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Topic Title: Why do we use B and C type MCB's?
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Created On: 29 June 2008 12:08 PM
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 29 June 2008 12:08 PM
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ericmark

Posts: 319
Joined: 12 February 2008

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?
Eric
 29 June 2008 02:32 PM
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Testit

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Originally posted by: ericmark

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?

Eric


Type B - trips between 3 and 5 time full load current

Type C - trips between 5 and 10 times full load current

Type D - trips between 10 and 20 times full load current

You are protecting the cable and if u wanted to use all type D breakers u r saying u want to keep EFLI 4 times lower to be able to trip the device then?

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 29 June 2008 02:45 PM
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ericmark

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EFLI would be handled by RCD and NFLI can be 4 times lower as it has to be that low to come with in volt drop rules of 5%.
NFLI = Neutral - Line Fault Loop Impedance maybe I have name wrong? Before RCD's we use to measure both and use highest value. With RCD's EFLI only needs to be 200 ohm so it will always be the neutral - line loop impedance which will be limiting factor.
Hope I have made my point plain.
Eric
 29 June 2008 03:06 PM
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Testit

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nope, still mud to me I'm afraid..

Firstly u dont always employ and RCD, and they can fail also..

Secondly the circit impedance would significantly reduce the size of circuit you could have and would likely be impractical...

Have to disagree I'm afraid..

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 29 June 2008 03:45 PM
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ericmark

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It seems from the one reply I did not make myself clear. When we for example install a ring main we need to limit the amount of cable used and hence the resistance so that.
a) A short circuit will open the MCB.
b) The volt drop does not exceed the 5%.
The maximum resistance between line and neutral to come within the 5% is the same as maximum resistance allowed for a D rated MCB. Before the 17th Edition we may have considered the Earth Loop Fault Impedance. Now an RCD protects for earth faults. We can allow that to go up to 200 ohms as being stable value so as far as earth is concerned either we have an earth or we don't. We are unlikely to be worried too much about the value if Zs is within 10 ohms or so of Ze. (more than 10 ohms we should be looking for bad connections) Many times I have seen the MCB trip before a 13 amp fuse blows. Looking at 314.1 we are to sub divide into circuits to stop one item affecting another and if we use D type breakers then there is less chance of a single item tripping the MCB rather than blow its 13 amp fuse. It would seem we might as well use D type breakers to protect the cable.
However, where we are protecting an appliance for example built in cooker or hob then we must abide by manufactures recommendations and quite possibly a type B or C would be called for.
There could be reasons why what I am saying could be wrong for example if the 13 amp fuse was unable to cope with the let through values of a D type breaker. However, as yet, I am unaware of any reason why we should continue to use B and C type breakers as a general rule.
Giving an example:-
D20 MCB needs from table 41.3 a loop resistance of no more than 0.57 ohms.
5% of 230 volt = 11.5 volt at 20 amp that's a loop resistance of 0.57 ohms.
So if the circuit is OK for volt drop it is also OK for a D type breaker. And of course well within the requirements for a C type breaker, so why ever use a B type breaker to protect cable, only reason would be to protect equipment like shower, cooker, hob i.e where only one item is supplied by that MCB.
So have I missed something? My whole point is the circuit impedance is good enough to satisfy volt drop it will also satisfy a type D MCB so would make no difference to the size of the circuit you could have.
Eric
 29 June 2008 04:07 PM
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GaryMo

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Volt drop is for the final circuit only, earth fault loop impedance also includes the live and earth loop on the suppliers side.
Also, it's my understanding that we should design final circuits to disconnect in the relevant time for the circuit protective device - B32 max loop impedance 1.15ohms, D32 max loop impedance 0.29ohms.
Imagine a TN-S supply with a Ze of 0.3ohms......
 29 June 2008 04:18 PM
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tattyinengland

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It is my understanding that type B mcbs do not allow for start up currents of motors or loads that innitially spike on start up. Type C and D mcbs are used to allow for starting currents. Type B mcbs are least tollerant of large start up currents and type Ds are most tollerant of start up currents. IE: a type D mcb will allow a large motor to start up (Innitially using way more amperage than the mcb is rated for, for a brief period of time) the motor runs up to its running current and by this time the amperage has dropped to a reasonable level and the mcb works in the normal way.
 29 June 2008 04:23 PM
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intrinsic4225B

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Originally posted by: ericmark

...I would have expected B and C type MCB's would have very little place and would just cause nuisance tripping?


