Will a 16 amp MCB give similar protection to a 13 amp fuse?

[tinnews]

I know this is a slightly odd question, I will try and explain.

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?

[dennis]

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]

[snip]

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:

16A MCB will be somewhat more tolerant to sustained overload than a

13A fuse. If you compare:

[David Robinson]

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]

In article , wrote: [snip background]

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.

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.

If you moor outside the 3 mile limit perhaps you could create your own regs ;-)

John

[Mike Tomlinson]

In article , [e-mail] writes

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.

got mine for 3.50 ish from Wilkinsons.

[JTM]

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

[tinnews]

All of the failures noted there seem to have been in the plug which I 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.

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.

[tinnews]

That's a useful guideline to throw into the mix, thank you.

[tinnews]

But they're going to be UK sockets, so rather difficult to buy in France or Belgium.

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.

[tinnews]

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).

[Windmill]

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).

[John Rumm]

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.

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!)

Probably a reasonable solution.

[John Rumm]

A bit more info here:

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.

No, the PDF was about investigations on trailing leads. (try right clicking and saving it before opening if you are having difficulty)

[John Rumm]

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:

Cartridge fuse: 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]

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.