The maths dont work out

[the__apprentice]

Sorry to blast you technical guys but wanted to keep this seperate,

If you have a 10 way split load board on the rcd side you have:-

3 x 32amp mcb's
1 x 40amp mcp
1 x 20amp mcb

On the non rcd side

4 x 10amp mcb's

In total Im determing the load to be 196A but the main switch is obviously only rated at 100A and would trip out if exceeded so how doe

 

[davy_owen_88]

Diversity - not every circuit will be running at full capacity all the time. Oh, a point to remember is that the main switch is only an isolator and will not trip even under short circuit conditions, same applied to RCD's.

An example of diversity in a domestic property:

You have 2 6A circuits but there are only 8x 100w lamps attached to each so only 3.5A per circuit. Also consider that only a few of those lamps are going to be on at the same time. 66% is the figure for this so for the load stated above you could assume the loading would be 2.31A per circuit instead of the full 6A.

 

[fattony]

diversty my dear friend, not all the load will be used at the same time, there is a table in the on site guide with th diversty factors in it.

Your asking some good questions and Im sure people here are happy to help

 

[Adam_151]

Main switches don't trip The main switch is protected by the company fuse which will be a BS1361 type II of max 100A for a domestic house in 98% of cases, you do have to be careful with older boards that have 60A main switch if the fuse is >60A though (though if the ratings add upto less than 60A there is nothing to worry about, like standalone extension board with 60A main switch and 30A rfc and 5A lighting fuses)

Load is not 196A btw, look up diversity in your onsite guide (and even that errs on the high side)

 

[RF Lighting]

Do you have a copy of the onsite guide?

If you do, look up diversity.


If you don't then it basically works as follows:

When you design an installation you assume that not every circuit will be loaded to capacity all the time.

For example if you have a 32A ring circuit, then this is the maximum that the ring can be loaded to, but in reality it is likely to be much less, and you are not going to run all the rings at capacity at the same time.


(Also if you are trying to pull more than 100A for a sustained period the main service fuse will pop)

 

[davy_owen_88]

At least no one can say we aren't keen

 

[the__apprentice]

Main switches don't trip

Wow some great help here guys, so let me get this straight, if a fault develops (short circuit) only the the mcb protecting that circuit will trip!

The main switch is just there as an isolator nothing else!

The rcd is there to protect against neutral to earth and live to earth faults!

Is the above correct?

 

[davy_owen_88]

Yup, spot on. The MCB for the faulty circuit will trip and if that doesn't then the service fuse will.

 

[RF Lighting]

Not bad really, 4 different people giving roughly the same answer in just 7 mins!!

 

[Lectrician]

Blimey - Everyones had a belly full of food and raring to go!

 

[the__apprentice]

pretty good I'd say, have a beer on me guys

 

[Adam_151]

MCBs and fuses generally all protect against all three types of overcurrent fault:

Overload - heathy circuit, but too many appliances plugged in
Short circuit - phase to phase, or phase to neutral fault of little impedance
Earth fault - A phase to earth fault of little impedance

RCDs are mostly for supplimentary protection (though in TT systems they provide the ADS in EEBADS instead of the overcurrent devices), they don't trip out on overcurrent, only on inbalence

Supplimenary protection against direct contact, extra fire protection, some stuff in some areas of bathroom, caravans, etc


an RCBO is an MCB with built in RCD

 

[plugwash]

Wow some great help here guys, so let me get this straight, if a fault develops (short circuit) only the the mcb protecting that circuit will trip!

It depends on short to what, if its a short live to earth there is a good chance the RCD will go first (or possiblly both will go)

If a fault happens so close to the CU and the fault loop impedance the short flows through is so low that the fault current is high enough to make the MCB arc over (unusual but it can happen) then the service fuse should blow. The service fuse also provides protection against overloading of the installation as a whole that isn't localised to one circuit.

Its pretty rare to blow even a 60A service fuse on a domestic though. Those things take a LONG time to blow at moderate overloads.