault protection is adequate now!)

But given that the SWA that is installed actually is four core, would it
still be bad practice to parallel the live and neutral cores *just* to
achieve equivalent of 32mm^2 thereby reducing voltage drop? To clarify,
the MCB(s) would not be uprated, just done to halve the current in each
core, if somehow one core were to go open-ciruit, the current in the
remaining core would end up the same as it presently is.

While a little non standard, I can't see any particular problem so long
as everything is labelled clearly, so a future maintainer can see what
is going on.

I can envisage that physically getting 32mm^2 of strands securely into
the henley blocks at the house end and the terminals of the consumer
unit in the workshop might be a challenge.

Ought not be a problem - 35mm^2 tails are not uncommon.

This would also involve switching the CPC from one of the cores to the
steel armour.

Nothing wrong with that. (I assume that the armour is connected to the
CPC anyway, even if paralleled with a core?)

MCB trip time calculations certainly aren't in my day job,
given this is not handheld equipment and is effectively a submain, does
a 5s disconnect time apply, rather than 0.4s?

Yup, OSG 3.5.2 (411.3.2.3 BS7671) would apply as the circuit exceeds 32A

I can see that 35m of 16mm^2 gives
R1 as 0.403 ohm for single core (or 0.202 ohm for double cores)
R2 as 1.085 ohm

Erm, well its late/early and I might be not thinking right, but it seems
to be you are a tad pessimistic on those figures.

16mm^2 should have a resistance of 1.15 mOhm/m, so R1 would be that
times 35, or 0.0403 Ohms - i.e. an order of magnitude better.

R2 for the armour would be 50 mm^2 of steel with a resistance of 3.1
mOhms/m - or 0.1085 ohms.

PME supply assumed Ze 0.35 ohm

With high current circuits, its probably better to actually measure
rather than work from the worst case standardised figures, since in
reality the value is often significantly lower, and this can make design
simpler.

Assume worst case where one core has failed
prospective fault current = 230V / (0.35 + 0.403 + 1.085) = 125A

However redoing that with the above figures: 230 / (0.35 + 0.04 + 0.11)
= 460 which is much closer.

Here is where you really want to use a lower measured Ze... ;-)

For a 50A type C MCB, a quick glance at the curves, I think this gives a
disconnect time of approximately 70 seconds, oh dear, I suppose that
alone is enough to rule out the changes he has in mind !!!

Well... if we go with the 460A, then that just about scrapes in at 5
secs on:

http://wiki.diyfaq.org.uk/images/d/d...e-MCBTypeC.png

You did not specify the material the SWA was insulated with, but if we
take it as a 90 degree C XLPE material, then we can check the adequacy
of the CPC with the adiabatic equation and a k factor of 143:

s = sqrt( 460^2 x 5 ) / 143 = 7.19 mm^2

Since we have a copper equivalent of 50 / 2.255 = 22mm^2 then we are
well in.

(Is this cable also exporting an equipotential zone to the room with the
kiln? Because that will complicate things if it needs to be a main
bonding conductor as well!)

Seeing that, I went on to check the existing situation which uses one
16mm^2 core each for live, neutral and CPC
fault current = 230V / (0.35 + 0.403 + 0.403) = 198A
from the 50A type C curve this still gives a disconnect time of 20s !

The dual core solution would perform better - even with the loss of one
core (since the loss will likely only affect half of the round trip).

So even the existing CPC doesn't meet the disconnect time for a 50A type
C MCB, even getting an actual measurement of Ze rather than assuming
0.35 ohms, couldn't possibly result in a low enough Zs to allow a type C
MCB.

It might not be as bad as that, since if you are using the armour in
parallel with a core then you have a better result than above.

But reverting to type B would meet the disconnect time (I think he only
recently changed to type C at the house end because of poor fault
discrimination).

Fault discrimination with what?

OK, so I think the disconnect time issue has trumped the original
question about using parallel conductors, any thoughts (or mistakes in
my calculations)?



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 21/11/2011 02:47, Andy Burns wrote:
Dad has a kiln in a workshop at the bottom of his garden, rated at
11.25kW, so maximum current 49A assuming 230V (perhaps notionally 47A
given its age, the rating plate mentions 240V/415V rather than
230V/400V, it can be wired for 1ph or 3ph, he runs it on 1ph).

The main part of the run from the house is 35m of 16mm^2 SWA, so
expected to give a voltage drop around 5V.

He's keen to reduce the voltage drop (thinks this will reduce the duty
cycle, I suppose it will, but I have my doubts if it will be a
significant effect).

Could you simply measure the voltage at each end of the cable, under
load...?
That would tell you what the actual volt drop is.
Then you'd know if it was worth worrying about...

I've heard that kiln elements go higher resistance with age - if it's
really old then it might be worth replacing the element(s) - though that
could result in an increased voltage drop.

What is it about the installation that is worrying your Dad ?
Adrian

On Nov 21, 7:44*am, Adrian Brentnall wrote:
On 21/11/2011 02:47, Andy Burns wrote:

Dad has a kiln in a workshop at the bottom of his garden, rated at
11.25kW, so maximum current 49A assuming 230V (perhaps notionally 47A
given its age, the rating plate mentions 240V/415V rather than
230V/400V, it can be wired for 1ph or 3ph, he runs it on 1ph).

