On Friday, 17 November 2017 12:28:04 UTC, John Rumm wrote:
So what happened ? a student was soldering and he said my soldering statio
n has stopped working.... this was on the same trip as the heaters who's LE
Ds had also just gone out. When we looked in the riser cupboard where the C
U is the MCB has tripped or cut out or whatever you prefer to call it. The
heaters (3 of them) were removed the other were switchd off, and the MBC sw
itch put to the ON position and those things that went off came back on aga
in so how did that happen ?
Measure what ?
2V and a soldering re-work station of about 160W max. ?
I'm not as current is current there is NO difference, it's just electrons a
Do you think the heaters or the soldering iron produced his fault current.
Yes and will this happen at 32A 40 A 50 A and how long will it take ?
what situation ?
So which tripped out ?
So are you saying that the professionals or the company employed to install
these MCBs installed cable that wasn't up to carrying the 32A 40A or 140A
or 160A you say the MCB is designed for ?
The inspection panel has a date of 16th june 2013.
The MCB tripped on its thermal response because the circuit was overloaded.
You need to understand that there are two different classes of over
current: overload current, and fault current.
Overloads are caused when a circuit is operating normally, but the total
load from the appliances connected exceeds the nominal rating of the
circuits protective device. Say for example, drawing 40A from a circuit
with a B32 MCB. The effects of this are to cause gradual heating of the
circuit wires and accessories. If left unchecked it could result in
cable damage or reduced life expectancy of the cables. The heating will
take time to reach a damaging level. So MCBs include a trip mechanism
based on a bi-metal strip, that heats in a way analogous to that of the
rest of the circuit. It will tolerate small overloads for a very long
time, and then ever reducing durations as the overload increases. So for
some overloads they may run for minutes or hours before tripping.
Fault currents however are a different class of fault. Here you have
something causing a short circuit, and the current that flows can be
100s or 1000s of amps. This results in very rapid heating of the circuit
wires (in some cases even explosive heating) - there is also no time for
this heat to be dissipated to the surroundings (i.e. its adiabatic
heating). This class of fault needs the kind of immediate response that
the bi-metal strip of the MCB can't provide. Hence it includes the
magnetic response that will react with the speed you would expect from a
As you would expect
You made the claim that you were not drawing anything close to 140A. I
agree with you, you weren't. I suggest that if however you were to drive
a nail through one of the circuit cables (i.e. to introduce a fault) and
then measure the current draw, you will see a *significantly* larger
current - hopefully only briefly.
Its may just be electrons, but that does not mean you can handle
situations where you are drawing 5A too much in the same way you handle
those where you are drawing 500A too much.
This is why circuit designers consider both scenarios, and the equipment
manufacturers design kit that behaves in an appropriate way to cope with
They produced an overload. However the voltage drop you witnessed during
this episode does cast doubt on the ability of the circuit to correctly
deal with faults.
No it won't happen at 40A or 50A. Those are not fault currents, those
You can get an estimate of the time to trip by looking at the response
The vertical bits of the curves represent the magnetic trip - that for
dealing with fault currents. The curved bits are the thermal response.
If you find 50A on the 32A curve you will see it intersects the time
axis at around 1000 secs. Real world conditions may mean you don't see
this - but you can be reasonably confident it will ultimately trip, but
it will likely take tens of mins to do so. If you are running on a
circuit also suffering an undervolt then the times will be longer.
The occurrence of a fault current.
The thermal one.
I can't say with any certainty what you have installed. I have never
seen it, or tested it. I can offer only educated guess work. I can say I
am suspicious that all is not well based on what you have told us, if
what you have told us is correct.
The fact that you have a voltage reduction device in place will make the
case more borderline. The installers *should* have checked that the
impedances of the circuits already installed were low enough for this to
be safely installed. I would not be surprised if this was not done.
There is a limit to the maximum length of cable that can be installed
for a circuit. This may have been observed as originally installed.
However it is not uncommon for extensions to be made later.
On Friday, 17 November 2017 18:42:48 UTC, John Rumm wrote:
Yes I assume that is what tripped it put.
