Help me keep my mother in law off my back.

[rjb]

For backgoround information. I am doing my C&G 2320 level 2 at present and I was warned that all sorts of electrical questions will come your way. Well this has turned out to be true. as I have suddenly become family electrical guru Yesterday my wife got an e-mail from my mother in law. She has an old re-wireable cartridge type fuse CU. I haven't worked on these before so perhaps a more inteligent person than me can help with this question.

Her e-mail is as follows. [By the way I am Richard, Simon is the handy man]

Here is a puzzle for Richard. Simon nailed some boards down in the loft
today.
Well, this evening when I want upstairs (no lights were on), I went into the
box room and switched the light on. The moment I switched it on, the bulb
went. I switched the landing light on, nothing dark. Well there was no
light in any room upstairs. I thought, well Simon must have been the
culprit - he must have nipped some wire up inthe loft.
I rang him, he came. He pulled the fuse for the upstairs lights out of the
fuse box, and the fuse wire had gone. As it happened I still had some fuse
wire stuck behind the electric meter, which was still there from Mr.
Bartlett when he rewired the house.
So Simon fixed the fuse, and now it is all working again.
How could this have happened. A fuse wire had never blown in 38 years,
since the house had been rewired. Could it have anything to do with Simon
fiddling around on the floor in the loft?
It's funny that it went the same day after he had finished and the fuse for
the lights under the loft floorboards.
What does Richard think, please?

Richard thinks.

a) I hope Simon has used correct rated fuse wire.
b) 38 years for a fuse wire to go is not bad. Probably coinsedence. It's working isn't mother in-law.
c) If fuse blows again. I should go all the way to London and test her lighting circuit for continuity insulation.

Am I the keen learning sparky correct with a,b and c. Or is there something further I need to check or ask my mother in law.

I don't want her to blow up just yet. . Also she never forgets! She can remember teh exact date when the house was re-wired Scarry!!

 

[JohnD]

a,b,c sound right.

"rewirable" and "cartridge" are not the same. If she has the cartridge type it is unsafe to try to mend themn with fusewire.

sometimes when a bulb fails it creates a momentary short circuit. which will trip a modern MCB in the time before the filament melts away. It is very unusual for it to blow a wired fuse though.

If you buy your MIL some CFLs (energy-saving bulbs) they will take away the problem, since they do not create this surge when they fail. they also save electricity and last a very long time. This is an advantage with the older homeowner who may not like scampering up stepladders.

If you feel like it, an IR test would be handy, in case there is any connection with the nailing which might have gone through a cable. I suppose it is just possible it has created a high-resistance contact between L&N or L&E though this seems unlikely and I can't see that it would be relevant to the circumstances described.

If your MIL does not like rewiring fuses (and, as you say, they are sometimes re-fused wrongly) and she has a Wylex Standard, which is the most common, it is easy to fit MCBs which are readily available to plug into the old CU very easily where the fuseholders went using similar copper prongs (though you have to change the plastic shield at the same time).

Then, if one trips, she just has to press a button or flick a switch to restore power to that circuit. They are also safer to switch on or off when the circuit has a connected load, which should not be done with the pull-out fuses.

If you are interested in this idea, ask her to read out any brand name she can see on the fusebox, and ask her if it is brown or white. Ask her if the fuseholders have coloured dots on, how many of which colour. If you feel like doing it, please reply for more useful info.

Beware of getting sucked into jobs beyond what you have learned already.

 

[rjb]

Thank you for the prompt reply and the sound advise. I think she wants reassurance she not up in a ball of flames!!

My feeling was a that a fuse wires got to blow at sometime and therefore following your point that perhaps the current surge, the bulb was the last straw for the fuse.

If a 1mm or 1.5mm cable has a very small nick how long would it take if it take (or ever at all) a correctly wired fuse to detect and blow. If it doesn't detect this then if the system still safe.

I just want to reassure her or she will keep phoning me

 

[JohnD]

a "small" cable damage could be anything from never to immediate

Inspection and IR test would tell you.

I should think a 1979 rewire would be PVC T&E, but it was about the time that lighting circuits started being earthed (some of them might not be). The installation is probably due for upgrading of things like:

main bonding
supplementary bonding in bathroom
G&Y sheathing missing on CPCs
"spur-on-spur"
unearthed metal switches and lamps
grommets missing from steel back-boxes
old thin pressed-steel conduit re-used and may be cutting into cables, especially switch drops from loft
Amateur modifications

5p says the main earth will be a bit of uninsulated bell-wire wrapped round the incoming cable armour

 

[rjb]

a,b,c sound right.

"rewirable" and "cartridge" are not the same. If she has the cartridge type it is unsafe to try to mend themn with fusewire....


