RCD nuisance trips

[Siddell]

Hi there,

I've got an old, Wylex, 4-way, rewireable fusebox, water mains providing my earth (PME to come) and a 100A service fuse. The circuits are:


i) ring main
ii) lighting (two circuits in single fuseway, one of which is a single light)
iii) immersion heater (not used)
iv) cooker and hob (actually leads to a double socket that feeds these appliances).


Once I've sorted my bonding, I'll get someone in to connect to the new PME terminal. What I need to decide, though, is what RCD protection to get. What I would like is to keep the fusebox and simply have someone install one, all-encompassing RCD. I am aware of regs 314.1 and 314.2 (which ordinarily even a dual RCD, split-load CU doesn't seem to satisfy) and that the only practical solution to that is to pretty much have each circuit individually protected by RCBOs, so would like to park that issue aside for now, given that I don't mind nuisance trips as long as they are infrequent, which brings me (finally) to my questions!


What is the likelihood and nature of any nuisance trips if one RCD were installed for the whole fusebox?


I know this may be something of a crystal ball question, but really I need to weigh up the chance and cost of problems (likely a new CU) against the cost of installing a single RCD. The only time a fuse has blown in the two years I've been in the house was on the lighting circuit, so no other obvious signs of earth leakage - that said, I probably have a sh*tty earth right now and any earth leakage I assume will be magnified once the PME connection is made.


To what extent could this be mitigated using a 100ma RCD (as opposed to 30ma)? Where is a 30ma RCD required? Is there an inherent firerisk with rewireable fuses slow fusing if only slightly overloaded?


Thanks for taking the time to read this and for your thoughts.

 

[malcolmsanford]

If you came to me with your plans, there would be a good chance I would carry them out for you. I'm sure any electrician worth his salt would do the same to be honest.

You are the client and if you want just a front end RCD on your installation then that is what you should have. If you then asked me if it was a good idea I would tell you that your barking.

There is no conceivable reason I would advise you that

1) You should keep rewirable fuses in a domestic installation in the 21st centuary, but it is you choice.

But if cost is your factor I would recommend the following

1) Get the sparks to replace the fuse carrier with MCB adapters they are easily found and will offer you better solution.
2) Get him to price you up RCBO's as well a let him give you the option
3) To get round section 314, which to be honest I don't think should be that widely quoted for domestic, fit a small non maintained emergency light above the CU, so in the case of a trip at least you will a light source to reset it.

As for nuisance tripping, that is impossible to predict, if your have rewirable fuses I would imagine the installation is 20+ years old, and so liable to have been added to and generally altered over the years. There is every chance that once you fitted this RCD, your real problems would start.

As said your the customer and within reason and the regulations you should get what you want, whether is is silly, well that is up to you. Would you ask a mechanic to fit an advanced breaking system to a 1980 Marina ...............doubt it

 

[spark 68]

I agree with Malcom,

As you say, even a Dual RCD split board does not fully comply with Regs 314, and it was a 'misnomer' calling them 17th Ed CU's, this was purely a 'cost based' exercise and was deemed arbitarily (I may add) as the minimum requirement.

The only way to fully comply with section 314 is by having seperate RCBO's.

My thoughts on this are if you want a single upfront RCD, and that is all you are willing to pay for, then I would fit it, subject to a couple of requirements* , as this still leaves the installation safer than it was.

*Incidentally, I would clamp the tails first with a Earth leakage clamp meter, before doing a couple of other tests, to see if I was likely to encounter tripping faults.

Edit: I would note this as a departure.

 

[Siddell]

Thanks for your input. I take your points.

What would the fitting of MCB adaptors provide that is not there now? Please excuse my ignorance.

Also, how would they or any RCBOs fit into the Wylex? I had thought that replacing anything in the fusebox unless like-for-like would not comply with regs and so I'd be back to fitting a new CU?

 

[telectrix]

you can replace your rewireable fuse links with MCBs. these are around £10 apiece. add £20 for your RCD, £5 for an enclosure to put it in, you are then up to £65 just for the bits. you can buy a dual RCD CU for £50 from b&q. so it's a no brainer.

 

[Engineer54]

Thanks for your input. I take your points.

What would the fitting of MCB adaptors provide that is not there now? Please excuse my ignorance.

Also, how would they or any RCBOs fit into the Wylex? I had thought that replacing anything in the fusebox unless like-for-like would not comply with regs and so I'd be back to fitting a new CU?

The old Wylex fuse boards had in later years, retro fit MCB conversions available to be fitted, they are both not a very good upgrade especially the older push button type MCB's. The main advantage is that the MCB's will have a known tripping curve, giving closer protection to circuits and a slightly higher fault level protection as apposed to re-wirable fuse elements.

RCBO's are not available for these old Wylex fuse boards, Malcolm was referring to any new CU you might consider installing...

My advise to you, is to go for a complete new CU installation. The protection it will offer to your installation is far and away superior to your present set-up, with or without an up front RCD. Bite the bullet and go for an all RCBO board, you then won't have nuisance tripping out of several circuits and not knowing which circuit is causing the problem without a series of tests being carried out!!

Look around and find an outlet that are selling RCBO's at reduced/on sale prices, the final cost may then not be that much more than what they call a 17th ed CU!! Up to you, but i think the small extra cost can more than pay for itself in more ways than one over and over again ...lol!!

