Adiabatic equation on distribution circuits

[moosey86]

Now I know this has probably been covered a few times but I'm struggling to come to terms with my own calculations.

I would appreciate it if someone could tell me if my calculations are correct.

So basically I've done a periodic and some of the CPC sizes for sub mains are less than half the size of the live conductors. I have gone through what I think is the correct procedure for the adiabatic equation and it is telling me that for a 200A TP+N supply the CPC can be 2.5mm, the live conductor is 95mm so I find this rather unrealistic.

This is what I have worked out:

S = square root of (l squared x t) / k

Now I have the Zs reading of the circuit which is 0.10 ohms.

So: 230 / 0.10 = 2300A (fault current)

Times this by itself: 2300 x 2300 = 5,290,000

Now I've got the schneider mccb tripping curves from here:-

http://www2.schneider-electric.com/...AQS/166000/FA166316/en_US/Tripping_Curves.pdf

So as it is a 200A MCCB (TM200D) of this range and the pfc was 2300A : 2300 / 200 = 115

Then according to the graph, anything over about 24A is <0.01 seconds (10 milliseconds).

The formula now looks like : S = square root of (5,290,000 x 0.01) / k

Then : S = square root of (52,900) / k

Then : S = 230 / k

As it is a separate earth cable run alongside the SWA I have applied the k factor of 143.

So : S = 230 / 143 = 1.6

Next size of cable up is 2.5mm.......... Surely not!

 

[Wilko]

Hi - 2300/200 = 11.5

 

[telectrix]

Now I know this has probably been covered a few times but I'm struggling to come to terms with my own calculations.

I would appreciate it if someone could tell me if my calculations are correct.

So basically I've done a periodic and some of the CPC sizes for sub mains are less than half the size of the live conductors. I have gone through what I think is the correct procedure for the adiabatic equation and it is telling me that for a 200A TP+N supply the CPC can be 2.5mm, the live conductor is 95mm so I find this rather unrealistic.

This is what I have worked out:

S = square root of (l squared x t) / k

Now I have the Zs reading of the circuit which is 0.10 ohms.

So: 230 / 0.10 = 2300A (fault current)

Times this by itself: 2300 x 2300 = 5,290,000

Now I've got the schneider mccb tripping curves from here:-

http://www2.schneider-electric.com/...AQS/166000/FA166316/en_US/Tripping_Curves.pdf

So as it is a 200A MCCB (TM200D) of this range and the pfc was 2300A : 2300 / 200 = 115

Then according to the graph, anything over about 24A is <0.01 seconds (10 milliseconds).

The formula now looks like : S = square root of (5,290,000 x 0.01) / k

Then : S = square root of (52,900) / k

Then : S = 230 / k

As it is a separate earth cable run alongside the SWA I have applied the k factor of 143.

So : S = 230 / 143 = 1.6

Next size of cable up is 2.5mm.......... Surely not!

something wrong here.

 

[Wilko]

I had a look and got 30ms for 2300A pfc. The very fast ads means a smaller CPC may work. But, if cpc really was 4mm we won't be getting 0.1 Ohms Zs I bet. And so the fault current would be less and the ads time would be more and the conductor size would go up. If that makes any sense ...
Anyway, I think the min size may come from reg 542.3.1 (likely 16mm?). And all bets are off if bonding is involved

 

[moosey86]

Hi - 2300/200 = 11.5

Ok fair enough

But even still the next rounded cable size up from that is a 4mm!

I still find that hard to believe...

I had a look and got 30ms for 2300A pfc. The very fast ads means a smaller CPC may work. But, if cpc really was 4mm we won't be getting 0.1 Ohms Zs I bet. And so the fault current would be less and the ads time would be more and the conductor size would go up. If that makes any sense ...
Anyway, I think the min size may come from reg 542.3.1 (likely 16mm?). And all bets are off if bonding is involved

reg 524.3.1 is for main earthing conductors is it not? I did a quick google search as I don't have my regs book on me. The circuit in question is a sub-mains supply to another dis board.

I'm guessing I'll have to work it out by the length of the cable to the size of what it currently is because like you say the zs reading will change once the cable size has changed!!

Haha I've never done this before it's a lot of fun isn't it

 

[Wilko]

reg 524.3.1 is for main earthing conductors is it not? I did a quick google search as I don't have my regs book on me. The circuit in question is a sub-mains supply to another dis board

Yes, that's true. I got there from section 543 Protective Conductors (that has the adiabatic stuff). The last para of 543.1.1 says "for a protective conductor buried in the ground Reg 542.3.1 for earthing conductors also applies".

Another thought : as Zs / mccb combo is giving us 30ms and 4mm from adiabatic and the installed CPC is bigger (?) it's ok. Then it's just the other regs and any bonding to consider.

 

[moosey86]

Yes, that's true. I got there from section 543 Protective Conductors (that has the adiabatic stuff). The last para of 543.1.1 says "for a protective conductor buried in the ground Reg 542.3.1 for earthing conductors also applies".

Another thought : as Zs / mccb combo is giving us 30ms and 4mm from adiabatic and the installed CPC is bigger (?) it's ok. Then it's just the other regs and any bonding to consider.

Ahh I see, it's all on ladder anyway.


It's like you said before, if you were to install 4mm for the CPC then the Zs would naturally be higher as there is a 35mm currently in place and it's not a short run... Off the top of my head it's probably about 50m.

 

[Wilko]

Next thought - what if Zs went up a bit? You measured 0.1, but if it was 0.13 say? Then PFC would be 1770A and I/Ir would be 8.8 and ads time would be from 3 to 30 sec, if I'm reading the graph right. At 3 sec I get 21mm. I'm just thinking from initial design perspective I would spec up the cpc to cover the possibility that Ze went off song, as we are on the knee of that mccb curve just now. Just some random thoughts, fwiw

Edit : ahh, with 35mm they will cover that.

 

[Risteard]

I haven't read all of the OP or replies but remember that you can't just calculate and use an earthing conductor or cpc bigger than the minimum given - it is an iterative calculation and you will need to recalculate based on the fault levels and Zs etc. of your newly selected size of protective conductor. The process needs to be repeated until such time as the minimum size calculated is less than or equal to the conductor size you have based your calculations on.

Otherwise the greater prospective fault current may increase the minimum size of conductor required depending on the disconnection time (with 0.01 seconds likely to be the definite minimum operating time for the device).

 

[The Ghost]

Pefc calc is now 230 x Cmin/Zs see guidance note 3.