UK Hotel extension question 5

[Thomasthebeast]

I have pasted the question in below, I have no Idea how to do this of someone could give me the pages and a step by step guide would be perfect, it's for my college hotel extension.
5. Determine, for one of the circuits supplying the fridges, the minimum possible cross-sectional area
of the CPC which will satisfy the requirements disconnection as ADS under earth fault conditions and the adiabatic equation as
Regulation 543.1.3 in BS 7671.

 

[pc1966]

What protective device is used and do you have the I2t let-through for that?

 

[Thomasthebeast]

What protective device is used and do you have the I2t let-through for that?

30mA rcd and I'm unaware what u mean by l2t

 

[pc1966]

An RCD is for "additional protection" and is not the over-current protective device. That is normally a MCB (or combined with the RCD as an RCBO).

The adiabatic equation is used when you have a short event that pumps energy in to a system, say clearing a fault, and for that you need the material constant 'k' which combines:

• Conductor resistivity (for your I2R heating effect)
• Specific heat capacity of the material (so you know how hot it gets for a given heat input)
• Maximum temperature rise (so you don't damage the insulation)

The BS regs or OSG has tables of them for typical conductor materials (Cu, Al, Fe) and classes of insulator (70C max or 90C max, etc) so you just use the k for your cable.

The I2t is the let-through "energy" that gives you the heat input for a given R. I2R is power (power = rate of delivering energy) and multiplied by time 't' it gives you total energy.

If you look up the OSG Table B7 they have worst-case I2t values you can use.

Chapter 8 of the IET's "Electrical Installation Design guide" has this covered quite well, and also has tables such as 8.5a showing values for specific manufacturer's MCBs.

 

[Thomasthebeast]

An RCD is for "additional protection" and is not the over-current protective device. That is normally a MCB (or combined with the RCD as an RCBO).

The adiabatic equation is used when you have a short event that pumps energy in to a system, say clearing a fault, and for that you need the material constant 'k' which combines:

• Conductor resistivity (for your I2R heating effect)
• Specific heat capacity of the material (so you know how hot it gets for a given heat input)
• Maximum temperature rise (so you don't damage the insulation)

The BS regs or OSG has tables of them for typical conductor materials (Cu, Al, Fe) and classes of insulator (70C max or 90C max, etc) so you just use the k for your cable.

The I2t is the let-through "energy" that gives you the heat input for a given R. I2R is power (power = rate of delivering energy) and multiplied by time 't' it gives you total energy.

If you look up the OSG Table B7 they have worst-case I2t values you can use.

Chapter 8 of the IET's "Electrical Installation Design guide" has this covered quite well, and also has tables such as 8.5a showing values for specific manufacturer's MCBs.

Okay thank you very much much appreciate the help

 

[pc1966]

I don't think I really explained what the I2t actually is very well.

Take a made-up example where you have a house on TN-C-S and the assumed 0.35 ohm Ze and your circuit has a run of 10m of 1.5mm T&E cable. What happens if there is a phase-to-CPC fault?

Cable R1 is approximately 10m * 12.1 mOhm/m = 0.121 ohm
Cable R2 = 10 * 18.1 = 0.181 ohm
Zs = Ze + R1 + R2 = 0.652 ohms
Fault current = U/Zs = 230 / 0.652 = 353A

Considering just the CPC, it is now going to dissipate I2R = (353*353) * 0.181 = 22.55kW for the whole cable, and so each meter of cable from the CPC alone is dissipating around 2.25kW. The same as a medium electric fire.

Not good! So what stops the house catching fire?

The answer is the Automatic Disconnection of Supply: most fundamentally the fuse or MCB that acts as over current protection, but if the fault is to Earth then any RCD should also trip.

And this leads on to the next common misconception - fuses, MCB, and RCD do not limit the current! At least in any significant level here (see below). What they do is limit the time during which fault currents are allowed to flow.

So going back to a typical electric fire, when you switch it on you normally can see a short delay until the element glows red and it if fully on due to the time it takes to heat up the metal element (and any ceramic former, etc). This is how you stop a fire and protect the cable from damage - you disconnect the supply quickly enough so the cable does not exceed a safe temperature for its insulation.

This is the 't' in your I2t value - the time at a given fault current. For fuses it is simpler to find from the current-time graphs, while for MCB you often find manufacturers do not give you the information for 't' once in the magnetic trip region, but sometimes they will give you a plot of I2t or a max value at some PFC (for example the 6kA limit for most domestic MCBs).

Going back to our example, if we had a 16A BS88-2 fuse from the plot below we see the fusing time is around 4ms:


So in this case our I2t is (353 * 353) * 0.004 = 498

Also in this case you might realise the fusing time is below a half-cycle of the 50Hz mains supply! Here the fuse is actually limiting the peak fault current.

The same data sheet (just search for F00192EN-08) also has curves for peak fault current which shows the 16A 10x38 fuse starts to limit at a PFC of around 150A RMS, and has a chart showing the fuse starts to melt at an I2t of around 200 and limits the let-through to no more than 1000 even for the huge rated PFC of 100kA.