MCB Trip time / current

[pipme]

Are the graphs for type B MCBs as shown on the TLC site http://www.tlc-direct.co.uk/Figures/3.18b.gif from 'Electricians guide ..16th Ed' by Whitfield, correct ?
Edit :- Just noticed no titles on graph. So :- Vertical axis = Time secs.
Horizontal axis = Prospective current , rms Amps.
Not a well printed graph but near enough !!

I am sure reading these topics that many of us diy persons are not really sure about the trip point of MCB or melt point of semi enclosed ( wire type ) fuses.
When not in a fault short situation, just a steady overload, of say 10% ... would it ever trip ?
Would it's performance (eventually) be degraded due to this overloading ?

I consider this to be an important general item which everyone using electricity, and perhaps being responsible for replacement of fuses of all household types, should be aware of ....

Not made too clear, gets clouded by 'fault' trip times and their closeness to 'instantaneous'.

P

 

[dingbat]

Are the graphs for type B MCBs as shown on the TLC site from 'Electricians guide ..16th Ed' by Whitfield, correct ?

Yes, although the size of the reproductions makes accuracy impossible. The originals are in BS7671 and, by the way, both axes are logarithmic.

When not in a fault short situation, just a steady overload, of say 10% ... would it ever trip ?

None of the fuses or MCBs normally used in domestic situations would operate at only 110% rated current, but the point at which they would operate varies widely with the device used.

Would it's performance (eventually) be degraded due to this overloading ?

Unlikely, but unwise to assume so.

I consider this to be an important general item which everyone using electricity, and perhaps being responsible for replacement of fuses of all household types, should be aware of ....

No need. All you need to know is that if a fuse or MCB is rated at 32A then the cable must be capable of carrying more and the design current should be less. You should never design outside the nominal capacities of protective devices and this is exactly the 'little bit of knowledge' that could become a dangerous thing.

 

[ban-all-sheds]

Protective devices have 3 values of interest here:

In: its nominal rating (e.g. 32A)
I1: its non-fusing/non-tripping current. For a Type B MCB this is 1.13In
I2: its fusing/tripping current. For a Type B MCB this is 1.45In, and it must trip within 1 hour

So if we have a circuit whose design current (i.e. the current that it is expected to carry in normal use) is Ib, then this should not be greater than the rating of the protective device:

Ib <= In

If Iz is the current-carrying capacity of the cable for continuous service allowing for the way that it has been installed, then this must be no less than the rating of the device:

In <= Iz

Finally, 1.45 times the current carrying capacity Iz of the cable must be no less than I2, the current causing effective operation of the device:

1.45Iz >= I2

So Ib <= In <= Iz

and

I2 <= 1.45Iz.


Or if you prefer pictures:

Running an MCB at or near its limit for long periods wll cause it to overheat, and eventually weaken. But you shouldn't do that - look at the diagram, Ib is lower than all the rest.