What HRC fuse holder fits MK Sentry II?
Due to plumbing & chimney repair I might have to move a group of 8
cables later this year.
I can a) either move the 8 cables downwards 4" behind quarry tile into
4 pairs of trunking with top/bottom battens over which new oak
skirting is attached with metal dowels (removeable) and MK Grid 2G
unpunched backbox vertically aligned with 25mm holepunched & cover
plate for zone... or b) replace all 8 cables with a 8 metres of 6mm 3c
SWA, downstream CU, upstream HRC fuse.
The latter is the better solution subject to guaranteed discrimination
of downstream 6A 20A Type-B RCBO - the problem is MK do not offer a
type approved HRC carrier for Sentry-II (all RCBO at present).
Fuse carriers seem to be getting hard to find. I can't say for sure, if
the reverse is true, but I have used the Hager fuse carriers along with
Sentry II RCDs interchangeably in a number of CU types. So there is a
reasonable chance of them fitting. However TLC don't seem to have them
I did wonder if Hager fit. If not it's go through the Schneider /
Merlin-Gerin catalogs (they are mechanically interchangeable although
some aesthetic differences).
Bizarre, I glanced at TLC tonight.
I suppose they expect people will use Henley blocks, but many
installations simply do not have the space for a) henley blocks &
associated cabling or b) henley blocks & switchfuse for remote CU (DNO
do not permit their BS1361/88 fuse to protect cabling beyond 2-3
Sorry, it was not clear.
At present the run of 8 cables are protected by 8 6A-20A Type-B RCBOs,
with 2 cables supplying up & down lights.
If I move those RCBO to a CU at the end of a sub-main, the sub-main
cable's protection must give discrimination. Trip characteristics for
quite a range of MCB Type-B & Type-C overlap, so I would prefer HRC
Obviously I do not want a 20A power radial trip at the end of the sub-
main to take out the sub-main's protection - ignoring 314.1 for the
moment it would also take out both lighting circuits. That MK do not
offer a HRC fuse carrier has always been odd, their technical
department must have a linking kit for two CU, henley blocks and a
large open wall :-)
Having experianced some MG stuff in the past, its not something I want
to repeat! ;-) (although I expect a fuse carrier even they could make
Indeed. Although if the total load of the downstream CU is less than the
overload rating of the sub main, then you could argue its protection
only needs provide fault protection for the sub main and not overload.
That may allow you to increase the size of its MCB so as to be clear of
It was the ^6A 20A" nomenclature I did not follow. Typically I would
expect a rating of say something like 6B 30mA meaning Type B with 6A
overcurrent with 30mA trip threshold RCD.
Or did you mean a mixture of 6A and 20A MCBs?
does actually have I^2t let through characteristics for their MCBs at
the end, and they would suggest that successive sizes of device ought to
discriminate, if my reading is correct. (a quick stacking operation in
photoshop suggests that the curves don't overlap when plotted on the
Or they believe that their MCBs will discriminate when cascaded. (must
admit I have always opted for a fused submain though)
MG make MK RCBO :-)
Not sure if MK post Dec 2009 "aesthetic change" are also MG.
I wish MK would make a CU like Clipsal series 56 backboxes, ie,
thickwall PVC rather than miserable-flimsy polycarbonate. That might
be due to the need for knockouts - but they could thread-n-plug or
self-seal inserts, solving the long running "IPx2 on top when regs
require IPx4" issue of MK consumer units.
With 8 downstream RCBO the sub-main MCB rating would need to be the
limiting factor. I think MK do 50A MCB, but not checked whether that
is "B" & "C".
A mixture of 6A and 20A RCBO.
Ooo, gone to see, just "planning" at this stage - hadn't checked.
- Type-B 20A trips instantly at 60-80 / 100A by chart / 5*In.
- Type-B 50A would trip instantly at 150-200 / 250A by chart / 5*In.
- Type-C 40A would trip instantly at 200-280 / 400A by chart / 10*In.
Thank you for digging the chart out.
I'm dubious about Type-B, but Type-C should make it possible.
Taking a lazy 20A Type-B (5*In) it is 100A, taking a most sensitive
40A Type-C is 200A.
Obviously the combined Zs for 6mm SWA + Ze (TN-C-S) would need to be
below 80% of the EFLI limit for a Type-C 40A (or whatever type/
rating). Cue the DNO to abandon PME because of cost and only give TN-
S :-) Gas mains are being laid up neighbours driveways so anything
seems to go these days.