The main part of the run from the house is 35m of 16mm^2 SWA, so
expected to give a voltage drop around 5V.

He's keen to reduce the voltage drop (thinks this will reduce the duty
cycle, I suppose it will, but I have my doubts if it will be a
significant effect).

Could you simply measure the voltage at each end of the cable, under
load...?

that method is asking for wrong results. Run a wire temporarily so you
can measure the Vdrop directly with a multimeter.

That would tell you what the actual volt drop is.
Then you'd know if it was worth worrying about...

I've heard that kiln elements go higher resistance with age - if it's
really old then it might be worth replacing the element(s) - though that
could result in an increased voltage drop.

What is it about the installation that is worrying your Dad ?
Adrian


NT

On Mon, 21 Nov 2011 06:39:17 +0000, John Rumm wrote:

Dad has a kiln in a workshop at the bottom of his garden, rated at
11.25kW, so maximum current 49A assuming 230V
snip
The main part of the run from the house is 35m of 16mm^2 SWA, so
expected to give a voltage drop around 5V.

He's keen to reduce the voltage drop (thinks this will reduce the
duty
cycle, I suppose it will, but I have my doubts if it will be a
significant effect).

Well there is 250W (ish) of loss in the cable out of 11250W or about
2% is that worth worrying about?

I'd measure the drop to start with to see what reality is. Just need
a meter at the kiln measure the volts with it off and then with it
on. The load might change as the kilns gets up to working temperature
so might be best to monitor the volts through a fireing.

--
Cheers
Dave.

All this does sound like overkill to me considering the minimal gains.
I'd not want to be the person paying the bill though!

Brian

--
Brian Gaff -
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
Blind user, so no pictures please!
"Dave Liquorice" wrote in message
ll.co.uk...
On Mon, 21 Nov 2011 06:39:17 +0000, John Rumm wrote:

Dad has a kiln in a workshop at the bottom of his garden, rated at
11.25kW, so maximum current 49A assuming 230V
snip
The main part of the run from the house is 35m of 16mm^2 SWA, so
expected to give a voltage drop around 5V.

He's keen to reduce the voltage drop (thinks this will reduce the
duty
cycle, I suppose it will, but I have my doubts if it will be a
significant effect).

Well there is 250W (ish) of loss in the cable out of 11250W or about
2% is that worth worrying about?

I'd measure the drop to start with to see what reality is. Just need
a meter at the kiln measure the volts with it off and then with it
on. The load might change as the kilns gets up to working temperature
so might be best to monitor the volts through a fireing.

--
Cheers
Dave.

In article ,
NT writes:
On Nov 21, 7:44*am, Adrian Brentnall wrote:
On 21/11/2011 02:47, Andy Burns wrote:

Dad has a kiln in a workshop at the bottom of his garden, rated at
11.25kW, so maximum current 49A assuming 230V (perhaps notionally 47A
given its age, the rating plate mentions 240V/415V rather than
230V/400V, it can be wired for 1ph or 3ph, he runs it on 1ph).

The main part of the run from the house is 35m of 16mm^2 SWA, so
expected to give a voltage drop around 5V.

He's keen to reduce the voltage drop (thinks this will reduce the duty
cycle, I suppose it will, but I have my doubts if it will be a
significant effect).

Could you simply measure the voltage at each end of the cable, under
load...?
that method is asking for wrong results. Run a wire temporarily so you
can measure the Vdrop directly with a multimeter.

You can meaure it entirely at the kiln end, by switching the kiln
on and off and looking at the difference.

You really want professional (fused) test leads for this though,
or if you have a socket too, use a plug-in power meter on its
voltage setting.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

John Rumm wrote:
On 21/11/2011 02:47, Andy Burns wrote:
Dad has a kiln in a workshop at the bottom of his garden, rated at
11.25kW, so maximum current 49A assuming 230V (perhaps notionally 47A
given its age, the rating plate mentions 240V/415V rather than
230V/400V, it can be wired for 1ph or 3ph, he runs it on 1ph).

The main part of the run from the house is 35m of 16mm^2 SWA, so
expected to give a voltage drop around 5V.

He's keen to reduce the voltage drop (thinks this will reduce the
duty cycle, I suppose it will, but I have my doubts if it will be a
significant effect).

Well its not going to be much even if you can halve the drop...

I would start by measuring Ze. Unless it is really low or the kiln is
running 24/7 then I would not bother trying to lower the voltage drop.

I'll have to check the calcs later, but it seems that they do meet the 5
seconds disconnection time.

--
Adam

* Sometimes I like to lay in my neighbours garden and pretend to be a
carrot *

John Rumm wrote:

Well its not going to be much even if you can halve the drop...

Yes, thats what I though

Since in this scenario you are
not using the parallel conductors to achieve a higher design current,
but only reduce the voltage drop, its not going to be an issue

thanks

I assume that the armour is connected to the
CPC anyway, even if paralleled with a core?

