On 23/11/2017 13:31, whisky-dave wrote:
On Wednesday, 22 November 2017 20:02:36 UTC, John Rumm wrote:
On 22/11/2017 12:36, whisky-dave wrote:
On Tuesday, 21 November 2017 20:13:50 UTC, John Rumm wrote:
On 21/11/2017 11:29, whisky-dave wrote:
On Monday, 20 November 2017 19:03:54 UTC, John Rumm wrote:

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.

Yes I know, but what I don't know is whether a 32A MCB would
be used with cable that the MCB is not up to protecting.

For a general purpose ring circuit,


I'm not sure if that is the case, we are an electronics teaching
lab.

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.

fine.


(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 design time)

How about 'cleaners' sockets that don't go through the lab filter ?
or so we have been told which is called the dirty mains.

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.

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 / suppressors.

I'm not sure if 40 or so PCs and dozens of SMPS would count.

Probably would.

But all the lab sockets did have a filter on them.

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).

Well here;s a link to our riser.
https://www.dropbox.com/s/u1rnoxretw..._0844.JPG?dl=0

the box opened is the one in the centre of the riser

https://www.dropbox.com/s/jr39axmtv0..._0845.JPG?dl=0

If that tells you anything.

Yup, normal memshield2 commercial style 3 phase CU. Which "room" is
yours?

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 phase installed.



However, some interesting things to note the

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 one.

Yes, but they have sfor the last 15 years+ just been refered to as
MCBs.

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.

Previuosly these were RCD and that;'s what they were called in the
days when it was my job to test them. Then that all changed with the
£30K upgrade 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 serious happens.




(I don't know if the memshield2 RCBOs have a different "RCD
tripped" dolly position from the normal overcurrent tripped one -
Adam might know?).

me niether AFAIK I shouldn't even be opening the box.

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 =30mA.

Yes I thought it was something like that and this is also more likely
to happen when dealing with students. Than a short between live &
Neutral.

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.

Yes I know I buy then, Quick blow, anti surge, time delay,
semi-delay, 'normal' I'm just glad I don't have to worry
about male and female and LGBTQ versions. Don't seem to nhave
those options with MCBs

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)

The memshield2 breakers are often 10kA rated BTW - so higher
breaking capacity than most domestic stuff.

I wonder why, I'd have thought the cable would have vapourised long
before 10kA .

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[1]. 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.

[1] Although your observations on voltage drop may suggest "big" and
"low impedance" are not words one would use for your supply!

I would assume that the installers that charged us £30k last
renovation had the sorted.

You know what they say about assumptions!

That they are like arse holes ?

No, that's opinions!

I've always thought that fault tripping was meant to go at 30ma
or less when there was an inbalance between of curretn detected
in the earth.

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)


So it looks like what's been installed is what's needed in the lab
and up to standard so everythings OK.

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 the one in the table on the RHS of the graph:

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

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.

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.

So as expected everything seems OK

Its less gloomy than it first appeared, but based on what currently
appears to be the situation, not all "ok".


Threre must be some reason why a B C or D would be installed.

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).

That's interesting would the wiring in a typical lighting ring 320A
or how about 40A .

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.

Type C is often used with high inrush loads (large transformers,
induction motors, large banks of strip lights etc).

I can imagine strip lights of the flourescant kind needing this, any
idea if its true of the recent LED tubes.

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.

Type D is only usually used in industrial settings for things with
very high inrush.

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.


what happened to A ?

It was left out to avoid any confusion with the current rating of
the device, which would often be specified with "A".

I keep forgeting why in ohms law current is I if a student should
ask.

Comes from the French where the translation would be something like
"intensity of current"

(Ampere was of the French persuasion as well!)

--
Cheers,

John.

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