Yes but knowing which tripped it isn't certain. It was running happily for
over two hours and only tripped after someone started using the soldering i
I doon;t believe circuit wire blow MCBs
I wpould expect the cables to be able to cope with such a thing the MCB lio
ke a fuse is meant to be the weakest link in the chain NOT the strongest.
Exactly and I would expect that to trip BEFORE cable damage is likely.
In a simialr way that a basic fuse is meant to protect the cable NOT the eq
Yes just like a fuse would.
called a fuse yes.
I thought they were meant to be faster than fuses.
Although there are differnt fuses and different MCBs I guess and then there
's the Bussmann fuse curves .....
Trouble was that the heaters while claiming they were going to draw about 8
amps for their 2KW capacity at about the 2+ hours mark they switched to 70
0W so about so about 3 amps rather than 8 amps. So with just ONE heater st
wiching down that is the 4 heaters running at 8 amps will reach the limit o
f 32 amps, the 5th heater running at 3 amps or was it two or 3 heaters swit
ching up of down from 3-8 amps that tripped the MCB or the constant 40 amps
Youy see trips and fuses both blow with a rap[id change or a significant c
hange in current so we don;t know why it tripped other than it's rating was
eventuallky surpassed snd you can;t say for sure what caused it.
Was it because of the soldering iron switching on or a heater switching....
along with sonme sparks perhaps, but I don't see how this would make a diff
ernce because if we had ZERO currunt draw and then put the nail through to
make a short circuit the MCB would have most likely tripped irrespective of
the current already flowing. If anyhting it;s make tripping slightly faste
r NOT slower.
And I believe that if a MCB 32 amp is installed the wiring in that ciruits
would be designed to take the current that a MCB of 32a could pass.
I'm not sure where you get that idea from.
I never said they were fault currents, the whole idea behind this excercis
e was to find the overload current NOT the fault current which is pretty mu
ch irrelivant to us.
Which is what happend and NOT at 50 amps but a littel over 40 if it was ove
and so will the heating efect on the cables.
Which we are NOT interested in.
Which is the thing we were intrested in which is why we set it up in this w
and left it.
Then that is down to those that installed it.
If a 32 amp MCB can rally passs 140 or 160 amps for 5 second what will the
state of the cabling be after this event ?
Seems silly to istall such a MCB doens't it if the cable can't handle the f
ault condition or an overload condition.
I would assujme the overload is there to protect the cable and the short ci
rcuit trip was there to protect equipment and possible lives when there is
a fault .
Off load the voltage is about 223V, although I can;t turn off everything ju
st the heaters I can't turn off the router/switch unit (not 240V as some ex
It has been upgraded a few times since the late 50s.
If its been running for hours and then trips, its an overload.
If it trips the moment you attempt to energise the circuit (quite
possibly with a "pop" from the MCB), then that *might* be a fault
current. If you plug your soldering iron it, and it goes bang, and
immediately trips, and you can't reset it while the iron is still
connected, then that would likely be a fault.
Its much the same situation with a fuse. Both will permit small
overloads for a long duration. In some cases (much depending on the
installation method used for the cable) even that may result in cable
damage, or at the very least premature ageing.
Generally it will, although there is a slightly grey area for small
magnitude overloads. Say running a 32A circuit at the low 40s. The MCB
will permit that pretty much indefinitely. For a ring circuit with all
the cable run in masonry, or clipped to the surface, there is unlikely
to be a problem. However where a circuit has cables running in less
thermally favourable environments you can get close to exceeding the
maximum conductor temperatures.
For the one at the origin of a circuit, or in a plug yes. (equipment may
have additional internal fuses for self protection though)
I think its generally accepted that if you circuit wires vaporise in the
event of a fault you can consider your circuit protective devices were
inadequate (or at least the operating characteristics of the circuit was
so far from ideal, that the CPDs are were operating out of spec)
For fault currents they are comparable for practical purposes, but quite
often a fuse will have a higher energy let through (I^2t) during its
pre-arc time. (which is sometimes you design cascaded systems with fuses
upstream of MCBs since they will usually discriminate)
Indeed, you can get "time delayed" and anti surge fuses, that allow more
inrush (and hence require larger fault currents to open quickly)
Is there any likelihood that your combination of loads will have
If the answer is no, then you did not trip the fault current detection
mechanism of the MCB - since that is the minimum required current for
that to happen (and indeed it would still be in spec if it required 160A
to trip using its fault current mechanism)
If your fault current is not high enough to trip the magnetic response
of the MCB, then it will still trip, but it will have to do so using the
thermal mechanism, and this may react *significantly* more slowly -
especially if it was not already right on the boundary of tripping
(which you can't assume - a fault can happen at any time).
