Which RCD's do I need to replace old fuses?

[pop80_uk]

Hi
I have an old fusebox with old style wire fuses. A friend has suggested I can replace these with more modern ones simply by plugging them in the place.

Can you help? Advise which ones I need?

Thanks

 

[JohnW2]

Hi
I have an old fusebox with old style wire fuses. A friend has suggested I can replace these with more modern ones simply by plugging them in the place.

If it's an old Wylex fusebox, that may be theoretically possible (they would be plug-in MCBs, not RCDs) - but there really isn't any point.

About the only real (slight) 'advantage' is that you can 'reset' an MCB just by 'flicking a lever', rather than having to re-wire a fuse - but if the need to do either is occurring at all frequently, you should be investigating (and rectifying) the cause, not change the fuses to MCBs.

Kind Regards, John

 

[ericmark]

The Wylex fuse box can be converted to miniature circuit breakers (MCB) it needs power off as to change over it exposes the live bus bars and the units are colour coded white = 6A, Blue = 16A, Yellow = 20A and Red = 32A so it is replace colour for colour.

In theroy you should test the loop impedance before changing. A 30A fuse will after many hours blow with 32A and in seconds with 100A and there is a curve linking the two. However with a 32A MCB there are two devices built into one. The thermal part works similar to a fuse but is slower than a fuse at high current so there is also a magnetic part which with the B rated MCB will trip between 3 and 5 times the thermal part so looking at 160A to trip it. To ensure a short circuit will trip the magnetic part we have to measure what the prospective fault current will be. Since ohms law comes in here we can actually measure the impedance (AC resistance) and that for a B32 should be 1.37Ω which allows for the volt drop of 5%.

So in theroy you should measure the earth loop impedance before changing them. Personally I think the possibility of wrong size fuse wire is a higher risk than having the wrong ELI so even if I could not measure it I would still change them.

However they are more expensive for the old Wylex than a new consumer unit so it may well be better to change whole consumer unit and also add RCD protection while your at it.

 

[ericmark]

Hi
I have an old fusebox with old style wire fuses. A friend has suggested I can replace these with more modern ones simply by plugging them in the place.

If it's an old Wylex fusebox, that may be theoretically possible (they would be plug-in MCBs, not RCDs) - but there really isn't any point.

About the only real (slight) 'advantage' is that you can 'reset' an MCB just by 'flicking a lever', rather than having to re-wire a fuse - but if the need to do either is occurring at all frequently, you should be investigating (and rectifying) the cause, not change the fuses to MCBs.

Kind Regards, John

Also slight disadvantage changing them. With my boxes once changed a light bulb blowing with ionisation would take out the MCB, however it never took out the fuse. In my case I have two old Wylex boxes both fed from a pair of 30 mA RCD's so no real advantage fitting a consumer unit for me unless I went down the RCBO route.

But with RCD's feeding the fuse boxes it is a clear advantage being able to flick off all the MCB's when the RCD trips to both assist resetting and to show which circuit has the fault.

 

[JohnW2]

Also slight disadvantage changing them. With my boxes once changed a light bulb blowing with ionisation would take out the MCB, however it never took out the fuse.

Indeed. Like you, I've never known a fuse to blow as a result of a bulb/lamp dying - but it was quite common for RCDs to trip when incandescent bulbs/lamps blew. However, I'm not sure I've ever experienced that since changing to CFLs/LEDs - probably at least partially due to the whole circuit now being much more lightlyt loaded, as well as due to the different 'dying behaviour' of these more modern lamps.

In my case I have two old Wylex boxes both fed from a pair of 30 mA RCD's so no real advantage fitting a consumer unit for me unless I went down the RCBO route.

As you say, no point unless you want RCBOs. I think I still stick with my view that, in the OP's case (one assumes with no RCDs), there really is little point in changing fuses to MCBs - it would be far better (and possibly even cheaper - certainly not a lot more expensive, materials-wise) to fit a new CU with RCD(s) and MCBs.

Kind Regards, John

 

[ericmark]

Just looked in screwfix consumer unit £17 and MCB £3.54 for old Wylex MCB £10 so even with just a 4 way consumer unit £8.80 cheaper to change consumer unit. Plus one then has the option of RCD protection. However swapping fuse for MCB can easy be done DIY. Changing a consumer unit is not really a DIY job so once you add labour costs plus an electrician would insist on RCD protection so really £38.90 for metal consumer unit with RCD's so parts will cost £13.10 more to change consumer unit including RCD protection for 4 way or £12 cheaper for 8 way once you include RCD plus labour charge which is unlikely to be under £100 so for the poster it would cost double to change consumer unit than to replace fuses with MCB's.

However if I was in his position I would swap consumer unit.

 

[Risteard]

Like you, I've never known a fuse to blow as a result of a bulb/lamp dying

It's not at all uncommon. Everytime I've had to replace a cartridge fuse in my house it has been as a result of a lamp blowing. On one occasion lamp failure caused the RCCB to operate.

 

[JohnW2]

Like you, I've never known a fuse to blow as a result of a bulb/lamp dying

It's not at all uncommon. Everytime I've had to replace a cartridge fuse in my house it has been as a result of a lamp blowing. On one occasion lamp failure caused the RCCB to operate.

Interesting. Do I take it that they were incandescent lamps?

I have to admit that I have zero peronal (domestic) experience of cartridge fuses on final circuits. However, very many moons ago I lived for a good few years with Wylex re-wireable fuses and incandescent lamps, and I don't recall one of those every blowing as a result of a lamp dying - that all started when I 'moved up' to MCBs (but still had incandescent lamps), and then went away again as the incandescents were gradually phased out.

Kind Regards, John

 

[Risteard]

Incandescent lamps indeed.

 

[JohnW2]

Incandescent lamps indeed.

Yes. I have to say that in the incandescent days, some of my lighting circuits could be pretty heavily loaded if lots of them were on at the same time - obviously meaning that the MCBs were running that much closer to their trip threshold than was the case after the incandescents were replaced with much lower power lamps/bulbs.

Kind Regards, John

 

[PBC_1966]

The Wylex Standard range will also take BS1361 cartridge fuses, which would be an improvement on rewireables but cheaper than the plug-in MCB's. I had some lighting circuits on 5A BS1361's at one time, I don't recall ever having to replace one due to a lamp burning out (incandescent).

 

[Risteard]

The Wylex Standard range will also take BS1361 cartridge fuses, which would be an improvement on rewireables but cheaper than the plug-in MCB's. I had some lighting circuits on 5A BS1361's at one time, I don't recall ever having to replace one due to a lamp burning out (incandescent).

The better quality incandescent lamps were often internally fused so it has become more of an issue since the banning of incandescents.

That said, I am happier to have HRC/HBC fuses than circuit breakers. They are frankly superior (although less convenient following a fault).

 

[ban-all-sheds]

Most reputable CU makers have compatible fuse carriers - POP to mix'n'match fuses & MCBs/RCBOs.