Type B MCB tripping at higher current rate than cable capacity

[Luka]

Can someone please explain me how type B MCB can protect a cable if it only trips when current is 3-5 times maximum load?
Let's say, 2.5mm radial circuit is protected by 16A type B MCB. Prysmian 2.5mm T&E current rating is 27A. How will 16A MCB protect the cable from overheating and burning if it is not going to trip below 48A (3x16A)? Sorry if my question is daft but it does not make much sense to me and despite making a research online I am unable to find the answer

 

[snowhead]

In your research, did you seen a reference anywhere as to how long the 16a type B will allow the excess current to flow?

 

[Luka]

In your research, did you seen a reference anywhere as to how long the 16a type B will allow the excess current to flow?

yes, it says it will trip within 0.4s of current exceeding 3-5 times maximum load. What about current exceeding let's say 2.5 times maximum load? Will it allow to flow? It is still like 40A, which is well above 2.5mm cable capacity

 

[Tom256]

Ok quick answer due to time of day.

Couple of points to keep in mind to help you understand:
Overcurrent can be due to a fault - known as Fault current - or
Overcurrent can be due to an overload - known as Overload current.
The curves in Appendix 3 work in accord with both and that should become clear in this answer.

Keep in mind also that a lot of the spade work in device design and cable construction and how they coordinate takes this into consideration as such you need to ensure you work in-line with that. See Regualtion 433.1.1 plus notes. Now also note regulation 433.1.201 and the relationship beween points (i) (ii) and point (iii). I will explain below.

In your example a BS 60898 16 Amp (In) MCB is required to have a 'non-fusing' current of 1.13 x In. That is below 1.13 times In it will not trip - see regulation 433.1 as to the significance this has on circuit design.
Your BS 60898 MCB has a 'fusing current' (I2) of 1.45 x In at which it will trip in 1 hour (conventional fusing time for this device) - have a look at the curve for this device in Appendix 3 and note this and how it is shown.

As as a side point, these values of fusing and non-fusing currents apply all the more when looking to coordinate devices.

So the cable needs to withstand the overload at 1.45 x In for 1 hour and this is satisfied in Iz is greater than In.
That is the reference I made earlier to the design work on cables being done, you just ensure that Iz is greater than In.
Iz is very often misused it is not It. Iz is for want of a simple phrase the 'get it done' value of the cable as installed.

The other consideration is that small overload of long duration greater than In but less than the 1.13.times In of the BS 60898 and that is satisfied if compliance with regualtion 433.1 is satisfied

Your 3 - 5 and [often much more] is the device startting to deal with larger currents. Question: When are you going to see that level of current in the life of the installation?

Anyway nearly 3 am if you have any more questions please ask away

 

[Tom256]

The other consideration is that small overload of long duration greater than In but less than the 1.13.times In of the BS 60898 and that is satisfied if compliance with regualtion 433.1 is satisfied

Just seeing small correction required to my post in that regulation 433.1 is not limited to currents of less than or equal to 1.13 x In.

 

[Yellowhammer]

Can't add too much to the above, a comprehensive answer. You have over simplified this though. 5x rated current is for "instantaneous" trip on a type B MCB. Look at appendix 3 of BS7671 figure 3A4 and you will see the time/current for the MCB in question.

That said the Max Zs for your device is such that it will ensure this 0.1 tripping time based on simple ohms law, so the cable will not see this current for a long time.

It is a matter of completing adiabatic for the circuit to determine whether or not the cable will withstand the thermal load of a fault for the duration (0.1s). This is explained in 434.5.2

If you are talking about overload on a circuit rather than fault, say on a socket circuit or something that does not supply a fixed load, then you need to ensure that Ib<=In<=Iz. A 2.5mm cable for example won't instantly catch fire or rise to 70 degrees if the current it is exposed to goes above Iz, the device just needs to trip in such time as to mitigate the cable getting over-heated.

My understanding anyway!

 

[Tom256]

Yes.
The BS 60898 can be used to provide protection against Short circuit (fault) current or Short circuit (fault) current and overload current.

If it is being used for Short circuit current only then the Conductor tabulated value (It) is calculated from design current (Ib) x the relevant correction factors in Appendix 4
If Overload and Short circuit current protection is required then the rating of the protective device (In) is used in the calculation of It.

As mentioned above by YellowHammer it is the effect of the current on the conductors insulation and its ability to withstand such for the time required before the device disconnects.
When an over-current protective device is providing BOTH overload and short circuit (fault) protection there is no need to calculate withstand if Ib < In < Iz < It and the required loop resistance conforms to manufacturers data or complies with relevant tables within section 41 BS 7671

Should the OCPD only be providing short circuit (fault) protection (Ib used rather than In) then the withstand must be confirmed using the formula in regulation 434.5.2. This is to ensure the conductors can withstand the current flow under the conditions . The manufacturers data of let-through energy can also be used in this confirmation calculation.

Again brief and ask if you are still not sure.

 

[Luka]

Ok quick answer due to time of day.

Couple of points to keep in mind to help you understand:
Overcurrent can be due to a fault - known as Fault current - or
Overcurrent can be due to an overload - known as Overload current.
The curves in Appendix 3 work in accord with both and that should become clear in this answer.

