UK MCB type confusion.

[David345]

I've noticed at my grandparents house that all the MCB's in the consumer unit are all type C, every single one of them. Including the ring final circuits and lighting (there's no fluorescent lights in the house at all). The house isn't that old as well. Maybe about 16 years old. With me being a trainee electrician I've always been taught that domestic dwelling ideally should use type B MCB's unless you have a whole row of fluorescent lights which is unlikely in a domestic dwelling. It's this not ok? Thanks.

And obviously the circuits will still work fine with a type C MCB but in the event of a fault the MCB might not trip in time.

 

[Flanders]

Sounds like someone working in the commercial sector fitted the consumers unit maybe had a load of spare mcbs on the van !! I would of thought disconnect time would be achieved as the cables runs would not be that long. Although the 32 amp circuits may be close to the maximum zs permitted. Was there any RCD protection

 

[David345]

Yeah if I can remember, there was a 30mA RCD installed there.

 

[Julie.]

There isn't any rule mandating type B or any other, as long as protection is achieved the installation is compliant.

It is perfectly acceptable to use C or B.

As long as the fault current is disconnected within the usual time, and the temperature rise (i^2t - adiabatic equation) is within limits then it is every bit as good as any other solution which meets the same requirements.

 

[David345]

Yeah that's fair enough. I was just confused, haha, I understand now. Thanks.

 

[static zap]

someone working in the commercial sector fitted the consumers unit

Have pondered this situation , will they last any longer,
built to a better quality ?
(Still expecting a bigger bang !)

 

[pc1966]

Have pondered this situation , will they last any longer,
built to a better quality ?
(Still expecting a bigger bang !)

As Julie already pointed out they are perfectly acceptable so long as you meet the disconnection times. And that is typically around half the Zs value for a given rating at the limits of the magnetic trip are 3-5*In and 5-10*In for B or C respectively.

However, the size of the BANG! is more complicated, or to be more technical, your I2t value is not always bigger for C than B unless you fall in to the region 3-5In where the B might trip "instantaneously" trip while the C trips on the thermal curve many seconds later.

Here are some examples from the Hager catalogue (one of the few that actually publish the data for their MCBs):

NOTE - they are not graphed on the same scales! Also the C-curve go down to 0.5A while they only go down to 6A in B-curve.

But if you look closely at, say, 63A (as top curve in both) you see the I2t let-through is practically the same except in that middle region (shown for mid-range currents, 4*In=252A to 7.5*In = 473A) where the B is massively less due to the great reduction in 't' from the magnetic trip's action.

 

[Kaiz Rxstr]

3 Types of MCB’s
The classification of Types B, C or D is based on the fault current rating at which magnetic operation occurs and their ability to handle surge currents without tripping. In = Nominal rating.
1. Type B breakers are designed to trip at fault currents of 3-5 times rated current (In). For example a 10 A device will trip at 30-50 A.
2. Type C breakers are designed to trip at 5-10 times In
3. Type D breakers are designed to trip at 10-20 times In

If you want to know more click here, Miniature circuit breaker types

 

[richy3333]

As an aside we had a house and barn that was wired with C curve MCB's and Schneider boards. When I asked the homeowner about its history they said that it was all their brother could nick from the stores in the factory where he worked as a maintenance electrician

 

[Julie.]

The characteristics PC posted are very useful - and once understood do illustrate how many people really do misunderstand how the "size of the bang" works.

At home I have three rfc (kitchen, upstairs and downstairs) - the Zs are 0.43, 0.84 and 0.67 ohms, (hager b-type RCBO btw) - under hot running and a supply voltage of 95% this would give 407A, 208A, and 261A fault currents.

If I used a B type - then in the first case 400A would have a let through of circ 1.3 kA^2S
But using a C would also give circ 1.3 kA^2S - EXACTLY the same! (within tolerance of reading the characteristics)

There would be a difference with the other two rings though
208A -> 0.35 kA^2S > 70 kA^2S
261A -> 0.5 kA^2S > 1.5 kA^2S

However in both these cases the type C wouldn't achieve the disconnect time in any case! - so shouldn't be used, and a big(ish) EICR fail if they were.

In general if you achieve the required disconnect time (0.1S) then the fault current and let through is the same type B, C, or D. - It's all about the level of fault and little to do with the protection itself.

Now the caviat - there are borderline conditions - just about make 0.1sec (318A on a 32A joby) - The B type will let through 0.9 kA^2S whilst the C type would let through 1 kA^2S - so slightly different.

Also if it was sized for a 5 second disconnect time then D type and B/C would be very different.

 

[Ian1981]

The graphs hurt my eyes and are a pain to read for me, unless I can print it out and use a ruler to draw in the lines.
I just use manufacturer data in table form where available ( lazy)
Hager and MK only shown.
Good for ensuring protective conductor sizes based on disconnection times of less than 0.1 seconds as required.

 

[pc1966]

I just use manufacturer data in table form where available ( lazy)

Which is perfectly correct and why the tables exist in the first place!

In reality few folk will be in a situation where they need the sort of detail the curves provide. I just used it to illustrate a point about the limited "region of difference" between the B & C MCBs.

But a lot of real-world electrical work is like that, if you are using conventional circuits in conventional situations you need only look up a couple of tables in the On Site Guide to get the key information for a safe & reliable design.