MCBs

[robinsoncrusoe55]

Whats the main difference between a type "b" or "c" mcb......Thanks![/b]

 

[HullMarc]

Time it takes to trip.

C type MCB will take currents over it's rating for longer than a B type will. Ideal when used on devices that have high inrush currents.

 

[RF Lighting]

//www.diynot.com/wiki/electrics:mcb

 

[ericmark]

The amps figure refers to the thermal part of the trip and it would need to exceed rated amps for quite a time to trip.
The 'B' 'C' and 'D' refers to magnet part of trip and once this current is exceeded it will trip in milliseconds.
'B' = 5 x rated amps
'C' = 10 x rated amps
'D' = 20 x rated amps
It is these figures that are used to determine maximum value so for example a B16 MCB the current required to trip is 80A so the cable resistance needs to be 230/80 or 2.875 ohms if over that figure even with a short circuit the MCB would not trip immediately. This is why when a new circuit is installed the electrician uses an earth loop impedance meter to check if there is a direct short the MCB will trip.

There is also one more rating on the MCB something like 4700 or 10000 and this is the amount of amps it can handle under fault conditions. The same meter is used but a different scale this measures the prospective short circuit current which is directly related to the ELI although measured both line to neutral and line to earth and highest reading is taken. This is measured at the consumer unit i.e. as close to incomer as possible where the ELI is measured at furthest point from consumer unit.

If you get the last installation certificate or periodic inspection report or even a minor works certificate you will see all these readings entered on the form.

This is why electricians say DIY is dangerous as it is very rare for any DIY guy to even know what the readings are never mind check them.

Same readings can also be used to calculate volt drop.

 

[DCC]

There is also one more rating on the MCB something like 4700 or 10000 and this is the amount of amps it can handle under fault conditions.

Out of interest, what's the time period for this rating?

 

[Adam_151]

BS7671 assumes that mcbs trip in 0.1 sec under fault conditions, however modern devices are likely to be far faster, manufacturers data does not typically list this as its not a particulaly useful figure itself, a graph of max I(sq)t against PFC is normally obtainable and this can be used to carry out the adiabatic equation. Knowing the tripping time would not allow this as at these times the fault current stops being a simple RMS value and becaomes asymetric

 

[ericmark]

As Adam has said once you start looking at let through figures etc it starts to get messy. If the PFC is above the MCB rating I have always used fuses instead. Of course one could use a moulded breaker but these are rather on the large size.

One time I had problems was in charging radio batteries for the banksmans radio in a crane. The 300A supply was too big for the RCBO we had intended to use so had to also include a 100A cartridge fuse like used in a house supply which had a low enough let through value to then use a RCBO.

 

[DCC]

BS7671 assumes that mcbs trip in 0.1 sec under fault conditions

Thanks Adam. 10kA even for 0.1sec is a reasonable chunk of energy, I'm fairly impressed they can survive that and live another day.

 

[ban-all-sheds]

//www.diynot.com/wiki/electrics:mcb

The following user says thank you to RF Lighting for this useful post:
robinsoncrusoe55 (yesterday)

Huh!

I write the Wiki article, you get the thanks for pointing someone at it.

There ain't no justice.

 

[ColJack]

The following user says thank you to ban-all-sheds for this useful post:
ColJack (today)

happy now?

 

[ban-all-sheds]

 

[rockdg]

Where and why should you use an RCD or MCB.


If i install a new circuit for a kitchen oven, should i use an RCD or MCB. Would it be safe to use either or am i breaking all regs picking the wrong one for protection

Thanks

 

[ColJack]

Where and why should you use an RCD or MCB.


If i install a new circuit for a kitchen oven, should i use an RCD or MCB.Thanks

yes..

new circuits that are buried less than 50mm from the surface of a wall, and are not in earthed metallic conduit or trunking need RCD protection..
this is in addition to overload protection which is provided by an MCB or fuse..
you can get combined RCD and MCB's and these are called RCBO's..

also, ALL socket circuits in a domestic situation need to be RCD protected also..