Plug-in MCBs

[meow2222]

If the OP has a rubber insulated installation, this would probably lose him all power.

NT

[John Stumbles]

IF: the OP said the installation was about 35-40 years old, not that it was rubber. My parents' house was rewired in the mid-late 60s in PVC.

[Andrew Gabriel]

Your inference that a rubber installation will be leaking enough to trip an RCD when it wasn't showing up any faults without an RCD is rather far-fetched. (Actually, rubber leaks less than PVC does.)

[John Rumm]

No guarentee that it is rubber at that age, although it is possible - especially on some of the larger radials.

True, although it is rather worse when under any sort of mechanical stress (conductors tend to work through the insualtion rather more easily IME).

[Patrick]

I've replied to myself because there have been so many replies I wouldn't know where to begin.

I never even considered that the wiring would be classed as too old for a modern CU with RCD. I have grown up in a house like this and so this wiring is all I know. I don't believe it to be rubber insulation, it does appear to have pvc characteristics.

My plan was to put plug-in mcbs in place of my rewireable fuses until I can get a new CU with RCD protection. I was then going to put the plug-in mcb into my parents house - that way there is no wasted money. However I only plan to stay in this house for another five or so years, so there is no way I'm rewiring the house so that rcd protection can be installed. I guess what I'll do it fit the plug-in mcbs (there on order anyway) and leave it at that. If nothing else they will allow my wife to restore power (if need be) when I'm not at home.

The funny thing is, I've been in this house for seven years without any leccy problems (I have had to rewire a fuse once maybe twice when a 240v halogen blew), and without even thinking about my fuse box. The thing that changed is having kids, When I was replacing some socket I thought about them and how I could make things safer for them.

Anyway, thank you to everyone who replied - it is most appreciated.

-- Patrick

[Christian McArdle]

It wouldn't. Any wiring that would trip a 30mA RCD should be condemned as dangerous. New rubber wiring would have no problem whatsoever passing that test. The problem is that rubber rots. Its 30 year life span, coupled with the fact that it was installed more than 50 years ago indicates that much of the rubber may have perished, leaving it in a dangerously leaky state. The perishing tends to happen at the ends, where the oxygen can get to it.

Christian.

[Patrick]

But my wiring wasn't installed more than 50 years ago, as I said earlier the house is about 35-40 years old.

[Christian McArdle]

Then it is probably PVC, unless you know otherwise. They haven't decided on how long PVC lasts. However, 100 years seems reasonable, provided the circuit is lightly loaded.

Christian.

[John Rumm]

I think he was explaining what the problem with rubber is - not suggesting that your house was wired with it. I raised it as a possibility since I have seen some instalations of that age that did still have some rubber.

Chances are your house *would* run with an RCD. If you restrict the number of circuits protected by the RCD to the socket feeding ones only then you also improve the chances of not having problems (and are also more compliant with the regs and minimise the dangers of having RCD protection).

My mums place for example had a full lighting circuit rewire back in the late 80's but the power circuits are still mostly 60's PVC (with a fair amount of additional stuff that is newer). There was some rubber on a few circuits that I had away over the years. At some point it acquired a "whole house" RCD (when she had a new cloakroom built which had a lekky shower IIRC), and to be fair has never experienced any spurious trips.

However adding selected RCD protection to some circuits is obviously harder to retro fit.

I understand the desire to add RCD protection WRT children being in the house - the same thought crossed my mind. Whilst I felt that we lived safely here for a good few years with a wylex rewireable CU and whole house ELCB (TT system), I also knew there was little chance that I or SWMBO would go sticking metal objects into sockets to see "what would happen" - whilst I hope the sprogs won't do this I can't guarantee it!

I also had a requirement to add new circuits for which I did not have sufficient spare ways on the exiting CU (loft conversion), so had to bite the bullet and go for a big upgrade. The upshot was that I removed all the old stuff, installed new CUs (five in total!) and RCDs as required, plus brought all the equipotential and supplementary bonding up to scratch.

[Jim Gregory]

What, rubber-coated in lead sheath? My house's old cables, dissed and abandoned in situ but replaced by T&E over twenty years ago, just before we moved in, were worth a bit as scrap when I eventually recovered them! Jim

[meow2222]

I'm surprised you say that. The old rubber installs I've seen typically have perished dirty wire ends, and the dirt causes leakage. They also are prone to having insulation fallen off inside the conduits, again giving opportunities for leakage. A shared or crossed neutral somewhere is not uncommon, which will also trip an RCD. Poor neutral to earth insulation again can cause RCD troubles. Old immersion heaters are often leaky. Then there are minor faults such as very slight charring on sockets, poor insulation on various fixtures, etc. Then there are the appliances, any one or two of which may have enough earth leakage to trip. None of these will pop a fuse, but they will trip an rcd.

NT

[meow2222]

or it might be ashathene, right period for it.

NT

[David Hansen]

On Mon, 19 Dec 2005 09:54:27 -0000 someone who may be "Patrick" wrote this:-

That is one of the few advantages of such things.

Did you order them before asking about them in this group?

MCBs will not make things safer for them. Assuming that the circuits are designed and installed properly, in most circumstances there will be little change in safety. In a few circumstances safety will be reduced a bit because of the slow operation of MCBs (it takes a relatively long time for a mechanism to operate compared to a wire melting). In a few circumstances safety will be improved a bit, where an MCB will operate before a rewirable fuse.