One can use the adiabatic equation to assess the effects on the cables.
Let's say you have a circuit wired in 2.5mm^2 T&E - that means your
smallest conductor is the pair of CPCs, totalling 3mm^2.
Let's say you have a fault current of 200A, and we can assume the MCB
will open the circuit within 0.1 secs. We have a minimum conductor CSA of:
MinCSA = sqrt( I^2 x t ) / k
(K will be 115 for PVC insulated cable)
So you get sqrt( 200^2 x 0.1 ) / 115 which means you need a conductor
CSA of at least 1.1mm^2 to survive the fault and not be damaged.
Now compare with a case when you can only muster say 130A of fault
current. That may take 25 secs to open the MCB. So run the sum again:
MinCSA = sqrt( 130^2 x 25 ) / 115
and you now need circuit conductors of at least 5.7 mm^2 to survive
You are misunderstanding what a MCB (or fuse) does.
MCBs have absolutely *no ability* to limit the current that passes
though them (save for a tiny internal resistance). Under fault
conditions, the current limitation is mostly down the the fault loop
impedance. If you have (say) a loop impedance of 0.05 ohms, then you
could see a 4600A fault current irrespective of the MCB's nominal trip
All a MCB can do is limit the time during which the fault current is
allowed to pass.
 The inductance of the supply transformer at the sub station will
slow the rise time somewhat for very high fault currents.
By doing sums with the data you provided.
Your exercise highlighted to Adam and I that there may be problem with
the circuit. We have simply tried to explain to you, why this *could* be
dangerous in some circumstances. Should our fears be correct, then a
genuine fault may not be cleared in time to prevent bad things happening.
Hopefully you are now aware of this and may choose to do with this
information whatever you like.
Ignorance is bliss huh?
They have probably gone home by now. I guess its now someone else's
Slightly shagged perhaps.
This is why when designing circuits you pay attention to things like
loop impedance. That way you can ensure that fault currents are large
enough to be disconnected quickly before damage occurs).
I am fascinated by the autotransformer the OP says is lowering the
supply voltage to his lab. Is it supplying the whole building or one
ring main? Given that its voltage output apparently drops 10% with a
40A load, are the assumptions about the supply under which fault
protection for the socket circuits was designed still true? Should
somewhat smaller current MCBs be installed? Should the whole
installation be condemned?
On Monday, 20 November 2017 20:02:32 UTC, Roger Hayter wrote:
The whole of the engineering building I think or perhaps just our part of t
Building 15 bottom mid right
> Given that its voltage output apparently drops 10% with a
Just for one circuit in the lab, we have two cicuits in the two labs on 2 p
This is one reason we tried 5 heaters on 1 circuit ring of one phase becase
we were told we were being given 20 2KW heaters.
The whole lab should be updated, yesterday we finally had some of the new t
riple glazed windows installed and with the warmer weather the heaters are
getting the temerature up to 20C by midday so we don't have to leave the he
aters (5x2KW + 4x1.6KW + a 2.5KW) on overnight on full I leave 2 or 3 on
the 1 element setting with the thermostat at about 70%.
This all should have happened before the end of august, we should have all
our windows and the 'central' heating system in operation.
On Monday, 20 November 2017 19:03:54 UTC, John Rumm wrote:
Seems reasonable but this trip has tripped before for no obvious reason lik
e 10KW heaters hanging off it, and as that is the test we were giving it.
Seems that whay as the calculated time is 2.7 hours. (10k seconds)
but the heaters were cutting back to 700W after about 2 hours.
When the trip goes we always diconnect everything that was connected before
resetting the trip and it reset immediatly.
Yes I know, but what I don't know is whether a 32A MCB would be used with c
able that the MCB is not up to protecting.
