Will a 16 amp MCB give similar protection to a 13 amp fuse?
tin...@isbd.co.uk
I am [re-]wiring a small boat that we have recently bought in Belgium,
I've done all the low voltage 12 volt (ELV strictly I guess) stuff and
am now moving on the to mains supply which (mostly) will only be used
when we're moored at places with shore supply available.
I'm fairly au-fait with all the issues with isolation from shore,
proper protection with RCD, etc. etc. I've already sorted out some
rather major fundamental nasties on that front (wrong way round
connectors leaving live pins when unplugged, no RCD protection, etc.)
but I am left with one nuisance problem that I haven't really solved
yet.
The boat has mostly french/belgian type sockets at the moment, most of
our mains electrical stuff however (of course) has standard UK plugs.
There are very few (if any) of the existing sockets in the boat which
are worth preserving and, after quite a lot of thinking, mucking
about, asking questions here, etc. I have decided that the easiest
solution will be to go to all UK sockets with adapters (or, possibly,
extension leads) for the equipment with french/belgian plugs, with a
longer term aim of changing plugs.
Finally to the question, space is limited on the boat and there are
very few places where flush mounted sockets are possible so the
neatest solution is to use two and four way 'trailing sockets' (as in,
the sockets of a multi-way extension lead). However these are
supposed to be protected by a 13 amp fuse in the plug, I really want
to wire them permanently and, if I do this, they will be protected by
16 amp MCBs. It feels to me as if this should be OK as 16 amp MCBs
will, in reality, probably trip at or close to the same current levels
as a 13 amp fuse.
Does this seem a reasonable/safe approach? It doesn't have to conform
to any specific wiring regulations (which it almost certainly won't),
no one is ever going to inspect the wiring of the boat at that sort of
level of detail. On the other hand I do want it to be safe for the
usual mix of family and friends as well as for me.
What does the team think?
--
Chris Green
dennis@home
<tin...@isbd.co.uk> wrote in message news:p89tq7-...@chris.zbmc.eu...
> Does this seem a reasonable/safe approach? It doesn't have to conform
> to any specific wiring regulations (which it almost certainly won't),
> no one is ever going to inspect the wiring of the boat at that sort of
> level of detail. On the other hand I do want it to be safe for the
> usual mix of family and friends as well as for me.
>
> What does the team think?
A lot of fourway strips have integral fuses.
If you use one of these then the MCB will only be protecting the flex.
The integral fuse is providing the protection that the manufacturer thinks
it needs.
Of course, as you say, the 16A MCB will probably offer better protection
than the fuse anyway.
John Rumm
> I know this is a slightly odd question, I will try and explain.
[snip]
> Finally to the question, space is limited on the boat and there are
> very few places where flush mounted sockets are possible so the
> neatest solution is to use two and four way 'trailing sockets' (as in,
> the sockets of a multi-way extension lead). However these are
> supposed to be protected by a 13 amp fuse in the plug, I really want
> to wire them permanently and, if I do this, they will be protected by
> 16 amp MCBs. It feels to me as if this should be OK as 16 amp MCBs
> will, in reality, probably trip at or close to the same current levels
> as a 13 amp fuse.
>
> Does this seem a reasonable/safe approach? It doesn't have to conform
> to any specific wiring regulations (which it almost certainly won't),
> no one is ever going to inspect the wiring of the boat at that sort of
> level of detail. On the other hand I do want it to be safe for the
> usual mix of family and friends as well as for me.
>
> What does the team think?
Short answer, I would use a 10A MCB rather than a 16. That will let you
draw 15A pretty much indefinitely.
There is evidence that some of the lower quality (i.e. probably cheaper)
4 way leads are not adequately protected by their 13A fuse - since this
will usually permit a sustained load of 20A and not all of them are up
to that. Read the report starting page 18:
http://www.esc.org.uk/pdfs/business-and-community/SwitchedOn-Issue-18.pdf
a 16A MCB will be somewhat more tolerant to sustained overload than a
13A fuse. If you compare:
http://wiki.diyfaq.org.uk/index.php?title=Image:BS1362FusingTime.png
with
http://wiki.diyfaq.org.uk/index.php?title=Image:Curve-MCBTypeB.png
--
Cheers,
John.
/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/
David Robinson
> a 16A MCB will be somewhat more tolerant to sustained overload than a
> 13A fuse. If you compare:
>
> http://wiki.diyfaq.org.uk/index.php?title=Image:BS1362FusingTime.png
>
> with
>
> http://wiki.diyfaq.org.uk/index.php?title=Image:Curve-MCBTypeB.png
That's an eye opener!
