Plug-in MCBs

Hi,

Just a quick question;

If I replace my rewirable fuses with plugin mcbs - do I use like for like? My fuses are 5 amp, 20 amp, and 30 amp. Should I get the same values, or should I replace the 5 amp with a 6 amp, and the 30 amp with 32 amp?

The reason I ask is that although I can get 5 and 30 amp MCBs from tlc, everywhere else seem to offer 6 and 32 amp.

TIA

Patrick

Reply to
Patrick
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I assume your skeleton modular CU system can be changed over from rewireable fuseholders to MCBs - with a 2-pole isolator and an RCD. Otherwise a new housing ideally catering for a split load is needed. Regarding32A, note that electric showers normally consuming 8kW are often protected by the hardier

45A devices. I think the ratings vary between main manufacturers, but the normal handling current should be less than its overload tripping sensitivity. Short circuits tend to approach infinity Amps in 20 milliseconds. Jim
Reply to
Jim Gregory

Normally the 5A/30A MCBs are intended for directly swapping into an old style fuse box but won't fit a modern consumer unit. The 6A/32A are usually modern pattern DIN rail types that fit modern consumer units, but can't be used in an old fuse box.

Christian.

Reply to
Christian McArdle

Thanks for the replies.

This is a Wylex (grey) fuse box. I do want to upgrade to a new CU split load CU with mcb and rcd. As a interim measure until I can afford to replace the the fuse box with a modern CU I thought I'd replace the rewireable fuse cartridges with Wylex plug-in MCBs. The house is about 35-40 years old and this is the original fusebox.

Screwfix sell the Wylex plugins with ratings of 6,16,20,32,40 amps. They state they are a 'Modern alternative to fuses for Wylex Standard consumer units' Most places I've found stock this range of ratings. However tlc stock this range of ratings 5,10,15,20,30,40 amps. They also state that these are a ' Direct replacement for rewireable and HRC fuse carriers in the Wylex standard range'.

Reply to
Patrick

If you order from TLC the ones that arrive will be from the series

6,16,20,etc. For all intents and purposes they are the same. MCBs should operate for overload quicker than the corresponding rewireable fuse. ie The overload performance of a 6a mcb will be better than that of a 5a rewireable.

Short circuit performance is another matter, but only on /very/ high PSCs, unlikely in a domestic situation.

Reply to
<me9

I'm not sure I see the point for a temporary situation, just replace the box when you get to that. If you want to spend something on improving safety, get some non-slip stair strips.

When you pick lighting mcbs, I'd suggest going with 6A type C rather than the more common type B, as Bs sometimes give a lot of nuisance trips.

NT

Reply to
meow2222

Only if you use dinosaur bulbs.

Christian.

Reply to
Christian McArdle

This is something I can do now.

I do want to change the box to a modern CU next year, but who knows what is round the corner - the car might fail its mot, the central heating might pack in, the dog could get sick. By doing this I'm spending a small amount of money as a 'temporary' solution, but as we don't know what the year will bring - 'temporary' might be longer than I'd wish.

Reply to
Patrick

Then buy anything that fits. The wireable fuses have such poor characteristics, that a replacement by cartridge fuses or MCBs will easily meet the requirements, even if the rating is increased, provided the circuit was already compliant. A 30A rewirable fuse requires either shorter circuits or thicker cables than a 32A MCB or 30A cartridge fuse.

However,

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you a split load consumer unit with RCD and 10MCBs for less than 60 quid. Why muck around with a temporary solution? You can do the job properly first time.

Christian.

Reply to
Christian McArdle

Yep agreed. Its not the price of the CU. I'm happy replacing fittings and have know doubt that I could fit a CU. I don't however feel entirely comfortable doing it, so it would be the cost of the CU plus the bill from a pro spark, which I've been led to believe will run into the hundreds.

Reply to
Patrick

The job will be not of disimilar magnitude to replacing all the fuseways in an old fuse box. A bit of extra work to be sure, depending on the available lengths of cable.

Christian.

Reply to
Christian McArdle

That what worries me. Starting the job only for it to turn into a nightmare. If I did go for it and all being well, afterwards do I just need to pay the leccy board to to test it?

Reply to
Patrick

No. You need to pay the local council to test it. Or a qualified electrician. Unless you happened to do it before the new rules came in.

Christian.