Isn't this why you shouldn't but a 13amp fuse in a lead that has 3amp flex
Probabely is but as I don't know the specs of the cable used.
70C cable as standard
for the equipment not the cable.
MCBs aren't there to protect the equipment, so what are they for.
well a short circuit is the most likely reason for a trip or rather a 'faul
mains adapted which has tripped it in the past.
Yes that is what I would have thought, if the wires get damaged then the MC
B isn't doing it's job maybe that's why they come in differnt amp ratings.
So an even better reason for testing this out.
Maybe it would have been good to see the wires ignite the wooden benches wh
ile we were present and not when no ones here to see it again a bit like th
e forest and trees.....
As long as the professionals but in what was required.
Yes I know I buy then, Quick blow, anti surge, time delay, semi-delay, 'nor
I'm just glad I don't have to worry about male and female and LGBTQ version
Don't seem to nhave those options with MCBs
No, we no longer have a power lab, labs here. well they are all done on th
e ELVIS system now or which we have about 30 in use.
I don't know what tripped it, I think switching tripped it as the '2KW' hea
ters went from 2KW down to 700W then back up to 1.6KW there were 5 of them
Perhaps it was the 60W soldering iron that was the 'last straw for the came
But at least we know the MCB actually tripped.
Previously we had RCD and I used a 10K resistor between earth and live in a
plug and used to go around testing the ciruits once a month. The H&S got i
nvolved and I had to stop testing and if the trips did trip, we had to call
maintaince who would then come and inspect & corect a few hours later, so
we don't tell them now, unless something really weird starts happening.
What current is that then ?
Is 32 amp OK and 33 a fault. ?
Only if you know the cables used.
Is this how say the wiring of a home is wired ?
i.e that the cable has to survive the fault current ?
Very similar to most fuses then.
But the fault curretn is a bit obsure because you;re factoring in time.
What is the fault current of a 32amp MCB.
Then perhaps we have a faulty instalation.
True but I've no idea what a genuine fault might be.
Nothing I can do with it.
So for me the MCB is pretty much working as expected.
I also expect or assume that if we are pulling 40A through a 32a MCB that i
t will trip just as fast if we shorted L-N with a 6 inch nail as it would i
f we just had one 60W soldering iron on the circuit.
cables runnig at 200V at 40A might get less hot than those running at 240V
at 40A. for same CSA.
What is the fault current then ?
So why have a 32amp why not a 20A ?
This is why we pay qualified electricical companies to come in and install
the cabling and do all woiring previously it was up to teh technicains to d
o it all on a strict budget but that;s all changed I am NOT meant to even '
untrip' a MCB let along decide what cable sizes we should have.
And I would expect a MCB of 32 amps to have cable that could support such a
In the same why if you look at the title/subject I was asking what is expe
cted of a 2KW heater it now seems that a 2KW heater is only expected to giv
e 2KW for about 2 hours then it switches down to 700W.
So if I were to run the heaters over night they wouldn;t be consuming 1.6KW
each as most of the time they's be consuming 700W, so 5 of these is 3.5KW
rather than 10KW , I would have expected both the MCB and the lab cabling t
o support this at least all night if not forever.
I think both, while the lab has stayed the same physical size we have had m
ore socket points installed. IN the old days we had power meters and ran va
riacs connected to rehostates and experimants like yuo;d expect in the 1960
Now we have soldering stations, LCD oscilloscopes and we run ardunios etc a
nd PCs, laptops so need more sockets. But I really don;t know if the POWER
requirement s are higher or lower.
For a general purpose ring circuit, the short answer is it should offer
both fault and overload protection. That means the cable in each leg (as
installed) has at least 21A of current carrying capacity, that loads are
not disproportionately bundled right at the end of the ring, and that
the overall loop impedance at the furthest reach of the circuit is low
enough to ensure "instant" disconnection in the event of a fault.
If all the fuse is doing is providing fault protection (i.e. any
portable appliance made in the last few decades) then it will usually
remain "safe" even with the wrong fuse. (flexes on modern appliances are
usually sized / length limited to ensure they are fault protected on
your typical Euro style 16A circuit with no additional fusing).