The number printed on the fuse (or MCB) is clearly proportionally
related to its characteristics, but what part of those response curves
does it actually relate to?! 1/5th of the instantaneous current?
2/3rds of the long term current? Why not an actual number that
actually sits on the response curve (somewhere!)?
(apologies if the PDF you linked to answered this - it won't open
here).
Cheers,
David.
JTM
<tin...@isbd.co.uk> wrote:
[snip background]
> Finally to the question, space is limited on the boat and
> there are very few places where flush mounted sockets are
> possible so the neatest solution is to use two and four
> way 'trailing sockets' (as in, the sockets of a multi-way
> extension lead). However these are supposed to be
> protected by a 13 amp fuse in the plug, I really want to
> wire them permanently and, if I do this, they will be
> protected by 16 amp MCBs. It feels to me as if this
> should be OK as 16 amp MCBs will, in reality, probably
> trip at or close to the same current levels as a 13 amp
> fuse.
If you buy the trailing sockets in France/Belgum they will
not have a fuse anyway, as the 13amp fused plug is only a UK
thing (in this context) so should not be a problem.
> Does this seem a reasonable/safe approach? It doesn't
> have to conform to any specific wiring regulations (which
> it almost certainly won't), no one is ever going to
> inspect the wiring of the boat at that sort of level of
> detail. On the other hand I do want it to be safe for
> the usual mix of family and friends as well as for me.
The more time you spend there, the more likley you are to
buy foreign stuff, then sourcing a different wall wart for
your latest gizmo becomes a problem. I'd suggest a mix of
trailing leads. (We have a block of UK sockets for the
g/children to charge their mobiles, game-boys etc. rather
than a load of adaptors which get lost in drawers or packed
in suitcases when they leave.
> What does the team think?
If you moor outside the 3 mile limit perhaps you could
create your own regs ;-)
John
--
John Mulrooney
NOTE Email address IS correct but might not be checked for a while.
Tidy desk tiny mind!
Mike Tomlinson
>Finally to the question, space is limited on the boat and there are
>very few places where flush mounted sockets are possible so the
>neatest solution is to use two and four way 'trailing sockets' (as in,
>the sockets of a multi-way extension lead). However these are
>supposed to be protected by a 13 amp fuse in the plug, I really want
>to wire them permanently and, if I do this, they will be protected by
>16 amp MCBs. It feels to me as if this should be OK as 16 amp MCBs
>will, in reality, probably trip at or close to the same current levels
>as a 13 amp fuse.
How about these? They have an integral 13A fuse, and while intended to
fit an existing sunken double backbox, could probably be adapted to fit
your boat. I have one at home waiting to be fitted and the gubbins
sticking out of the back needs only a shallow recess.
http://ebulbshop.com/acatalog/Converter_Socket_1_Gang_to_4_Gang.html
I got mine for 3.50 ish from Wilkinsons.
--
Mike Tomlinson
JTM
<RIudnddbdLNKnUHR...@brightview.co.uk>, John
Rumm <see.my.s...@nowhere.null> wrote:
> Short answer, I would use a 10A MCB rather than a 16.
> That will let you draw 15A pretty much indefinitely.
and up to 8 per 16A circuit, but never any advice about max
no of trailing sockets or adapters
--
John Mulrooney
NOTE Email address IS correct but might not be checked for a while.
Knowledge is knowing a tomato is a fruit; Wisdom is not putting it in a fruit salad
tin...@isbd.co.uk
am going to remove, thus it would seem that my usage will actually be
safer, I just need to keep an eye on the sockets.
> a 16A MCB will be somewhat more tolerant to sustained overload than a
> 13A fuse. If you compare:
>
> http://wiki.diyfaq.org.uk/index.php?title=Image:BS1362FusingTime.png
>
> with
>
> http://wiki.diyfaq.org.uk/index.php?title=Image:Curve-MCBTypeB.png
>
It's difficult to compare but if my interpretation of the fuse curves
is right (I'm assuming a 13 amp fuse lies in the *band* between the
two RH curves) then a 13 amp fuse can be distinctly worse than a 16
amp MCB can't it? E.g. at 30 amps a 13 amp fuse could take up to 400
seconds to blow whereas a 16 amp MCB will trip after about 200
seconds. It's not a big difference though.
I do have some 10 amp MCBs so could go with them I guess, the supply
as a whole can only provide 16 amps. Maybe I could put in one set of
full sized sockets where, if/when needed anything that draws close to
the limit can be plugged in.
Thanks for the links and advice.