Where do you propose that such high current transients such as to instantaneously operate a Type B or Type C miniature circuit breaker (MCB) would exist in for example, a domestic installation?
 29 June 2008 05:42 PM
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ericmark

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I seem to be having a problem between earth and neutral loops. As I was lead to understand the earth loop impedance where using RCD's can be very high and is no longer a limiting factor. As such Ze and Zs are out of the picture and we are now interested in R1 + Rn shown as Step 2 in my book. I seem to recall having to make up the lead to measure R1 + Rn myself as only the lead to measure R1 + R2 (Zs) was supplied. To measure R1 + Rn both earth and neutral leads are connected to neutral.
As I see it the R1 + Rn values are now what limits the length of cable we can use on a circuit. On lighting circuits, I have seem many comments, of how once the fuse in the consumer unit has been changed to a MCB, the trip has opened each time a bulb has blown, yet it never blew the fuse. Using C or D type MCB's could reduce this problem, but in the past we did not have RCD's on the lighting circuits, so the EFLI would have drastically reduced the cable length we could use. With 3% volt drop the max R1 + Rn value for a 6 amp MCB is 1.15 ohms but the R1 + Rn values for a D type MCB are 1.92 so the D rating is not the limiting factor.
I will freely admit where RCD's are not used then R1 + R2 could become the limiting factor and then the B type makes sense but I am talking about where RCD's are used so even with a TT supply and a Ze of 150 ohm R1 + R2 would not be limiting factor.
Sorry I am dyslexic and it seems I have not explained myself well enough on previous posts from all the references to Earth Phase Loop Impedance.
Eric
 29 June 2008 07:00 PM
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andardry

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Hi eric , I'm nowhere near qualified enough to make a comment, but I do understand where you're coming from and agree. Bit like the recent thread on Zs readings, are they relevent. I can see a point where they do away with earthing altogether and rely totally on the RCD

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Andrew
 29 June 2008 07:15 PM
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alancapon

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Originally posted by: ericmark
. . . . . As I see it the R1 + Rn values are now what limits the length of cable we can use on a circuit. . . . .


This is only half of the circuit. The other half is the resistance between the phase and neutral conductors at the consumer unit, looking back into the supply. This resistance (which will be slightly different on every installation) when added to (R1+Rn) will tell you if you really can operate the mcb with a fault at that point.


Regards,

Alan.
 29 June 2008 08:04 PM
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Testit

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Hellooo eric..
A circuit cable is limited by:

Volt drop
Earth impedance
P/N impedance

in relation to its loading.......

The higher the P/N impedance the greater the volt drop as it naturally means the resistance is higher in the phase due to increased length of the circuit.

RCDs dont limit the current to 30ma.. but trip as low as 30ma or below.. i.e they trip when the current reaches up to 30ma but actually let through the full fault current on faults, tripping within a short time as the 30ma is reached when the fault occurs.

If you short the Phase and earth you will find that both the RCD and MCB trip, as happened yesterday when the mrs allowed the metal door to swing shut on an extention lead i was using on a cement mixer in the garden infact...

If you employ type D MCBs then you may find that in such circumstances the mcb wont trip. Admittedly the RCD may well trip then in its place. For lights I personally find it is the rcd that rips when lights blow, but a P/N short would obviously do it..

Currently using an RCD doesnt actually allow high impedances in the circuit, just higher impedances in the Ra of TT installs. The circit impedances for EFLI are still required to be low, i.e subtractng your Ra for your EFLI will give you figures within the normal circuit tolerances. You couldnt argue safety for higher reading in any circuit as higher resistances would mean increased heating and probably poor connections with possibility of fire risk.

You could calculate circuits on P/N impedances to employ an RCD and type D breaker,,, but naturally the higher currents for tripping the mcb would still mean shorter circuits or increased cable size in order to allow for the higher currents required to trip the mcb on a P/N short and dissipate the energy. Having said that I've not looked at a D type breaker characteristic in a while... but I recall its 10 - 20 times the load current as a general rule... sooooo... 640 amps for a 32amp MCB needs to be achieved on the PSCC at the furthest point of the circuit.. as opposed to 160amps with a type B mcb.

I dont have any figures on me or books but discriminating with intake fuse, and the larger fault currents employed requiring larger general cable requirements for suppliers, more energy needing to be dissipated through the device which is never good, are some of the factors that i think make the choice of D type breakers an exception rather than the norm...

Edit: Oh you would need to make the swiches and RCDs capable of withstanding the higher currents also..

All IMHO

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Experience can sometimes show that cost prevails over quality and safety, such little self-value that people hold.

Edited: 29 June 2008 at 08:53 PM by Testit
 29 June 2008 09:21 PM
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kaichung

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You will never use a D type breaker in a domestic install - it will cause too many instances of blown DNO mains fuses, and your supplier won't like it too much!

Besides, it will tend to make the install very expensive, as heavier cable will be needed to run the circuits than otherwise required.