For a general purpose socket circuit, then the MCB will serve two
purposes. Protecting against faults, and protecting against excessive
The one it *must* provide is the fault protection. In some circumstances
the overload protection can be provided elsewhere and/or by other means
(the most obvious example of which is a spur from a ring - where the
maximum current carrying capacity of the spur cable is less than the
nominal trip of the MCB - the overload protection then comes from the
limitation to only a single socket (unfused spur). It will still
adequately offer fault protection though)
You have a similar choice (at least for the larger loads): Common
nominal ratings of 3, 6, 10, 16, 20, 32, 40, 45, 50, 63 (and possibly
others depending on range and brand)
Three different fault / inrush characteristics: Types B, C, & D
And often a range of maximum breaking currents, typically 6kA, and 10KA,
but again there are others. (those plug in wylex 3036 rewireable
replacements often only do 3kA)
Yup, cute, but not really "power electrics" is it? ;-)
(I recall an ex Marconi college engineer lamenting the lack of exposure
to things over 5V by most of the current generation of new engineers -
he used to like demoing drawing an arc a couple of metres long from the
output of a high power transmitter!)
For a B32 MCB the minimum fault current to be *sure* of getting an
instant trip would be the 5x In rating, or 160A
Its the one in the table on the RHS of the graph:
You can use the equation to indicate the minimum size of cable required.
If those used are equal or greater, then you are happy. If they are
smaller, you have a problem.
(I would be very surprised if anyone would have installed undersized
cables initially - generally if you install one of the standard circuits
(say 2.5mm^2 ring for a 32A protected ring) all will be fine unless you
have excessive de-rating factors to take into account.
Yes. The "On Site guide" has a table that gives a maximum length of
cable permitted for each of the standard circuit types to save needing
to do the sums. In some cases the limitation is that of voltage drop,
and in others its maximum earth loop impedance. Where the limitation is
the latter, the level is set so that the cable should always have proper
Nominally 160A for a type B device. 320A for a type C, and 640A for a D
The most likely case is when a cable gets physically damaged - say
penetrated, crushed, burnt etc.
Probably nothing you can do directly...
For those wishing to don the tin knickers, a memo to "the powers that
be" would be about all you could do.
i.e. When we did this, we observed that. It has been drawn to my
attention this may indicate a problem, but I am no expert and can't say
for certain. You might want to consider having the wiring checked etc.
If it then blows up later, or someone gets electrocuted, you can put on
the "I told you so" tee shirt and wash your hands of it.
On Tuesday, 21 November 2017 20:13:50 UTC, John Rumm wrote:
I'm not sure if that is the case, we are an electronics teaching lab.
I would assume this had been checked and done and was even safer than a hom
e system as we are dealing with students.
and in our case ?
Well here;s a link to our riser.
the box opened is the one in the centre of the riser
If that tells you anything.
I would assume that the installers that charged us £30k last renovatio
n had the sorted.
Yes it is for teaching purposes we run a course on it and it's the standard
kit for teaching power at degree level anyway.
5V ! we're trying to keep them to 3.3V. ;-)
I've always thought that fault tripping was meant to go at 30ma or less whe
n there was an inbalance between of curretn detected in the earth.
I wonder if 160A going through a student would cause any damage or would it
even be noticable, woul,d oit wake them up ?
To me it seems a bit late to trigger a fault if it takes 160A.
Not sure I should start measuring things if size is important.
I would hope a teaching lab would come up to those standards.
I would assume that each MCB would be used with the appropriate cables in
place. Threre must be some reason why a B C or D would be installed.
what happened to A ?
Just as well we've been promised a new lab then for the past 10+ years.
Unlikely in this lab.
'Powers' interesting phrase.
I wonder if these are the powers that women lack and that is why they don't
reprot sexual misconduct.
If it has a ring, and lots of sockets into which you can plug any device
you fancy, then its classed as a general purpose circuit.
(a non general purpose socket would be circuits for individual bits of
kit, like an immersion heater circuit or a fire alarm one - i.e.
situations where you know all about the specific equipment/load at
The only thing you might encounter different is in IT labs where the
large quantities of switched mode PSUs would often require that high
integrity earthing be used as well since there is often quite high earth
leakage when in normal operation as a result of all the input filters /
However the circuit protection would still be the same.
