TYPE B / TYPE C MCB'S

[ROBOROB]

I'm sorry if i sound like a novice, maybe its because I am!

Whats the difference between a type B mcb and a type C mcb?

 

[ban-all-sheds]

Different response curves - in a nutshell Type Cs will withstand a short duration surge better than Type Bs.

See http://www.tlc-direct.co.uk/Book/3.6.4.htm for a lot of info.

 

[Millennium_Boy]

The Type B is the one you must use at home,

the type C is a Commercial "Time delay" type of mcb, unsuitable for domestic circuits, as it does not offer a "fast enough disconnection time, as a reasonable earth loop impedance", to be suited to household use.

 

[ROBOROB]

Thanks to you both.
It looks like a really useful site, and confirmed what you are saying. (would I doubt it?).

So then, .......... its off to the shops for me. Type B's, here I come.

Thanks guys

 

[ban-all-sheds]

The Type B is the one you must use at home,

the type C is a Commercial "Time delay" type of mcb, unsuitable for domestic circuits, as it does not offer a "fast enough disconnection time, as a reasonable earth loop impedance", to be suited to household use.

Surely the potential application for a Type C in the home is for a lighting circuit where fluourescent startup surges or filament break surges would cause a Type B to trip. Lighting circuits need a 5s disconnect response.

Genuine, not rhetorical or sarcastic question - (as I don't have all of the tables), what are the maximum Zs values for 0.4s 6 & 10A Type Bs compared to those for 5s 6 & 10A Type Cs?

And anyway - isn't the difference between Type B and Type C that the point at which the response curve goes vertical is further to the right with the latter type, and that in the 0.4 - 5s range there is virtually no difference?

In other words, for fixed appliances where the disconnect requirement is 5s, aren't Type Cs just as effective?

 

[spark]

b-a-s has got the right idea. MCB's have the same maximum earth fault loop impedance for both 0.4 and 5 second disconnection times (in fact all breakers will trip within 100ms, hence why the requirements of both disconnection times are met). Due to the nature of breakers they have an either 'will trip' or 'won't trip' character. They don't just trip slower when you reduce the magnitude of the fault current, as fuses do (until you reach disconnection times in excess of 10 seconds for B's, 4 seconds for C's and 3 seconds for D's which is then generally for clearing an over load current not a fault current).

My understanding of the reason breakers have this characteristic is because they work on a thermal-magnetic basis (as opposed to fuses which are purely thermal). By changing the type of breaker you are changing the nature of the magnetic trip within the device, hence why b-a-s noticed that the tripping curves on the graphs are pushed further over to the right as you change from B up to D type, increasing the amount of fault current required to trip the device within 5 seconds. As an example a B6 requires 30A fault current to operate, the C requires 60A and a D requires 120A. This is reflected in the maximum tabulated Zs, these being 8 ohms for B6, 4 ohms for C6 and 2 ohms for D6.

In simple terms, the thermal trip (denoted by the number) is designed to operate the device in the event of longer term overload and magnetic trip (denoted by the letter) handles the fault currents.

b-a-s, maximum Zs values for B6 is 8 ohms & B10 is 4.8 ohms. As for using a type C for a fixed appliance, there's no reason you couldn't use it, as long as the maximum Zs is within limits. As explained above, a type C is not a 'time delayed' device, but for a given fault current, you may find that a B will trip but a C won't.

 

[ROBOROB]

I thought it was a straight question with a simple answer...... obviously not!!

Let me give you some background info.
I've recently bought a house that requires a complete re-wire.
obviously Im going to replace the consumer unit and my original query was re: what type of mcb's to use. I saw one in screwfix complete with type b mcb's for £59.99 which looked like a good buy, but I wanted to ensure it was suitable for the intended use. what do you think?

Thanks in advance.

 

[masona]

To help the electrician,is it this one ? if not,ref or model number would help.

 

[supersparks]

To help the electrician,is it this one ? if not,ref or model number would help.

Volex is **** gear!

AR

 

[masona]

 

[Scoby_Beasley]

Volex is **** gear!

So whats the best stuff ? (like Crabtree meself)

 

[supersparks]

Volex is **** gear!

So whats the best stuff ? (like Crabtree meself)

boards:
MK,
MEM,
wylex and
hager are the best makes

AR

 

[Scoby_Beasley]

Glad to see our wylex is on your list

 

[masona]

Glad to see our wylex is on your list

Are you sure you hic read that hic right?

First 2 letter wy could be reading vo,wait 'til you soder up and have a another look If it is a volex,get rid of it

 

[Scoby_Beasley]

Glad to see our wylex is on your list

Are you sure you hic read that hic right?

First 2 letter wy could be reading vo,wait 'til you soder up and have a another look If it is a volex,get rid of it

Nah read it right