I think so, long time since I looked at the setup

I can see that 35m of 16mm^2 gives
R1 as 0.403 ohm for single core (or 0.202 ohm for double cores)
R2 as 1.085 ohm

Erm, well its late/early and I might be not thinking right, but it seems
to be you are a tad pessimistic on those figures.

Ah, the table I used was ohms/km rather than milliohms/m, and it was
even later/earlier when I tried it, so I think zero hit the deck
somewhere, I did say I wasn't used to the calcs!

With high current circuits, its probably better to actually measure
rather than work from the worst case standardised figures,

Yes I appreciate that, but a) it's 50 miles away, b) I don't have the
kit, c) with my wrong numbers it looked like even if it was zero it
wouldn't help

redoing that with the above figures: 230 / (0.35 + 0.04 + 0.11)
= 460 which is much closer.

Here is where you really want to use a lower measured Ze... ;-)

OK.

if we go with the 460A, then that just about scrapes in at 5
secs

sounds better

You did not specify the material the SWA was insulated with, but if we
take it as a 90 degree C XLPE material, then we can check the adequacy
of the CPC with the adiabatic equation and a k factor of 143:

I had assumed it was XLPE, he's not replied to a few of my Q's yet.

s = sqrt( 460^2 x 5 ) / 143 = 7.19 mm^2

Since we have a copper equivalent of 50 / 2.255 = 22mm^2 then we are
well in.

I figured the 22m^2 equivalent, but wasn't aware of the adiabatic
equation at all.

(Is this cable also exporting an equipotential zone to the room with the
kiln? Because that will complicate things if it needs to be a main
bonding conductor as well!)

Yes, I don't think he has an earth rod for the workshop, can't remember
if he has plumbed water down there either ...

It might not be as bad as that, since if you are using the armour in
parallel with a core then you have a better result than above.

Ah yes forgot to count that.

But reverting to type B would meet the disconnect time (I think he only
recently changed to type C at the house end because of poor fault
discrimination).

Fault discrimination with what?

He has 50A type B MCB in the C/U down at the workshop end, and was
getting trips, but the house end was tending to trip every time, so he
replaced house end with a type B. In the end he found the reason for
that (elements wired in parallel that should have been in series, were
tripping after several hours, I think his controller uses slow start to
build up to full heat overnight).

OK, so I think the disconnect time issue has trumped the original
question about using parallel conductors, any thoughts (or mistakes in
my calculations)?

Thanks again John.

Dave Liquorice wrote:

Well there is 250W (ish) of loss in the cable out of 11250W or about
2% is that worth worrying about?

I don't know, I think he's under the impression that the elements are
ending up running at full load for long periods, he started off thinking
he had over 10V of drop, I think he's probably only got about 5V anyway,
and could maybe get it down to 2-3V ... I'm dubious about what effect
it'll have

I'd measure the drop to start with to see what reality is. Just need
a meter at the kiln measure the volts with it off and then with it
on. The load might change as the kilns gets up to working temperature
so might be best to monitor the volts through a fireing.

Yes, he's asked me to take a meter with me when I next visit, I'm
tempted to take a spare UPS, and monitor it from a gash laptop overnight

Brian Gaff wrote:

All this does sound like overkill to me considering the minimal gains.
I'd not want to be the person paying the bill though!

He assures mum that it doesn't take much once it gets up to temperature,
but then he tells me a slightly different story, I've given him my old
CurrentCost clamp meter, but he's never told me how much a firing costs ...

He does sell some of his work, and makes other work to order, but I
suspect it's not fully costed!

Adrian Brentnall wrote:

What is it about the installation that is worrying your Dad ?

He thinks it will be struggling to sustain top temperature, and
shortening the life of elements more quickly in the process of trying.

Andrew Gabriel wrote:

You can meaure it entirely at the kiln end, by switching the kiln
on and off and looking at the difference.

Not sure if his controller insists in ramping up over several hours, I
realise the same measurement still applies, just less convenient to take
readings,

You really want professional (fused) test leads for this though,
or if you have a socket too, use a plug-in power meter on its
voltage setting.

I've conveniently ignored the lighting circuit (lower tolerance on
voltage drop, probably scrapes in even with existing configuration) and
sockets (I think he avoids running the pugger on its 1ph-3ph convertor
while the kiln is running and I doubt the wheel uses much power).

Anyway, yes there is a socket circuit, so I could leave a meter (or UPS)
plugged in there rather than poking about with unfused leads.

Dave Liquorice wrote:
On Mon, 21 Nov 2011 06:39:17 +0000, John Rumm wrote:

Dad has a kiln in a workshop at the bottom of his garden, rated at
11.25kW, so maximum current 49A assuming 230V
snip
The main part of the run from the house is 35m of 16mm^2 SWA, so
expected to give a voltage drop around 5V.

He's keen to reduce the voltage drop (thinks this will reduce the
duty cycle, I suppose it will, but I have my doubts if it will be a
significant effect).

Well there is 250W (ish) of loss in the cable out of 11250W or about
2% is that worth worrying about?