If working correctly then it should be on par with that done in domestic
Commercial installs often tend to make use of larger numbers of radial
circuits with smaller MCBs - usually just to provide better
discrimination in the event of a fault, and the ability to isolate
smaller sections of the infrastructure.
Yup, normal memshield2 commercial style 3 phase CU. Which "room" is yours?
However, some interesting things to note there:
Many of those MCBs are not actually MCBs but RCBOs - i.e. they include
RCD and MCB functionality in one unit (this is good for a situation like
yours since any earth leakage faults will only take out the affected
circuit and not others). Although this does mean that when seeing a
trip, you need to decide if its an over-current one or an earth leakage
(I don't know if the memshield2 RCBOs have a different "RCD tripped"
dolly position from the normal overcurrent tripped one - Adam might know?).
The other interesting thing is that most of those socket circuits are
protected by C type devices. That means that the fault current required
to open one is double that which we have been discussing! However the
good news is that only applies to Live to Neutral faults and not Live to
Earth faults since the RCD part of the RCBO will take care of those at
The memshield2 breakers are often 10kA rated BTW - so higher breaking
capacity than most domestic stuff.
You know what they say about assumptions!
(to be fair, the kit they have used is decent stuff)
Mmmm smell the ozone, feel the hairs on your arms perk up in the
Given your pictures above, make that 320A... however:
With a RCD or RCBO yes. Since you have RCBOs on the socket circuits,
then faults to earth will be cleared even if the circuits don't meet the
maximum allowed earth loop impedance.
That makes the whole situation less worrying, since the only fault that
won't be cleared is the less common L to N fault. (although given the
type C device there is probably no chance of clearing such a fault on
the instant part of the trip if you ever did get one of those)
No, that's fine. You don't want it too low, or it would trip with switch
on surges. Remember the magnetic trip facility in the MCB/RCBO is only
there to deal with short circuit style situations, and currents of
hundreds of amps are normally commonplace in these situations.
The thermal part of the trip takes care of the everyday overloads like
someone plugging in a bunch of radiators.
And the RCD trip response will take care of most electrocution risk
events (unless you manage to get a student between L & N while not earthed!)
Pretty much all wiring should.
Since you have RCBOs on each circuit, that generally makes things much
Usually yes. (In many cases they could be the same cables).
Yup, B is general purpose and what you see in most domestic installs
(although I usually use type C on lighting circuits to minimise nuisance
trips on filament lamp failures).
Type C is often used with high inrush loads (large transformers,
induction motors, large banks of strip lights etc).
Type D is only usually used in industrial settings for things with very
It was left out to avoid any confusion with the current rating of the
device, which would often be specified with "A".
Unlikely in most cases - but unlikely is not the same as never.
Well the presence of all those RCBOs changes the picture somewhat. The
electrocution risk is very much lower (from direct contact anyway). I
would expect there is still a fire risk in the even of a L to N short
On Wednesday, 22 November 2017 20:02:36 UTC, John Rumm wrote:
How about 'cleaners' sockets that don't go through the lab filter ?
or so we have been told which is called the dirty mains.
I'm not sure if 40 or so PCs and dozens of SMPS would count.
But all the lab sockets did have a filter on them.
I assume so.
which is why we have our benches split into seprate rings and 2 phases.
251 PC lab, 252 Now the PCB room (this year),
253 the hardware lab (my lab), 254 my office, 255 ex power lab with 3 phas
Yes, but they have sfor the last 15 years+ just been refered to as MCBs.
Previuosly these were RCD and that;'s what they were called in the days whe
n it was my job to test them. Then that all changed with the £30K upgr
ade to the labs electrics.
I was then told NOT to test them and that if they tripped we'd need to call
the 'estates team' in to come and sort any problem out.
Which I must admit we tend to ignore, unless of course something really ser
me niether AFAIK I shouldn't even be opening the box.
Yes I thought it was something like that and this is also more likely to ha
ppen when dealing with students. Than a short between live & Neutral.
I wonder why, I'd have thought the cable would have vapourised long before
That they are like arse holes ?
That's nice to know, so thanks for that info.
