Circuit Breakers

[DominicB]

Was just looking at circuit breaker types and I was a little bit confused so wondered if anyone could help clear it up.

Basically say If I determined my current would be 26a before taking into account any rating factors and chosen a 32a circuit breaker for my protective device. After taking into account my rating factors for length etc the actual current demand would be up to 43.7A (Which would exceed the 32a rating) so how does this work?

 

[Leesparkykent]

I think you've got things a little muddled here Dominic. These factors will decrease the current carrying capacity of the cable.

 

[DominicB]

Thanks for the reply I think i didn't explain my confusion very well basically how for example a type b 32a circuit breaker would actually trip at up to three to five times the rating (say 160a) but if the cable isn't rated that high then how comes this is okay?

 

[telectrix]

basically, the cable can withstand a massive overcurrent for the short duration that it takes the MCB to trip.

 

[DominicB]

Right that makes sense although if the sockets were protected by a 32a MCB and say a load of appliances were plugged in that drew 50-60a but the mcb would trip at a minimum of 96a (three times the 32a) wouldn't that be dangerous or is that okay for them to draw more then that because the cable should be able to handle more?

 

[telectrix]

a 32A type B breaker handling 50A would trip in approx 16 minutes.( see app.3 in BS76571).

 

[DominicB]

Right okay gotcha in my book it gives an example saying that the design current of a 6kw heater would be 26a (6000/230) so the nominal protective device would be 32a.

It then sase as the circuit is 21m in length installed via method C at 35 degress surrounded by 100mm of thermal insulation the cable must be capable of carrying a current of 43.7a as a minimum? If the cable is calculated to carry 43.7a then why is a 32a chosen?

 

[telectrix]

a 32 A breaker is selected because its rated current (In) is greater than the design current (Ib) so it will not trip in normal use. However, it's value must be less than the cable's max. current (Iz). the formula is Ib < In < Iz.

so we have here 26 < 32 < 43. i.e. it complies.obviously due to the installation method ( insulation) a 6mm or even 10mm cable would be appropriate, going by your 43A .

 

[DominicB]

So the 43.7a is the maximum current that would pass through the cable at normal conditions so if it did carry that much it would trip the circuit breaker in a certain amount of time? Thank you for your help

 

[davesparks]

Right okay gotcha in my book it gives an example saying that the design current of a 6kw heater would be 26a (6000/230) so the nominal protective device would be 32a.

It then sase as the circuit is 21m in length installed via method C at 35 degress surrounded by 100mm of thermal insulation the cable must be capable of carrying a current of 43.7a as a minimum? If the cable is calculated to carry 43.7a then why is a 32a chosen?

The 43.7A is not a real world value, it is a calculated value used to de-rate the cable.
What is confusing about the way the calculation works is that instead of actually reducing the current carrying capacity of the cable by calculation we instead multiply the nominal current (or design current in some cases) by factors which actually increase the value. This gives us the Iz value, which is not the current that will flow in the cable, but actually the current rating the cable is required to have to safely handle the nominal current (or design current) when subjected to the external factors such as temperature and grouping.

The example you have given is a pretty poor one in this case as an electric heater will almost certainly be able to be calculated based on its design current rather than the nominal current. So you would apply factors to the 26A rather than the 32A to get Iz.

 

[davesparks]

So the 43.7a is the maximum current that would pass through the cable at normal conditions so if it did carry that much it would trip the circuit breaker in a certain amount of time? Thank you for your help

No, see my above post, the 43.7A is a theoretical value only, it won't flow through the cable (except in a bizarre and very specific fault situation)

 

[DominicB]

Thanks for your replies, im just wondering why its only a theortical value if its based on external factors which have been calculated like the highest ambient temp and thermal insulation?

 

[Pat H]

It is an odd way of calculating things.
A cable that is in insulation has a de-rating factor. But what actually happens is the cable will heat up and its resistance increase.
So the current flowing will actually drop (voltage is nominal and fixed). However based on our de-rating value and subsequent thicker cable requirement the resistance will be lower. So less heat generated and less loss.
Its a sort of backwards way of achieving the same result.

 

[DominicB]

Sorry im still confused, whats the difference between the It and Iz because in the wiring regulations it says It is the current carrying capacity without correctional factors or installation conditons yet the calculation for It appears to take these into account

 

[gazdkw82]

As previously mentioned, these factors will decrease the amount of current the cable can safely carry. The de-rating factors and information on CCC’s can be found in Appendix 4 (BS7671)


The current carrying capacities can be found on page Appendix 4 (page 332 BS7671).


So first find the CCC of the chosen cable for the chosen circuit. Apply the factors, if the total CCC is now below the trip current of the protective device then the CSA will need to be increased.

 

[telectrix]

i have always found it far easier to use tables to obtain the rating of the cable according to the installation method. and instead of multiplying by a derating factor e.g. for ambient temp, divide to get the reduced ccc of a particular cable.

 

[Richard Burns]

Start from the beginning again.
In your example you have a 6kW load that will take (6000/230)=26A.
Therefore the design current Ib is 26A.
Therefore the next highest rating of circuit breaker is 32A so this is the In selected.

You then need to have a cable that can handle the current that could be drawn.
The design of circuit breakers and the tabulated current ratings of the various cables are coordinated such that a cable that has a current carrying capacity of 32A (according to the installation method listed in the relevant table) is protected by a 32A circuit breaker under all conditions of overload.

However the tables do not account for all factors that may affect the cables capacity to carry current.
The tables state that the ambient temperature is assumed to be 30°C and that the installation method is as stated in the table.

The limits are there to ensure the cable does not overheat, so if the current carrying capacity is 32A then the cable can take 32A on a continuous basis without overheating. This is the Iz.
However if the cable is buried in 100mm of insulation then the heat cannot leave the cable so easily and it would get hotter than expected, so if it were carrying 32A it could overheat.
Similarly if the ambient temperature is 35°C then it is harder to transfer the heat of the cable to this hotter air and so the cable could overheat again.

To take this into account the current carrying capacity of the cable is reduced by a correction factor e.g. the cable can take less current than the tabulated value; so because of the higher temperature it may only be able to take 29A.
This would then prevent this cable from being used with a 32A breaker, however a cable with a greater csa which may be tabulated as being able to carry, say, 35A would have the current carrying capacity reduced to 32A which would be OK.
Therefore you would be looking in the tables for a cable csa that is capable of carrying 35A (the It) because this value is going to be reduced by the ambient temperature to 32A (the Iz).

In your case the tabulated value that you are looking for is 43.7A because the ambient temperature and the insulation will reduce this value down to 32A (or more) which is the limit for the circuit breaker required.

The way the regulations calculate this value is to apply the correction factors as divisors to the In so that you get a higher value of current carrying capacity to look up in the tables, but this tabulated value is only to allow you to look up a cable that is suitable once the CCC has been reduced, it has no bearing on the actual current in the circuit.

 

[telectrix]

Richard. you have missed your vocation. as you explain things so well, you should have become a tutor.

 

[Pat H]

Thanks Richard a very clear explanation. I often find myself explaining it (not as well) as its easy for it to "appear" as if the correction factors are increasing the current in the cable rather than indicating a cable CSA size that will be ok with those correction factors at that current.

 

[DominicB]

Thanks guys for for your help that all makes perfect sense now, very much appreciated!