Can anyone pls explain 431.1.1 current cant exceed 145% of the smallest wire on the circuit?

[1capybara]

This can have several interpretations.
1. Break trip rating cant be above 145% of the ampacity rating of the wire? That doesnt make sense because the breaker should be protecting the wire by tripping first.
2. Load cant exceed 145% the ampacity rating of the wire? Load should be less than the ampacity of the wire.
3. If a 32 amp breaker feeds a 4mm circuit, no hot or neutral wires on the circuit shall have ampacity less than 1/1.45 = 72.5% of 32 amps. Not even on un-fused spurs.
4. Can someone please explain? TIA !

 

[Ian1981]

I2 must be equal to or less than 1.45x Iz

This equation is satisfied without having to think about it if Ib<In<Iz (bs 7671 gives examples of certain OCPD)

As an example a 2.5mm protected on a 20amp mcb
1.45 x 20 =29 amps(the mcb should trip at this current within 1hour).
A 2.5mm clipped direct ccc is 27amps
27x 1.45= 39.15 amps
Therefore 29amps the I2 is less than 39.15amps or the Iz of the cable x the 1.45 factor

The 1.45 factor is only applicable for overload for certain OCPD listed within bs7671 regarding that regulation.

 

[Richard Burns]

Ian has covered it really.
Presumably you mean 433.1.1.

I2 is not the load current, the circuit breaker rating or the current carrying capacity of the cable; it is a current that will cause the protective device to operate within the conventional time. i.e. the effective operation of the protective device.
This current would only be experienced when there is an error of some sort in the circuit as it will be above the design current of the circuit.
Presumably it is there to ensure that during an overload situation the protective device will not remain closed long enough to cause deterioration in the cable due to high temperatures.

 

[1capybara]

How can there be a current of 29 amps for 1 hour on a circuit with a 20 amp MCB? Are we assuming the MCB fails to operate at 20 amps?

 

[Ian1981]

A 20 amp mcb will not operate when it reaches 20 amps nor will it operate when it reaches 21 amps.
We are talking about overload only.

 

[telectrix]

How can there be a current of 29 amps for 1 hour on a circuit with a 20 amp MCB? Are we assuming the MCB fails to operate at 20 amps?

if you look in app.3 of BS7671, it will show the time taken for a MCB to operate for a given current. most will generally cope with 1.45 In for a bout 1 hour before tripping.

 

[PEG]

The OP could do worse,than have a scan over the time/current graphs,and get a general feel for how near or far,various OCPD's are from from operating,with varying constant current scenarios.

Some are more satisfied than others,about being in an unsatisfactory state

 

[Richard Burns]

I rather like this diagram from Eaton, albeit only for a 60898 MCB, but it describes all the characteristics for B, C and D and also shows the 1.13In, 1.45In, 2.55In values.