Are YOU willing to pay for your lighting circuits to be run in 6mm sq cables, or the ring final circuit in 16mm sq cables?? - and you will have a very tough time getting these large cables into standard wiring accessories.


Edited: 29 June 2008 at 09:22 PM by kaichung
 29 June 2008 11:12 PM
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rocknroll

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The reason behind lamps causing MCB's to trip is due to high arcing currents at the time of the fault, this is often associated with cheap bulbs, the type B may be substituted with a type C if tripping becomes a regular occurrence as you would in commercial applications, often this can be cured by changing to better quality bulbs or changing from 6 to 10A, obviously the Zs parameters must comply.

Type D circuit breakers are not suited and not recommended for domestic applications due to the low fault currents and can easily cause damage to cables and possibly fires, they are designed for specialist applications and require a much more critical approach in their implementation.
regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 29 June 2008 at 11:15 PM by rocknroll
 30 June 2008 10:05 AM
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ericmark

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Thank you Testit & kaichung your point about incoming fuse is very valid and I will agree other than for lights with only 6 amp there could be a problem. As to rocknroll we can't use 10 amp MCB's on lights 6 amp is max as the ceiling roses are only rated at 6 amp. I expected I had missed something which I had otherwise I would have seen them used more in the past. Where we have multiple limiting factors it is easy to miss one.
My thanks again for all your help.
Eric
 30 June 2008 11:42 AM
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mapj1

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Although - a D type is not much worse than a 3036 hot wire fuse, and ther are plenty of installs protected by them, and not that many more failed company fuses.
Effectivly you are saying you would rely on the slower acting thermal part of the breaker, rather than the "instant" magnetic part, to clear all practical fault conditions.
Certainly for high current circuits like 40A showers and so forth, I think it might be difficult in real installations to go bigger than C curve and meet a 0.4 secs disconnection. 5 secs disconnection can be met with the thermal part of the curve, so B,C or D curve are identical..
As someone who says all but the longest lighting circuits that used to be fused, can be rewired with a 'C type' 6A, and neatly sidestep the blown lamp puts out whole house problem, I agree the idea has some appeal.
However, I think the voltage drop meets the spec assumption is a bit dangerous - there is often more drop (although more current too, so still lower impedance)in the supplier's side than there is in the final circuit.
regards M

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regards Mike
 30 June 2008 01:55 PM
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ericmark

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Thank you mapj1 I was exploring what benefits there may be now near everything will be RCD protected and was making an Excel spread sheet to both produce a printable schedule of test results and auto check if all reading complied with the regulations using both calculations and lookup tables and noted nothing ever failed on neutral line loop impedance unless feed by a fuse and with a RCD nether did they fail on earth line loop impedance and the main failure point always seemed to be volt drop. I wondered if I had made a mistake with my calculations so looked further which was when I realised the D Type MCB and the 5% volt drop both required the same neutral line loop impedance. With the RCD taking care of the earth line loop impedance requirements there at first look seemed little requirement for the B and C type MCB's which seemed odd but explained why everything failed on volt drop well before any other limiting considerations. But thanks to this post attention was brought to the limitations of the incoming fuse by Testit and kaichung with the fuse data being in the form of a graph as yet I have now worked out how to bring it into my spread sheet. But I have not given up. The other issue I have is where the MCB is not just protecting the wire but is protecting the appliance as in showers, hobs, and built-in ovens or some other part of the installation as with ceiling roses being rated at 6 amp so in spite of 16 amp limit in the BS 7671:2008 in real terms it is still 6 amp.
Any ideas are welcome.
All best Eric
 30 June 2008 02:08 PM
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rocknroll

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As to rocknroll we can't use 10 amp MCB's on lights 6 amp is max as the ceiling roses are only rated at 6 amp.


Duh! What about a 13A socket on a radial with a 20A MCB then

C'est la vie, t'es pas~?


regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 30 June 2008 at 02:30 PM by rocknroll
 30 June 2008 05:34 PM
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AJJewsbury

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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.

Humm - well observed!

But as others have noted "resistance" for VD doesn't include Ze, whereas loop impedance does.

Other point is our (UK) habit of using reduced CSA CPCs - which means we can hit Zs limits long before VD ones.

- Andy.
 01 July 2008 09:59 AM
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mapj1

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Although note that the Original Post refers to RCDs, so the reduced CPC is less serious. IF the RCD jams, then it will still fail to safe - just not quite within the 0.4 secs the reg writers intended.
My concern is more that the supply may not be as good as the nominal impedance of 0.3 ohms or whatever - R1/R2 may be fine, but in rural areas in particular, the the DNO's length of wet string may be less than perfect in fault?
regards
Mike.

-------------------------
regards Mike
IET » Wiring and the regulations » Why do we use B and C type MCB's?

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