Keep in mind also that a lot of the spade work in device design and cable construction and how they coordinate takes this into consideration as such you need to ensure you work in-line with that. See Regualtion 433.1.1 plus notes. Now also note regulation 433.1.201 and the relationship beween points (i) (ii) and point (iii). I will explain below.

In your example a BS 60898 16 Amp (In) MCB is required to have a 'non-fusing' current of 1.13 x In. That is below 1.13 times In it will not trip - see regulation 433.1 as to the significance this has on circuit design.
Your BS 60898 MCB has a 'fusing current' (I2) of 1.45 x In at which it will trip in 1 hour (conventional fusing time for this device) - have a look at the curve for this device in Appendix 3 and note this and how it is shown.

As as a side point, these values of fusing and non-fusing currents apply all the more when looking to coordinate devices.

So the cable needs to withstand the overload at 1.45 x In for 1 hour and this is satisfied in Iz is greater than In.
That is the reference I made earlier to the design work on cables being done, you just ensure that Iz is greater than In.
Iz is very often misused it is not It. Iz is for want of a simple phrase the 'get it done' value of the cable as installed.

The other consideration is that small overload of long duration greater than In but less than the 1.13.times In of the BS 60898 and that is satisfied if compliance with regualtion 433.1 is satisfied

Your 3 - 5 and [often much more] is the device startting to deal with larger currents. Question: When are you going to see that level of current in the life of the installation?

Anyway nearly 3 am if you have any more questions please ask away

"Question: When are you going to see that level of current in the life of the installation?". That was exactly my point and my concerns was that CMB would not trip until then but current will be still high enough to damage the cable. I think I do understand it now, at 3-5 x In it will trip within 0.4s, at 1.45 x In it will trip after 1 hour, at any overcurrent between those two, time of tripping will depend on level of overcurrent (the more overcurrent, the sooner it trips), but it will still trip soon enough to avoid damage of the cable. Is it fair to say it this way? Thank you for taking time at 3am to explain it to me

 

[Tom256]

"Question: When are you going to see that level of current in the life of the installation?". That was exactly my point and my concerns was that CMB would not trip until then but current will be still high enough to damage the cable

Under short circuit fault conditions a fault of negligible impedence is assumed as such for your 16 Amp device you could reasonably see >20 x In would flow. 100's of amps so far more than the 16 x 5 figure.
The time the device operates is always limited by its physical make up, the speed it can physically operate in, yet times of 0.1 seconds or less to disconnect are realistic.

During the time it takes the device to break the fault it will 'let-through' some energy and the conductors subject to that energy let through by the ocpd must be able to 'withstand' the rise in temperature that results.
The copper conductor will be fine on the whole, it is the insulation that is affected and can break down catastophically or be somewhat compromised so as to reduce the life of the installation.
You will note looking through the tables in Appendix 4 that different cables are able to tolerate different operating temperatures.

Now a lot of the work has been done for you on this. The short circuit fault current the cable will withstand has been confirmed providing the rules as set out in this thread, on device and cable selection, are complied with.
As a further check you can confirm the ability of the cable to 'withstand' the energy let-through by using what is called the adiabatic equation as set out for you in regulation 434.5.2.
Confirmation of this is a required check under some circumstances, one such example I gave previously in this thread although there are more.
The adiabatic equation assumes all heat generated stays within the conductor [that is the meaning of the word Adiabatic] and as such is only really applicable for fault duration of no more than ~5 seconds.

When we look at regulation 433.1 and the small overload of long duration this must be designed out for this long period where the insulation is subjected to higher operating temperatures without the ocpd disconnecting can greatly reduce the life of the installation. It is quite surprising see the data on this by just how much a small overload over a long period can affect the life span

 

[Yellowhammer]

So I would say the most common example of a circuit requiring overload protection is going to be a radial or ring socket circuit. You are correct that the circuit can sit a reasonable percentage over the OCPD's rating for 10,000+ seconds. I am not an oracle and this is from my mind, rather than documentation, but on the socket circuit for example you will likely overload the circuit by plugging in and running too many devices, duration of this load is going to play a part.

So say we had a 20A radial circuit wired in 2.5mm cable in a non-insulated ceiling void (I am going to assume the loading of grouped circuits is suitably low to exclude them from de-rating in this instance). On that circuit you are sat with a 2kw heater plugged in, while a couple of kitchen appliances run away at say 2.5kw for a tumble drier and dishwasher at 1.8kw. You now have 6.3kw, so 27.3A- ish, using Uo. At this the OCPD is probably going to pop in a couple of hours- I would hazard a guess both the dishwasher and the tumble drier are not going to be running for this duration, the tumble drier, probably nowhere close, dishwasher perhaps half. The heater is thermostatic so will again not be constant.

Please let me know the It for the cable etc. I don't have the BBB on me right now, but this is probably above what it's rated for as a constant load. I have not factored in the resistance of the cable itself in my calculations, but it will realistically be negligible..

The loading on these circuits, by my understanding, by design is not constant. For items likely to pull high current for prolonged periods a good designer will add another circuit (Immersions?).

It's not to say it's impossible, but as a designer, which extends to being an electrician doing small alterations within a domestic property, it is your responsibility to design circuits to try and mitigate these things. Division of circuits such that you won't overload (multiple socket circuits, kitchen specific circuits for appliances etc.).

This is all somewhat off the top of my head and rushed, so please pull me up on anything that is questionable here!