You could turn that into underfloor heating:-)

--
Adam

* Sometimes I like to lay in my neighbours garden and pretend to be a
carrot *

ARWadsworth wrote:

I would start by measuring Ze.

That's easy for you, with your shiny new toy. I only have a multimeter
(not true RMS) and an ancient wind-up Robin insulation/continuity tester
(nearly a decade out of calibration).

Unless it is really low or the kiln is
running 24/7 then I would not bother trying to lower the voltage drop.

It's a bee he's got in his bonnet ... If I can persuade him it'll only
make a 2-3V improvement he might drop the idea, but then again, since it
seems allowable to double-up in these circumstances, he might want to
suck it and see.

I'll have to check the calcs later, but it seems that they do meet the 5
seconds disconnection time.

Thanks, It did seem counter-intuitive that a dead short on such a fat
cable (even of that length) wouldn't pop the breaker in under a minute,
serves me right for trying to think at 3am.

On Nov 21, 10:38*am, Andy Burns wrote:
Adrian Brentnall wrote:
What is it about the installation that is worrying your Dad ?

He thinks it will be struggling to sustain top temperature, and

He thinks it will. Is it or isnt it?

shortening the life of elements more quickly in the process of trying.

Running them undervoltage extends their life.


NT

Andy Burns wrote:
ARWadsworth wrote:

I would start by measuring Ze.

That's easy for you, with your shiny new toy. I only have a
multimeter (not true RMS) and an ancient wind-up Robin
insulation/continuity tester (nearly a decade out of calibration).

And a clamp meter?

You can guestimate the Ze.

Measure the voltage at the house CU with no load. Then measure the voltage
at the house CU and with a known load. Ze = (Vnoload-Vload)/current

It's how I calculated Tim Watts Ze and was within 0.02 ohms.


Unless it is really low or the kiln is
running 24/7 then I would not bother trying to lower the voltage
drop.

It's a bee he's got in his bonnet ... If I can persuade him it'll only
make a 2-3V improvement he might drop the idea, but then again, since
it seems allowable to double-up in these circumstances, he might want
to suck it and see.

The problem is that if you have a Ze of say 0.15 ohms then there will always
be a voltage drop
of around 7V on your incoming supply with a 47A load even if you moved the
kiln to next to the CU. I realise that you do not pay for this voltage drop
as you would on your SWA cables.

Now if your Ze is 0.02 ohms it would be worth doing something about the SWA
voltage drop.

--
Adam

* Sometimes I like to lay in my neighbours garden and pretend to be a
carrot *

ARWadsworth wrote:

Andy Burns wrote:

That's easy for you, with your shiny new toy. I only have a
multimeter

And a clamp meter?
You can guestimate the Ze.

I could borrow one, I was thinking you'd need the proper kit to measure
it, rather than estimating it that way.

Now if your Ze is 0.02 ohms it would be worth doing something about the SWA
voltage drop.

Yes, I see ...

NT wrote:

On Nov 21, 10:38 am, Andy wrote:

Adrian Brentnall wrote:

What is it about the installation that is worrying your Dad ?

He thinks it will be struggling to sustain top temperature, and

He thinks it will. Is it or isnt it?

Dunno, will have to ask. I think he has decent thermocouple to measure
that, a while back he was disappointed with repeatability of results,
maybe this all stems from trying to 'fix' that.

shortening the life of elements more quickly in the process of trying.

Running them undervoltage extends their life.

I think he's concerned that they're on for a longer duty cycle, I don't
know if that would tend to cancel out the lower power from being
undervolted?

On 21/11/2011 10:38, Andy Burns wrote:
Adrian Brentnall wrote:

What is it about the installation that is worrying your Dad ?

He thinks it will be struggling to sustain top temperature, and
shortening the life of elements more quickly in the process of trying.


I'd say it was unlikely that a few volts drop would make any significant
difference..

I'm guessing it's a pottery kiln ?
Does it have a temperature controller, or does it rely on cones to
control the firing ?

My glass kiln (computer-controlled - and smaller at only 14kw) uses a
surprisingly-small amount of electricity per firing - as the heating
elements are turned on and off to achieve the required ramp and hold times.

I would think that he's probably worrying about nothing... g

Adrian

On 21/11/2011 10:38, Andy Burns wrote:
Adrian Brentnall wrote:

What is it about the installation that is worrying your Dad ?

He thinks it will be struggling to sustain top temperature, and
shortening the life of elements more quickly in the process of trying.


forget that about the power on my kiln - it's 3kw or thereabouts -
duty cycle comments still apply!
Adrian

On Mon, 21 Nov 2011 12:12:57 +0000, Andy Burns wrote:

I think he has decent thermocouple to measure that, a while back he was
disappointed with repeatability of results,

Is there some other factor affecting the performance of the kiln.
Like it being windier one day compared to another and thus more heat
loss from the shed.

--
Cheers
Dave.

Adrian Brentnall wrote:

I'm guessing it's a pottery kiln ?

It is.

Does it have a temperature controller, or does it rely on cones to
control the firing ?

Has a controller, he uses it to ramp up over several hours so that peak
load should be on E7 rates, he's talked about the thermocouple for it,
and calibrating using cones.

