Adiabiatic & voltage drop

[brman]

If the PFC was 2090A, then you would use the manufacturers data and realise that the tripping time would then be closer to 0.001s.

The higher the PFC, the faster the breaker will trip. That is the slight flaw in your logic

Ok. I am a catching up slowly

The only trouble I have with that is the data I have just looked at (MK sentry) stops at 0.01s. I do see that the time would be less than that but I am not convinced I could reliable guess how much lower as it will be limited mechanically (just thinking that a lot of RCDs I test go at around 0.01s regardless of current). Plus the graph for MK definitely shows the curve tending to horizontal below 0.1s. Doing to sums in reverse and using the cable size from the "appendix 3" results (0.87mm2) then the real trip time at 2090A would have to be 0.0023s. Feasible but I'm still (edit) not convinced.

The thing is (and I know you guys are going to disagree with me) I think this is a misuse of the tables in appendix 3. I believe these tables are giving the minimum current required to achieve a certain trip time. i.e they are to calculate the max Zs. I don't believe they hold up in this case where you are effectively assuming the breaker will limit the current to 320A for 0.1. I know you guys don't believe this is actually what happens but it is what would have to happen for the adiabatic results to make any sort of sense.

 

[brman]

(The posts are also confusing the various opds., Ipfs. and Zss. - nevertheless -)

The overcurrent protective device breaking does not limit the Ipf to 315A.

The full IPF will flow before the opd breaks the circuit - however quick.

that is my point. I get what DS is saying about t being much less that the tables (or even the manufacturers specs) but...........

 

[Guest55]

If the PFC was 2090A, then you would use the manufacturers data and realise that the tripping time would then be closer to 0.001s.

The higher the PFC, the faster the breaker will trip. That is the slight flaw in your logic

you are not obliged to use the manufacturers data beyond that which is given in the regs tables.
i'm with brman on this , for all intent and purposes 0.1 secs or less can be considered "instantaneous" in operating a protective device and i wouldnt really look for any quicker times unless the cable was huge lol.

 

[D Skelton]

you are not obliged to use the manufacturers data beyond that which is given in the regs tables.
i'm with brman on this , for all intent and purposes 0.1 secs or less can be considered "instantaneous" in operating a protective device and i wouldnt really look for any quicker times unless the cable was huge lol.

i agree

 

[brman]

Just one last thing (and then I will shut up for the sake of the OP!). I've just realised I have been ignoring the let through energy (I2t) specs of the breakers. I'm not very clear how they actually spec these (and how that relates to the real world) but if I have read a couple right then the breaker will limit I2t to less than that expected from Ipf and the t-I curves. That could be a flaw in my argument.........

 

[Deleted member 26818]

Instananeous operation is determined by the a.c. frequency (50Hz).
50Hz means that the voltage cycles form 0V up through +230V, back down through 0V to -230V and back up to 0v again.
In effect the voltage reaches + or - 230V 100 times a second. 1s / 100 = 0.01s or 10ms.
There are no figure below 0.01s because it is not known at which part of the cycle the fault will occur.
It could be that the fault may occur at 0v, thereby taking one half cycle before peak voltage is reached, or it could be that the fault occurs half way between 0V and +230V taking only a quarter of a cycle for peak voltage to be reached.
Conversely the fault could occur after peak voltage (+230V) is reached, and it will take 3/4 of a cycle before peak voltage of -230V will be reached.
Of course, depending upon the value of current, the device may operate before peak current is reached.
It's just not possible to determine, so the value of 0.01s is used.

 

[Silly Sausage]

230 x root 2 = 325V peak.

 

[Deleted member 26818]

Here is a little example for you where using values determined from different sources willl cause problems.
A standard 32A Radial uses 4mm² T&E with a 1.5mm² CPC.
With a fault current of just 1.6kA, the adiabatic equation can show that the minimum CSA for the CPC should be greater than 4mm².
In fact the circuit conductors would not even be adequately sized for the fault current.
1600A² = 2560000
2560000 X 0.1s = 256000
√256000 = 505.9644
505.9644 / 115(k) = 4.4mm².
However if we conduct the adiabatic equation correctly using the values from Appendix 3 for a Type B 32A MCB:
160A² = 25600
25600 X 0.1s = 2560
√2560 = 50.5964
50.5964 / 115(k) = 0.44mm².
The CPC in 4mm² T&E will be sufficient and so will the circuit conductors.

You can use whatever values you wish.
However using a mishmosh of values from different sources will often result in an oversized CPC being determined.
In some cases it will even mean that the circuit conductors are not adequately sized.
To my mind it is always better to either use the tabulated values, or actual values, not a mishmosh of both.

 

[brman]

yes, I can see that. I've been doing my own sums and it does appear to work as long as you are happy to extrapolate curves beyond that give. For example I found some curves for hagar MCBs which show clearly that t is never less than 0.01s (the line is horizontal at that point). However if I then look at the I2t curves for those devices it implies that actually the let through energy will indeed be limited to the level that makes the sum work.
I am not sure I understand it but I reckon I might have to accept it :lol: