Circuit Wire For Long Run

[Cookie]

No, the UK regulations generally do not specify how you achieve disconnection, only the times, etc, needed for safety. But they do list approved device standards for doing so (e.g. BS / EN ones on fuses, MCB, RCD, etc).

That section you quoted (411.3.2.5) has been updated but really it was a clause to deal with cases where the usual fault disconnection (OCPD or RCD) are not feasible so other means of protecting against shock need to be considered (supplementary bonding, etc).

The note about "disconnection may be required for reasons other than protection against electric shock" is (I think) to point out you might need it for overload/fire protection anyway, even if that fails to meet the maximum disconnection times discussed in that section about shock protection.

But beyond that, there are other rules permitting no OCPD or special cases when disconnection might be more dangerous, e.g. support electromagnet in factory, etc.

Well, even in an IT system you still need to open device under a multi phase fault.

Thank you for explaining this to me, but I still think disconnection should not be accomplished via RCD, as RCDs can fail.

 

[Cookie]

At AC power frequencies the skin depth in copper is about 9mm, so any conductor that is less than around 20mm diameter it has quite a small effect. Somewhere there will be equations to allow its computation for different conductors but most folks in engineering take the easy route of getting a feel for when you can ignore something, and ignoring it.

For example it is unlikely you will know any AC system parameter to better than 5% uncertainty, let alone 1%, so once effects get down to the 5%-ish region they can be often ignored for most purposes.

The temperature coefficient for copper is around 0.00393 so each 10C increase in temperature has a 3.93% increase in resistance. Which is about 4%. Which is why the UK regs have a 0.8 factor for going from measured at 20C to Zs value working at 70C.

Yes, our regs do have detailed formulae for non-standard cases such as measurement very cold/hot and/or operation at unusually high temperatures, but that vast majority of industrial cases are just like commercial and domestic in the 20C test & 70C assumed max operation is perfectly applicable.

In terms of generic AC values we have tables in the regulations with the values to assume. For example this if for single cables in various configurations giving both the DC and AC "resistance" values (more correctly the voltage drop due to cable impedance):
View attachment 84241

Alright, because in all honesty I'm trying to get these values down to 30*C:

 

[pc1966]

Just a tad bigger than I'm used to thinking about.... ?

To be fair it is a lot bigger than I ever have to deal with.

Also as you get to such large conductors there are good reasons to consider parallel sets instead. Not only are they a bit easier to wrangle, they have a higher current rating for the same overall CSA as cooling is usually better. However, you do have to consider cycling the sets to keep current flow, etc, balanced and you also need to look carefully about the OCPD implications for a fault half-way along one of the set, etc.

 

[pc1966]

Well, even in an IT system you still need to open device under a multi phase fault.

Thank you for explaining this to me, but I still think disconnection should not be accomplished via RCD, as RCDs can fail.

As I said, I would consider RCD as my 2nd choice unless there is no reasonable alternative. So for a TT set-up you have no reasonable alternative so it is a RCD.

Also you can take steps to improve the reliability:

• Having two RCD in series is on obvious case (so domestic TT example with a 100mA delay incomer and 30mA per circuit RCBOs).
• Fitting SPD to reduce the chance of RCD electronic damage due to lightning surges, etc.
• Having and enforcing a 6 month periodic trip-test is another good idea.

 

[pc1966]

Alright, because in all honesty I'm trying to get these values down to 30*C:

If it is 75-30 = 45C colder then it would be 17.7% lower for the resistive part.

For big cables with X dominating Z the overall difference is going to be less, as that won't change much with temperature.

 

[Cookie]

If it is 75-30 = 45C colder then it would be 17.7% lower for the resistive part.

For big cables with X dominating Z the overall difference is going to be less, as that won't change much with temperature.

So my ignorance will show, but if I type in 17.7% of 2.0 in Google I will have the 30*C Ac resistance of 12 gauge wire?

 

[pc1966]

So my ignorance will show, but if I type in 17.7% of 2.0 in Google I will have the 30*C Ac resistance of 12 gauge wire?

No, you need it reduced by 17.7% so multiply by 0.823

 

[Cookie]

No, you need it reduced by 17.7% so multiply by 0.823

Will this work with all wires sizes from 2.08mm2 to 1013mm2?

 

[pc1966]

Will this work with all wires sizes from 2.08mm2 to 1013mm2?

Yes, but only the the R term, the X term won't vary much with temperature and for large conductors that inductance starts to dominate the overall Z.

E.g. looking at post #40 you see X becomes comparable to R at cables around the 150mm or so size.

 

[Cookie]

Yes, but only the the R term, the X term won't vary much with temperature and for large conductors that inductance starts to dominate the overall Z.

E.g. looking at post #40 you see X becomes comparable to R at cables around the 150mm or so size.

Last question- I promise ?

Any idea what the AC R would be at 150*C and 250*C? I'd like to use that as a basis to calc final short circuit conditions.

 

[pc1966]

Last question- I promise ?

Any idea what the AC R would be at 150*C and 250*C? I'd like to use that as a basis to calc final short circuit conditions.

The basic assumption is copper is fairly linear w.r.t. temperature up to around 1000C (so anything that might be usable for a cable) so you have:

R = R20 * (1 + alpha20 * (t - 20))

Where R20 is the temperature at 20C (e.g. from tables) and alhpa20 is the coefficient at 20C which is around 0.00393 (depending on copper purity and annealing, etc, but not far off this).

So if looking at 250C and the 20C tabulated values for UK cables, then the adjustment factor is:

1 + 0.00393 * (250 -20) = 1.9039

So basically almost double the cold resistance.

 

[Cookie]

What about 150*C?

 

[Soi disant]

What about 150*C?

Ummm......

R = R20 * (1 + alpha20 * (t - 20))

Where R20 is the temperature at 20C (e.g. from tables) and alhpa20 is the coefficient at 20C which is around 0.00393 (depending on copper purity and annealing, etc, but not far off this).

So if looking at 250C and the 20C tabulated values for UK cables, then the adjustment factor is:

1 + 0.00393 * (250 -20) = 1.9039

 

[Cookie]

Ummm......

I'm still getting the hang of this.

 

[Cookie]

The basic assumption is copper is fairly linear w.r.t. temperature up to around 1000C (so anything that might be usable for a cable) so you have:

R = R20 * (1 + alpha20 * (t - 20))

Where R20 is the temperature at 20C (e.g. from tables) and alhpa20 is the coefficient at 20C which is around 0.00393 (depending on copper purity and annealing, etc, but not far off this).

So if looking at 250C and the 20C tabulated values for UK cables, then the adjustment factor is:

1 + 0.00393 * (250 -20) = 1.9039

So basically almost double the cold resistance.

If curious here is why I ask:

Parallel Arc Fault Resistance Found to be 0.03 Ohms | on ElectriciansForums

Find Parallel Arc Fault Resistance Found to be 0.03 Ohms Advice and Help. How-to Parallel Arc Fault Resistance Found to be 0.03 Ohms in the UK Electrical Forum advice boards on ElectriciansForums.net Est.2006 | Free Electrical Advice Forum. Parallel Arc Fault Resistance Found to be 0.03 Ohms on...

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UL and the NFPA are using 25*C for their loop impedance calcs and a Ze of 0.25 ohms.

It is the belief of many electricians that the fault current is universally much higher than 500 amps.

If this is the case, that arc fault coverage reaches much further into the circuit. If we can prove that to be the case, much of 210.12 is nullified.

There is more, but thats the general part of it.