The best combination of cost and protection is generally High Braking Capacity fuses, which are a plug in replacement for your rewirable fuses. Their only real disadvantage is replacing blown ones, which means using a screwdriver with some designs of board and carrier. Resetting an MCB is easier.

[zymurgy]

I have very early PVC on my upstairs lights. Grey twin core with no earth strand. House is 1958.

What does this 'ashathene' look like ?

Cheers

Paul

[John Rumm]

I have only very rarely seen that. Most of the rubber I have come across is just plain rubber sheath on both the inners and the outer. Some like the external downwire that feeds our head end from the aerial power cable are individually covered PBJ (i.e. jute secondary outside sheath over the rubber).

[Martin Angove]

The Wylex replacements also come with a baseplate. The units have differently-spaced pins to the rewireables they replace and so they are not a simple drop-in replacement; you'll need to dismantle the CU. Be very careful as sometimes the insulation is missing from the meter tail connections. I believe the 5A/30A variants are designed the same as the fuse carriers, but I've never used them.

MCBs will be safer than rewireable fuses in practically all circumstances. In fact if your ring cable is 2.5mm2 with 1mm2 earth (common in the 1970s and 1980s) then MCBs are definitely preferred. Modern 2.5/1.5 isn't such a problem.

But since you're looking at Screwfix, why are you worried about "affording it later"? How many circuits do you have? At £7.25 each, even a minimal 4-circuit CU is going to cost £29 to upgrade which is fractionally less than half the cost of Screwfix's Volex 12-way split CU complete with 63A RCD and 10 varied MCBs and a free pack of screwdrivers (82204, £58.98).

Remember to notify Building Control before you start - this kind of work

*definitely* comes in as "notifiable" under Part P.

Hwyl!

M.

[<me9]

You don't need to remove the CU cover, just remove the fuses and bsseplate. replace baseplate from MCB plug in mcb. Presto!

High PFC situations (unusual in domestic unless next door to substation) may make MCBs response poor because of slowness in clearing dead short. Backup fuse in supply cutout may operate first.

I started my rewire 15 years ago (as part of a building project nortified to BCO). I completed it (with replacement of rewireables by MCBs at the end of March 8-).

[meow2222]

Wouldnt bco want it tested? A new CU on an old install is then likely to fail the testing.

As I've said, there may be more to it than meets the eye if the OP replaces the CU. Given the OP's limited knowledge I would hesitate to replace it if I were him/her. And the old CU is really not the issue on a 60s install anyway.

NT

[Andrew Gabriel]

Funny you should say that. When I tested my parents' late 1950's install in 2000, the Wylex CU was the only faulty item. The ring circuit fuse carrier had been making a poor contact, resulting in overheating of the bakalite and insulation melted off the wires.

Since the CU is in the coat cupboard and surrounded by hanging fabric, I replaced it with a high quality industrial metal MEMshield2 consumer unit.

[Martin Angove]

The whole point of notifying BC is so that they can arrange for it to be tested. Judging by 1960s-vintage wiring I commonly meet it is likely that if the system fails inspection it will be down to one of:

• missing cpc on lighting circuits (not universal, but reasonably common before the mid 1960s)

• main EP bonding not to current regs

• supplementary bonding not present or not to current regs

The lack of RCD protection for sockets likely to feed equipment used outdoors may also be noted, and supplementary protection on TT systems (RCD, ELCB or worse) may not be up to scratch.

The 1960s cable I've met tends to be in pretty good nick and easily passes insulation tests if there is no physical damage. As someone noted elsewhere we do not have accurate figures for the real lifespan of PVC, but it certainly outlasts rubber and there are many 40 or 50 year-old PVC installations out there working perfectly well.

Having read the whole thread I'd tend to agree that replacing the CU isn't necessarily the best option for the OP. I'm afraid I was thinking as "me" and couldn't see the point in spending £30 now for a "temporary" solution when another £30 would get me a "permanent" one. Although MCBs are better than rewireables under most circumstances the biggest "safety" advantage to going the CU route would have to be the possibility of installing a split unit with RCD.

However, a secondary point I was trying to make (and probably didn't do so very clearly) was that replacing rewireables with MCBs is (from a Part P point of view) exactly the same as replacing the whole CU. Anyone doing so *should* treat the exercise in exactly the same way, satisfy themselves that cable ratings are not exceeded with the new devices, upgrade certain parts of the system as required and perform a complete test.

To be honest, the biggest issue on a 1960s install (apart from the three mentioned above) is likely to be the sheer lack of sockets. My parents' house was built in 1967 (IIRC) and is a reasonably large four-bedroom semi. The downstairs was originally completely open-plan except for the hall. The kitchen had (again, IIRC) two single sockets and a cooker point while the whole of the rest of the downstairs had three single sockets and two fused outlets.

In fact, when my parents moved in, there were only two sockets downstairs apart from those in the kitchen as the house had been built without central heating and the previous owners (we moved in in 1972) had installed a radiator directly covering one of the three sockets.

It's not a simple job to add more either, as the single "ring" is run under the floor upstairs, and the sockets downstairs are *spurs*.

Hwyl! M.