I too have always gone for fuses at the head end as fuse trip curves
are way slower than the fast electromechanical side of an MCB. I might
telephone MK tomorrow and see what they say - it must guarantee
I'm at the "Planning-2010" stage :-)
Discrimination applies to RCD's rather than MCB's. The overload
protection is to protect the cables and equipment, and are sized
Doesn't the Sentry have a full DIN rail though? If so then anything will
fit, just clip any HRC carrier in and link the busbar across using a
suitably rated link if the bottom terminal doesn't line up exactly with
the existing busbar position.
No, discrimination is equally applicable for MCBs and the MCB component
of an RCBO. Note that this is relation to fault currents and not
So for example a 200A fault current would place both a 6A MCB and an
upstream 16A MCB in the instantaneous part of their trip response curves.
Even then you can have difficulties with the projecting part not
aligning with the other devices, and that preventing the cover going
I have a suspicion the problem will be the busbar-alignment, any link
strictly needs to be 25mm, bit bodgy, no type approval. Projection is
usually limited for HRC carriers (no MCB toggle), but 32A is not
really like enough for 8 downstream circuits. Larger than 32A might be
even harder to fit. I knew I should have used Hager for this
Looks like option a).
Move the 8 cables down 4" behind quarry tiles into 4 pieces of
trunking with top/bottom battens over which new oak skirting is pushed
onto dowels. Easily removed, easy to replace a cable (compared to 3
rooms under plaster). MK Grid 2G backboxes are available unpunched,
use one vertical to define the Zone with 25mm Q-Max punched holes. I
just realised I could at last run 3C+E to link the kitchen heat alarm
to the main interlinked alarms, so some benefit.
What I meant was, it is less likely to be a major problem in practice in
the application set out in this thread. I was more concerned about
Thunderbird being terrible as a newsreader TBH than proofreading my
rather brief reply.
That is a minor concern, the MK and Hager range will fit any standard
DIN enclosures, so are therefore the same as far as the front of the CU
is concerned, so a Hager HRC should drop straight in.
Did you mean to say discrimination applies to overload currents not
fault currents as per 17th Ed. Sec. 536?
Yes, but a big-time fault on (say) an electric shower is likely going to
put both the 45A mcb and the 60A House Service Cut-Out fuse in the
instantaneous part of their trip curves...
Of course, I may be missing the point completely. :-)
Lets see, the trip curve has a band of time...
B20 --- 0.01s = 114A ... 0.01-5s = 80A ... 0.01-3s = 70A
B45 --- 0.01s = 257A ... 0.01-5s = 180A ... 0.01-3s = 158A
C40 --- 0.01s = 480A ... 0.01-5s = 200A ... 0.01-3s = 130A
BS -- 0.01s = 900A ... 0.01-5s = 270A ... 0.01-3s = 240A
BS being BS1361 60A.
So a slow B20 could take as long as 3s to trip at 70A, but 0.01s at
It does show how discrimination at large values despite changing type
is barely a guarantee particularly when considering EFLI.
Well it does apply to overloads as well obviously, but the point I was
trying to make is that while overload currents in cascaded sequences of
MCBs will generally result in the devices discriminating without too
much extra consideration, one needs to pay more attention to fault
currents in the magnetic part of the trip curve.
Looking at the MK doc I linked to, it suggests that they have ensured
that the let through energy on each device is less than that required to
trip the next size up the range - which ought to ensure fault current
discrimination as well. However when cascading devices from different
manufacturers that may not always be the case. (or having a type C
downstream of a type B protected submain)
Indeed it would. However the MCB will _usually_ get there first
(although as people have found, that ain't always the case!)
Setting aside the back-up issue, which is an absolute requirement...
The point that was confusing me slightly is that the 17th Ed doesn't
care much about discrimiation for fault currents per se. It just says
"Where co-ordination of series protective devices is necessary to
prevent danger and where required for proper functioning of an
installation..." and leaves it entirely to the installation designer to
make a value judgement without giving any further guidance.
I'm struggling with criteria for "...required for proper functioning of
an installation" and I don't see any particular "danger" other than the
old saw of being "plunged into darkness by an unrelated fault".
 Where an upstream device with higher breaking capacity protects a
downstream device with breaking capacity less than max. PSCC.
On Mon, 25 Jan 2010 23:32:46 +0000, John Rumm wrote:
Not too much of a problem in my experience, John. We've successfully
loaded all sorts of DIN compatible boxes with DIN modules and in
virtually all cases we've had no problems at all. When there has been a
problem it's usually been with a specialist piece of kit that, for
example, needs additional insulation over its terminals for a particular
project. The mechanical relationship between the DIN rail and the front
cutout is part of the DIN spec, I think. Occasionally some kit does
extend out slightly further than others though.
Mick (Working in a M$-free zone!)
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