My glass kiln (computer-controlled - and smaller at only 14kw)

larger, his is 11kW

uses a
surprisingly-small amount of electricity per firing - as the heating
elements are turned on and off to achieve the required ramp and hold times.

Yes, it is quite well insulated, as I mentioned I gave him an energy
meter which can clip on the tails from the Henley blocks, but he's never
mentioned how much it costs per firing ... though it did show him the
cost of leaving the pond pump/UV treatment on 24x7

I would think that he's probably worrying about nothing... g

Maybe, I was more concerned that he doesn't do anything daft with the
wiring, that what results that achieves g

Adrian Brentnall wrote:

forget that about the power on my kiln - it's 3kw or thereabouts -
duty cycle comments still apply!

Yeah, no probs.

When he upgraded the controller a couple of years back to one that uses
external solid state relays (DIN rail mounted TRIACs or similar I
suppose) they recommended special fast acting fuses to protect them,
turned out the fuses were more expensive than replacing the 'relays'

Dave Liquorice wrote:

Is there some other factor affecting the performance of the kiln.
Like it being windier one day compared to another and thus more heat
loss from the shed.

Possible, it's a metal skin building, ply-boarded internally but I think
kingspan (or equivalent) between, alu double glazed windows.
Doors and draughts are probably the weakest link, the kiln is nearish a
door FWIR.

In article ,
Andy Burns writes:
Andrew Gabriel wrote:

You can meaure it entirely at the kiln end, by switching the kiln
on and off and looking at the difference.

Not sure if his controller insists in ramping up over several hours, I
realise the same measurement still applies, just less convenient to take
readings,

You could use another known load, such as a kettle and a 2kW heater,
to work out what the supply impedance is at the kiln, or at least to
cross-check the results if you think the kiln might not have been
running at full blast.

You really want professional (fused) test leads for this though,
or if you have a socket too, use a plug-in power meter on its
voltage setting.

I've conveniently ignored the lighting circuit (lower tolerance on
voltage drop, probably scrapes in even with existing configuration) and
sockets (I think he avoids running the pugger on its 1ph-3ph convertor
while the kiln is running and I doubt the wheel uses much power).

If you're using CFLs or fluorescents with electronic control gear,
that won't notice. Filament lamps OTOH will amplify the effect.

Anyway, yes there is a socket circuit, so I could leave a meter (or UPS)
plugged in there rather than poking about with unfused leads.


--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On 21/11/2011 10:28, Andy Burns wrote:

With high current circuits, its probably better to actually measure
rather than work from the worst case standardised figures,

Yes I appreciate that, but a) it's 50 miles away, b) I don't have the
kit, c) with my wrong numbers it looked like even if it was zero it
wouldn't help

Yup, I appreciate the logic of c ;-)

Re b, then an estimate with current and voltage drop will do. Easiest if
you have a clamp meter.

redoing that with the above figures: 230 / (0.35 + 0.04 + 0.11)
= 460 which is much closer.

Here is where you really want to use a lower measured Ze... ;-)

OK.

if we go with the 460A, then that just about scrapes in at 5
secs

sounds better

You did not specify the material the SWA was insulated with, but if we
take it as a 90 degree C XLPE material, then we can check the adequacy
of the CPC with the adiabatic equation and a k factor of 143:

I had assumed it was XLPE, he's not replied to a few of my Q's yet.

Even if you use a 70 degree PVC k factor of 100, then its still big
enough, so it does not really matter in this case.

s = sqrt( 460^2 x 5 ) / 143 = 7.19 mm^2

Since we have a copper equivalent of 50 / 2.255 = 22mm^2 then we are
well in.

I figured the 22m^2 equivalent, but wasn't aware of the adiabatic
equation at all.

Bit more detail he

http://wiki.diyfaq.org.uk/index.php?...abatic _Check

and here

http://wiki.diyfaq.org.uk/index.php?...e#CPC_siz ing

(Is this cable also exporting an equipotential zone to the room with the
kiln? Because that will complicate things if it needs to be a main
bonding conductor as well!)

The main issues is if there is access to an independent earth at the
workshop (so unfinished floor, metal services entering the building from
the ground, metal frame etc). The since you would have that in proximity
to the PME earth, you would need a main equipotential bond between them,
which typically means a copper equivalent of 10mm^2 for the bonding over
and above that required for the CPC. (which given the substantial size
of armour with 4 core SWA, I think you can actually meet the requirement
with just the armour).

Yes, I don't think he has an earth rod for the workshop, can't remember
if he has plumbed water down there either ...

Probably worth checking for the fault condition where it is the
suppliers PME earth introducing a dangerous potential into the workshop.

It might not be as bad as that, since if you are using the armour in
parallel with a core then you have a better result than above.

Ah yes forgot to count that.

But reverting to type B would meet the disconnect time (I think he only
recently changed to type C at the house end because of poor fault
discrimination).

Fault discrimination with what?

He has 50A type B MCB in the C/U down at the workshop end, and was
getting trips, but the house end was tending to trip every time, so he
replaced house end with a type B. In the end he found the reason for

Type C I presume you mean?