 

[Deleted member 9648]

Plug in MCB's will provide more 'convenient' and certain fault protection than rewirables,in that you just reset rather than having to replace a fuse wire,and the risk of incorrect fuse wire and hence unsafe fault protection is reduced....there are no other advantages.
You cant fit RCBO's to the type of fuseboard you have...that would need a CU change to implement.

EDIT....another point with the old wylex CU's is that if the top fuses cover is missing (which they often are) live parts can be exposed.....take E54's and Tels advice and bite the bullet,you know it makes sense!

 

[Deleted member 26818]

Are you sure it's a 100A fuse? Seems rather high for a four way board.
Do you not have an upfront RCD at present?

 

[ackbarthestar]

Considering a 4-way Wylex box you will need to check the following

1/ Size of Service fuse
2/ size of tails - usually 16mm2 but sometimes smaller if the service fuse is 60A
3/ size of earthing conductor - 4-8 way boards are likely to have a small number of earthing terminals for the number of ways available plus earthing and bonding, and certainly unable to take anything greater than a 10mm2 earthing conductor
4/ Type of material supporting the plastic CU - wooden patrices are regarded as a fire hazard, although I have never seen one with fire damage caused by overheating of the fuse busbars
5/ Condition of fuse carriers, enclosure -IP2X at least
6/ Internal protective parts are still in place - you'll know what they are when you find you fingers on them without realizing it (isolate first)

I'd follow the advice of the 'Gandalfs' from this forum and consider, at least a basic RCD 6-way board using with 6kA MCBs or better, a DP Iso. 6-way board with your 4 RCBOs and a couple of blanks.

 

[Deleted member 26818]

You want to be carefull, the Tolkien Society will be on your case for use of their word.

 

[ackbarthestar]

You want to be carefull, the Tolkien Society will be on your case for use of their word.

Good grief - being hunted down and cleaned out by a bunch of Hobbits with big hairy feet sounds exciting

But Sparks loose their sparkle and fade away into middle earth.....

 

[Siddell]

Thanks for all the info. Great stuff.

Just to answer the two questions - new service fuse sticker says "100A Type II b series 8 BS1361" so I presume 100A fuse inside and no, I have no RCD protection at all yet.

All your points make a lot of sense - it's difficult to argue with things that make your installation a lot safer. What is nagging me is cost and nuisance trips.

Cost: well, you can't put a price on life I guess. Having said that I have been quoted £475 plus VAT to "remove CU, test circuits, adapt wiring as necessary, provide and install 6 mod MK RCD/RCD split load DB with MCBs". Is that a bit steep?! If, in the end, it is the right thing to do I will do it, even at that price. As telectrix pointed out, new CU is similar price to MCBs plus RCD, so must be labour intensive quote I've had. I also presume that to install a single RCD the same tests and therefore labour charge would apply as for a new CU? In which case, it really would be a no-brainer unless I was considering going without RCD protection altogether. That would be crazy right? The only reason I ask is because it's only recently that most circuits need RCD protection.

Nuisance trips: it would be annoying if I did all that, only to have trips all the time even though the installation is in reality safe - of course, if they trip because of a serious fault then that is a different story.

Thanks for your thoughts - I promise I'm not going round in circles, just trying to get all the info.

 

[telectrix]

475 seems a bit steep IMO, unless the spark who quoted has seen a need for upgrading earthing/bonding/shared neutral/possible IR faults. my advice would be to get another quote, but be sure to use a registered spark, as the work is notifiable to LABC ( unless you are in scotland or ireland\

 

[GLENNSPARK]

Hi there,

I've got an old, Wylex, 4-way, rewireable fusebox, water mains providing my earth (PME to come) and a 100A service fuse. The circuits are:


i) ring main
ii) lighting (two circuits in single fuseway, one of which is a single light)
iii) immersion heater (not used)
iv) cooker and hob (actually leads to a double socket that feeds these appliances).


Once I've sorted my bonding, I'll get someone in to connect to the new PME terminal. What I need to decide, though, is what RCD protection to get. What I would like is to keep the fusebox and simply have someone install one, all-encompassing RCD. I am aware of regs 314.1 and 314.2 (which ordinarily even a dual RCD, split-load CU doesn't seem to satisfy) and that the only practical solution to that is to pretty much have each circuit individually protected by RCBOs, so would like to park that issue aside for now, given that I don't mind nuisance trips as long as they are infrequent, which brings me (finally) to my questions!


What is the likelihood and nature of any nuisance trips if one RCD were installed for the whole fusebox?


I know this may be something of a crystal ball question, but really I need to weigh up the chance and cost of problems (likely a new CU) against the cost of installing a single RCD. The only time a fuse has blown in the two years I've been in the house was on the lighting circuit, so no other obvious signs of earth leakage - that said, I probably have a sh*tty earth right now and any earth leakage I assume will be magnified once the PME connection is made.


To what extent could this be mitigated using a 100ma RCD (as opposed to 30ma)? Where is a 30ma RCD required? Is there an inherent firerisk with rewireable fuses slow fusing if only slightly overloaded?


Thanks for taking the time to read this and for your thoughts.

what you`v got to remember with these BS3036s is their a bit on the strong side....so any minor earth faults will not necessarily be being picked up by them...together with the earthing arrangements here at the mo.....if theres any issues regards insulation then an RCD should sense it.....and act accordingly....these final circuits need subjecting to both continuity and IR tests first before anything else.....you need to confirm that they are good for continued service before even contemplating introducing an earth leakage device into the mix....

 

[Siddell]

Spot on.

I'll get someone else round to discuss and quote.

Thanks guys.