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Why do we use B and C type MCB's?

Postby ericmark » Sun Jun 29, 2008 11:16 am

Why do we use B and C type MCB's?
A D type MCB trips at 20 times the rated value.
So 230/(Rated value x 20) = Max resistance of cable.
Also 230 x 5%/Rated value = Max resistance of cable.
Since divide by 20 = 5% the limit due to current required to trip a D type MCB is same as the limit due to volt drop of 5% before the 17th Edition B and C type MCB's would be used because of the earth loop impedance but with RCD's taking care of that I would have expected B and C type MCB's would have very little place and would just cause nuisance tripping? Especially in lighting circuits with 3% volt drop and often tripping the MCB when the light emitting devices blow.
I can see to protect equipment we may need B and C types but not to protect the installation.
So what have I missed?
My error has been pointed out using D type could blow the incoming fuse so other than for very low rated MCB's such as for lights we should not use C or D rated. I still invite any comments I want to design a spreed sheet to make the Schedule of test results and at the same time test for anything which is wrong with the results which is why I posed the question.
Last edited by ericmark on Sat Jul 12, 2008 6:15 pm, edited 1 time in total.


Simply Build It

Postby sparx » Wed Jul 02, 2008 10:49 pm

Hi Eric,
I think if you look at those boring I2t let through charts on page 248-230 you will see the amount of energy passed by say a 32A CB varies between 160A, 320A, 640A so under short circuit conditions their are good reasons to use the lowest practical rating ie B type unless tripping too easily. This has led to the practice of changing 6A type B's to type C's too eliminate the 'Blown bulb trips', however on the other side of the equation is the Zs factor ie a circuit designed for a type B may have too high a circuit impedance to comply with a C rated breaker.
Eg a 6A type B will stand a Zs of upto 7.67 Ohms so a circuit of say 4.6Ohms would be well in, BUT if then replaced with a C6 only allowed 3.83Ohms so would not comply, ( a D type only allows 1.92Ohms) so a simple swap to cure a lamp problem could cause a failed future PIR ,
regards SPARX
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Postby ericmark » Thu Jul 03, 2008 12:56 am

Thank you sparx seems two main reasons for not using D Type is:-
1) It may allow incoming fuse to blow.
2) BS 8436 will not take the current so may not open MCB.
As to your example with 230 volt and 6 amp the R1 + Rn must be less than 1.15 Ω to keep within the 3% volt drop limit. Although R1 + R2 may be less than R1 + Rn one would expect 1.45 Ω + Ze assuming no parallel paths. For a D Type the permitted limit is 1.92 Ω so Ze would need to be more than 0.47 Ω before one would have a problem. This is unlikely on a TN-C-S supply. On a TT supply 200 Ω is the permitted limit with the RCD being the limiting device for earth loop impedance.

I was making a spread sheet so I could enter details and they would be auto checked. To begin with I thought I had made an error in that the earth leakage never seemed to be limiting factor except with some fuses. It was then I realised the volt drop was a bigger limiting factor than the earth loop impedance. Yet as my son so rightly points out there is no provision in the Schedule of test results for R1 + Rn and most electricians only measure the bits they need to record. When I pointed out 39 meters was maximum length of 1.5mm² cable that could be used on a 6 amp supply at 3% volt drop. He was shocked. Also 79 meters with 2.5mm² on a 32 amp supply at 5% volt drop and he surmised that some one had miss read the 100m² area covered as being a 100 meters of cable.

I am not suggesting we should use all D Type MCB’s but like you have just done many times I have told people they can’t use D Type because of earth loop impedance problems and you must check earth loop impedance before extending any cables but now I realise that it is the R1 + Rn reading which is much more likely to limit cable length and this reading is very rarely made directly.

Mind you it seems very few checks are made on the reading submitted anyway my son found another electrician working for the same company was not disconnecting the earth wire to the earth rod before measuring the earth rod resistance and as a result had been entering readings for Ze on a TT supply of around the 1 Ω which had never been queried. Only when he tested a house with no gas and got 9 Ω did the electrician ask my son the pass level and realised his error. The paper work is supposed to be checked back in the office and with clearing agent and then will LABC and non had picked up on error. Only a paper work mistake I know and no real danger but what is point of Part P if errors like that are missed. My son also tells me he has entered the R1 + R2 reading in R2 slot by mistake and as a result was over permitted Zs but again no one noticed.

So I guess there are many houses with too much cable in circuits so using Type B will at least reduce the risk where mistakes have been made. I think maybe I need to hide the regulations book I think I am guilty of following the book too much rather than using common sense.

After writing this I noted the same questions were raised in the IET forum with the point being raised on the number of reading which should be checked yet the results are not entered in the test results which must mean in the main they are never checked.

73 Eric

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