It probably would not help anyway since it sounds like this was an
overload related trip and not an fault current one. Hence both type B
and C breakers would have the same thermal response anyway - so its pot
luck as to which goes first. (sods law always dictates its whichever is
the biggest PITA to go and reset!)

that (elements wired in parallel that should have been in series, were
tripping after several hours, I think his controller uses slow start to
build up to full heat overnight).

Yup, that could do it ;-)



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 21/11/2011 12:58, Andy Burns wrote:
Adrian Brentnall wrote:

forget that about the power on my kiln - it's 3kw or thereabouts -
duty cycle comments still apply!

Yeah, no probs.

When he upgraded the controller a couple of years back to one that uses
external solid state relays (DIN rail mounted TRIACs or similar I
suppose) they recommended special fast acting fuses to protect them,
turned out the fuses were more expensive than replacing the 'relays'

Hmm - the standing joke in the power electronics I was involved with
(fork trucks) - was that the semiconductors were there to protect the
fuses! - it always seemed to work out that way g

Without actually running some sort of data logger that shows you 'time
on' and 'time off' for the elements it's hard to be certain, but my gut
feel is that the odd 5v subtracted from 240v (say 2% difference) isn't
going to make that much difference to the power from the elements. I
guess w=vi.... but then, the element's R will vary with temperature....

Adrian

John Rumm wrote:

On 21/11/2011 10:28, Andy Burns wrote:

John Rumm wrote:

With high current circuits, its probably better to actually measure
rather than work from the worst case standardised figures,

an estimate with current and voltage drop will do. Easiest if
you have a clamp meter.

Yep, can arrange one.

The main issues is if there is access to an independent earth at the
workshop (so unfinished floor, metal services entering the building from
the ground, metal frame etc)

No ... concrete base, wooden frame, corrugated metal cladding to
outside, if there's water plumbed in, I'm pretty sure it'll be in
plastic, more questions that'll probably take days for a reply!

Probably worth checking for

"checking for"?

the fault condition where it is the
suppliers PME earth introducing a dangerous potential into the workshop.

I can see if the supplier's earth was faulty, there'd be plenty of
metalwork to touch, the case of the kiln is all metal, which I assume is
earthed rather than double insulated, he'll tend to have damp clay in
hand on the wheel and the pugger

replaced house end with a type B.

Type C I presume you mean?

I did.

(elements wired in parallel that should have been in series

Yup, that could do it ;-)

Always happened overnight when it'd had a few hours of intermittent
operation to warm up the MCB ...

In article ,
Andy Burns writes:
Adrian Brentnall wrote:

forget that about the power on my kiln - it's 3kw or thereabouts -
duty cycle comments still apply!

Yeah, no probs.

When he upgraded the controller a couple of years back to one that uses
external solid state relays (DIN rail mounted TRIACs or similar I
suppose) they recommended special fast acting fuses to protect them,
turned out the fuses were more expensive than replacing the 'relays'

I've use FF fuses to protect mains switching power MOSFETs in one
of my designs. Not yet had the race to see which blows first though.
Heating elements don't normally have any switch-on surge, so you can
fuse the current quite closely, but pay attention to the fuse's own
power dissipation when close fusing.

MCB's are pretty damn fast too if you run into the fault protection
zone (magnetic trip area). Where I had an expensive X10 DIN rail
dimmer, I put a 3A Type B MCB next to it. However, it's no longer
dimming mains filament lamps (they're now all low voltage), so that
shouldn't be an issue anymore.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On Nov 21, 12:12*pm, Andy Burns wrote:
NT wrote:
On Nov 21, 10:38 am, Andy *wrote:

Adrian Brentnall wrote:

What is it about the installation that is worrying your Dad ?

He thinks it will be struggling to sustain top temperature, and

He thinks it will. Is it or isnt it?

Dunno, will have to ask. *I think he has decent thermocouple to measure
that, a while back he was disappointed with repeatability of results,
maybe this all stems from trying to 'fix' that.

shortening the life of elements more quickly in the process of trying.

Running them undervoltage extends their life.

I think he's concerned that they're on for a longer duty cycle, I don't
know if that would tend to cancel out the lower power from being
undervolted?

no. If you run a filament lamp at half rated power, you get way more
than twice rated life. Same principle with heating elements, even
though its primarily corrosion thats the issue rathr than evaporation.


NT

"NT" wrote in message
...
On Nov 21, 12:12 pm, Andy Burns wrote:
NT wrote:
On Nov 21, 10:38 am, Andy wrote:

Adrian Brentnall wrote:

What is it about the installation that is worrying your Dad ?

He thinks it will be struggling to sustain top temperature, and

He thinks it will. Is it or isnt it?

Dunno, will have to ask. I think he has decent thermocouple to measure
that, a while back he was disappointed with repeatability of results,
maybe this all stems from trying to 'fix' that.

shortening the life of elements more quickly in the process of trying.

Running them undervoltage extends their life.

