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.

(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)

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.

However the circuit protection would still be the same.

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.

I would assume this had been checked and done and was even safer than
a home system as we are dealing with students.

If working correctly then it should be on par with that done in domestic
situations.

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.

For a general purpose socket circuit, then the MCB will serve two
purposes. Protecting against faults, and protecting against
excessive overload.

and in our case ?

Same.

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)

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?

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.

(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
=30mA.


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

You know what they say about assumptions!

(to be fair, the kit they have used is decent stuff)

Is there any likelihood that your combination of loads will
have exceeded 100A?

No, we no longer have a power lab, labs here. well they are all
done on the ELVIS system now or which we have about 30 in use.
http://www.ni.com/en-gb/shop/select/...ab-workstation


Yup, cute, but not really "power electrics" is it? ;-)

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.


(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!)

5V ! we're trying to keep them to 3.3V. ;-)

Mmmm smell the ozone, feel the hairs on your arms perk up in the
electric field!

If your fault current is not high enough to trip the magnetic
response of the MCB,

What current is that then ? Is 32 amp OK and 33 a fault. ?

For a B32 MCB the minimum fault current to be *sure* of getting an
instant trip would be the 5x In rating, or 160A

Given your pictures above, make that 320A... however:

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)

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.

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!)

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 fault protection.

I would hope a teaching lab would come up to those standards.

Pretty much all wiring should.

Since you have RCBOs on each circuit, that generally makes things much
simpler.

But the fault curretn is a bit obsure because you;re factoring
in time. What is the fault current of a 32amp MCB.

Nominally 160A for a type B device. 320A for a type C, and 640A for
a D type D.

I would assume that each MCB would be used with the appropriate
cables in place.

Usually yes. (In many cases they could be the same cables).

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).

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
high inrush.

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".

Then perhaps we have a faulty instalation.

Possibly.

Just as well we've been promised a new lab then for the past 10+
years.

;-)

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.

True but I've no idea what a genuine fault might be.

The most likely case is when a cable gets physically damaged - say
penetrated, crushed, burnt etc.

Unlikely in this lab.

Unlikely in most cases - but unlikely is not the same as never.

Hopefully you are now aware of this and may choose to do with
this information whatever you like.

Nothing I can do with it.

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.

'Powers' interesting phrase. I wonder if these are the powers that
women lack and that is why they don't reprot sexual misconduct.


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.

That's OK in theory but not in practice.

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
though.


--
Cheers,

John.

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