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Welcome to the forum mate.
There are 15 mentions of 'unwanted tripping' in BS7671, all do to with RCD's
Can you please expand on your question.
 
For over-current it is usually the switch-on surge that can trip a breaker, even though the running current is within normal ratings. Usual suspects are motors and lighting system (filament bulbs with low resistance when cold, and starters for florescent, along with LED drivers).

Ideally you know the load's current/time characteristics and you look at selecting a MCB that is outside of that envelope. So either increasing the nominal current and/or going B => C => D curve to increase the "instant" trip point.

But in all cases you still need to see if the resulting Zs requirement for disconnection times are met, and if the wiring, etc, is adequately protected by your choice (for overload, and maybe adiabatic limit for fault disconnection).
 
Beat me to it. Was half way through writing what @pc1966 said ^^^^^^^^^ (but not as well)

Type B will trip immediately between 3-5 times the rating
Type C are 5-10 times
Type D are 10-20 times

Then the even less common Type K will trip 8-12x, and Type Z will trip 2-3x.

You occasionally see a Type C MCB used in domestic settings, often a C6 added to avoid tripping when a load of downlights were turned on.

.
 
As @Dartlec mentioned it is unusual to see a trip on any single lamp (outside of a lighthouse...) so it is often a large number simultaneously switched that does it.

When installing places like showrooms, open plan offices, etc, that have lots of lights it pays to divide them in to several circuits that have separate switches (maybe a grid switch plate or similar) off a common breaker. As well as avoiding too many being switched at once, you make life easier for maintenance and energy saving (e.g. if lights next to window can be off on a dull day but those further in need to be on, etc).
 
Three socket outlets in three different rooms fed with a 4.0mm twin and earth cable.
When all the students come home at the same time, the 20amp circuit breaker tends to trip!
 
Three socket outlets in three different rooms fed with a 4.0mm twin and earth cable.
When all the students come home at the same time, the 20amp circuit breaker tends to trip!
Depends entirely on the loads that they have plugged into them! If it's kettles then there's your answer
 
Three socket outlets in three different rooms fed with a 4.0mm twin and earth cable.
When all the students come home at the same time, the 20amp circuit breaker tends to trip!

If the cable is definitely 4mm then have a word with an electrician about changing the 20A breaker for a 32A breaker.
 
If the cable is definitely 4mm then have a word with an electrician about changing the 20A breaker for a 32A breaker.
4.0mm twin & earth only has a current carrying capacity of 36amps, a 32amp BS EN 60898 circuit breaker has an I2 value of 47amps!
 
4.0mm twin & earth only has a current carrying capacity of 36amps, a 32amp BS EN 60898 circuit breaker has an I2 value of 47amps!

Check out Appendix 15 of the regs mate. This does depend on the installation method.

What is "unwanted tripping"? 1603797660566 - EletriciansForums.net
 
Interesting:
Not only are you to 'protect' 36amp cable with a circuit breaker that won't operate at less than 47amps, but you are also trying to 'protect' a 27amp cable with a circuit breaker that won't operate at less than 29amps!

"Small overloads of a long duration"

Do you realize that if the rate of rise in over-current (in a faulty appliance) is as great as 100mA per hour, if the over-current protective device is only half an amp too large, it's still going to take five hours (exceeding the current carrying capacity of the cable) before the device will notice that there is a problem?
 
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But this is accounted for in the current carrying capacities of the cables, listed in the tables of Appendix 4 of BS7671.

Have a read of paragraph/section 4 of Appendix 4 (page 377 of the big blue book). This is why Cf = 0.725 if the protective device is a 3036 fuse (see section 5), as they don't blow until nearly double their rating.
 
^^ this. You seem to be overlooking the definitions. Iz is not the absolute maximum current the cable will carry without damage. The Iz value for a cable calculated with the necessary correction factors is the current at which it is deemed to be protected by an MCB of equal or lower In. Therefore if your installation method gives you 36A for 4.0mm², it will carry whatever a 32A MCB will let it have, and live out its expected lifespan. FWIW 2.5mm² will take about 60A clipped direct before it actually starts melting.

The small, long duration overloads that are to be avoided are those where the load exceeds In, which is down to inadequate design, not a failure of coordination between cable and MCB characteristics. Running a 20A circuit at 26A continuously for 30 years will prematurely age the MCB as well as any effect it has on the cable.
 
Also remember that is largely the reason for the thermal trip in the MCB - it is roughly an inverse-time curve in line with the cable overload characteristics. Then the point when the magnetic trip kicks in it is deemed a "fault" rather than an "overload" so rapid disconnection is necessary for the ADS approach to shock protection.

The choice of B/C/D curve allows that broad definition of overload/fault point to be moved to some degree if you know specific loads (e.g. motors, large banks of LED lights, etc) have normal inrush currents that might present problems for good coordination of cable protection.
 
So many things to consider.
I think there may be a case of 'departure with tail between legs', here. Nice one.
 
Yes, the figure of 1.45 is a fusing factor.
Why would I consider using a 'fusing factor when calculating the size of a circuit breaker?

A fusing factor is used to increase the size of a fuse in order to absorb initial starting currents!
If I wish to absorb initial starting currents when using a circuit breaker, I can simply choose a C or D Type of the same nominal value.
 
No, the 1.45 fusing factor applies to all OCPD under 433.1.1 (iii) and it relates to the slightly vaguely worded "effective operation" of the OCPD, but it leads to 3 rules for overload protection:
  • In >= Ib (i.e. you don't deliberately overload the OCPD)
  • In <= Iz (i.e. the OCPD is not above the cable rating)
  • I2 <= 1.45 * Iz (i.e. the OCPD is effective at 45% above cable rating)
If you look at the current-time plots of typical MCBs you see they operate in under an hour at 1.45 rating. For example 6A * 1.45 = 8.7A and a 6A MCB around 8.7A from Fig 3A4/5/6 (as the B/C/D are identical in the thermal trip region). So a cable rated at 6A in whatever installation method applies is then adequately protected by such a MCB.

Rewirable BS3036 get slightly special treatment due to the characteristics (and generally poorer tolerance on manufacture/implementation). So a 5A * 1.45 = 7.25A and Fig 3A2(a) shows about 9.2A for 1 hour (i.e. at about 84% above rating, not 45% above).

Thus 433.1.202 overrides the (ii) and (iii) rules of 433.1.1 with In <= 0.725 * Iz so if you have a 5A fuse then the cable rating must be at least 6.9A to allow for the uncertainty of such fuses. Really this only applies for EICR now as no one in thier right mind would be installing BS3036 fuses, even if you could still buy them!
 
Really this only applies for EICR now as no one in thier right mind would be installing BS3036 fuses, even if you could still buy them!

course you can. fuse wire on a card still available in wilko.
 
Really this only applies for EICR now as no one in thier right mind would be installing BS3036 fuses, even if you could still buy them!

course you can. fuse wire on a card still available in wilko.
The replacement wire is, but I don't think Wylex, etc, have made the boards or BS3036 holders for many years now!
 

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