I think he's concerned that they're on for a longer duty cycle, I don't
know if that would tend to cancel out the lower power from being
undervolted?

no. If you run a filament lamp at half rated power, you get way more
than twice rated life. Same principle with heating elements, even
though its primarily corrosion thats the issue rathr than evaporation.


If you run a 240v 60w bulb at 110v does that mean that the bulb as well as
having a longer life will run at something like 30w ?

the_constructor wrote:
"NT" wrote in message
...
On Nov 21, 12:12 pm, Andy Burns wrote:
NT wrote:
On Nov 21, 10:38 am, Andy wrote:
Adrian Brentnall wrote:
What is it about the installation that is worrying your Dad ?
He thinks it will be struggling to sustain top temperature, and
He thinks it will. Is it or isnt it?
Dunno, will have to ask. I think he has decent thermocouple to measure
that, a while back he was disappointed with repeatability of results,
maybe this all stems from trying to 'fix' that.

shortening the life of elements more quickly in the process of trying.
Running them undervoltage extends their life.
I think he's concerned that they're on for a longer duty cycle, I don't
know if that would tend to cancel out the lower power from being
undervolted?

no. If you run a filament lamp at half rated power, you get way more
than twice rated life. Same principle with heating elements, even
though its primarily corrosion thats the issue rathr than evaporation.


If you run a 240v 60w bulb at 110v does that mean that the bulb as well as
having a longer life will run at something like 30w ?

A bit more than 15 watts. Half the volts means a quarter of the power,
modified by the lower filament resistance at the lower temperature.

You'd get very little useful light per watt out though, as a filament
that glows white at 240V will be glowing red or possibly yellow, with
the vast majority of energy being released in the infra red spectrum.

Reducing the voltage by 10% will increase bulb life by well over 20%,
and light output by, as a guess, 30 - 40%.

--
Tciao for Now!

John.

On 21/11/2011 17:25, Andy Burns wrote:
John Rumm wrote:

On 21/11/2011 10:28, Andy Burns wrote:

John Rumm wrote:

With high current circuits, its probably better to actually measure
rather than work from the worst case standardised figures,

an estimate with current and voltage drop will do. Easiest if
you have a clamp meter.

Yep, can arrange one.

The main issues is if there is access to an independent earth at the
workshop (so unfinished floor, metal services entering the building from
the ground, metal frame etc)

No ... concrete base, wooden frame, corrugated metal cladding to
outside, if there's water plumbed in, I'm pretty sure it'll be in
plastic, more questions that'll probably take days for a reply!

Probably worth checking for

"checking for"?

Checking if there is a way of accessing an independent earth in the
workshop. If there is, then whatever it is (pipes etc) need to be
included in the main equipotential bonding. Hence that will also need to
be carried on the SWA CPC.

the fault condition where it is the
suppliers PME earth introducing a dangerous potential into the workshop.

I can see if the supplier's earth was faulty, there'd be plenty of
metalwork to touch, the case of the kiln is all metal, which I assume is
earthed rather than double insulated, he'll tend to have damp clay in
hand on the wheel and the pugger

There is a (rare) failure mode with PME supplies, that a loss of
suppliers Neutral (the so called Protective and Neutral conductor that
comes into the building), then your installations neutral and hence
earth will be left floating, and connected to live via all your appliances.

This could result in live casework on the kiln. This becomes dangerous
if you can touch something else that is still connected to a real earth.
If all the stuff around you is only earthed via the PME earth, then it
should all be sat at the same elevated voltage, and hence not pose a
direct shock risk.


replaced house end with a type B.

Type C I presume you mean?

I did.

(elements wired in parallel that should have been in series

Yup, that could do it ;-)

Always happened overnight when it'd had a few hours of intermittent
operation to warm up the MCB ...

Yup, especially with slow start - that could push it well into the cycle.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Nov 21, 11:57*pm, John Williamson
wrote:
the_constructor wrote:
"NT" wrote in message
....
On Nov 21, 12:12 pm, Andy Burns wrote:
NT wrote:
On Nov 21, 10:38 am, Andy wrote:
Adrian Brentnall wrote:
What is it about the installation that is worrying your Dad ?
He thinks it will be struggling to sustain top temperature, and
He thinks it will. Is it or isnt it?
Dunno, will have to ask. I think he has decent thermocouple to measure
that, a while back he was disappointed with repeatability of results,
maybe this all stems from trying to 'fix' that.

shortening the life of elements more quickly in the process of trying.
Running them undervoltage extends their life.
I think he's concerned that they're on for a longer duty cycle, I don't
know if that would tend to cancel out the lower power from being
undervolted?

no. If you run a filament lamp at half rated power, you get way more
than twice rated life. Same principle with heating elements, even
though its primarily corrosion thats the issue rathr than evaporation.

If you run a 240v 60w bulb at 110v does that mean that the bulb as well as
having a longer life will run at something like 30w ?

A bit more than 15 watts. Half the volts means a quarter of the power,
modified by the lower filament resistance at the lower temperature.

You'd get very little useful light per watt out though, as a filament
that glows white at 240V will be glowing red or possibly yellow, with
the vast majority of energy being released in the infra red spectrum.