.....If you are interested in this idea, ask her to read out any brand name she can see on the fusebox, and ask her if it is brown or white. Ask her if the fuseholders have coloured dots on, how many of which colour. If you feel like doing it, please reply for more useful info.

1. The fuse is the re-wirable type BS3036 I believe. Sorry, my rubblish describtion.

2. She tells me it is Wylex - Black. If you can help with infomation about updating the protection then any information would be useful. However, does changing fuses come under part P? If so Maybe I will leave it until I am fully qualified. But the infomation required would be good to learn anyway.

Thanks

 

[JohnD]

'mmmm, black? i wonder if it might be a brown one that is very dark? i have an idea I have seen a black one though.

It is very old and probably has a 60A main switch.

I expect it looks like an older version of this

"How it's done"

To swap the fuses for MCBs, you turn off the main switch and check for dead

Do not take off the CU casing. On these old CUs there are exposed brass terminals on the main switch which are live even when the main switch is off.

But undo the fuse holder cover and remove it.

If the fuses are not already well-identified, label the fuse ways with fuse rating as already fitted, and name of circuit if you know it for sure. You can make a reasonable label strip with white PVC electrician's tape and a permanent pen.

The fuseholders pull out and you will observe they have copper prongs that fit through slots in the plastic covers behind. Each fuse rating has its own coloured cover with prongs at different spacings which are not interchangeable to prevent you putting one rating of fuse in the incorrect slot.

Only take out one at a time to avoid the risk of getting them mixed up.

You will observe that each plastic shield has a retaining screw. If you take this out it will expose the busbars and if you are careless with the metal screw it might touch them (so it is vital to turn off power first)

If you are changing from fuses to MCBs, you can undo a screw to one of the plastic shields, and lift it out. You then fit and screw down the new plastic shield for the corresponding MCB, check that it is secure, and plug in the MCB (it will never need to be removed again). Then continue one at a time until they are all done.

If you can lay your hands on a scrap Wylex CU, you can practice this at the workbench. There are millions about.

As it is work on a CU, I suppose it is notifiable. However it would not be worthwhile as a notifiable job, since the time and expense would be similar to fitting a new CU and doing a proper job. I suspect that 99.99% or more of fuse to MCB swaps on these CUs are not notified.

If it was not done with proper care, there would be risk of shock.

As the MCBs are slightly bigger than the fuseholders, the fuse cover will not fit back on. For some reason Wylex never bothereed making a cover to fit. It is sometimes possible to fabricate a cover with a suitable plastic box, or to cut a hole in the old one, but many are left without covers.

New Wylex Standard MCBs cost about £7 to £8 each, but if you happen to want them, lots of people on here will have a box of Wylex MCBs removed from replacement work, available at modest cost. I know I have.

 

[THRIPSTER]

I think the clue here is probably 'nail' and 'floorboards' - I would be very keen to find whether one of the nails intermittently pierced a cable and whilst functioning correctly at the moment may present some future safety/fire hazzard.


Regards

 

[Sarahspark]

Yes - the immediate 'pop' might have blown the fuse and also blown some of the 'shorting copper' away from a nail.This might be a problem if the cable is running on reduced CSA at the point where the fault was (resistance = heat).If the earth wire has blown away some fittings could be left with no earth to them (problem if they are Class 1) I suggest a thorough inspection of loft wiring.

Sarah

 

[bernardgreen]

Well, this evening when I want upstairs (no lights were on), I went into the box room and switched the light on. The moment I switched it on, the bulb went. I switched the landing light on, nothing dark. Well there was no light in any room upstairs. I thought, well Simon must have been the culprit - he must have nipped some wire up inthe loft.

The question to ask the good lady is this,

When the bulb blew did is do so with a flash or didit simply not come on at all.

If it did blow with a flash then there is almsot certainly nothing to worry about. The lighting circuit was intact and the fuse was not blown otherwise the lamp would not have had power to blow out on. So the work in the loft did not blow the fuse. The fuse blew when the vaporised metal of the failing lamp created a highly conductive ( almost plasma ) short circuit in the bulb. The very aged fuse wire went before the buld went open circuit. For wire fuses that is un-common but not un-heard of.

But if the lamp did not come on at all when it was swtched on then yes there has been a prblem with the wiring in the loft that blew the fuse BEFORE the box room lamp was turned on.

 

[JohnD]

and in the second case, the old bulb would be working again after the fuse was changed (or at any rate, the old bulb would not be burnt out).

the fuse might still have blown due to the light switch being operated, if there was a nail in the Switched Live.

 

[rjb]

Just had mother in law on the phone (how my Autumn evenings just fly by ) it did blow when turned on .

When I've saved up my 850 pounds for a Megger once I've finished the City and guilds 2320 I think I'll have a house to test on for the 2391!!

 

[Symptoms]

Does the Mother-in-Law have life insurance and house & building cover? More important, is your wife the only beneficiary under the old girl's Will?