--
Chris Green
tin...@isbd.co.uk
> In article <p89tq7-...@chris.zbmc.eu>,
> <tin...@isbd.co.uk> wrote:
> [snip background]
>
>
> > Finally to the question, space is limited on the boat and
> > there are very few places where flush mounted sockets are
> > possible so the neatest solution is to use two and four
> > way 'trailing sockets' (as in, the sockets of a multi-way
> > extension lead). However these are supposed to be
> > protected by a 13 amp fuse in the plug, I really want to
> > wire them permanently and, if I do this, they will be
> > protected by 16 amp MCBs. It feels to me as if this
> > should be OK as 16 amp MCBs will, in reality, probably
> > trip at or close to the same current levels as a 13 amp
> > fuse.
> If you buy the trailing sockets in France/Belgum they will
> not have a fuse anyway, as the 13amp fused plug is only a UK
> thing (in this context) so should not be a problem.
>
France or Belgium.
> > Does this seem a reasonable/safe approach? It doesn't
> > have to conform to any specific wiring regulations (which
> > it almost certainly won't), no one is ever going to
> > inspect the wiring of the boat at that sort of level of
> > detail. On the other hand I do want it to be safe for
> > the usual mix of family and friends as well as for me.
> The more time you spend there, the more likley you are to
> buy foreign stuff, then sourcing a different wall wart for
> your latest gizmo becomes a problem. I'd suggest a mix of
> trailing leads. (We have a block of UK sockets for the
> g/children to charge their mobiles, game-boys etc. rather
> than a load of adaptors which get lost in drawers or packed
> in suitcases when they leave.
>
I was originally going to have a mix of trailing blocks but I then
decided that there would always be the wrong one in the wrong place.
I think keeping things flexible (no pun intended) is probably a good
idea though.
--
Chris Green
tin...@isbd.co.uk
> In article
> <RIudnddbdLNKnUHR...@brightview.co.uk>, John
> Rumm <see.my.s...@nowhere.null> wrote:
> > Short answer, I would use a 10A MCB rather than a 16.
> > That will let you draw 15A pretty much indefinitely.
> French regs allow up to 5 sockets on a radial 10A circuit
> and up to 8 per 16A circuit, but never any advice about max
> no of trailing sockets or adapters
>
--
Chris Green
tin...@isbd.co.uk
the other side of that panel you can see is either water or another
cabin (or inside a storage space).
--
Chris Green
John Rumm
> In article
> <RIudnddbdLNKnUHR...@brightview.co.uk>, John
> Rumm<see.my.s...@nowhere.null> wrote:
>> Short answer, I would use a 10A MCB rather than a 16.
>> That will let you draw 15A pretty much indefinitely.
> French regs allow up to 5 sockets on a radial 10A circuit
> and up to 8 per 16A circuit, but never any advice about max
> no of trailing sockets or adapters
That is really just a way of codifying an allowance for diversity into
relatively low power circuits. We used to have a similar rule for old
15A socket circuits that permitted three sockets per radial circuit.
These days normal power circuits are large enough (i.e. 20 - 32A) that
its easier to just specify a maximum floor area served.
John Rumm
You are right, it is shown as a band for the fuse. You read your current
off the horizontal, and then use the two curves to give a range of
disconnection times.
> amp MCB can't it? E.g. at 30 amps a 13 amp fuse could take up to 400
> seconds to blow whereas a 16 amp MCB will trip after about 200
> seconds. It's not a big difference though.
The lowest trip current for the 16A MCB is a bit higher than the lowest
for the fuse - generally the fuse will take 20A continuous or a fraction
over, the MCB will probably manage 23 - 24 or so. The higher limit on
the fuse is however very similar. They are similar in response, and
hence the same question arises about whether the socket is really up to it.
(this is possibly a moot point, since I doubt you will be plugging 20A
sustained loads into the sockets anyway!)
> I do have some 10 amp MCBs so could go with them I guess, the supply
> as a whole can only provide 16 amps. Maybe I could put in one set of
> full sized sockets where, if/when needed anything that draws close to
> the limit can be plugged in.
Probably a reasonable solution.
> Thanks for the links and advice.
--
John Rumm
> On Nov 11, 2:42 pm, John Rumm<see.my.signat...@nowhere.null> wrote:
>
>> a 16A MCB will be somewhat more tolerant to sustained overload than a
>> 13A fuse. If you compare:
>>
>> http://wiki.diyfaq.org.uk/index.php?title=Image:BS1362FusingTime.png
>>
>> with
>>
>> http://wiki.diyfaq.org.uk/index.php?title=Image:Curve-MCBTypeB.png
>
> That's an eye opener!
>
> The number printed on the fuse (or MCB) is clearly proportionally
> related to its characteristics, but what part of those response curves
> does it actually relate to?! 1/5th of the instantaneous current?