Reply to
Christian McArdle

Thanks for that Christian

Reply to
Patrick

Yup, the rewireable fuse replacement MCBs are not always quite as "plug in" as the name suggests. Depending on the type and range (and the trip current) they may come with a new base section which needs to be substituted for the original base. Installing this can require a certain amount of reconnecting wires in the CU itself - so if you end up replacing all of them, it is task not that different from doing the whole CU from scratch.

Reply to
John Rumm

The replacement "base" for fitting MCBs in a Wylex consumer unit takes seconds and doesn't need any wires disconnecting - just a little care if the switch is left "on"...

Reply to
Frank Erskine

There isnt a problem to solve Patrick. Wire fuses are legal for new installs today, and meet all the 16th edn requirements. Theyre 100% compliant.

MCBs do give faster trips and lower overload margins, but fuses are nothing to get worried over, as long as some idiot hasnt replaced the fuse wires with lumps of copper etc. If its a new-to-you house, its probably worth 50p to replace all the fuse wires with ones you know are the right rating, as thats where the risk lies with rewirable fuses.

Re who knows whats round the corner, yes, and the one solution to that is to keep some money in the bank. Wasting it on MCBs youre going to throw out soon wont achieve a lot imho.

However I do feel from other posts you're being encouraged to do something thats liable to turn into a nightmare, ie replace your CU yourself. While it may all go fine, IRL putting in an RCDed CU is a recipe for trouble, and youre liable to then have to work out why half of the system wont stay switched on, and then how youre going to keep the freezer food from spoiling and so on. Just be aware that shared neutrals, minor leakages and so on, all of which are fairly common, would cause your new CU to not work. Then you got to find the problem and sort it.

I'm not against diying it, just I think you need to understand whats involved and prepare for it, rather than getting dumped in it. If you do some reading here you should be able to learn what the problems are and how to sort them so youre upto speed.

NT

Reply to
meow2222

Patrick,

I would tend to agree with most of that. Much depends on what actual "problem" you are attempting to solve. MCBs are "better" in many cases - but not necessarily that much so as to offer huge advantages just as a straight swap for fuses. Including a RCD in at least some circuits has a bigger implication on overall safety of your system, although there are both pros and cons. Making sure you have RCD protection for equipment you use outside however is highly desireable.

You mention that the install as a whole is some 35 to 40 years old - that may in itself have a much bigger implication on the overall safety than the type of overcurrent protection used in the CU. At that age there is a fair chance (unless the system has been widely extended) that there are insufficent sockets fitted and hence you have to rely on extension leads etc, and that circuits will not have been designed with the typical usage of todays homes in mind. There is also the possiblity that old rubber insulated cable may have been used for some of the circuits - if so there is a real and present danger posed by this. There is a fair chance that the main equipotential bonds to the other services (gas, and water) are missing or undersized.

Knowing what type of earthing arrangement you currently have would also help your decision making process.

(see the FAQ here:

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Reply to
John Rumm

lack of mcbs. I hadnt really taken in that this was an old rubber insulated install.

You've likely got a waterpipe earth, and if the supply has ever been changed to plastic, maybe no earth at all. If its a TT supply and any connection associatde with the ELCB has come adrift, as I've seen happen, you may have an earth that will go live at the first low impedance fault opportunity. I still remember my surprise at being bitten by what ought to have been earth.

Presumably you've got 2 core cotton rubber pendant flexes, now with bare live bits at the holders, and maybe very overloaded socket wiring.

Just echoing what John says, that the mcbs arent the issue. Nor is the CU. Those old Wylexes are basic but adequate.

And nor is replacing the CU a solution. If you fit an RCD'ed CU it almost certainly will not work, and you'll be stuck with no socket power until its all rewired.

If it were mine, I'd check the earthing is adequate, inspect and fix any wiring problems, then leave the whole lot as is until its all rewired. Then again if its all in rubber, it may be unwise to remove any socket or switch to check anything, as donig so tends to cause shorts / fuse pops / fires. It doesnt sound good.

The moral of the tale is tell us what youre trying to achieve in post

1, as often people get the diagnosis or cure wrong, or there are just better options.

Someone will now tell me I've misunderstood the whole thing probably :)

NT

Reply to
meow2222

I must admit I'm still running with the 'temporary' solution I put in when I moved in to my house some ... er ... [mumble] 10 or so years ago and that was to stick a 30mA RCD between the meter and the CU. (It was made easy in my case by the installation having a Henley block between the two.) The only trips I've had have been water getting into electrics e.g. leak from bathroom into ceiling rose below.

Reply to
John Stumbles

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