Adiabatic Equation

[trebor]

does anyone know what the current limiting class 3 means on an mcb, for eg does this mean a max of 3ka let through energy ?

if it does it makes a difference when using the adiabatic equation for sizing cpc`s

 

[JUD]

It may be a type 3 MCB. Does it have an M number on it? e.g. M3 would be 3kA breaking capacity.

To work out the adiabatic you divide the supply voltage by the Zs for the circuit which gives you the fault current.

Example

32A Type B 60898 MCB
Supply Voltage = 230V
Zs = 0.5

If = V ÷ Zs
If = 230 ÷ 0.5
If = 460A


A 32A Type B MCB will operate in 0.1 seconds with a fault current over 160A so the adiabatic would be:

S = √(I² x t) ÷ k
S = √(460² x 0.1) ÷ 115
S = 1.26mm² (1.5mm²)

 

[Guest123]

does anyone know what the current limiting class 3 means on an mcb, for eg does this mean a max of 3ka let through energy ?

if it does it makes a difference when using the adiabatic equation for sizing cpc`s

Energy limiting class number
An energy limiting class number (1,2 or 3) is marked on circuit-breakers of Types B and C having rated current up to and including 40 A, and rated short-circuit capacity of 3 000 A, 4,500 A, 6,000 A and 10,000 A. The number denotes the maximum I2t let-through by a circuit-breaker under short-circuit or earth fault conditions. This information may be used by the electrical installation designer for the same purposes as the information obtained from the I2t characteristic.

 

[Guest123]

Got a bit more info.......


With regard to the Energy Limiting Class......


Class 1 (1 in the box) = no limit

Class 2 (2 in the box)
Rated short circuit capacity 10kA
Type B 240000 As
Type C 290000 As

Class 3 (3 in the box) - (most are now these)
Rated short circuit capacity 10kA
Type B 70000 As
Type C 84000 As


All the info can be found in BS 60898, appendix ZA

 

[Jurasic Spark]

does anyone know what the current limiting class 3 means on an mcb, for eg does this mean a max of 3ka let through energy ?

if it does it makes a difference when using the adiabatic equation for sizing cpc`s

I thought it was the impulse withstand voltage, tab 44.3 and tab 44.4 page 85 of BS7671

 

[Guest123]

I thought it was the impulse withstand voltage, tab 44.3 and tab 44.4 page 85 of BS7671

Those tables refer to overvoltage situations rather than the fault current energy let through (I2T).

I dont have access to BS 60898 and I sure aint shellin out £160 for it but all the info is in there.

 

[Jurasic Spark]

Those tables refer to overvoltage situations rather than the fault current energy let through (I2T).

I dont have access to BS 60898 and I sure aint shellin out £160 for it but all the info is in there.

Yes, but where you see a number 3 on a circuit breaker isn't this refering to cat 3 from the tables I mentioned? So this 3 would indicate the c/b is capable of of withstanding 4kv.
Tab 44.4 implies that c/bs are normally cat 3.

 

[BigBreakfast]

Hi Trebor,

Minimum protective conductor size for energy limiting class 3 - B & C type devices are shown in the OSG page 104, table 2D, which refers to regulation 434.5.2 of BS 7671.

 

[Guest123]

Yes, but where you see a number 3 on a circuit breaker isn't this refering to cat 3 from the tables I mentioned? So this 3 would indicate the c/b is capable of of withstanding 4kv.
Tab 44.4 implies that c/bs are normally cat 3.

Sorry but I disagree.

The number 1, 2 or 3 on the front of an MCB underneath the short circuit capacity can be used the same way as the manufacturers I2T values i.e energy let through, nothing to do with voltage withstand.

For a "B" type device with a class 3 rating the As value would be 70,000 and for a "C" it would be 84,000.

I am of course happy to be corrected but this is my understanding and indeed was my 2400 tutor's also.

 

[Jurasic Spark]

Sorry but I disagree.

The number 1, 2 or 3 on the front of an MCB underneath the short circuit capacity can be used the same way as the manufacturers I2T values i.e energy let through, nothing to do with voltage withstand.

For a "B" type device with a class 3 rating the As value would be 70,000 and for a "C" it would be 84,000.

I am of course happy to be corrected but this is my understanding and indeed was my 2400 tutor's also.

Interesting, when I did the regs course the tutor said it was to do with the impulse withstand voltage from lightning strikes. I'm not saying your wrong but it would be nice to get it cleared up. I've looked on the net and haven't found anything definitive but there's some info' here:
http://www.beamainstallation.org.uk/assets/pdfs/CircuitBreaker.pdf

 

[Guest123]

Energy limiting class number
An energy limiting class number (1,2 or 3) is marked on circuit-breakers of Types B and C having rated current up to and including 40 A, and rated short-circuit capacity of 3 000 A, 4,500 A, 6,000 A and 10,000 A. The number denotes the maximum I2t let-through by a circuit-breaker under short-circuit or earth fault conditions. This information may be used by the electrical installation designer for the same purposes as the information obtained from the I2t characteristic.

This was the only info I found....and now I cant find it again.

I'll do a bit more research and try and get a definitive answer.

 

[Guest123]

I managed to find it again...

 

[Jurasic Spark]

This was the only info I found....and now I cant find it again.

I'll do a bit more research and try and get a definitive answer.

I've found a bit more info'. The very last line mentions Polution Degree 3?
Have we both got it wrong??
http://www.electrika.com/products/m/man-0170/pdftech/07-207.pdf

 

[Jurasic Spark]

I managed to find it again...

Circuit Let Through Energy Calculations - XS Engineering

I found another article which backed up what you have said Lenny, I could only view it once without signing up!!
Thanks for that, you learn something every day

 

[sparkylad]

I may as well add my thoughts on the subject.

My understanding was that it referred to the max Ka rating that the c.b could safely handle and still be of serviceble use afterwards , as opposed to the main 6KA [ domestic] short citcuit handling rating whereby the c.b can open on this fault but is not guaranteed to be still in a serviceable condition if reset--thats why you are given the two values---hope that makes sense