> 2/3rds of the long term current? Why not an actual number that
> actually sits on the response curve (somewhere!)?
A bit more info here:
http://wiki.diyfaq.org.uk/index.php?title=MCB#Types_B.2CC.2CD
Generally its worth noting the curves show a pessimistic case in terms
of trip times. Various factors may shorten the times - but obviously one
would design to cope with the pessimistic case.
In the case of the MCBs the vertical bit of the response curve relates
to the magnetic trip. This is specced at 3-5 x In for a type B MCB.
That section of the response curve is for dealing with fault currents
(i.e. very high instantaneous currents resulting from shorts, nails
through cables etc). These thresholds are significantly higher than the
nominal current since you don't want them tripping with every switch on
surge. All that needs to happen is prevent the cable from melting (or
most likely the earth wire in the cable) in response to a fault.
The thermal bit of the curve needs to protect the circuit cable from
overload damage (i.e. sustained high temperature). The thresholds
specified in BS EN 60898 say that In x 1.13 shall not cause a trip, and
that a current of In x 1.45 and over shall cause one (the implication
being that currents between these ranges may cause a trip). Since
circuits have a thermal mass, they will take time to reach operational
temperature for a given load current. Even moderate overloads will
generally not result in cable damage if they are not sustained for two
long. So the trip curves reflect that. Needless to say they are affected
by external influences - so higher ambient temperatures or higher than
nominal supply voltages may shift the curve to the left. A MCB that is
"warm" (i.e. one that has been carrying a load for a period of time)
will be more sensitive to overload than a cold one.
> (apologies if the PDF you linked to answered this - it won't open
> here).
No, the PDF was about investigations on trailing leads. (try right
clicking and saving it before opening if you are having difficulty)
Windmill
>Boats tend not to have anywhere one can mount 'flush' boxes because
>the other side of that panel you can see is either water or another
>cabin (or inside a storage space).
They (Homebase, B&Q, CPC) sell triple 13 A. sockets with one integral
13a. fuse which are capable of either surface mounting using the
supplied plastic back box or recessed mounting into an MK triple metal
pattress (though the latter is difficult to find).
Primarily intended to convert an existing single socket to a triple
socket, and the built-in fuse protects against overload.
Though of course that doesn't help much if you also want European
sockets.
(This is irrelevant to your query, but it's interesting to compare
fusing curves for old rewireable consumer unit fuses with those for
the cartridge fuses which for a time replaced the rewireables.
The rewireables seemed to blow more quickly so better protection.
But of course more risk that someone would then replace the fuse
wire with a chunk of heavy copper wire from a cable, which would be
a very dangerous thing to do).
--
Windmill, Use t m i l l
Til...@Nonetel.com @ O n e t e l
. c o m
John Rumm
> (This is irrelevant to your query, but it's interesting to compare
> fusing curves for old rewireable consumer unit fuses with those for
> the cartridge fuses which for a time replaced the rewireables.
> The rewireables seemed to blow more quickly so better protection.
> But of course more risk that someone would then replace the fuse
> wire with a chunk of heavy copper wire from a cable, which would be
> a very dangerous thing to do).
With high fault currents the rewireables actually blow more slowly than
the cartridge fuse equivalent. So in addition to the risk of being
re-wired with the wrong wire there are other knock on effects. The two
most notable being the poorer fault current protection requires that the
cables for a circuit protected by one to be de-rated by 7/10ths of
nominal capacity, and it also means that spurs on ring circuits using
older spec T&E cable with a 1mm^2 earth conductor may not actually be
fault protected at all.
Rewireable:
http://wiki.diyfaq.org.uk/index.php?title=Image:Curve-BS3036.png
BS1361 Cartridge fuse:
http://wiki.diyfaq.org.uk/index.php?title=Image:Curve-BS1361.png
So for a 30A fuse, note the current required for a 0.1 sec disconnect is
280A for the cartridge fuse, and a massive 450A for the rewireable.
However at the other end, a 5 sec disconnect only takes 87A on the
rewireable, but 125A on the cartridge.
BruceB
"John Rumm" wrote in message
news:UJmdnbhOXZIS60HR...@brightview.co.uk...
These days normal power circuits are large enough (i.e. 20 - 32A) that
its easier to just specify a maximum floor area served.
********************************************
Although there is no longer a maximum floor area specified in 7671. The
latest amendment uses the phraseology of for instance, "Historically, a
limit of 100m2 has been adopted" in the 'informative' appendix 15 to 7671.
Regards
Bruce
John Rumm
To an extent its a moot point, since the maximum cable length limit will
impose a practical restriction on what you can actually do.