Reducing the voltage by 10% will increase bulb life by well over 20%,
and light output by, as a guess, 30 - 40%.

Resistance shifts about 8:1 over the operating range of a GLS lamp, so
power used will be more than 15w.

At around 2700K, the operating temp of a GLS lamp,
life expectancy is proportional to voltage^13

and
light output is proportional to voltage^3.4

Speed of chemical reaction doubles for every 10C rise, so losing 5v
can make quite a difference to element life, or at least its the wire
corrosion failure mode.


NT

In article ,
John Williamson wrote:

[Snip]

A bit more than 15 watts. Half the volts means a quarter of the power,
modified by the lower filament resistance at the lower temperature.

You'd get very little useful light per watt out though, as a filament
that glows white at 240V will be glowing red or possibly yellow, with
the vast majority of energy being released in the infra red spectrum.

Reducing the voltage by 10% will increase bulb life by well over 20%,
and light output by, as a guess, 30 - 40%.

In our theatre, I often wondered why the houselights were rather dim. I
discoverd that, on installation, no-one had bothered to set the output of
the master dimmer. As a result the lamps were getting 160v. Some of them
are still working after 13 years!

--
From KT24

Using a RISC OS computer running v5.16

On 21/11/2011 02:47, Andy Burns wrote:
Dad has a kiln in a workshop at the bottom of his garden, rated at
11.25kW, so maximum current 49A assuming 230V (perhaps notionally 47A
given its age, the rating plate mentions 240V/415V rather than
230V/400V, it can be wired for 1ph or 3ph, he runs it on 1ph).

The main part of the run from the house is 35m of 16mm^2 SWA, so
expected to give a voltage drop around 5V.

He's keen to reduce the voltage drop (thinks this will reduce the duty
cycle, I suppose it will, but I have my doubts if it will be a
significant effect).

I know it is normally frowned on to parallel-up conductors to give
increased current carrying capacity ...

But given that the SWA that is installed actually is four core, would it
still be bad practice to parallel the live and neutral cores *just* to
achieve equivalent of 32mm^2 thereby reducing voltage drop? To clarify,
the MCB(s) would not be uprated, just done to halve the current in each
core, if somehow one core were to go open-ciruit, the current in the
remaining core would end up the same as it presently is.

I can envisage that physically getting 32mm^2 of strands securely into
the henley blocks at the house end and the terminals of the consumer
unit in the workshop might be a challenge.

This would also involve switching the CPC from one of the cores to the
steel armour. MCB trip time calculations certainly aren't in my day job,
given this is not handheld equipment and is effectively a submain, does
a 5s disconnect time apply, rather than 0.4s?

I can see that 35m of 16mm^2 gives
R1 as 0.403 ohm for single core (or 0.202 ohm for double cores)
R2 as 1.085 ohm
PME supply assumed Ze 0.35 ohm

Assume worst case where one core has failed
prospective fault current = 230V / (0.35 + 0.403 + 1.085) = 125A

For a 50A type C MCB, a quick glance at the curves, I think this gives a
disconnect time of approximately 70 seconds, oh dear, I suppose that
alone is enough to rule out the changes he has in mind !!!

Seeing that, I went on to check the existing situation which uses one
16mm^2 core each for live, neutral and CPC
fault current = 230V / (0.35 + 0.403 + 0.403) = 198A
from the 50A type C curve this still gives a disconnect time of 20s !

So even the existing CPC doesn't meet the disconnect time for a 50A type
C MCB, even getting an actual measurement of Ze rather than assuming
0.35 ohms, couldn't possibly result in a low enough Zs to allow a type C
MCB.

But reverting to type B would meet the disconnect time (I think he only
recently changed to type C at the house end because of poor fault
discrimination).

OK, so I think the disconnect time issue has trumped the original
question about using parallel conductors, any thoughts (or mistakes in
my calculations)?


Is there any reason why you can't use an RCD to provide protection and
use a local earth?

I guess I would "weigh" up the cost of cables csa vs cost of lost power
over a number of years, given expected use of the kiln, to pick the
optimum size of cable in terms of cost.

Andy Burns wrote:
ARWadsworth wrote:

Andy Burns wrote:

That's easy for you, with your shiny new toy. I only have a
multimeter

And a clamp meter?
You can guestimate the Ze.

I could borrow one, I was thinking you'd need the proper kit to
measure it, rather than estimating it that way.

You can use the old CostCurrent clamp meter. You just need to convert from £
to Amps:-)

--
Adam

* Sometimes I like to lay in my neighbours garden and pretend to be a
carrot *

Fredxx wrote:

Is there any reason why you can't use an RCD to provide protection and
use a local earth?

That would be quite straightforward, if it there was any benefit.

I guess I would "weigh" up the cost of cables csa vs cost of lost power
over a number of years, given expected use of the kiln, to pick the
optimum size of cable in terms of cost.

Replacing the existing cable is not really on the cards, its been buried
for many years, the workshop and garden have grown up around it.
Besides buying a replacement with a total CSA greater than 4x16mm^2
wouldn't be cheap.

The question was really around whether it was allowable to do the
doubling up, or just to stick to the status quo, rather than anything else.