suppliers main fuse (100a)

[j.capes]

Hi,

I have a question which is annoying me a bit and no one seems to be able to answer my question very well so i hope someone here can help me !!

Ok, so at home the suppliers fuse is 100a, which is the norm as far as i understand. There are two consumer units, the first has a main switch which is 63a and the second has a 63a RCD. So this all seems ok however on the first consumer unit the individual breakers add up to more than the 63a main switch???? I understand that diversity is applied in these situations sometimes but surely if everything draws its full load there is no protection as the main switch wouldn't trip because that offers no overload or fault protection and the next thing in line is the 100a suppliers fuse which is rated higher than the 63a main switch anyway which means the 63a mains switch would burn out?? Am i making sense?? I know also that the 63a mains switch can take more than its raiting but deffinaty not 100a???

Also i understand that because there are two boards (both 63a protection) that means the load is split and comes over the 100a suppliers fuse as 126a which makes sense but the individual boards dont seem to be protected.

To me it would make sense to put a 100a main switch, so that if the installation was used to its max the switch would allow that current to flow which in turn would blow the suppliers fuse and allow safe protection for the whole installation?

I hope im getting across my point and that you understand what im saying. Any answers would be appreachited. Thanks

 

[matt.leung]

dont forget what type of 100a fuse it is.... it will not blow at 100a's.

it could be an HRC fuse....

 

[Evans Electric]

First thing , J Capes, is welcome to the Forum , I get the impression you are, maybe, an apprentice ???

1. You will find that older C/units will have 60/63A main switches.

2. Diversity has a huge bearing on domestic elects. and is ever present. There is no real point in adding up the values of the MCBs as it gives no useful information to be honest. Hardly any domestic ring main will ever carry 32A for a start. Yes ,a shower will draw its full 35 - 40 A but only for say 10 minutes then its off again . Imm. Htr. will draw its full current ( 10 - 12 A ) but again the stat will cut in once tank is up to temp.

So 98% of domestic main fuses either 60A or 100 A usually sit there with no problem . If you have a clamp meter you will see the current climb when cooking and showring is taking place but there is also a time element where a HRC fuse will carry a load for a certain time .

The whole supply network is based on this happening .

Edit :- You may not be an apprentice but possibly a bit older ?? I summise this, Watson, because your spelling is quite good and so is your setting out , and there is a notable lack of "textspeak" . ?:|

You will get a much better answer shortly when others log in .

 

[steptoe]

one question,

what makes you think you have 100A main fuse?

have you actually seen it?

do not go by what is printed on the enclosure, that simply is an indication of what can be carried by said holder, it may well be more than likely a 60 or perhaps an 80A fuse.

 

[SPECIAL LOCATION]

Hi,I have a question which is annoying me a bit and no one seems to be able to answer my question very well so i hope someone here can help me !!

Ok, so at home the suppliers fuse is 100a, which is the norm as far as i understand.

erm.. NO 100A is NOT the norm....

Many new builds and service cut-out replacements are fitted with 80A.

and..

Many older properties are still only fitted with 60A!

There are two consumer units' date=' the first has a main switch which is 63a and the second has a 63a RCD. So this all seems ok however on the first consumer unit the individual breakers add up to more than the 63a main switch???? [b']I understand that diversity is applied in these situations sometimes but surely if everything draws its full load there is no protection....

Actually the above statement shows that you DO NOT understand diversity...

An individual circuit MCB rating is NOT the load current of that circuit!!

(that would be bad design with risk of nuisance tripping!?)

Multiple MCB rating added together are NOT the maximum demand.

....as the main switch wouldn't trip because that offers no overload or fault protection and the next thing in line is the 100a suppliers fuse which is rated higher than the 63a main switch anyway which means the 63a mains switch would burn out?? Am i making sense?? I know also that the 63a mains switch can take more than its rating but defiantly not 100a???

Also i understand that because there are two boards (both 63a protection) that means the load is split and comes over the 100a suppliers fuse as 126a which makes sense but the individual boards don't seem to be protected.

To me it would make sense to put a 100a main switch' date=' so that if the installation was used to its max the switch would allow that current to flow which in turn would blow the suppliers fuse and allow safe protection for the whole installation?

I hope im getting across my point and that you understand what im saying. Any answers would be appreachited. Thanks[/quote']

You need to step back and reconsider your scenario from a different angle..

First:- Consider a scenario of a bit of basic circuit design...

We have a hypothetical load that typically draws 'X'amps occasionally going to 1.5'X'amps at its peak load.

Due to thermal insulation all around the cable route we install a bit of cable that can carry 2.5'X'amps

and the nearest MCB rating that is bigger than our peak 1.5'X' and smaller than our cable rating 2.5'X' happens to be a 2'X'amps rated MCB..

So we now end up with an MCB (2X) that is larger than the max load (1.5X)..

AND

the max load rarely happens cuz most of the time the load is only 'X' amps..

So in this example.. if we just blindly look as MCB rating we get a figure that is double what the normal everyday load is!

Second consideration:-

1/

go around you house and write a list of every electrical item you have that could possible be switched on...

2/

Tell me how often per day, week, month or year,

EVERYTHING is actually switched on?

I guess there will NEVER be a time like this...

(We have been married now for 25 years and I have NEVER had a single minute when everything is switched on!!!)

3/

Draw a bar graph time line with a bar for each appliance..

divide you time line into 15 minute segments...

0:00,0:15,0:30,0:45,1:00,1:15,1:30,1:45,2:00,2:15,2:30,2:45.. etc..

Now shade in the real time periods during the day when a particular appliance is likely to be in use (AT FULL POWER)..

remember kettles, toasters, washing machines are probably at full power for less than 5mins, while they just heat something up...

Look back at you graph and see what time of day and duration that you get FULL MAX LOAD being drawn!!?

- - - - - - - - - - - - - - - - - - - - - -

By now you are hopefully starting to get a REAL grasp of diversity...

It is not just max power ratings.... time of day & durations also come into to play!. :|

And it certainly is not JUST mcb ratings!!