Yesterday an IoT (internet of Things) student asked for a 5V battery, when
I said I don't have any he asked "well how am I supposed to work on my pro
ject at home then"
So it looks like what's been installed is what's needed in the lab and up t
o standard so everythings OK.
So as expected everything seems OK.
Looks like I may have to test for that situation :-D
Even the students seem simplier ;-)
That's interesting would the wiring in a typical lighting ring 320A or how
about 40A .
I can imagine strip lights of the flourescant kind needing this, any idea i
f its true of the recent LED tubes.
Sounds sexy I wish we had kept some of the old power lab and HV stuff.
I remmeber being told how I'd have to phone up the royal London hospital if
we started up our HV lab as it cause interference at the hospital and the
generating board needed to be informed in advance.
I keep forgeting why in ohms law current is I if a student should ask.
Still general purpose, since you don't know what will be powered from it
as such - i.e. its not dedicated to running a particular bit of equipment.
Yup, nothing to do with high integrity earthing though.
(High integrity earthing just dictates that the earth be connected in a
ring (which on a ring circuit it naturally would be) and that separate
connections are used for each termination on the sockets - so sockets
with a pair of earth terminals are used).
Perhaps, but that ain't what they are, and the difference is
significant. (you do have some normal MCBs in there, and also a three
phase switch (the one with the three dollies linked). All the RCBOs have
the test buttons.
Its more likely in general. The design of most flat cables with the
earth in the middle makes it harder to create a LN short without also
creating a LE or NE one.
Nothing to do with the cable - more to do with the "stiffness" of the
supply. A big low impedance mains feed close to a substation can provide
very significant fault current. This can pose a problem for the MCB
because there is an upper limit to the current flow it can successfully
interrupt. A typical domestic MCB will normally be rated for 6kA. So any
fault current less than that it should be able to open without
sustaining damage or welding its contacts together. More than that and
it may fail to disconnect or get destroyed while trying.
The memshield ones are aimed more at industrial use, and hence many can
cope with higher fault currents.
 Although your observations on voltage drop may suggest "big" and
"low impedance" are not words one would use for your supply!
No, that's opinions!
With the exception of the loop impedance and voltage drop. The voltage
drop under load (combined with the voltage reduction device) means you
could damage equipment that is sensitive to undervolt. The loop
impedance (combined with the type C RCBOs) means its unlikely you would
clear a L to N fault without having to rely on the thermal trip
mechanism of the RCBO. So an increased fire risk.
Its less gloomy than it first appeared, but based on what currently
appears to be the situation, not all "ok".
The nominal rating for a lighting MCB would be 6A. So the fault current
required to "instantly" trip a type B is 30A, or 60A for a type C
60A of fault current should be easy to realise on most lighting circuits
if the total circuit impedance is 3.8 ohms or less.
Generally less so... the strip lights (especially the older ones with
magnetic ballasts) can take quite a surge on startup and can present
quite and inductive load.
Comes from the French where the translation would be something like
"intensity of current"
(Ampere was of the French persuasion as well!)
On Thursday, 23 November 2017 21:19:50 UTC, John Rumm wrote:
General purpose for a lab I'd guess, other wise we wouldnlt have had ro spe
cify we wanted an extra 4 to 8 sockets extra installed on each of the bench
So I wouldn't consider it general purpose as in home general purpose.
I wasn't aware they was a connection, and I don't think many homes have suc
h a filter anmd consoder it general purpose.
Not to me since we've told this riser has nothing to do with us and if ther
e is a problem we have to get someone in to sort it.
But every single one of our cables is round, never seen a flat one used in
the lab adn we have 100s of them, I have 50+ unused still with the twist w
ore around them, 3 pin plug to IEC (mistakenly called a kettle lead) and th
ey are all round cables.
So this doesn't sound much lioke a genral purpose setup to me.
well that I don't know btu I expected a voltage drop as we exceeded the 32A
I wasn't suprised that my 2KW heater only measured 1.6KW at 202V.
why aren't assumptions the same, are yuo saying not everyone can have assum
I don't believe we have such equipment, so I'm not particually worried.
But we have had problems in the PCB room but as yet it;s rare and hasn't be
en looked at closely.
