ratings

[snowplough]

Hi Guys,

One of many questions for you,

Can any one explain to me for instance, if i have a 100amp main fuse , then why is the c.u ive got (REWIREABLE) say on it 60 amps , i thought that the fuse needs to be the same size or bigger then the equipment it is protecting, i.e main switch on consumer unit and all the buzz bars etc.
If this is correct should i replace the c.u with one that as at least 100amp load capacity.

If the case is that its ok to have a lower load rating then the main fuse then in the case of a overload or short circuit say before the small fuses in the c.u ,whats to stop the c.u cathing fire .

Many thanks for reading this ,

Snowpiece

 

[Johnny-G]

The main switch is rated higher than the protective device of any circuit so, theoretically, the device should trip long before 60a+ is drawn long enough through the main switch and when it was installed it was unlikely you'd use more than 60a on the installation. The 100a fuse is the property of the DNO and is only there to protect their equipment as that's all they care about.

 

[mechelec]

the DNO fuse is for preventing customer installation overload and short circuit faults damaging their supply cable but they do allow it to protect the meter tails (up to 3m max length usually) not the consumer unit or distribution board

 

[Deleted member 26818]

How do you know that the rating of your fuse is 100A?

 

[snowplough]

Hi Guys,

Thanks for your reply,

I appologise, i meant to say , the fuse is always smaller then the device its protecting.

If the DNO fuse is 100amps ,and my C.U says 60amp max load then dosent that mean anything above 60 amps but below 100amps could flow melting the metal in the main switch or something causing fire , why else would it tell you not to have more then 60amp load on it .

I have 2 x30amp, 2x 15amp, 1x5amp , and a spare fuse way which adds upto 95amp which wouldnt blow the main DNO but what about the 60amp max C.U rating ? .

Your Thoughts much appreciated

Snowplough

 

[telectrix]

these rewireable fuse boards were installed before we had huge KW showers and cookers. 60A was considered to be well above any anticipated load. ( that's why the DNO fuses were generally 60A at the time. ). now we have DNO uprating their fuses but old boards with 60A main switches left in situ. when fitting high current loads, therefore, we have to consider if we are going to overload that 60A main switch and upgrade ( usually by means of replacing the CU ).

in the scenario you have just posted, all circuits will not be fully loaded simultaneously ( diversity ) and so in real life that 95A would not be attained

 

[snowplough]

May Thanks Telextrics,

I can understand maybe all the 95 amps may not be used at the same time in our home as it would mean all lights on ,heavy equipmeant on sockets kettles etc emergenheater on etc, .. but in thwe scenario where access current was drawn over a long period above 60 amp but below the 100amp DNO, would this melt the main switch on causing fire?.

Another question for you , i was looking at the Split board C.u in wickes today with the rcds protecting both both sets of breakers , the C.U MAX LOADING SAID 100AMP , when i added up each side ,they both came to alot more then 100amp which brings me back to the same question ,why have a C.u maximum when there would be a possibility of exceeding that and what would protect the C.u in that case if the main DNO DIDNT bLOW.

Many Thanks

Snowpiece

 

[telectrix]

it's called diversity. mentioned in several previous threads. look at your dual RCD CU again. say you've got a 40A MCB for a shower ( largest load in the installation.) so then you add all the other MCBs up, and multiply that figure by 0.4, then add the result to the 40A. i.e 100% of largest load + 40% of remainder.

 

[snowplough]

Many thanks Telectrix,

I did diversity i level 3, but i thought it just applied to the main incoming tails , ill have a look in the On Site Guide to see if i can see this ruling so you dont overload the C.U.

I want to replace my C.U with one of these new split boards thats why i have asked these questions regarding C.U maximums etc.

Another question to bother you with , my current C.U IS rewireable , the 30 amp breaker for the ring main as a 32 amp rcd protecting it which is fixed next to the fuse board downstream from the 30 amp breaker rewiwerable breaker which makes sense to me as this will protect the rcd from overload an short circuit, now going back to this split board we been talking about , why is the rcd upstream of the set of breakers its protecting as the line goes from the main switch straight into the rcd first.? as mine as mentioned is down stream as i mentioned,
and secondly the rcds are rated at 80amp max load ,and again the breakers the rcds protect on the split board add up to more then this on each side? is this also worked out wth good old diversity.

Thanks for taking time to read this as every answer seems to throw out more questios to me,

Kind Regards,

Snowpiece

 

[telectrix]

you're correct there. diversity applies to RCDsas well. don't forget that the A rating of an RCD is it's rated current capacity, not to be confused with overcurrent protection.the RCD you have in now has obviously been added to give protection to socket circuits as a cheaper alternative to replacing the CU, using the 30A rewireable as overcurrent protection. this RCD can be removed when you upgrade the CU.

 

[snowplough]

ThanksTelectrix,

I had A look in the O.S.G P95 onwards trying to understand the tables , i thought that you added diversity to each individual circuit i.e 66% lighting , cookers with the 30% +10 rule etc then just total it up to get your new total with diversity, but in the tabe on P97(O.S.G) it mentions the 100% of total plus o.4 as you said in your previous reply. so i am a little confused ,is the 100% of hihest load plus 0.4 a rule of thumb way of working it out for consumer units such as mine bearing in mind that if i took my 30amp shower as my higest load which is for my ring final circuit, i disregard the 66% rule for my lights, and cooker 30%,then just times these and any other load regardless weather or not it is in the table on p97 with diversity allowed for it.


Another thing Telectrix, you said diversity also applies to tHE C.U load sizing as well as for the main tails , on the bottom of page 96 (O.S.G) it is important to ensure C.U are of sufficient rating to take the total load connected to them without the application of diversity . So is this then not contradicting what we have been saying and should be able to take all the breakers added together ,im getting mix messages,hope you can clarify.


Many Thanks


Snowpiece

 

[telectrix]

confusing innit. thats BS7671 for you. but , remember that the rating of a MCB is >the design current of the circuit. e.g. a RFC design current is generally around 24A. you have a 6A MCB on a lighting circuit that never draws more than 3A.

 

[snowplough]

You said it Telectrix, (lol)

I thought passing my 2391 was difficult , i can,t get my head round this logic,


I understand the logic where 1b must be less or the same as In, In must be the same as or less then the cable current carrying capacity, and all three, ie 1n,ib,Cable current carrying capacity can all be the same theoretically.

Which takes me right back to my initial theory as i thought the Main fuse should be the same rating as the C.U rating, or at least the C.U MAX load rating should always be more then the main fuse so the main fuse will blow protecting the C.U from damage and even hving all main isolating switches as a mcb as it would be cheaper to reset a switch on the C.u rather themn calling out the Local elec company everytime you blow the main fuse.

Many Thanks as always Telectrix for your comments ill keep scrtching my head.


Kind Regards


Snowpiece

 

[malcolmsanford]

View attachment BS60439 UK Annex ZA.pdf

Have a read of this and it might help you regarding new Consumer Units.

The majority of older consumer units are not obviously now suitable for the modern installation as Tel as pointed out, and so why often you will see old wylex BS 3036 CU's and perhaps a modern shower enclosure next to it.

 

[snowplough]

Many thanks Malcolm,

Ive got to get my head around this on site guide as well i think,

Kind Regards

Snowpiece