For me it is.
So would the lighting cable be OK at these currents ?
So I would assume if e replaced our 70 odd floursenant tubes with LED versi
on we wouldn't need to change the woring in the lihting circuit of teh lab.
Bloody french, after 2 french GFs and a french flatmat think I've heard eno
ugh of the french ;-)
its can be general purpose and have high integrity earthing - the two
are not exclusive.
Lots of places use mains filters of various types for IT kit. They are
(mostly) useless to be fair.
If you want proper protection for IT stuff, then either a line
interactive or online UPS is the way to go.
Well its very relevant to the conversation we have been having and to
some of the questions you asked. It may not be your responsibility to
know about these things as a part of the job, but that does not mean you
have to pretend to be ignorant of the actual details.
Those are flexes not cables. The cables form part of the fixed wiring
between the CU and sockets etc.
Flexes are far more vulnerable to L to N faults than cables, but do have
the advantage of dedicated fault protection from the plug fuse.
If you look at that photo of your CU, all the "Ring room nnn" circuits
are general purpose circuits. The ones labelled things like "Door spur",
"door bell", "security panel" etc are not general purpose circuits.
(Note that general purpose has specific meaning in this context)
Indeed you would expect a drop, but the amount you got was rather large.
A typical bit of kit to suffer would be something with a compressor
driven by an induction motor. Some fridges / freezers can be quite
sensitive, either having insufficient torque to start and stalling, or
drawing too much current and overheating.
Well that's ok then... for you.
We can do an adiabatic check to be sure:
S = sqrt( 60^2 x 0.1 ) / 115 = 0.16 mm^2
Since the smallest lighting cable used is 1mm^2 (and 1.5mm^2 being
common in larger installations), you would have loads of headroom.
The existing wiring as far as the fittings would be fine. With some
replacement tubes you would need to alter the wiring in the lamp fitting
to wire the ballast out of circuit.
(Although you can get some "straight replacement" LED retrofit tubes
that will work with the ballast)
On Friday, 24 November 2017 18:30:24 UTC, John Rumm wrote:
Well as long as someone else knows when and when not to install high integr
ity earthing, I've no idea.
Are yuo sugegsting managment could be fooled by the company installing thei
r prodicts ;-)
But what if you have a mix of uses in a lab.
This isn't an IT lab, we have aa whole 3 floor building for that.
There are NO problems that I can see.
Whatever happened to trip the MCB, now if the MCB hadn't tripped would that
have been a problem ?
We call them cables not flexes.
Our students don't get access to them as they are in conduit.
Students don't get access to them.
So the only flat cables our students ever see are IDC ribbon cables, for th
em that is what a flat cable is.
Strangly enough none of those are in use, haven't been for years.
Well that woukd depend on the 'Zero' no load point wouldn't it I mean if th
e voltage off load starts at 223V rather than 240 or even 230......
Interesting you should ay that as on friday or CNC machine cut out, this ha
s a densist grade compressor attacheted to it adn because the room was cold
a heater too room 252. Today no ones in their it's cold nothing is on and
the mains voltage measure 216V .
So that could be whats been going wrong.
Yep, as I've said and the previous managers have saiud the heating in the l
ab or department hasblt worked in 35+ years why don't you fix it.
I wonder why they don't give similar headroom for general purpose mains.
We replaced 4 tubes will LED versions without rewiring they seem to work fi
I thought that's where we started weeks ago, that you expected to be
able to trip a 32A breaker with the heaters, and I gave you the 2.7
As John has calculated, if the supply is so "soft" that it drops to
202V, it might only be able to supply 140A, so a short circuit could
last long enough to cause serious damage ...
I do not think your heaters are taking 40A. We were discussing why the
MCB didn't trip, because a 32A breaker can supply more than 40A for
hours without tripping.
You just glibly replied to Johns message where he showed you ... he
wasn't saying your heaters are taking 140A, rather that in the even of a
short circuit (where ideally you'd want many hundreds of amps available
to ensure the MCB trips in under half a second) your 202V supply could
only supply 140A which would take it 20 seconds to trip.
140A at 202V is 22kW heating up your cabling, your lab should be toasty
until the fire service arrive.
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